Unit 3 - Population Growth
and Regulation
In this unit you will learn about the
fundamental factors which influence how populations grow. You will also learn
about the internal and external factors that regulate growth. Sometimes the
factors that affect population growth are environmental, such as the presence
of limited resources. Other times it is the trade off between factors such as
survival and reproduction, or the number of young, and the size of each young
produced. Species have evolved as a result of these environmental pressures,
and this often gives rise to predictable patterns of survivorship and
reproduction. For example, habitats that are complex and stable may favor
organisms that produce fewer, larger offspring, whereas less crowded environments
may favor organisms that produce smaller, but more numerous offspring.
It is possible to model the growth of
populations over time. Simplistic models give explosive, exponential growth
predictions. Complex models, which take resource availability into account,
show that populations are limited in the total number of individuals that can
successfully survive and reproduce.
Outcomes from these complex models indicate that there are important
repercussions for humans as we begin to reach our carrying capacity here on
earth. Because of the lack of clean water, food, or clean air, scientists
predict that human population size will soon reach its limit. The consequences
that will follow this event remain debatable.
Why should you care about population biology? Many of these theories and models
apply to everyday life. If you like to vacation in the mountains or at the
beach, you need a healthy ecosystem to keep the place looking vacation-like. If
you get sick, you want antibiotics that effectively reduce the population size
of germs inside you. And if you live in
Read Concept 52.1 How
biological processes affect population density… on pp. 1136-1139
Unit 3 outline:
3.1. Population characteristics
l. definition
ll. density
lll. dispersion
3.2 Patterns of dispersion
l. clumped
ll. course grained versus
fine-grained
lll. uniform
lV. random
3.3 Demography
l. definition
ll. demographic study factors of
populations
3.4 Life tables
l. definition of a life table
ll. application to real populations
3.5 Survivorship curves
l. type I
ll. type II
lll. type
III
3.6 Life history traits (LHT)
l. variation in LHT patterns
ll. environmental factors
lll. life
history traits and extinction
lV. life history tradeoffs
3.7 Reproductive episody
l. semelparity
ll. iteroparity
3.8 Population growth models
l. birth rate
ll. death rate
3.9 Methods for studying population growth
l. simple mathematical models
ll. comparing simple models to
natural populations
3.9 Experimental population growth
l. logistic population growth-
theoretical
ll. logistic growth and real populations
lll. r and K-selected traits
3.10 Population limiting factors
l. density dependent factors
ll. density independent factors
3.12 Human population growth
l. current growth rates as
exponential
ll. limits to human growth rates
3.1 Population Characteristics
l. definition
A population can be defined as a group of conspecifics (members of the same
species) that use the same niche. Recall that a niche is an organism's
"address"- everything the animal uses to survive and reproduce successfully.
This can include food, shelter, water, nesting sited etc. Two important
characteristics of any population are the density, or the number of individuals
per unit area, and the dispersion, or patterns of spacing of individuals in a
given area.
ll. density
Your book lists several different methods
for measuring the density of a population. Some method are better suited for
small organisms confined to an isolated habitat, such as a starfish or hermit
crabs in a tide pool, while other methods are best for larger, migrating
organisms such as caribou. It is important to realize that each method has its
advantages and disadvantages. For example, counting all the individuals in the
population is extremely important when studying rare and endangered species
since habitat conservation plans are often based upon these estimates. The
Peregrine Falcon is about to be removed from the
- visual counts, such as raptor fly-overs or
Christmas counts or songbirds
- traps ( live, snare, pheromone for insects, pitfall
for crepuscular species)
- vocalization frequency (for nocturnal or arboreal species)
- fecal pellet (for mammals)
- nests or dens
- pelt records (especially good for historical work)
- percent ground cover (for plants)
- transect counts ( where the number of animals and plants are counted along a
line. Good for sessile organisms)
If the data are accurate
one can also estimate the frequency of juveniles, adults, larvae and other
types of immature organisms. The
data are then used to construct life tables (see section 3.5 below). Whatever
sampling method is used, the method itself must avoid all biases such as the
tendency to over, or undercount certain individuals. If every time you hear a golden cheek warbler sing you count one male in your
sample, then you may be estimating the density of golden cheek warblers if the
male moves frequently within his territory. Sampling from a population is an
advantageous method when there are limited people or funds to do the work, and
if the species in question appears to have a uniform distribution throughout
its habitat. This sampling technique is less useful when habitats begin to
vary, and the distribution of individuals is lumped.
Marking and recapturing individuals is another way to estimate population size.
This would be most applicable to migrating species, such as butterflies and
birds that follow highly predicable patterns of migration. Initial marking and
recapturing of individuals must be based on random sampling techniques however,
and must include a sufficient number of individuals to avoid any biases. Mallet
et. al. (1987) found that mark-recapture techniques
influenced the behavior of Helconius butterflies, and thus population
estimates, since the butterflies avoided areas where they had been previously
captured.
lll. dispersion
Populations can vary their dispersion
patterns. Often the distribution of resources, and the
type of resource can influence how a population distributes itself in its
environment. Local distributions often are limited by physical or abiotic
factors of the environment. These may include: temperature, moisture, light,
pH, soil quality, salinity or water currents. For example, trees in the
SEPARATION OF TREE SPECIES IN THE UNITED STATED BY SHADE AND NITROGEN TOLERANCE |
||
TREE SPECIES |
SHADE TOLERANCE |
NITROGEN TOLERANCE |
|
Tolerant |
Tolerant |
WHITE OAK |
Tolerant |
Intermediate |
BIG TOOTH |
Tolerant |
Intolerant |
BEECH |
Intermediate |
Tolerant |
BASS WOOD |
Intermediate |
Intermediate |
TREMBLING |
Intermediate |
Intolerant |
SUGAR MAPLE |
Intolerant |
Tolerant |
WHITE ASH |
Intolerant |
Intermediate |
TULIP POPULAR |
Intolerant |
Intolerant |
3.2 Patterns of dispersion
l. clumped
A clumped distribution of organisms may
occur when the resources themselves are patchy. For example, salvia plants may
group together in places where the soil is alkaline and free of salt. Organisms
often clump around the most limited of resources or factors: temperature,
moisture, light, nutrients (nitrogen and phosphorous) and oxygen content. It
may be that one factor such as temperature, is most important. Saguaro cactuses
can withstand a frost if the plant can thaw out during the day. Plants will die
however, if temperatures remain below freezing for more than 36 hours. The
distribution of saguaro cactuses in
Individuals may also group together for
mating and social purposes, or because there are safety in numbers. Individual
fish swimming in a school in the open ocean are less likely to be eaten than
fish that swim by themselves. It is harder for a predator to pick off
individuals in a group since it is more difficult to focus on a single
individual. Imagine trying to catch a tennis ball when someone is throwing five
of them at you at once. You will notice how difficult it is to keep your eyes
on the ball. Indeed it is much harder than trying to catch a single ball.
ll. course grained versus fine-grained
Resources may vary in the size and number of
clumps that are formed. This will dramatically affect the distribution of the
animals using the resources. A coarse-grained environment may have large
patches, such stands of acacia trees in the African grasslands. The trees may
be lumped together around a limiting resource such as water and the clumps of
trees may be very spread out across the savanna. A large numbers of
individuals, such as monkeys can use the trees simultaneously for sleeping, and
shelter from predators. Thus the distribution of monkeys and acacias in this
case will be into a few, large patches. In contrast, a fine-grained environment
will have smaller patch size and thus can accommodate fewer individuals per
patch.
lll. uniform
Uniform environments are ones in which the individuals are evenly spaced
throughout the habitat. Plants may be evenly spaced to avoid competition for
resources such as sun and water. Other organisms, such as penguins may space
themselves out to avoid aggression in a limited area.
lV. random
A random distribution occurs when there is
no distinct pattern or predictability to the spacing. This is most likely to
occur when conspecifics do not have an influence on each other (either for
resources or social interactions). Trees in a forest often fall into this
category.
Clumped or uniform patterns appear to be
most common in nature, while random distribution appears to be more rare. Whatever the distribution of individuals is,
however, it is important to study these patterns. By examining the distribution
of individuals we can analyze how populations change over time. For example, we
can determine whether populations of an endangered species are growing in size,
or declining towards the edge of extinction.
3.3 Demography
l. definition
Demography is the study of factors that
affect the growth and decline of a population. Additions to a population can
occur through birth or immigration, while declines in a population occur
through death or emigration. The birth and death rates of a population depend
upon the age of the individuals in the population. For example, the death rate
will be lower in a population of young organisms than in a population of older
organisms.
ll. demographic study factors of populations
An age structure in a population is created
when you have an overlap in generations. It is defined as the age of all the
individuals in the population. Each age group will have its own birth rate,
(for example 5 year olds versus 20 year olds), and death rates (20 year olds
versus 60 year olds). Since certain age groups are more likely to reproduce
young and other age groups are more likely to die, the total number of
individuals in each age group will have direct effects on the growth and
decline of a population. For example, a population of young rats will have a
faster growth rate than a population of old rats whose birth rate has declined,
and whose mortality rate has increased. While it sounds easy to create age
structures for populations of organisms, it can be quite difficult to
accurately age wild organisms. There are a number of ways to age wild-caught
organisms and the most accurate method may be to mark and recapture
individuals. For example, nesting and fledgling golden cheek warblers are
captured in Central Texas each spring, and each is given an individual colored
band combination and identification number. In succeeding years, biologists
attempt to recapture every individual in the population (as well as band any
new fledglings). In this way long-term (10-year) age profiles and can be
collected to create age structures for a localized endangered species. While it
takes a tremendous effort to mark and recapture the majority of individuals,
the results can be impressive. In the case of golden cheek warblers, Dean
Keddy-Hector's study recaptured 90 percent of all individuals in the
population. Other, less accurate methods of estimating age include observing
the wear and replacement of teeth in deer, or the annual growth rings in sheep
horns and trees.
3.4 Life Tables
l. definition
Once you have collected information on the
age of individuals in your population, you can begin to estimate the likelihood
of an individual surviving from one year to another. For example if you mark
one hundred nestling birds in year one and only recapture 80 individuals the
next spring, you can estimate the chance of surviving as a yearling. Thus Life
Tables are created from age data to estimate how long an individual of a given
age is likely to live. Data for the life tables can be collected by following a
specific group of individuals from birth to death, or through a census of
living and dead individuals. Information on human age is used constantly by
insurance companies that to figure out how long you are likely to live. Doctors
use it when trying to predict how long someone can live once they have been
diagnosed with cancer or heart disease. Life tables are important to have when
estimating how well and threatened or endangered species is doing. They can
help determine whether a population is able to maintain itself or whether individuals
are dying and the species is going extinct.
ll. application to real populations
Life tables have been created for thousands
of different species. If you look on page 1139 in
Plant life tables are more complex. Age is
more difficult to determine and it is difficult to identify separate individuals.
While the "parent" plant may die, it also lives on through the
sprouts and suckers that come off of the roots of the plant. Thus demographers
have to deal with mortality on two levels-one for the individual (or parent)
plant, and one for the clones.
Plant mortality life tables can be useful
when asking specific types of questions. For example, data on seed mortality
and survival, life expectancies of perennial plants marked as seedlings, and
life cycles of annual plants can be fairly easy to collect and fairly accurate.
3.5 Survivorship Curves
Data on another species, the cactus ground
finches, would show that individuals do not necessarily survive at a constant
rate. Life tables can be used to create mortality curves and survivorship
curves. A mortality curve plots the rate of death against age. Often it may
have several phases. For example in the finches we would see three phases: a
juvenile phase where mortality rates were high, an adult phase where overall
mortality were lower, and finally a "post adult" phase when mortality
rates increased once more for older individuals. In this case, the change in
mortality rate over age results in a "J" shaped curve. This pattern
of survivorship is common for some birds and many mammals.
l. type 1
Survivorship curves can be plotted in a
number of ways. Typically, the logarithmic number of survivors is plotted
against time, or the age of the survivors. The accuracy of the table will
depend upon the accuracy of the original data, and whether or not environmental
conditions have changed. It turns out that species have different patterns of
living and dying (see figure 52.5 on page 1140 in Campbell)) Type I
survivorship curves depict a species with high survival rate through most of
its lifetime, and a high mortality rate at the end. Animals (typically
vertebrates) that have few offspring and invest heavily in their young fall in
this category. Humans who have access to medical care and preventative medicine
also follow these patterns of survivorship.
ll. type II
Type II organisms have a steady rate of
mortality throughout their life span. Small animals such as squirrels, hydra,
and migrating songbirds often follow this trend.
lll. type III
Type III organisms suffer tremendously high
rates of mortality at an early age. Once individuals get through the
bottleneck, however, they can live for a long time. Barnacles, for example,
produce millions of young and release them into the sea. Most of the young
drift along ocean currents and are eaten by predators. A few manage to settle
in the rocky intertidal zone where they can find food and shelter. Those few
individuals that make it there have an excellent chance of survival. Trees,
insects, weeds and many aquatic species like clams would also be examples of
type lll organisms. They make hundreds, even thousands of offspring but invest
little energy in each. As a result many offspring do not find a favorable
environment and die at an early age.
Note that while these 3 survivorship curves
fit a great number of species, there are still exceptions to the rule. For
example, some species exhibit a stepwise survivorship progression. There would
include invertebrates that experience high survivorship during most of the year
followed by high mortality during periods of molt. During a molt an animal's
exoskeleton, such as a lobster's shell, is soft. This makes the organism very
vulnerable to predation. Indeed, organisms like the lobster often hide during
this period. Another exception to the typical survivorship patterns is
exhibited Atlantic mackerel (Scomber scombrus). Mackerel tend to exhibit a type
III curve with a high juvenile mortality rate.
Read Concept 52.2 "Life
History Traits" on pp. 1141-1143
3. 6 Life History Traits (LHT)
l. variation in LHT patterns
In addition to births and deaths there are
other factors that shape the population dynamics of a given species. These
traits often affect the timing of reproduction and death.
ll. environmental factors
Often the life history patterns that arise
coincide with specific types of environments. For example, David Lack's studies
of songbirds in the 1940's indicated that temperate species often produce more
offspring than tropical species. This pattern appears to hold true for mammals
and lizards as well. Certain life history traits tend
vary with one another. For example and increase in fecundity coincides with an
increase in adult mortality.
lll. life history traits and extinction
Many biologists believe there are certain
life history traits that increase a species' susceptibility to extinction. If
this is so, it would be critical to know whether a
endangered species, such as the whooping crane, posses these traits. Some of
traits that have been identified include:
a) dispersal ability. Species that are capable of migrating between fragmented
habitats stand a better chance of finding mates and maintaining genetic
diversity. Thus if one fragmented population goes "extinct' it can be
replaced by migrating individuals (see unit 4).
b) Degree of specialization. Organisms who specialize
on only one type of resource, like panda bears that will only eat a specific
species of bamboo, or wasps that will only eggs on one species of fig, are more
likely to go extinct. Animals with more generalized diets can switch between
food items if there is a loss of habitat.
c) Rates of reproduction. Species, like rats, that can produce a lot of
offspring at an early age are more likely to bounce back after a population
decline. An endangered species which can produce 100 offspring per year can
replace itself more quickly that a species which only produced 1 offspring per
year.
d) Longevity. A species that lives a long time, such as an
elephant or a parrot (both which can live 70 or 80 years) may be able to
withstand poor environmental conditions for several years and forgo
reproduction during that time. In contrast, a species, which reproduces
early, but dies quickly (such as opossums that typically live less than 3
years) may be unable to "wait it out" and die without replacing
themselves.
lV. life history tradeoffs
No matter what the fundamental life history
pattern a species follows, there appear to be several fundamental tradeoffs in
nature. Since most organisms compete for limited resources, there is a finite
amount of energy available to an organism over its lifetime. Decisions must be
made about how to invest this energy. One of the most important tradeoffs is
the investment of energy in survival versus an investment of energy in
reproduction. Should one put all of one's energy into reproducing now, or put
energy into surviving until the next breeding season when the conditions may be
more favorable. Investment in the offspring includes not only the weight of
offspring (ex. energy stored in eggs or seeds) but energy for nursing, or
brooding, or caring for the young. Herbaceous perennial plants, for example,
invest 15-20% of annual energy in reproduction whereas as wild annuals expend
15- 30%. Domesticated grain crops such as corn invest 30-40% of their energy in
reproduction. Lizards like Lacerta vivipara invest 7-9% of their annual energy
in reproduction where as
Mountain salamanders (Desmognathus ochophaeus) spend 48% of their energy on
eggs and brooding.
3.7 Reproductive Episody
l. semelparity
Reproductive episody is the frequency of
offspring production over one's lifetime. Semelparity (semel = once, parity =
to beget) describes species such as salmon that have one large brood of
offspring and then die. Annual plants and century plants also follow this
pattern, as well as most insects, and some species of fish like the salmon. An
extreme example would be some species of bamboo live for 100 to 120 years
before the produce seeds and die.
ll. Iteroparity
Iteroparity (itero= to repeat), in contrast,
describes species who have fewer offspring over many seasons. Thus perennial
plants and most vertebrates would fall into this pattern. It is important to
recognize that the total number of surviving offspring produced by a
semelparous or iteroparous species may be the same. It is the timing of
reproduction that varies. Conditions favor semelparity when the cost of
surviving between broods is very high and there is a large tradeoff between
fecundity and survivorship. Conditions favor iteroparity when infant morality
is high, when parents are not present to raise their young, and established
individuals have higher rates of surviving. This coincides with many of the
Type III survivorship conditions.
Iteroparous species face another trade-off.
An organism can produce several small offspring or a few large ones. Thus organisms
can vary the number, and size of the offspring they have within a given
breeding cycle. In general, the number of offspring an organism has will
increase, as the cost of survivorship from one year to the next decreases. For
example, organisms that experience high predation rates or overwintering
moralities often fall into a Type III patterns of
survivorship. As the size of offspring increase, the number of offspring
decreases. Codfish lay millions of eggs that float in the ocean with no
parental care. Bass, in contrast, lay 100's of eggs and provide some parental
care. Most amphibians and reptiles provide little or nor care for their
offspring and lay many more eggs than birds which actively care for their
young. The exception to this rule is crocodiles that actively defend the nest
and care for young. Crocodiles tend to lay fewer eggs than other species of
reptiles.
Read Concept 52.3- 52.4"Population
Growth Models" on pp. 1143-1167
3.8 Population Growth Models
l. simple mathematical models
Recall that the two factors that affect the
growth of any population are the overall birth rate and death rate. A
population's growth can be examined through observation, experimentation, or
through mathematical modeling.
It is extremely difficult (if not impossible) to model an experimental
population, that contains all the factors and variables that are found in a
natural population. Yet models should not be ignored. They can be quite useful
in identifying the important principles that affect the growth of most populations.
Thus a useful starting point when modeling is to start with simple assumptions
and add assumptions one at a time. We will start off in our model of population
growth by assuming that the population size is small, and that resources are
unlimited. Under these simplistic conditions:
a change in populations size = BR (birth rate) -DR
(death rate)
(during a given time interval )
or N/ t= B-D
where N= the population and t= time
It is not very useful to look at the birth
rate for an entire species when what we are studying is a sample population.
Thus it is more useful to study a per capita birth rate (b) and death rate (d)
than an absolute birth and death rates. A per capita birth looks at the birth
rate per a certain number of individuals. For example, if the population size
is 1000 and 20 offspring are born into that population, then the birth rate b=
20/1000 or .020. (If you are unsure with these calculations you may wish to
look at Unit 2 and go over frequencies and percents again).
Thus we can now restate our original
equation using per capita birth and death rates as
N/ t= bN - dN
We can also symbolize the rate of population
growth (r) as the per capita birth rate minus the per capita death rate, or r =
b-d. A positive r indicates that the population is growing while a negative r
indicates that the population is declining.
Thus
N/ t= rN
Finally, we may wish to look at a particular
segment of time during which the population is growing or declining. We call
this segment of time the instantaneous rate of growth or decline (rmax). Using
the instantaneous rate of growth or decline will allow us to create an equation
that is essentially the same as the preceding one, but with smaller time
intervals, Thus we arrive at our final equation:
dN/dt=
rN
Under the experimental growth conditions
which were outlined above (a small population size and unlimited resources),
rmax is the intrinsic rate of population increase and dN/dt = rmaxN . This is the called the Exponential Population Growth
Equation and gives us a J shaped curve. You can see an example of a two
different J-shaped curves on page 1160 in
ll. Comparing simple models to natural populations
The exponential growth curve gives us a good
starting point, and may be useful in describing populations that have move into
new or empty environments. Recall that a natural disaster such as flooding may
"empty" a habitat. Populations will expand quickly as they capitalize
on the unlimited resources. It is under this period of time that we will see
rmax (the intrinsic rate of increase) at its maximum. However, the exponential
growth model is unrealistic under most natural conditions because of its
starting assumptions. Most populations are limited by resources such as food,
shelter, water etc. The carrying capacity of any population is defined as the
maximum number of individuals that can be supported over time in a given
habitat. We symbolize the carrying capacity as K. The carrying capacity (K) for
each population is determined by its most limited resource. What limits a
natural population will vary from species to species, and between environments.
Light availability may be the limiting resource for a plant in a cold climate,
while water may be the limiting resource for that same species in a warmer,
dryer climate. Thus K is environmentally determined. An increase in the number
of individuals, or a decrease in the birth rate will both lead to a decrease in
the intrinsic rate of growth, and a decrease in the populations
growth.
3. 9 Experimental
Population Growth
l. logistic population growth- theoretical
To make a more useful model, we will now
change one of our starting assumptions. We have seen that unlimited resources
are rare in nature and that population growth is affected by the amount of
available resources. In logistic growth equations we need to allow for a change
in r, the intrinsic rate of growth as we get changes in N, the populations size. This will allow us to incorporate the idea
that as the population size grows, (and reaches its carrying capacity (K))
resources will be limited and the rate of increase (how fast the population can
grow) will also be limited. Thus as our population size approaches the K, we
will get a decrease in the intrinsic rate of growth:
dN/dt = rmax N (K-N/K)
This will now give us a more realistic model
of change in population size over time. Instead of giving us a J-shaped curve
as we saw in the exponential model, our new logistic growth equation gives rise
to a sigmoidal shaped curve (see page 1162, figure 54.11 in
ll. logistic growth and real populations
We have now included more realistic
assumptions into the population growth models. How well do the modified models
mimic population growth among species in the real world? Data from a variety of
species suggests that the annual plants and small animals, such as beetles,
crustaceans and microorganisms fit as S- shaped growth curve fairly. Exceptions
occur when the starting population is too small. A small starting population
may not have enough genetic variation to keep the population viable, or enough
individuals to meet the demands of social breeding and cohabitation. Growth
along the S- shaped curve may not occur smoothly if there are lag times in the
appearance of resources and the start of the breeding cycle.
lll. r and K selected traits
The different life history
"strategies" used by species are sometimes referred to as
"r-selected" and K-selected" strategies. Table 52.3 illustrates
how groups of behaviors often cluster together, depending upon whether the
species is opportunistic and lives in an uncrowded environment. Species living
in a simple, spacious environment can grow rapidly with little competition.
Thus traits which favor early reproduction with lots of young are more likely
to arise. A perfect example would be an annual plant. In contrast, species
living in complex, crowded environments face severe competition. Young must
receive large amounts of parental investment to be successful in this crowded
market. Thus traits which favor delayed reproduction and heavy investment in a
few offspring are strongly favored.
Read Concept 52.5"Population
Limiting Factors" on pp. 1148-1152
3.10 Population Limiting Factors
l. density dependent factors
It is clear that the environment can have a
potent affect upon a species ability to survive and successfully reproduce.
Let's examine these factors in more detail. There are two basic categories of
population limiting factors. Density dependent factors are those that intensify
as the size of the population increases. This is due to the fact that
individuals are competing for limited resources. This limiting resource,
whether it is food, water, shelter, etc. sets "K" the carrying
capacity. Density can also increase the mortality rate when overcrowding leads
to toxic buildup, and physiological stress. A good example would be what
happens in your garden when you have planted your plants too close. At first
the small plants have plenty of sunlight, room and nutrients to grow. As the
nutrients are used up and the plants begin to crowd one another, their roots
compete for water and the leaves may compete for sunlight. Predation rates can
also change with density. When a prey species becomes common, predators often
switch and focus on the most common species. Thus a grasshopper that had been
eating other things now switches over to your prized snapdragons. No matter
what the cause, density dependent factors are ones that have an increased
percentage of mortality as the population size increases.
ll. Density independent factors
Density independent factors are different.
Here, natural disasters such as storms and droughts affect the population
equally. A bad storm may kill 80% of a bird species whether or not there are 50
or 1000 individuals. Most organisms experience both types of mortality over the
long term.
Finally, while both exponential and logistic
models predict a smooth change in the intrinsic rate of increase, populations
in nature often fluctuate. Some population fluctuations are so regular that
they are now perceived as cyclical population cycles. The life cycle of the
cicada, which complete a life cycle every 13 or 17 years, is the best known
example. For predatory species that heavily depend on one prey item, the size
of the prey population has dramatic effects on the size of the predator
population. Data collected over the past 100 years indicates that snowshoe
hares and lynx have regulate boom and bust population
cycles that take place every 10 years.
Read Concept 52.6"Human
Population Growth" on pp. 1152-1156
3.12 Human Population Growth
l. current growth rate as exponential
It is important that to consider human
growth patterns when studying the population growth and decline of other
species. While the human population remained relatively stable and small for
most of the history, explosive growth in the last few hundred years has had an
affect on every other species on the planet. Humans out compete most species
for almost every resource- food, water, land, etc. And this means there are
fewer resources available for other organisms. Thus their carrying capacity is
dramatically affected our presence. In addition, as human populations increase,
the amount of toxic build up, whether through pesticide run off, cargo ship oil
spills, or acid rain, not only poisons humans, but other animals as well.
Ironically, there are some species that benefit by the growth of the human
population. Edificarians, species such as house sparrows and cockroaches use
human shelters for homes are one example. Some species such as bacteria and
amoebae's can use humans as an intermediate host in their
life cycle and others, such as lice, thrive in humans. Thus no matter
what the species, humans have an impact of their ability to survive and
reproduce.
Ironically, the exponential growth model
ideally fits the explosive growth of humans. This is a unique case among larger
animals and is most likely due to our capacity to simply out compete other
organisms. As Fig. 52.20 indicates, population growth remained low (500
million) until the mid 17th century and they began to double at a phenomenal
rate. The world population is now estimated at 6 billion and will rise to 8
billion in another 17 years.
The two most important factors that have
lead to this explosive growth are a decrease in the mortality rate- primarily
among infants and children and a relatively stable birth rate in developing
countries. Age structure also plays an important role in determining which
countries are growing the fastest. While industrialized countries such as
ll. limits to human growth rates
An essential question remains. What is the
carrying capacity for the human race? Some population estimates that the human
population will double to 12 billion by the year 2050. Others suggest that the
human population will peak at about 10.6 billion and then begin a slight
decline as it stabilizes. What ever the number is, humans will reach a carrying
capacity, and it is still unclear what the most limiting factor will be. Many
people have predicted that food resources will limit population growth. Already
we see that a disturbance to food production due to floods in
It may be that food is not the limiting
resource at all. Instead clean, drinkable water may be most in peril. Already
most streams in the
No matter what the ultimate cause, humans in
some parts of the world are already reaching their carrying capacity and we as
a global species will reach this point in the next two generations. Not only
does it affect humans but the other species who share this planet. Humans will
have to make conscious decisions about the state of population growth through
social change and individual reproductive decisions, or endure the inevitable
consequences.
UNIT 3: Study Terms:
populations, density, dispersion, clumped
dispersion, uniform dispersion, random dispersion, census, density estimates,
mark-recapture, line-transect method, spot-mapping, grain, course grain, fine
grain, demography, age structure, sex ratio, fecundity, death rate, generation
time, life table, cohort, survivorship curve, Type I curve, Type II curve, Type
III curve, life histories, semelparity, iteroparity, age of first reproduction.
exponential growth, dN/dt, B (per capita birth rate), D (per capita death
rate), r (per capita growth rate), rmax(Intrinsic Rate of Natural Increase),
Logistic population growth, K (carrying capacity), r-selected species
(opportunistic populations), K-selected species (equilibrium populations),
density dependent factors, density independent factors, cyclic population
growth, lag effect, age pyramids and population age structure, age of first
breeding.
Tables: 52.1, 52.3, 52.3
Figures 52.3,52.5, 52.7, 52.9 52.10, 52.11, 52.12, 52.13
52.14, 52.15 and 52.18-52.27
Short Answer and Study Questions
1. List and explain the various types of dispersion
patterns found in populations of organisms. Explain what factors might cause
these types of distribution patterns.
2. Compare censuses and density estimates in terms of accuracy and difficult of
accomplishing.
3. Explain how the mark-recapture system of estimating population size works.
Show the formula used to calculate potential population size.
4. Explain the concept of ecological grain. Give examples of what kinds of
environmental factors would be fine or course grain factors for a large animal
like a Bison.
5. Draw a graph, which shows the three types of Survivorship curves. Label each
axis and give one example of an animal that shows each type of survivorship.
6. Create a simple life table in which no individual lives beyond the age of 4 years
and only half of each cohort survives into the next year.
7. Describe how resource limitation, survival ability and fecundity all
interact to influence how many young an animal will have in one year, versus in
its entire lifespan.
8. Contrast semelparity and iteroparity. Under what conditions would you expect
organisms to use each reproductive pattern?
9. Graph the relationship between population size and number of generations for
a species like a bacterium that grows exponentially but with none overlapping
generations. In your plot (graph) be sure to label each axis, and show clearly
how many cells exist at the end of each generation over 4 generations of
growth.
10. Explain the meaning of all of the components of the equation of exponential
population growth. As a special case, explain the circumstances under which
growth will be most rapid.
11. Graph the relationship between population size and number of generations
for a species with overlapping generations in which r=1.0. Label each axis, and
draw the relationship so that it is clear how many individuals exist at the end
of each generation. Under what circumstances would you expect to find this kind
of populations growth?
12. Graph the relationship between generation time and rmax for bacteria,
protists, insects and vertebrates.
13. Graph the relationship between population size and number of generations
for a population that shows logistic growth. In your graph, show where 'r' will
be maximal, and where 'r' will be minimal. Also, show the location of the
carrying capacity, and show accurately how growth rate changes as population
size approaches the carrying capacity.
14. Explain the meaning of each term in the equation for logistic population
growth. Show what happens to this equation when (a) population size approaches
the carrying capacity, and (b) population size is close to zero.
15. Explain two ways in which real populations deviate from logistic growth
curves.
16. Give four characteristics of r-selected and four characteristics of K-selected
populations. Under what conditions should each kind of population do best.
17. Describe the two categories of population limiting factors. Give at least
two examples of each kind.
18. Give two possible explanations for cyclic population growth such as that
documented in the 10-year snowshoe hare Canadian lynx cycles.
19. Describe the characteristics of rapidly growing and slowly growing human
populations.
this page last updated: November 2006
send comments to: akeddy@austincc.edu