Unit 4 -Community Interactions

In Unit 3 we examined how populations change over time. Sometimes the presence of another species, or sometimes the abiotic environment itself affects a species. But the focus of the unit was on the individual, and on populations of a given species. In this unit we are going to switch our level of investigation to the community level. Instead of examining a single species, we are going to examine how species interact to create a complex network of interdependent relationships.

A good example would be a comparison with human communities. You live within a community where you do business: your bank, the grocery store, your school, and the restaurant where you work. At each place you have a specific type of interaction that allows you to live within this community. A change in one relationship, such as an increase in tuition fees, may affect the amount of time you now spend at the restaurant.

Organisms have different types of relationships with other organisms that share their habitat. Some of these relationships are beneficial, while others are not. Humans become keenly aware of these complex interactions when we alter ecosystems. Adding or removing key species can set off a chain of reactions that can permanently damage an ecosystem.

References to sources are indicated by numbers in parentheses [example: (3)] in the text. The list of sources is at the end of the unit before the study questions.

Read "What is a community " on pp. 11.59

Unit 4 outline:

4.1 Introduction

l. definition of a community
ll. species richness
lll. species diversity

4.2 Interspecific interactions

l. coevolution
ll. predation

a. carnivory

b. herbivory

c. parasitism

lll. competition

lV. mutualism

V. commensalism

4.3 How interactions affect community structure

l. consequences of community interactions
ll. key stone species

4.4 Disturbance and non-equilibrium

l. definition of disturbance
ll. ecological succession
lll. ecological stability
lV diversity and stability
V. non equilibrium model
VI. intermediate disturbance hypothesis

4.5 Island Biogeography

l. definition
ll. equilibrium and island biogeography

4.6 Conservation and Biodiversity

l. introduced species
ll. fragmentation and metapopulations

4.7 Human impact of ecosystems and communities 

4.1 - Introduction

l. definition of a community

A community can be defined as a group of organisms that interact. This is different from a population, which is a group of individuals of the same species. Communities may exist on a small scale, such as an suburban garden with, or a large scale such as the Brazilian rain forest.

ll. species richness

There are different ways to examine variation within a community. One is to examine the total number of species that a community contains. This measure is known as species richness. Species richness has been investigated for many years and a number of conclusions can be made. One is that tropical communities are richer in species than temperate communities. For example, the number of ant species that exist in different regions of the world appear to vary with latitude:



 Species Number

 Degrees Latitude



  60 N



  45 N



  20 N



  10 N



  15 S

Topographic features such as peninsulas, however, can diminish the richness in a local area by reducing the chances of migration.

There are many hypotheses that attempt to explain this variation in species richness, but why should we care? Because if we can understand the factors that promote and protect diversity, then we can use our conservation efforts more efficiently to preserve global biodiversity. One million dollars spent to protect a rainforest with 50,000 beetle species may be a more prudent investment than protecting a taiga environment with only three types of beetles. In general, explanations for species richness fall into one of two categories: ones that are biotic (organismal) based and those which are abiotic (nonorganismal) based.

Biotic hypotheses state that it is the interaction among the species themselves and the complexity of the environment that promotes diversity in the tropics. For example one hypothesis, called the animal- pollinator model, argues that the winds are less frequent and intense in the tropics. Dense vegetation helps diminish the winds as well. Thus most plants in the tropics are animal pollinated (bats, birds and insects). Coevolution of the plants and animals in this region has resulted over time in an increase in pollination efficiency, thus leading to a high degree of organismal specialization. As more and more specialists evolve, more habitats exist, and thus more species can co-exist (Waser, 1996)(1). While this may hold true for differences among terrestrial environments it does not explain any differences that may exist among aquatic ecosystems. Thus the animal-pollinator model can explain not all variation in species richness.

Alternative explanations are based on abiotic factors. These explanations suggest that differences in environmental characteristics (instead of organisms) drive the system and result in parallel changes in species richness. For example the abiotic productivity model suggests that the greater the productivity of a region, the greater the amount of plant and animal material that can be produced and thus the greater the number of niches and species which can be supported.

There are over 28 species richness models in all. Each one has its supporters, its critics and has produced conflicting evidence. It is still unclear whether one model- biotic or abiotic, can explain all the patterns seen in the living world. Instead different combinations of models may explain diversity in each biomes. Whatever the ultimate factors are that promote species richness, conservation which preserves areas with the greatest number of individual species will also protect biodiversity.

lll. species diversity

A problem exists when only using the number of species to describe an ecosystem- it does not take species abundance into account. For example, it may be important to know not just that an ecosystem contains tigers but also how many tigers exist. This may help us determine the health of an ecosystem. Species diversity is a better measure than species richness because it incorporates both the number, and abundance of a given species. Abundance is the number of individuals per a given species.

A number of indices have been created to measure species diversity. Recall that an index is a relative measure of comparison. Say you wish to compare the sweetness of three oranges. You create an index of sweetness that ranges between (bitter) 0 and 1 (sweet), and rank each orange accordingly. A ranking of .9 means nothing unless you realize that the index only goes to 1, and thus your orange is very sweet. Indices can also be created to measure how many different animals exist within a community. Two examples will suffice. One group of indices, called dominance indices, is primarily influenced by the number and abundance of common species and less by the number and abundance of rare species. This bias may have drastic consequences when these indices are used by governmental agencies. For example, park managers who use this method may conclude that an ecosystem is maintaining its diversity when its rare species are actually dying out. Thus we need to ask the question: should all species be counted equally, or should rare species (such as Bengal tigers) be given more "weight" than common species such as house sparrows? Ordinal indices are ones that rank the importance of a species as well as its abundance in the environment. This would make bumble bees, which are widely distributed on a global basis, less important than mountain gorillas, which have a very small geographic distribution.

There is no perfect index of species diversity- each has its own built in assumptions. Choosing the most appropriate measure may depend upon the types of questions being addressed. For example, conservation biologists would be prudent to use ordinal indices, which favor rare species, when trying to preserve endangered species habitat.

Read Concept 53.1- 53.2" Community interactions" starting on pp. 1159-1170



4. 2 - Interspecific interactions

No matter what the actual number of species, species will end up interacting with one another. Everything from two species sharing a burrow to a lion eating a mouse would be considered an interaction. We can divide the types of interactions that exist among species into those that have a positive effect, those that have a negative effect, and those that have a neutral effect upon the participants.

Type of Interaction 

 Effect on Species 1

 Effect on Species 2













In general, when the effects of an interactions are particularly positive (leading to an increased reproductive success among individuals) or negative, (leading to a decreased reproductive success among individuals), this selective pressure favors traits that lead to coevolution.

l. coevolution

Coevolution can be defined as a genetic change in two species as a result of their interactions with one another. When individuals benefit or suffer from interacting with another species, those individuals who reap the most benefit (such as the harvesting of energy) or suffer the least amount of harm (by escaping predation) are more likely to survive and reproduce, adding their genes to the next generation of individuals. Over time the frequency of a trait may increase or decrease in a population as a direct result of the interspecific interaction.

Coevolution is well documented in the literature. One of the best known systems is that of figs (genus Ficus) and fig wasps. Over 900 species of Ficus exist, and each one is pollinated by its own species of fig wasp. In return for pollination, the female wasps have a place to lay their eggs. The interaction between the two species is so close that male wasps never even leave the fig. Once they are hatched, they locate the females within the plant and inseminate them. The males then die. When Smyrna figs were introduced from Turkey into California in the late 1800's the crops repeatedly failed until the obligate pollinating wasp species was introduced as well.

Occasionally coevolved species attempt to "cheat" on one another. Orchids are often pollinated by specific species of bees. Some orchids even mimic female bees: males pick up and transfer pollen while attempting to copulate with the flowers. In one species of orchid (Ophrys) males prefer the flowers to real female bees. While this is highly advantageous to the orchids (since males spend a lot of time visiting them and transferring pollen) it is highly disadvantageous to the bees that fail to reproduce! In contrast, some species of male Bombus bees "cheat" by biting through the flower petals to get the nectar and avoid entering the flower and picking up the pollen.

ll. predator-prey

The first type of interaction we will examine is that in which one species greatly benefits at the expense of a second species. These are generally called predator-prey relationships and can be broken down into four basic types:

1) Carnivory: Animals feeding on herbivores or other carnivores.
2) Cannibalism: Where predator and prey are of the same species.
3) Herbivory: Animals feeding on plants.
4) Parasitism: Animals or plants feeding on other organisms without (typically) killing them.

There are a variety of strategies that organisms that have evolved in organisms that reduce their risk of being eaten. This suggests that predation is a strong selective force. Listed below are some of the most common traits found in prey species that reduce the risk of predation:

1) Aposematic coloration: coloration that advertises a distasteful or toxic prey. Ex. Monarch butterflies accumulate poison from milkweed plants and are consequently distasteful to birds and other predators.

2) Mimicry: animals that look, sound, or behave like another animal. Mullarian mimicry occurs when one distasteful species looks like another. This reinforces the basic noxious "appearance" of both species. An example where some butterflies and wasps have coevolved to look like one another. Batesian mimicry occurs when a tasty or harmless species evolves to mimic a noxious species. An example would be venomous coral snakes and the innocuous milk snakes, which mimic the coral snake color patterns. Note that this type of mimicry only works when the "tasty" species is less common than the noxious species. Under these conditions, predators who select a prey are more likely to bite into a noxious species and be deterred from future nibbling.

3) Crypsis: is the development of a "frozen posture" so that the prey species is camouflaged against its background. Often the appendages are retracted to avoid detection. Ex. Chameleons whose skin tone can be adjusted to match the background.

4) Intimidation displays: behavior on part of the prey species to decrease the likelihood of a predator attacking. Ex. Toads swallow air to appear larger.

5) Polymorphisms: when genes arise in a population such that there is more that one distinct physical type (morph) within a prey species. Ex. Green and red morphs of pea aphids can be maintained by the action of predators. Predators who selectively prey on green bugs are less likely to go after the red individuals. Thus the frequency of green individuals will go down, while the frequency of red individuals goes up. Note that at some point the predator (who is having a hard time finding green individuals) will begin to prey on red individuals and the green morphs will begin to increase in frequency, and so on….

6) Chemical defenses: chemicals that can be excreted to ward off potential predators. Ex. Toads have salivary glands called parotid glands located in their head region. These glands secrete noxious substances onto their body surface when the toads are disturbed by predators. Sometimes these chemicals can make the predator physically sick or mentally impaired. Once the predator learns to associate the prey the noxious taste, it is often a lifetime association. Just think back to a food you really hate and think why! Often it is because you got sick on that item and have developed a strong aversion for that food item.

7) Masting: the synchronized production of all prey young within a short time period such that a predator is satiated and cannot eat all of the young. The result is that some young always manage to survive to reproduce. Ex. 13 - year and 17- year cicadas emerge all at once. There are too many insects to eat at once. As a result, some cicadas to escape predation and survive to reproduce. Many species of predators have coevolved in response to this feeding opportunity and may time the production of their own young to capitalize on this bonanza. In birds of prey, such as great-horned owls, eggs hatch in January. The young raptors are now ready to feed on the young of prey species that hatch in the spring.

How common are these defense mechanisms? Witz (1989)(2) surveyed 354 papers, focusing primarily on insects. He found that the most common antipredatory defense mechanism used by prey was chemical defense (46%). Studies of natural systems indicate that the effects of predation are significant and have greatly influenced the evolution of prey species. Thus coevolution is likely to be a strong force in predator- prey relationships.

ll. herbivory

Herbivory is the act of an animal, whether invertebrate or vertebrate, consuming a plant. The variety of plant defenses which exist suggests herbivory, like predation, is a strong selective force that shapes plant evolution over time. Plant defenses can be divided into two basic categories: chemical and structural.

Chemical defenses include nicotine and caffeine, tannins and resins, toxic compounds such as atropine, (found in deadly nightshade plants), and chemicals that mimic insect hormones to disrupt insect molts. This means that insects can not properly develop and reproduce. Many modern insecticides such as Logic, which we use to kill imported fire ants, are based upon the defense systems found in nature. Logic affects ants by sterilizing the queen. Since she can no longer produce offspring, the colony eventually dies out. This is why Logic does not kill off the colony immediately, but takes several weeks to have an effect. Since chemicals are expensive for the plant to produce they are often allocated to the most valuable tissues (such as new and tender growth) or are produced only in response to herbivory. On average, a plant loses 7-10% of its vegetation to herbivores. This amount of loss may be manageable for older, larger, mature plants, but not to younger, smaller plants. Plagues of locusts and other swarming insects can overwhelm even the biggest plants however, and reduce them to stalks in a matter of minutes.

lll. parasitism

Both parasitism and herbivory are often considered special subdivisions of predation. This is because the "prey" species of a parasite of herbivore does not have to die as a consequence of the interaction. None the less, parasitism in particular has a powerful selective pressure on the evolution of species, including our own.

Up to 50% of all organisms that inhabit the earth are considered parasitic. This includes species that feed on plants as well as tapeworms and leeches. Recall that parasites typically feed on their hosts without killing them. If a mechanism has evolved in a parasite that allows it to shed its eggs or young to another host (to continue its life cycle), the need to keep the original host alive becomes irrelevant. Under such conditions the parasite can become deadly. Parasites, which are sub-lethal to healthy adult individuals, may also be lethal to the young, sick or very old individuals.

lV. competition

So far we have concentrated on interactions where one species benefits at the expense of another. Competition is a different type of interaction altogether. Here both species are interacting to obtain resources, and the interaction can be highly detrimental to both species. Competition interactions can be categorized as follows. Resource competition, is said to occur when organisms interact in order to gain access to resources or mates. This type of competition is most often seen in invertebrates. Interference competition occurs when organisms interact with each other physically. In vertebrates this interaction has often been ritualized into a sequence of escalating threat behaviors between organisms on adjacent territories. The winner of the competition typically takes the best of both territories while the loser either dies, leaves, or takes a suboptimal territory. Interference competition can also occur among plants. For example allelopathy occurs in plants where one plant produce chemical substances that keep other plants growing near them. These substances may be acids, and bases, or organic compounds that limit light and nutrients. Bracken ferns (Pteridium aquilinum) are a common vascular plant found around the world. It produces toxins that accumulate in the topsoil. These toxins then kill the germinating seedlings of other plants, especially conifers. Recall that conifers are larger than the ferns and if they established themselves they would block sunlight and diminish the success of the ferns.

The competitive ability of any given species may vary with environmental changes. Factors such as temperature, humidity and oxygen availability may affect how well one species can do against another. For example, Park and his colleagues found that the competitive ability of two species of flour beetles (Tribolium sp.) was significantly influenced by climate.(3)

 Temperature (C)



  Percent (%) Wins by T. confusum

 Percent (%) Wins by T. castaneum































Tribolium confusum, which did well in dry conditions against Tribolium castaneum, did more poorly in wet conditions. Moreover, these abilities seemed to be exacerbated by warmer temperatures.

Reviews on the frequency of competition in nature find that 55-75% of all species investigated exhibit some type of competition. Here are 6 types of competition and their definition:

Resource Competition

1) Consumptive or exploitative: where individuals compete for resources such as food and water.

2) Preemptive: where individuals compete for space.

3) Overgrowth: where one species is overgrowing or blocking the light for another species

Interference Competition

4) Chemical: competition which uses the production of toxins (allelopathy)

5) Territorial: behaviors such as fighting used to defend space

6) Encounter: temporary, infrequent interactions directly competing for a specific resource. An example may be a permanent water source which individuals fight over when prolonged drought dries up intermittent water sources.

It turns out that exploitative competition is the most common, occurring in 71/188, or 37.8% of the species studied (4). Preemptive and overgrowth competition is most often used by sessile (e.g. non-moving) organisms, while territorial and encounter competition is more likely to be used by active mobile organisms. Chemical competition is used by terrestrial plants and not by aquatic plants since chemicals become diluted and ineffective in water.

V. mutualism

Species interactions that result in a net benefit to both species are considered mutualistic. Classic mutualistic associations include plants and their pollinators, such as birds, bats and insects. Typically the animals gets a tasty nectar meal out of the interaction (while the plant gets a chance to pick up pollen from another plant and transport some of its own pollen to another member of its species). In one species of euglossine bees, males do not collect food from the flowers but collect fragrances, which they then turn into a sexual attractant for females of their species.

Several generalizations have been made about mutualistic interactions:

1) The need for mutualism decreases as resource availability increases. Thus mutualism may have first evolved, not a result of gracious altruism, but as a reciprocal parasitism where two species "get the best of each other".

2) Mutualism is more common in stressful environments.

3) In large populations, the benefits of mutualism per individual are reduced.

Obligate mutualism occurs when the relationship between two species has become so tightly linked that one species can not survive with out the other.

Vl. commensalism

Commensalism occurs when one species benefits from an interaction with another species, but the second species is neither harmed nor helped. The effect on the second species is neutral. The most common type of commensalism is a condition called phoresey, which is the passive transport of one organism by another. An example of phoresey would be sea anemones growing on the back of hermit-crab shells. The hermit crab is unaffected by the hitchhiker, while the sea anemones benefits by being exposed to new food resources. Note the distinction between different types of interactions can be blurred. Some hermit crabs use the sea anemone as camouflage. In this case the interaction would no longer be considered commensalism but have changes into a case of mutualism.

4.3 How interactions affect community structure

l. consequences of community interactions

It is important to recognize that mutualistic (and commensalistic) interactions can have consequences that extend beyond the two species to the community level. In other words these species do not live in a vacuum but operate with in a web of species interactions. Pull on the web in one location, and the effects of that tug can be felt in many places. An example of this can be seen in a salt marsh community. Marsh elders (Iva frutescens) and black grass (Juncus gerardi) exist mutualistically in salt marshes. The interaction between the elders and black grass leads to a decrease in soil salinity, and an increase in oxygen levels surrounding the plants. When one of the mutualistic species- black grass is removed from the salt marsh, the action affects other members of the community. Aphids that lay their eggs on the elders (who benefited from the black grass, are unable to find suitable egg habitat. Aphids now decline in number. Populations of Ladybird beetles that fed off of the aphids now decline as well. Thus a cascade of events are stimulated by the loss of mutualism.

ll. keystone species

As we have just seen, communities tend to have complex interactions and the addition or removal of a given species can have far reaching effects. But do all species have such an impressive impact on their neighbors? Studies suggest no. It turns out that the presence of some species within an ecosystem is much more important than the presence of others. A keystone species is one whose affect upon a given ecosystem far outweighs either its biomass or its geographic distribution. It is important to realize that a keystone species is not the same as a dominant species, one that is important due to its relative abundance. A lion may have greater effect on the savanna than gazelles, which are more common. Keystone species can be predators, prey, or species that in some way modify their habitat.

Certain starfish and sea otters have been described as keystone predators. Another example is the large mouth bass found in Northern U.S. lakes. The removal of bass from a lake in Michigan led to a significant increase in the abundance of planktivorous fish (e.g. those that eat tiny one celled organisms), the disappearance of all large zooplankton and the appearance of small plankton species. (5) Thus the entire distribution of organisms was disturbed by the removal of one species. When the large mouth bass were reintroduced to the lake, the lake returned to its original state. Another good example is found with the reintroduction of wolves into Yellowstone Park. The presence of the wolves caused changes in the populations of mountain lions coyotes, elk and raptors.

Read Concept 53.3"Disturbance influences species diversity…" on pages 1171-1173

4.4 - Disturbance and non-equilibrium

What happens to communities over time? Do they sit there, and remain at the status quo with the same species eating one another year after year? Or do they follow patterns of growth and decline similar to the rise and fall of human cultures and civilizations. Do changing ecological conditions, such as changes in sunlight availability and nutrient availability over time affect which species thrive and which do not? Moreover, what happens when density independent events such as floods, drought, fire, and storms interrupt an ecosystem? Do the communities pick up where they left off, with all the same species in place? And if they change, do they follow predictable patterns of change?

l. definition

There is a whole division of ecology devoted to the investigation of these questions. An ecological disturbance can be defined as any event, whether biotic or abiotic, which disrupts a community and its current structure. As with other things, disturbance can be small scale, such as the introduction of a exotic plant into a pond, or large scale such as the massive 1998 flooding in Central America following Hurricane Mitch. Ecological succession is then defined as the sequence of chance that emerges as a result of the disturbance.

ll. ecological succession

If you look at a forest that has been heavily burned, you will notice that a strange pattern of life begins to emerge. Soon after the fire, small grasses and abundant wildflowers may bloom and cover the ground. As the flowers fade away taller grasses and small shrubs may make an appearance along with the seedlings of small trees. Continue to watch this patch of land over the years and you may see a forest eventually return. This pattern of change is called ecological succession. Succession may be defined the pattern of recolonization by organisms. It may or may not occur after a disturbance. Disturbances can include events such as fire, storms, overgrazing, and erosion. Primary succession occurs when there is an invasion of plants into an area where no plants have gone before … (sorry, too much star trek). Thus there is no soil for the plants to grow on, and colonization is slow. It may take hundreds or even thousands of years to build up sufficient soil. An example of this would be the many lakes created in North America when glaciers receded at the end of the Ice Age. Primary succession accounts for only a small fraction of succession on the earth today- such as that following volcano eruptions and the spreading of the sea floor. Over geological time however, primary succession accounts for the progression of organisms on continents, islands and the ocean floor. Secondary succession occurs when a disturbance destroys the organisms within an area but leaves the soil intact. In a way, secondary succession is really a temporal "blip" in a longer-term primary succession pattern.

Early views of succession believed that the organisms were replaced through a process known as facilitation. This view argues that the presence of the first organisms somehow prepares the environment and makes it easier for succeeding organisms to inhabit the land. An example would be an invading plant that fixes nitrogen in a nitrogen poor soil. More nutrients are now available so that a small bush may be able to thrive. The bush now provides shade and protection from herbivory for young tree saplings, and so on. The most extreme form of facilitation is called enablement, when the survival of a particular species depends upon the colonization of an earlier species. Under such conditions communities tend to follow very specific patterns of succession since only certain plants can arise at any given time.

Another type of succession has been found called inhibition. Here the presence of a first species actually prevents the development of certain subsequent organisms. Thus whoever colonizes an area first determines how a community will develop, and what species will be present. A good example of this is found in European sand dunes. In general, sand dunes begin to form around clumps of grass and then spread as different species of grass invade the area and hold the sands in place. In Europe, sand dunes begin to form around clumps of marram grass, followed, in sequence, by fescue (another grass), sand sedge, and sea couch. Van der Putten and co-workers transplanted plants of each species into pots containing soil from either it predecessor or successor. They then observed whether a grass would thrive or fail in the soil from another species. They observed the following results.


 in Marram grass soil

in Fescue soil

  in Sand sedge soil

  in Sea couch soil

 Marren grass










 Sand sedge





 Sea Couch





Van der Putten argued that plants were harmed by the soil-borne diseases of their successors, but not by the diseases of their predecessors. Thus once a plant established itself in a community, it rapidly out-competed its predecessor and thrived until its successor comes along. Marrem grass can only thrive in new habitats that have not been compromised by the pathogens of the other grasses.

Another example of inhibition can be found with sunflowers- a mid-successional plant. Decaying sunflower leaves produce compounds that inhibited the growth of early successional plants seedlings such as Amaranthus. In contrast, sunflower leaves did not inhibit the growth of three-awn grass, a species that typically replaces sunflowers.

A third type of succession has been discovered. It is called tolerance succession and appears to be intermediate between facilitation and inhibition succession. Here any species can start the colonization process. Thus a grass seed and a beauty-berry bush seed could both invade a new area and establish them selves. But once the new plants have colonized the area, the process of succession proceeds in a somewhat reliable fashion until a climax community is reached. A climax community is comprised of species who are not easily replaced tend to dominate an older mature ecosystem. For example, Gray Birch trees tend to dominate younger forests while Oak and Beech trees tend to dominate older forests in the Eastern United States. An example of this is presented in the following table which describes the predicted percentage of tree species in a forest over time, and the actual data from a 200-year-old forest in New Jersey.

 Predicted Percentage of Different Species of Trees after 200 years

 Tree Species

  0 years

 50 years

 100 years

  150 years

 200 years

  Actual Data

 Grey Birch














 Red Maple














Note that ecosystems can show a mix of succession processes, and herbivory, disease, and human interference can alter patterns of succession. Thus stochasticity can have significant effects of the species composition of an ecosystem.

llI. ecological stability

For a long time, researchers believed that communities as a whole were stable and that disturbance only came from outside forces such as hurricanes. That is unless there was a disturbance, the community would remain the same indefinitely. Long term data records, such as the historic annual bird counts, seemed to support this idea. But how can we test the idea of inherent stability? Two different methods have been suggested, depending upon what it is we wish to measure.

To measure how well a community resists change we could

1) apply a force or pressure (such as over watering a field, or introducing rabbits)

2) see if the community changes

3) repeat the experiment in different communities (or in our case different fields)

To measure how well a community bounces back after a disturbance (what could be called community resilience) we could

1) determine a stable point where the population levels of different species appear unchanging

2) apply a force or pressure

3) measure the time it takes for the community to return to its original stable point

4) repeat the experiment in different communities

Unfortunately there is no easy answer, or set of conclusions that arise from these experiments. There are however, some patterns that emerge based upon the type of biome involved in the disturbance. For example, lakes tend to be weakly resistant and weekly resilient since there is no easy way to wash pollutants away. Rivers may not be resistant (since they receive so much run off from the land), but are more resilient than lakes since the moving waters can carry pollutants away faster.

IV. diversity and stability

The traditional view, called the equilibrium hypothesis, argues that most communities are stable, and the interspecific forces such as parasitism, predation and competition help maintain predictable number of species and individuals. One factor that would help maintain this equilibrium would be the diversity of the community. For a long time people believed that communities with more diversity should be more stable. This was based upon the notion that with more diversity there would be more resources at hand. Thus the community could remain stable because of all the possible interspecific interactions and niches available. It is similar to the idea that the more self sufficient you are as an individual - for example the greater number of tools and resources you have, the more resilient and thus stable you are.

Unfortunately the inherent belief that diversity corresponds to stability is not supported by experimental evidence. There is anecdotal evidence both for and against links of stability and diversity but experimental manipulations do not show a strong correlation.

V. non-equilibrium model

What is the evidence for equilibrium with or without diversity? A British woodland bird community was studied for 22 years from 1971 until 1992. They found that the amount of variability in the community increased the longer the community was observed. This may be because over the span of 22 years, overall environmental variability happened to increase. A more modern view, called the non-equilibrium hypothesis, argues that disturbance is a frequent and naturally occurring phenomenon. As a result, species composition is ever changing. There is no one stable point that a community reaches and maintains since a constant state of change is the normal pattern.

VI. intermediate disturbance hypothesis

A modern, alternative approach to both of these ideas is offered by the intermediate disturbance hypothesis. This suggests that the most diverse communities, such as coral reefs and tropical rain forests, are kept diverse because they have multiple disturbances. Instead of destroying a community, frequent modest disturbances actually diversify a habitat and allow a maximum number of species to thrive and reproduce. To study this idea, Sousa carried out an elegant experiment in a marine intertidal zone. He found that small rocks easily moved by waves had a mean density of 1.7 sessile plants and animals. Intermediate-sized boulders, which were occasionally moved, have a mean species number of 3.7 species and large, immovable boulders had a mean of 2.5 species. Were there fewer species on the smaller rocks because they were disturbed more often or simply because they were small and thus had less surface area? To test this idea, Sousa cemented the small rocks to the ocean floor. Species density increased on the small rocks, indicating that it was the level of disturbance, and not the rock size that kept species numbers down (6).

Other studies like the one described above have supported the intermediate disturbance hypothesis. Their only drawback is one of scale: most intermediate disturbance experiments are carried out in small patches of forests or intertidal zones. It is unclear whether these patterns will exist in large ecosystems.

Read Concept 53.4 "Biogeographic factors affecting...." on pp. 1175-1177

4.5 Island Biogeography

l. definition

Biogeography is the study of the distribution of species and entire communities over time. For example, some of the most complete studies of ecological succession have occurred when volcanoes completely denude offshore islands. One of the most famous examples was the island of Krakatau in 1883. All life on the island was destroyed by the eruption. Scientist were then able to observe each species that recolonized the island, and the effect the new species had on the island's community. In less dramatic fashion, scientists can observe the present distribution of species and show how their present distribution reflects distant evolutionary history. The fact that Australia broke off from Pangaea before the evolution of mammals explains why there are no true mammals on that continent.

Because islands are small and often isolated, they serve as good models for the study of biogeography. The term "island" not only includes places like Krakatau, but any habitat where the species of the "island" are surrounded by unsuitable habitat. Mountains, rivers, deserts and canyons can all create islands which, for all intense purposes, species cannot cross. Thus these mini ecosystems allow us to observe community interactions in a simpler environment. The hope is that the generalizations that arise from these "simpler" systems can then tell us something about more complex environments.

ll. equilibrium and island biogeography

In the 1960's MacArthur and Wilson tried to develop some generalizations about the factors that determine species diversity on islands, Their hypotheses have undergone several modifications in the last 35 years as researchers have expanded on the original model. Here are some of the basic generalizations:

1) The number of species on an island will tend towards equilibrium. This is a result of a balance between the rate of immigration, and the rate of extinction on the island (see figure 53.21 on page 1127 in Campbell).

2) The equilibrium number of species on any given island will be determined by the island's size, and distance from a potential pool of colonists. These factors determine the rates of migration to and from the island.

3) While a particular species may come and go from an island due to emigration or extinction, the total number of species on the island should remain constant (hence the idea of an equilibrium).

4) Distance from the mainland to an island affects not only the rates of colonization, but the rates of extinction. This is because immigration of new individuals can slow down the rate of extinction by replacing the individuals who die.

5) Rates of immigration and extinction will be affected by the number of species already present on the island. The greater the number of species on the island, the less likely that a new immigrant will represent a new species. Extinction rates will increase as species number increases since of the competitive exclusion principle indicates that no two species can occupy the same niche.

There are strong data to support the idea that species richness increases with island size. For example, there are more land plants on the larger Galapagos Islands and more amphibian, reptiles and beetle on the larger islands in the West Indies. There is little consistent data, however, to support the other predictions of island biogeography, and there ideas remain controversial.

Read Concept 55.1"The Biodiversity Crisis" on pp. 1209-1212

4.6 Conservation and Biodiversity

We just learned that biogeography is the study of the distribution of species and communities over time. We approached this topic from a theoretical point of view with the island hypotheses of MacArthur and Wilson. Biogeography also gives us vital information about the overall diversity of life on the planet. It is in the field of conservation biology that biogeography can make its most important contributions. Recall from unit 1 that conservation biology is the area of science that focuses the management of biodiversity. And as biogeographers have discovered, we are in a global crisis.

Crisis is an overused term these days. It seems like every time you turn on the news you hear about a crisis in Russia, or Serbia, or someplace new. We have become habituated to these extreme words. So what do we mean by saying there is a biodiversity crisis and what does it matter?

Unit 1 states that there are over 1.5 million described species and up to 80 million species in total. The greatest numbers of species have been located in the tropics, and in coral reefs. It is now estimated that humans have artificially altered over 50% of the land surface on the planet and we use over 50% of the accessible fresh water. That is a lot of activity for one species. Not only do we take over land to build houses, malls, and everything in between, but we alter geological and chemical cycles as well (note these topics will be discussed at length in Unit 5).

So what? The reason that human activity is important is that we are accelerating the loss of biodiversity through the accelerated extinction rate of organisms. In other words our activities are resulting in the extinction of entire species. Extinction is a natural process and all species eventually go extinct or evolve into something new. Though out evolutionary history species have come and gone. But up to now the overall rates of extinction have appeared fairly constant. Now it is estimated that the rate of species extinction may be 50 times higher than in any time in the last 100,000 years. In the tropics it is estimated that the rate of extinction is 1000 to 10,000 higher that the normal "background" rate. This change appears incredible at first, and one may be suspicious of such high estimates. But several independent sources are supporting these estimates:

1) 11% of the 9040 known bird species are endangered

2) 680 out of 20,000 plant species may be extinct by 2000

3) 20% of known freshwater fish are endangered or already extinct

What are the causes of current species extinction and how much is linked to human activity? There are three main threats to species:

1) competition by introduced species

2) habitat destruction

3) overexploitation

l. introduced species

The following data have been collected on 484 extinct species and the causes for their extinction:

 Causes of 484 Cases of Animal Extinction by Activity

 Cause of Extinction


 Cause unclear


 Introduced animals


 Habitat destruction




 Other causes


It turns out that the introduction of other animals, and direct habitat destruction, such as deforestation are the primary known factors. The fact that 56% of the cases were unclear does not mean that human activity was unimportant in these cases. Indirect human activities such as pollution can be important, but they are difficult to measure directly.

Introduced or "exotic" species can mean many things. They may be the bushes growing in front of your apartment building or the majority of flowers growing in your neighbor's yard. Indeed most of plants and even grasses we have growing in our yards and cities are completely exotic. The fire ants, which plague us each year, are an introduced species. The effects from introduced species can take many forms:

1) Predation: Think about your cat that gobbles up all the birds in the yard.

2) Disease and parasitism: American chestnut trees, and European and American elm trees have all suffered tremendous losses through disease. The State of California bans the import of many fruits because they are worried they will carry the "medfly" a small insect that can wipe out the citrus industry.

3) Competition: Introduced cats, rats, and mongooses have accounted for 43.4% of bird extinctions on islands.

ll. fragmentation and metapopulations

Habitat destruction can occur in many ways: agriculture, mining, forestry, urban development and environmental pollution. Even if the entire forest or coral reef has not been destroyed, the habitat may be so fragmented by human activity that the remaining pieces of habitat are unsuitable to sustain life (see figure 55.7 on page 1161 in Campbell). Since organisms may be unable to move between these fragments, and unable to survive on the edges of these fragments, the chunks of environment resemble islands. The rules of island biogeography that were discussed above now apply to these fragments or islands of suitable habitat.

We can call these populations of a single species which are separated metapopulations. The quality of the habitat "island" that each metapopulation lives on may vary significantly. A patch with high quality resources may be able to sustain a metapopulation and produce more offspring that a poor quality patch. A poor quality patch is more likely to go extinct, and will only be repopulated if immigrants from a high quality patch find the island. Furthermore, the more isolated a patch is, the more likely that the individuals in the patch will be cut off from the larger genetic pool and become genetically extinct. Cricket frog (Acris crepitans) vocalizations, and female mating preferences were studied in ponds in the Central Texas area. It was found that males in different ponds (in this case a pond is equivalent to an "island) evolved different frequency mating calls. Thus 2.2 cm long male from a pond in Bastrop. TX had lower frequency mating call than a 2.2cm male from a pond in Wimberley. Furthermore, females from each pond had different abilities to hear the male's call. Females from the Bastrop pond were more attuned to hear low frequency mating calls and females from the Wimberley pond were more attuned to high frequency mating calls. This suggests that over time, isolated metapopulations can evolve away from each other until they reach the point where they no longer recognize each other as potential mates.

As mentioned, habitat quality differences can lead to reproduction rate differences between different patches. A source habitat is one in which reproduction rates exceed mortality rates. This means that this metapopulation can be used a source of migrants to new habitats. A sink habitat is the opposite. Here the mortality rate exceeds the reproduction rate and the population is in decline. It is vital to recognize which habitats are source habitat and which are sink habitats if conservation efforts are going to succeed. There is little use in reintroducing individuals into sink habitats since the animals will die before the population can grow. It is estimated that as little as 10% of metapopulations may be source habitats. Life tables (see Unit 3) are necessary to distinguish the growing from declining populations. Work on the Peregrine Falcon and the northern spotted owl are good examples of the interactions between source and sink habitats.

4.7 - Human Impact on Ecosystems and Communities

Every time a new shopping mall is built, a new road is cut, or a park is created, we drastically alter the natural environment. While building a mall certainly destroys almost all life, roads and parks are also destructive. Even lands that were once used for agriculture and then abandoned are no longer natural ecosystems. Restoration ecology is a branch of ecology that attempts to return disturbed ecosystems to their former natural state. Two examples of restoration ecology will demonstrate how difficult yet important a process it is.

First, let's turn to the problem of humans as competitors. We can out-compete almost every other organism for food, water, and other necessary resources. We do this every time we take ranch land and develop it into a subdivision. Animals who live on this land may migrate to adjoining lands, or die if there is no place to go. This is the case with the top trophic-level competitors here in the United States. We have driven animals such as bears, wolves, bald eagles, condors, and mountain lions to the brink of extinction. Not only do we take away their lands, but we also kill them since we believe they compete for our cattle and sheep. In the process of removing these top predators, however, we drastically alter the ecosystem. An example of this was seen in the case of removing the large mouth bass from Northern U.S. lakes. By removing top carnivores, such as wolves and bears we cause the number of small predators to increase dramatically. For example the numbers of opossums, coyotes, and raccoons in Texas have risen sharply. These mid-sized predators no longer compete with top predators for food resources such as snakes, mice and squirrels. In addition the number of mid-sized predators has increased because no one, in turn, is eating them. The numbers of herbivores, such as deer, also dramatically increases because their population size is no longer controlled by predation (while coyotes may be able to prey on small deer, raccoons and opossums can not). Instead deer under go huge population blooms and crashes as their food resources fluctuates. This winter in Central Texas is a case in point. Many white tail deer are currently dying in the Hill Country due to the extended drought and human activity. A relatively wet winter last year coupled with low predation caused an increase in deer numbers. The drought, and the recent building explosion in Hays county, leads to minimal grass in the Hill Country. As a result deer are either starving, or coming up onto the road shoulders at night to eat grass that grows in ditches and roadside culverts. An average of two or three newly killed deer have been found each week on a ten-mile stretch of highway between Kyle and Driftwood Texas this fall.

Restoration ecologists try to reintroduce top predators to rebalance ecosystems. Wolves have been reintroduced to places like Yellowstone National Park, and Arizona, while California Condors have been reintroduced to Southern California. The results have been mixed at best. Many ranchers do not want the wolves in the area where they may threaten their livestock. They not only shoot wolves that have wondered off the protected lands but illegally shoot wolves within the parks and protected areas as well. Condors that have been raised in captivity and then reintroduced into the wild are imprinted on humans. This means that their natural fear of humans has been decreased and instead they are conditioned to expect food from humans. Recently several condors have broken into houses in the Southern California in search of food. This may seem innocent until you realize that a full-grown condor stands over 4 feet tall and they use their feet and beaks to rip open carrion and screen doors alike. Homeowners have come in to find condors tearing through the house.

One promising solution is to condition predators (like wolves) before their release to avoid livestock. John Garcia found that rats who have ingested food, and then have gotten physically sick after ingesting the food, will avoid eating the same food again in the future. Indeed rats will forgo eating then eat that same type of food again. This behavior, called taste aversion conditioning, may be familiar to you. If you have ever gotten sick on a food (for me it is hot oatmeal) you know that you would rather starve than to eat the food. Even the thought of the food is distressing! Researchers have been able to take sheep and cattle carcasses, lace them with a poison and let wolves feed on the remains. Not only will these individual wolves avoid sheep and cattle, but pups that have been feed by their mothers (through regurgitation) develop the aversion as well. If predators such as wolves can be trained to avoid livestock, perhaps ranchers can be persuaded to tolerate these important creatures.

Restoration ecologists also attempt to restore complete habitats. For example after strip mining a track of land is ruined, since most of its animals and its topsoil have been removed. By reintroducing topsoil and planting native seeds, the land can potentially be coached back into its natural state. A working knowledge of succession is vital in order to bring back a portion of the original grasses, shrubs, trees and animals. Some ecologists are worried that this ability to restore land will be abused. Fragile ecosystems such as taiga and tundra in Alaska could be destroyed as lands are opened for pipelines and oil exploration. Habitat such as the tundra can not be restored in a matter of years. Instead it may take hundreds or even thousands of years. Large-scale restoration of an ecosystem is immensely expensive, and it is unclear who would pay for the rehabilitation. Even if the monies are available, restoration ecology is still in infancy as a science and the techniques are modest at best. After the wreck of the oil tanker the Torrey Canyon in 1967, some of the clean up methods such as suction devises and scrapers did more damage to the habitat then the oil itself. The Exxon Valdez disaster ten years ago is another case in point.

Read  Concepts 55.2-55.5 pages 1215-1229


1) Waser, N.M. et al. (1996) Generalization in pollination systems and why it matters. Ecology 77: 1043-1060.

2) Witz, B.W. (1989) Antipredator mechanisms in arthropods: A twenty-year literature search. Florida Entomolgist 73: 71-99.

3) Park, T. (1954) Experimental studies of interspecies competition. Physiological Zoology 27: 177-238.

4) Stiling, P. (1999) Ecology. Prentice-Hall, New Jersey.

5) Mittleback, G.G., et al. (1995) Perturbation and resilience: A long term whole-lake study of predator extinction and reintroduction. Ecology 61: 2347-2360.

6) Sousa, W.P. (1979) Disturbance in marine intertidal boulder fields: the nonequilibrium maintenance of species diversity. Ecology 60: 1225-39.

Unit 4 Study Terms:


 symbiotic relationships

 interspecific competition

 Non-equilibrium Model



 intraspecific competition

  Dynamic equilibrium

 species richness


 competitive exclusion principle

 keystone species

 species abundance


 ecological niche

 Intermediate disturbance hypothesis

 diversity indices

 cryptic coloration

 fundamental niche



 warning coloration

 realized niche



 aposematic coloration

 resource partitioning




 community stability



 Batesian mimicry

  ecological succession



 Mullerian mimicry

 climax community



FIGURES: 53.2, 53.2, 53.4, 53.11, 53.16,  53.25, 53.26, 53.27, 53.29

Unit 4 Short Answer and Study Questions

1. Draw a rank abundance diagram for a community in which one species comprises 80% of the individuals in a community. One species comprises 10% of the individuals. One species comprises 5% of the individuals. One species comprises 3% of the individuals, and the remaining species comprises 2% of the individuals. Label each axis.

2. List and define all of the types of interspecific interactions.

3. Name and describe the various types of predation.

4. Describe at least two examples of interspecific interactions that have lead to the evolution of coevolved adaptations.

5. Name and describe the various types of anti-predator defenses.

6. Describe the parasite-host relationship. What is the typical function of a primary or ultimate host? What is the typical function of the secondary or intermediate host or hosts?

7. Name and describe the two basic kinds of interference competition. What is the competitive exclusion principle?

8. What is the ecological niche? How is the fundamental niche different from the realized niche? Why does competition sometimes favor species partitioning resources?

9. Name and describe the different types of symbiotic relationships. Describe in detail one type of coevolved relationship that benefits both participants.

10. Explain how predation can influence community structure. Describe one example from your textbook of how predation can influence community structure.

11. Explain how disturbance can influence community structure. Explain why succession does not always lead to only a single climax community.

12. Describe how fire influence community structure in central Texas woodland-grassland communities. How does the frequency of fire in this context, influence the type of plant community that should dominate in one area?

13. Describe the various ways in which grazing by domestic livestock influence natural ecosystems.

14. Explain the Non-equilibrium model of community composition. How does it compare with the equilibrium model of community composition?

15. Compare the dynamic equilibrium model of community composition with the intermediate disturbance hypothesis in terms of the intensity and frequency of disturbance. What role do keystone species have in communities?

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STUDY GUIDE EXAM 4- UNIT 5 - Trophic Structures and Nutrient Cycles

Perhaps the most fundamental "need" of any organism is the need to procure food. Not just calories, but a balanced diet sufficient in energy, vitamins, nutrients, and trace minerals. Without this balance animals can not successfully grow and reproduce. As you have seen in the last unit, obtaining these nutrients means very different things for an oak tree and a bird of prey. Organisms are linked by more than the fact that they eat each other. The very calories and nutrients an organism requires are locked up inside the leaves and roots, or muscle and fat of the prey species. The laws of thermodynamics demonstrate that the transfer of energy is never perfect, and a large amount of energy is lost each time one organism consumes another. Thus a smaller and smaller amount of energy is available for the animals at the top of the food pyramid.

There is a finite amount of energy and nutrients in the world. Thus it is imperative to cycle these nutrients through the bodies of organisms and back into the earth and water if we are to keep ecosystems in equilibrium. Not only is it important to keep nutrients moving, but to keep the relative amounts of nutrients in balance. Too much nitrogen or phosphorous in a habitat can have as damaging effects as too little.

Read overview and Concept 54.1- 54.3 on pp. 1184-1194

5.1 Ecosystems
l. definition
ll. trophic structure
lll. connectance and linkage
         lV. food chain and food webs

5.2 Trophic levels
l. primary producers
ll. primary consumers
lll. secondary and tertiary consumers
lV. detritivores

5.3 Energy flow in Ecosystems
l. primary productivity
          ll. secondary productivity
          lll. ecological pyramids

5.4 Chemical Cycling
l. definition
ll. forces that move nutrients
lll. carbon cycle
lV. nitrogen cycle
V. phosphorus cycle
Vl. sulfur cycle

5.5 Human Impact of Nutrient Cycles
l. human impact on food webs
ll. acid rain
lll. eutrophication

5.1 - Ecosystems

l. definition

Ecosystem was a term first created by a British plant ecologist named Tansley in 1935. The term was created to define an environment, which included both the organisms and complex physical forces. Thus it included not only organisms like bacteria, plants, fungi and animals but their abiotic world, and the energy and minerals that move through that world. An ecosystem can be small- such as a rock crevice filled with water after a rain, or as large as the Amazon forest. There are no definitive boundaries for most ecosystems.

ll. trophic structure

The trophic structure is the set of dietary relationships between organisms in an ecosystem. In other words- who eats whom. Typically these relationships are not a simple linear chain. Instead a better analogy would be a food web or net where simultaneous interactions occur between a number of species. Organisms are classified by function levels rather than by species. Thus bullfrog tadpoles are considered herbivorous since they eat algae while the adults are ravenous carnivores.

lll. connectance

Connectance is a measure by which we can determine the relative complexity of a food web. In other words, how many species are interacting with one another simultaneously. It is defined as

Connectance = actual # of interspecific interactions
                            potential # of interspecific interactions

such that for any given number of (n), the total number of potential interactions is


The number of links one species has with other species is called the linkage density (d).Linkage density is calculated as follows:

Linkage density (d) = the number of actual interactions
        the total number of species

We can use a case history of insects on pitcher plants to illustrate our point. A study of the food web of insects on a single pitcher plants 3 found a total of 19 species visited the pitcher plant and 33 different interspecific interactions occurred. Thus in the case of the pitcher plant food web:

(19 x 18)/2 = 171 potential interactions existed so

Connectance = 33 (actual # of interspecific interactions)
           171 (potential # of interspecific interactions)

and connectance is equal to 0.19. The linkage density is therefore 33/19 = 1.74.

Linkage density describes how tightly interconnected species are within a given community. The higher the linkage, the more dependent species are upon one another. Thus the extinction of a species in a tightly liked ecosystem may have profound repercussions on many organisms. Moreover, if a pollutant enters the ecosystem, the higher the linkage, the more destruction will occur. The lessons learned from the pesticide DDT are an sobering example of what happens when toxins enter a tightly linked ecosystem These will be discussed below when we examine human impacts on the environment.

It is difficult to know which links in a food web are most important. One way to investigate this question is to determine how much energy flows between two links. An experimental approach is to manipulate a link (ex. remove a predator) and watch the results.

An example of this can be found in Texas by studying the Attwater's prairie chicken. Attwater's prairie chicken, a brownish chicken-sized bird was once quite common in the Southern portions of Texas, and the species numbered in the millions at the end of the 19th century. Now there are fewer than a dozen birds alive and they are all in captivity. A reintroduction program was started at the Attwater's prairie chicken refuge near Eagle Lake, Texas. One important question refuge managers have been asking is to what extent do great horned owls feed on the prairie chickens? Moreover, what would happen to the populations of reintroduced birds if you first remove the great horned owls? Great horned owls probably feed on the prairie chickens if the opportunity arises. But in all likelihood, great horned owls also feed on opossums and raccoons who prey on the chicken. Thus a "simple" solution of removing the owls may have unseen and unfortunate consequences.

lV. food chain and food webs

There are a number of generalizations that have been made about food chains and food webs. Some of the most important are as follows:

1) Cycles where species A eats B who eats C who in turn eats A are very rare.

2) The average proportion of top predators, intermediate and basal species remains roughly constant in webs independent of the number of species. This is due to the percentage of energy available at each level (see Energy flow)

3) Linkage density is often constant except for webs with very large numbers of species.

4) Omnivory (where an organisms feeds on both plant and animals) is rare.

5) Food webs are more complex in complex environments.

6) Top predators tend to be large and sparsely distributed, whereas herbivores tend to be small and more abundant.

5.2 Trophic Levels

l. primary producers

We can now organize organisms into who eats whom. Primary producers are defined as organisms that produce their own energy. Autotrophs, which use the sun to photosynthesize organic molecules such as glucose, make up the largest percentage of primary producers. Phytoplankton, tiny one-celled plants that live in ponds and oceans, are a good example. Chemoautotrophs, which turn chemicals into food, are another, rarer type of primary producer. These organisms tend to live in deep-sea vents. Instead of using sunlight, chemoautotrophs use sulfur and oil that bubbles up from the earth's mantle. In general, the ability of any primary producer to produce food is limited by sunlight, temperature, water and nutrient availability.

ll. primary consumers

Primary consumers, as the name implies, are the organisms that eat the primary producers. This would include any invertebrate or vertebrate that feeds off of plants. Snails, insects, zebras, frugivorous bats and seed eating birds would all be examples of primary consumers. You find primary consumers in every habitat- in other words every place food is produced. Primary consumers are often generalists, and opportunistic. They forage on different plant species as food availability waxes and wanes. A deer that only eats tender young grass shoots in the springtime may forage on twigs in the winter when grass is unavailable. Primary consumers may also feed up and down the food chain: squirrels for instance may eat both acorns and bird eggs.

lll. secondary and tertiary consumers

Secondary consumers are those that eat the primary consumers. A spider who eats the aphid feeding off of a pine tree would be considered a secondary consumer. The chickadee that eats the spider is now the tertiary consumer. Often times the higher level consumers are larger animals that have extensive territories or home ranges. The large predatory cats, raptors (birds of prey), bears and wolves and even humans can fall into this category.

lV. detritivores

This trophic level category is often over looked. Detritivores are the organisms that feed off dead and rotting tissue- whether it is a dead deer or a dead plant. For example, it is important to realize how much primary producer vegetation lies uneaten on the ground in the form of leaves, stems and roots. Any one who gardens knows that old crops that are plowed back into the ground, or compost which is added to the soil, produces far more fruits and vegetables than ground which has been stripped clean of plant material. It is the detritivores, such as bacteria and fungi, which break up dead plants and animals and return the raw nutrients back to the soil. The rate of mineral availability is often determined by the decomposition rate of detritivores. Warm moist conditions favor this rate. Thus the decomposition and turn over rate of nutrients is much faster in the tropics.

5.3 Energy Flow

When an armadillo eats a grub, some of the energy that was trapped in the grub's body is now available to the armadillo. But most of the energy from the grub (90%) will be lost as heat, crumbs spilled, or incomplete digestion. Thus 90% of the calories in the grub will be "lost " and only 10% will be turned into muscle and tissue in the armadillo's body. Now only a small fraction of the energy from insect can be carried up to the next level- the red-tail hawk that eats the armadillo. Most energy is lost as energy moves through trophic levels, and this tends to keep food chains (i.e. who eats who) very short.

l. primary productivity

The bulk of life on Earth consists of plants- only a small fraction (less than 1 % by weight) comes from animals. Gross primary productivity is defined as the amount of energy fixed by photosynthesis. This number is huge, but does not take into account the amount of energy the plants need to use to respire, grow, and reproduce. Thus net primary productivity is a more useful measure of productivity. Net primary productivity is equal to the gross productivity minus the amount of energy used by the plant. Measurements from different habitats suggest that the primary productivity is about 50% to 90% of the gross primary productivity. Large plants, such as trees that have more structure and surface area to support, tend to have reduced net productivity. Harvesting raw plant material and weighing the biomass is the simplest way to measure productivity. Two types of loss must be incorporated into this measure of overall biomass: biomass lost to primary consumers (in other words herbivores eating the plants) and biomass lost due to death of the plant.

People have used various methods to measure productivity in different ecosystems around the world. If you look in your book at Figure 54.4 you can see there is a tremendous amount of variation in biomass production. For example, algal beds and reefs are the most productive while extreme deserts are the least. Why is this important? As human beings gobble up more and more lands to live and play on, we are running out of fertile land to grow crops. It is essential to protect productive habitats such as temperate grasslands -not only for their diversity but also for the number of food resources that we depend upon.

What are the limits to primary production? Water is the limiting resource in most terrestrial environments. There is a direct correlation between the amount of rainfall an area receives, and the amount of biomass it produces (think about the tropical rain forest versus the extreme desert). Nutrient availability is also important. Not only do nutrients need to be in the soil, but also they need to occur in a usable form. For example, calcium may be present in the Texas hill country soils, but if the pH is too high (above 7) the calcium may be bound to other minerals and unavailable to the plants. Thus the pH of the soil (whether it is acidic like New England or alkaline like the Texas hill country) affects whether some nutrients are available for plant use.

Light and nutrient availability primarily limit the productivity of aquatic environments. Water easily absorbs light. Thus while it is light at the surface of a lake or the ocean, below 20 meters it becomes very dark. In contrast, too much light can raise the temperature, overheat the plants and kill them. Thus in the tropics primary productivity is greatest a few meters below the surface where light is still available and the temperatures are cooler.

ll. secondary productivity

Secondary productivity is defined as the rate at which consumers (such as herbivores) convert food into their own biomass. Thus it is the weight or bulk of energy in the animal's body. Since we know that energy transfer is far less then efficient, we can see that there is a lot less secondary productivity than there is primary productivity. This is especially true since we look at different categories of secondary producers. Herbivores like zebras, and carnivores like lions are endotherms. This means they spend a tremendous amount of energy maintaining their own body temperature. This is compared to ectotherms (such as snakes) that do not regulate internal body temperature and require far less energy.

lll. ecological pyramids

Ecological pyramids (see Figure 54.11-54.14 in Campbell) are a useful method to describe how much energy is transferred up through the trophic levels. Anywhere from 5 % to as much as 20% of the productivity of one level can be transferred up to the next trophic level. Typically pyramids become very small at the top as the biomass is contained in a few larger animals. There are usually 5 or fewer top predators in any ecological pyramid- while the bottom of that same pyramid contains millions and millions of plants. One of the most important lessons to be learned from these pyramids is the fact that many more individuals can exist if they eat lower on the food chain. An acre of land can feed one cow that can feed one person for a year. That same acre of land can feed 22 people for a year if it is planted in foods (for example grains, and vegetables) that people eat directly. As more and more people go hungry in the world, there are very compelling reasons to limit our intake of meat and make more food available.

Read Concept 54.4"Cycling of Chemical Elements in Ecosystems" on pp. 1195-1199

5.4 Chemical Cycling

l. definition

It is important to reemphasize the fact that nutrients often act as limiting factors in the environment. McNaughton (1988) studied grasslands in the Serengeti and found that areas with high levels of magnesium, sodium and phosphorous supported more herbivores than areas with low mineral concentrations.

Minerals do not dissipate like other resources but tend to clump, and accumulate in individuals or specific species. This creates a "pool" of resources. Flux rate is defined as the rate at which these resources move from one pool to another pool in the environment. This is the theory behind using plants, such as mustard plants, to remove minerals or hazardous materials from the environment. By growing mustard plants on a particular patch of toxic soil, heavy metals can be absorbed by the plants, thus shifting the pool of resources from the soils into the plants. In the case of heavy metals, the plants can now be transported away from the site and the environment can be made safe to grow other crops or simply as a place for people to live. This process of using plants to removing radiation was used in Chernobyl in the 1980's after the nuclear power plant leaked contaminants into the surrounding habitat.

ll. forces that move nutrients

Factors that affect the rate at which minerals are transported include meteorological, geological and biological factors. Meteorological factors are those caused by minerals being dissolved in rain, snow, atmospheric gas and dust. Acid rain in a forest or on a lake would be an example of meteorological movement. Geological movement would include surface and subsurface drainage. An example would be minerals that are pulled down into the soil after a rain. Biological movement includes the movement of minerals through animals via digestion, predation, or the accumulation of minerals in the animal's body.

Nutrient turnover rates can vary dramatically between biomes. In rainforests, the turnover rate can be a short as 10.5 years. In contrast the turnover rate in the taiga of the Soviet Union it may take as long as 42.7 years. One good way to study nutrient rates and flow patterns is to use radioactive tracers to label the elements as they move through the environment.

There are two basic types of nutrient cycles. Local cycles have no mechanisms of long distance transfer. A good example would be the turnover of resources in a pond or lake. Global cycles are those that have an interchange with the atmosphere or the ecosystem. Minerals that melt off a mountain and run into a river would be a good example. Global cycles especially apply to the movement of nitrogen, carbon, and oxygen, as we will see next .

lll. carbon cycle

The levels of atmospheric carbon are normally quite low (0.03%). Autotrophs incorporate some of this carbon as biomass via photosynthesis- (about 1/7th of the atmospheric CO2). Plants return some of this to the atmosphere via respiration and decomposition. Fires increase the rate significantly.

Volcanoes significantly increase atmospheric carbon levels, while seasonal fluctuations exist as well. The rates of CO2 in the atmosphere decrease significantly in the Northern Hemisphere summers while increasing in the winter. Since there is more land in the Northern Hemisphere, photosynthesis ties up CO2 in the summer. Photosynthesis rates decrease in the winter but cell respiration rates by plants are still high. Cellular respiration by animals appears to have a minimal effect on the rates of atmospheric CO2 (remember that animals make up less than 1% of global biomass).

lV. nitrogen cycle

The nitrogen cycle is a global cycle. While some of the details appear complicated, the cycle itself can be broken down into 5 basic steps: 1) nitrogen fixation whereby bacteria can take nitrogen from the air and reduce it to ammonia. This is only carried out by certain bacteria such as Rhizobium, and a few algae, 2) nitrification where species of bacteria such as Nitrococcus and Nitrobacter can take the ammonia from the soil and convert it into nitrates- a usable form of nitrogen, 3) assimilation where plant roots assimilate nitrogen as nitrates. Animals can then assimilate the nitrates by eating the plants, 4) ammonification, where the decomposition of plants and animals and the release of animal waste form ammonia in the soil. Detritivores such as bacteria and fungi out this process. Since ammonia is not generally used by plants, however, valuable stores of nitrogen are now unavailable, 5) denitrification, the reduction of nitrates into gaseous nitrogen. Note that the process of denitrification by bacteria is almost the reverse process of nitrogen fixation.

Nitrogen in the soil and water is more important to organisms (in terms of its short-term availability) than atmospheric nitrogen that is abundant, but unavailable. Atmospheric nitrogen is important on a evolutionary time scale, however, as quantities of usable nitrogen move between the air and the terrestrial environment. Nitrogen availability is often the critically limiting factor that affects individual species and population cycles. In modern agriculture vast amounts of biomass are removed from the soil. With it goes the nitrogen that is trapped in the plant. Thus the land is stripped of its nitrogen and farmers must add nitrogen back to the soil in the form of fertilizers to renew the land. By adding additional fertilizer, modern humans have doubled the amount of nitrogen input in the terrestrial portion of the nitrogen cycle. Much of this added fertilizer is then lost again through run off and erosion. The result is eutrophication as vast amounts of nitrogen are dumped into localized water sources. Algal blooms and other destructive consequences often follow as we will see below when we look at human impacts on the environment.

V. phosphorus cycle

Phosphorus is different from other nutrient cycles in that phosphorus fluctuates between geological and biological pools. It is a simple cycle, however, because it does not contain an atmospheric component. The Earth's crust is the primary source of phosphorus; it tends to stay localized and cycle quickly. The exception is when phosphorus is carried off of the land by erosion and runoff, and ends up in the sea as sediments on the ocean floor. Geological events such as uplift and the movement of the earth's plates may eventually redeposit the phosphorus back into the terrestrial environment.

The most important form of phosphorus is phosphate, which plants can easily absorb from the soil. They do this so quickly that often soils are depleted of their phosphorus. Animals then obtain their phosphorus by eating other organisms. Animals excrete phosphorus as a waste product in their urine and feces. It also returns to the soil when plants and animals decompose.
Phosphorus is the limiting nutrient in aquatic environments where plants can assimilate phosphorus even faster than terrestrial environments.

Vl. sulfur cycle

Human activity has had more effect on the sulfur cycle than any other nutrient cycle. Sulfur is important because it has a direct effect on soil pH levels. Many nutrients that may occur in soil are chemically unavailable to plants if the pH level of the soil is too high or too low.

Much of the sulfur on the planet is bound up in geological deposits of organic matter such as coal, peat, oil, and in organic matter such as rocks. Weathering of these materials releases sulfur in a salt solution. Volcanoes and decomposition, especially in wetland ecosystems, releases sulfur in a gaseous form of hydrogen sulfide (H2S). Hydrogen sulfide quickly oxidizes into sulfur dioxide (SO2). Sulfur dioxide in turn is soluble in water, and chemically converts into a weak sulfuric acid (H2SO4). Some bacteria can now use this sulfuric acid and convert it back into hydrogen sulfide, which starts the process all over again.

Read Concept 54.5 "Human Impact on Ecosystems" on pp. 1200-1206

5.5 - Human Impact on the Environment

Humans have a tremendous impact on the world around us. Our impact is due to a number of factors including our sheer number, our distribution around the globe, our use of resources in different environments, and the outcome of our conspecific and interspecific interactions. The three examples shed some light upon the consequences of our actions.

l. human impact on food webs

DDT was first created in the late 19th century and its usefulness as a pesticide was first recognized in 1939. It was used in World War II and the Korean War to treat lice amongst the troops in the foxholes and trenches. In the 1950's truck regularly came through neighborhoods in Texas (Austin) spraying DDT on the cars, children, and the foliage to control mosquitoes. By 1970, the global production levels of DDT peaked. Most industrialized countries eventually realized its harmful effects however, and DDT use was banned in the U.S. in the mid 1970's. DDT is still banned for use in the United States but is currently manufactured in south Texas to sell for use in Mexico.

Why is DDT so destructive? First of all, it does not decompose easily. DDT can last for 10 years or longer in the soil- long after the "pest species" has died or become resistant to it. DDT has low solubility in water, but high solubility in fats and lipids. Most fats, in any environment, are found in living tissues of organisms. Thus DDT has a great affinity for living organisms and tends to concentrate (biological amplification) in the cells and tissues of individuals as one organism eats another. Once DDT works its way up the food chain, it ends up in organisms such as birds where is causes eggshell thinning. Egg shells become so delicate that the weight of an adult bird brooding its eggs will cause the eggs to crush. DDT ravished the Peregrine falcon populations in the 1960s as breeding pairs failed to produce young year after year.

ll. acid rain

As stated earlier, humans have had a massive effect on the sulfur cycle. While we have increased the emissions of carbon dioxide and nitrogen by 5-10 percent (with disastrous effects), we have increased sulfur emissions by 160%, primarily through modern industrial activity. Two activities that emit the greatest amounts of sulfur dioxide (SO2) are smelting of non-ferrous ores, and the burning of coal and oil. Typically the rocks that overlay deposits of oil and coal are also rich in sulfur. Mining activities (such as strip mining) disturb these layers of rock and soils exposing them to weathering agents such as air (which oxidizes them) and water. Erosion then carries off the sulfur-laden soil. Mining can pollute lakes, rivers and streams hundreds of miles away from the original source.

Smelting activities that start off as a local problem around the mine or smelter soon turn into a widespread disaster. For example, to avoid local polluting, smelters in Anaconda, Montana and other places now build taller smokestacks. Prevailing wind currents can now carry the sulfur emissions to other states and other countries, spreading the effects of the gas.

Gaseous sulfur can mix with rainwater, resulting in acid rain. Recall that natural rain water varies in its acidity and can be as low as 5.6 (recall that a neutral pH is 7.0). Most plants do best in soils with a neutral pH but can withstand slightly acidic or alkaline conditions. Acid rain has been measured as having a pH of 4.1 to 4.5. This is over a 100 times more acidic than natural rain water. (Recall the pH of milk is 6.6 and vinegar is 2.7)

What are the effects of acid rain? Most studies have focused on aquatic ecosystems where organisms are extremely sensitive to changes in pH. In some lakes harm begins when the pH levels drop below 6. Direct exposure to acid rain damages fish such as rainbow trout and brook trout. While adult fish may be able to withstand fluctuations in pH, small fry (newly hatched fish) are more sensitive. Thus an acid rain during the breeding season may kill off all the young and decimate a species.

Acid rain causes minerals that are toxic organisms to leach out of the soils surrounding the lakes and rivers and flow into the water system. Fish and aquatic organisms that do not die from the acidic water can be killed indirectly through aluminum, mercury and lead poisoning.

Terrestrial systems also suffer when acid rain causes "tree die back". Trees waste away from the inside out as the external foliage and branches die off. Branches harmed by the initial rains fail to leaf out the following spring, and the entire tree eventually dies due to lack of foliage (needed for photosynthesis). Trees weakened by acid rain are more vulnerable to insect attack, disease and harsh weather. Many scientists suspect the maple groves of Vermont and Southern Canada have been experiencing tree die back since 1980.

Countries that are most vulnerable to the effects of acid rain are those areas of the world composed of pre-Cambrian Shield bedrock. These bedrocks of granite and quartz found throughout North America and Scandinavia are naturally acidic and lack the ability to neutralize acid rain. Strata made of limestone, such as the Hill Country region of Central Texas, are naturally alkaline and can even benefit from the additional acidity. Prevailing winds from the United States often dump our smelting and industrial gases on Canada and Northern Europe- the very regions most vulnerable to harmful emissions.

lll. eutrophication

Eutrophication is the enrichment of water with excess nutrients, primarily phosphorus and nitrogen. Recall that lakes that are low in nutrients are called oligotrophic whereas lakes rich in nutrients are called eutrophic. Increased nutrients lead to swift agal blooms as the algae take the available nutrients and use them to rapidly reproduce. This in turn increases water turbidity, decreases the levels of O2 in the water, and decreases the water suitability to many indigenous organisms such as fish.

There is wide spread global variation in lake eutrophication levels. Seventy-five percent of Canadian lakes are still oligotrophic. In contrast, 70% of the U.S. lakes may be dangerously eutrophic.2 This is most likely due to the fact that farms surround many smaller lakes in the United States, and the farm run off that wind up in the lakes contain excess fertilizers.

Eutrophication in rivers is harder to measure since nutrients quickly wash away. Man made reservoirs on lakes show higher levels of eutrophication than natural lakes. The degree of human induced eutrophication coincides with the areas of highest human population density. Humans exacerbate eutrophication through 4 pathways:

1) urban waste (ex. detergents).

2) rapid industrialization leading to industrial waste build up.

3) intensified use of fertilizers in agricultural settings. These fertilizers tend to have a clumped distribution.

4) distribution of livestock and their waste.

There are two general methods for controlling eutrophication. The first one is preventative:

1) treat waste water to remove phosphorus and nitrogen.

2) divert waste water from natural areas.

3) management to limit water and fertilizer usage. Remove phosphates from detergents.

Secondly, corrective measures can be takes to clean up those lakes and natural areas that are already polluted:

1) physical manipulation (by removing polluted water, aerating the remaining waste).

2) chemical manipulation (by applying chemicals that kill the algae).

3) biological manipulation (by manipulating the food chain or web in lakes by the introduction of exotic species).


Unit 5 Study Terms

abiotic factor
trophic relationships
energy flow, trophic levels
autotroph (primary producer)
herbivore (primary consumer)
secondary and tertiary consumer (upper trophic level consumer or predator)
food chain
trophic web
primary productivity (PP)
gross primary productivity (GPP)
net primary productivity (NPP)
standing crop biomass
limiting factors
secondary productivity
ecological efficiency
biomass pyramid
biogeochemical cycles
nutrient pools
nitrifying bacteria
denitrifying bacteria
cultural eutrophication
acid rain
greenhouse effect

Figures 54.2, 54.4, 54.5, 54.6, 54.9, 54.11-54.14, 54.16, 54.17, 54.18, 54.19, 54.21, 54.22, 54.24, 54.25, and 54.27

Short Answer and Study Questions:

1. List the types of abiotic factors that might influence living organisms within an ecosystem.

2. Describe using a series of simplified chemical reactions what happens to the energy from sunlight has it "flows" from species to species within a community. Label each trophic level in this food chain.

3. Draw a sample food chain and trophic web. Explain how the two differ from each other.

4. Explain how gross primary productivity is related to net primary productivity. How does the concept of net primary productivity relate to standing crop biomass?

5. List the five ecosystems that have the greatest primary productivities, and explain why their productivities are high.

6. Explain what happens to energy in a green plant when the plant is eaten by a herbivore; i.e. how is the energy used by the herbivore.

7. Draw a pyramid of productivity. Label each level. Why does this have a pyramidal shape? Why is this pyramid shape sometimes inverted when standing crop biomass is plotted for aquatic ecosystems?

8. Draw and label a generalized model of a biogeochemical cycle.

9. Explain what happens to nitrogen, sulfur, or phosphorus in its own cycle.

10. Describe what happens to water and mineral cycling in forested watersheds when these are deforested.

11. How does erosion trigger the process of eutrophication?

12. Explain how biomagnification can lead to high concentrations of persistent pesticides and heavy metals in upper trophic level predators.

13. Draw a simplified sketch of the carbon cycle. Use this cycle to explain where the carbon comes from that is building up in our atmosphere. How does this excess carbon effect global climate?

this page last updated Nov 2006
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