Biology 110, Section 11
J. Greg Doheny
Chapter 53
POPULATION ECOLOGY
Quiz Questions:
1. What is the Mark-recapture method of estimating population size? (4 marks)
2. What is the Carrying Capacity (K) of an environment? (2 marks)
3. List the three main types of Population Dispersion, and indicate which are dependent on the
behavior of the organisms, rather than random environmental factors. (4 marks)
4. What is a Cohort, and what is a Cohort or Longitudinal Study? (4 marks)
5. Is a Survivorship Curve an example of a longitudinal Study or a Cross-Sectional Study?
6. Do Humans exhibit a Type I, II or III Survivorship Curve?
7. Do Mollusks exhibit a Type I, II or III Survivorship Curve?
8. Do Bears exhibit a Type I, II or III Survivorship Curve?
9. Do Squirrels exhibit a Type I, II or III Survivorship Curve?
10. Do Queen Bees exhibit reproductive Semelparity or reproductive Iteroparity?
11. Do Humans exhibit reproductive Semelparity or reproductive Iteroparity?
12. Is production of massive numbers of small offspring an example of a K-Selected or an r-Selected
reproductive characteristic?
13. Is production of only a few offspring, which have a highly developed ability to compete for
limited resources an example of a K-Selected or an r-Selected trait?
14. Is Fecundity (the average number of offspring a female has every time she reproduces) a
Density-Dependent or a Density-Independent regulator of population density?
NOTES
Ecology deals with how populations of animals, plants, insects and other organisms live in the
environment, interact with each other, and compete for resources. Biologists who study how
organisms interact with other organisms and with their environment, rather than simply
studying how they work are called Ecologists. Some ecologists study one or more species of
animals (or other organisms) within a specific area (ie-studying the wolves and cougars, and
their competition for food in British Columbia), while others are concerned with populations all
over the world (ie-studying the decline of Blue Whales in the oceans of the world). Population
Ecology generally deals with only one type of animal in a specific area, while Community
Ecology deals with the interactions and competition between several types of animals, insects,
fish etc., in a certain area. These are some of the things that Population Ecologists are
concerned with.
CARRYING CAPACITY:
The ‘carrying capacity’ of an environment (an area or ‘parcel’ of
land or water) is the maximum number of organisms (animals, insects etc.) that the area can
support. (For example: one square kilometer of forest being able to support a maximum of 100
squirrels, due to the availability of nuts to eat.)
POPULATION MEASUREMENT:
In order to study populations of animals, insects,
plants, etc., you need to know how many of them there are. (ie-How many humming birds are
there in a 10 square kilometer area of forest?) If you’re studying a small population in a small
area, it may be possible to count all the organisms you wish to study, but this is usually not the
case. Instead, most ecologists rely on sampling techniques to estimate the total population.
They take one or more samples of a population from one or more areas they are studying. For
mobile organisms, the Mark-Recapture method is a good way of estimating a total population.
In the Mark-Recapture method, you capture a sample of animals (for example), mark them
(also known as ‘tagging’), and then let them go again. At some time in the future you capture
another sample of animals, and see how many of them are animals that you had marked the
first time. The total number of animals in the population (N) can be estimated as follows:
Where (x) is the number of marked animals captured in the second sampling, (n) is the total
number of animals captured in the second sampling, and (s) is the number of animals marked in
the first sampling. (In the humming bird example listed above, you would take samples by
setting out humming bird TRAPS, allowing you to capture, tag, and then release the humming
birds.)
PATTERNS OF POPULATION DISPERSION:
Knowing the number of organisms in
an area doesn’t give you the complete picture. You must also know how that population of
organisms is distributed in the area. There are three basic dispersal patterns (see Figure 53.4):
1.
Uniform: Where the animals (for example) are dispersed evenly over the area.
Uniform dispersal patterns of animals are sometimes caused by territoriality, where
animals do not like to live in close proximity to other animals of the same species.
Presumably, this is an adaptation that takes the carrying capacity of the land into
account. (ie-if animals lived too close together, they would have to compete too hard
for the land’s limited resources. This would only be true of animals that compete with
each other for resources (like bears), rather than cooperate with each other for
resources (like humans, or wolves).
2.
Clumped:
3.
Random:
With clumped dispersal pattern, the animals are not evenly distributed.
They live together in clumps. Clumping sometimes helps with reproductive patterns, or
cooperation for resources. (ie- It is easier to find a mate, if several organisms of the
same species congregate together in clumps. This ease of finding a mate, however,
must be balanced against competition for resources, and the desire to increase genetic
diversity. If a group of organisms lives together, it may be easier to find a mate, but the
odds of that mate being related to you will increase, thus lowering genetic diversity.)
With random dispersal patterns, organisms are not as uniform as with
uniform dispersal patterns, and not as clumped as with clumped dispersal patterns. The
intermediate clumping patterns are due to random environmental factors like wind and
water availability.
The main difference between the first two dispersal patterns and the third is that the first two
patterns are determined more by the organisms, and their behavior patterns than by the
environment. The third pattern type is determined more by the environment than the
organisms.
POPULATION DEMOGRAPHICS:
Knowing the total population size and its
distribution patter still does not give you the whole picture. You must also know how fast the
population is growing or shrinking, how many of the organisms are being born vs. being killed
by predators or dyeing of old age. You would also want to know the ratio of males to females,
how many of the females are in an age where they are capable of reproducing, and how the
animals can be broken down into age groups etc. Measuring the number of males vs. females,
the birth and death rates, and the number of organisms in each age group is part of a science
called Demographics. Measuring these ‘vital statistics’ is called Demography (‘vital’ means
‘life.’ Therefore, these are the statistics of living organisms).
Vital statistics are compiled in things called ‘Life Tables’ (Figure 53.1). These statistics are often
obtained by marking (tagging) and then watching a Cohort of animals over time. A ‘cohort
study’ is where you look at a group of animals (or insects, or even people) that are all the same
age, and are all in the same environment or situation, and keep track of what happens to them
over a long period of time. This is also sometimes called a ‘longitudinal study.’ A cohort or
longitudinal study differs from a ‘cross-sectional study’ in that a longitudinal study watches the
same animals as they age over time, whereas a cross-sectional study counts and looks at
several individuals of different ages and sexes at a single moment in time, rather than watching
the same individuals as they age over time. Longitudinal Cohort Studies do not just apply to
animals and population ecologists. These studies are also used by pharmaceutical companies
when they are testing the effects of a new drug; and by Epidemiologists, who are medical
researchers who study patterns of illness and disease. Politicians also use vital statistics and
population dynamics to determine which parts of a population are growing fastest vs. dying
fastest in order to plan how many elementary schools vs. hospitals will be needed etc.
Another way to look at survivorship over time is to look at things called ‘Survivorship Curves.’
Survivorship Curves are drawn by marking a group of individuals at birth (marking 1000
individuals, for example), and then re-counting them at regular intervals to see how many are
still alive. The long-term survivorship of an organism will generally fall into one of three types
of survivorship curve: (see Figure 53.5)
1. Type I: Almost all of the offspring survive, live a full life, and only start to die off
towards the end of their natural life span. (A natural life span is how long you would live
if you had everything you needed to live, and had no predators to kill you off. This is the
case for most humans in the developed world. It is not the case for most other types of
organisms.) Type I Survivorship Curves are flat from the beginning, and remain close to
100% survivorship until they start to drop off towards the end of the natural life span.
Type I curves are typical of animals that produce only a few offspring, care for them
extensively, and only release them when they are well prepared to fend for themselves.
Humans display Type I curves, as do other closely-related primates. Bears, Lions and
other large, successful hunters, who have no predators to worry about also display Type
I curves.
2. Type II: The survivorship curve shows a constant rate of mortality (death) over time.
Organisms that show Type II curves (such as rodents) are cared for until they are able to
survive on their own, but have to worry about predators throughout their whole lives.
Thus, the death rate is due mainly to the probability of being eaten by a predator (For
example, Gophers being eaten by Hawks). The longer you live, the greater your chances
of being eaten, thus leading to a survivorship curve that declines at a constant rate.
3. Type III: The survivorship curve shows a rapid decline, and then flattens out, with the
few survivors living for a long time. This type of curve is typical for organisms that
produce a large number of offspring that are not yet prepared to survive on their own,
and are rapidly eaten by predators. The few that survive, however, will grow large
enough to not have to worry as much about predators, and will survive for a long period
of time. Arthropods and Mollusks are good examples of this, where huge numbers of
offspring are produced, most of which are eaten by small predators fairly quickly. Those
that survive will grow large enough, and have a protective shell, so that only a few, large
predators are able to eat them. Some organisms that have protective shells have to
periodically shed their shell (or ‘molt’) in order to grow. They are not vulnerable to
predators when they have a shell, but are vulnerable when they molt. These organisms
have a modified Type III survivorship curve that looks like a downward zig-zag line, due
to the fact that they are vulnerable to being eaten during molts.
Reproductive Rates and Population Growth: The population of animals in an area at any given
time will be a combination of A) the death rate, B) the rate of emigration vs. immigration, and
C) the birth rate. Statistics for birth rates are compiled in tables called Reproductive Tables
(Figure 53.2), also known as a ‘Fertility Schedule.’ The number of male animals is usually
ignored, because all they do is inseminate females. It is the number of female offspring each
female of a certain age can produce that counts.
Per Capita Rate of Increase: The phrase ‘per capita’ means ‘how many per person,’ or ‘how
many for each individual.’ For example, the ‘per capita income’ in Canada is calculated by
dividing the total amount of money that is made by the total number of people with jobs.
Likewise, the per capita birth rate is the average number of births per individual. It is calculated
by taking the total number of births and dividing it by the number of individuals who are of
reproductive age. The per capita rate of increase is how quickly a population is growing when
the birth rate exceeds the death rate.
Two mathematical models can be used to describe birth rates:
1. The Exponential Growth Model (Figure 53.7): is applicable when all the
organisms have enough to eat, and they simply reproduce as fast as they are capable of
reproducing. Under these conditions, the population will increase exponentially at the
maximum possible rate. Obviously, organisms that produce more offspring each time
they reproduce will multiply more rapidly than organisms that produce only a few
offspring (rats vs. humans, for example). The Exponential Growth model is described as
the change in the number of individuals (N) over a fixed amount of time (t) according to
this formula.
2. The Logistic Growth Model (Figure 53.9): The Logistic Growth Model takes the
carrying capacity of the environment into account, and assumes the rate of population
growth will be zero once the maximum carrying capacity (K) for the environment has
been reached. Both N and K are a specific number of individuals.
Life History Traits: As mentioned above, different organisms have developed different
methods of survival and perpetuation. Some organisms produce offspring that are not well
suited to survive at first, and compensate by having lots of them. Others produce only a few
offspring, but make sure that they will survive and thrive. These variables (how old an
organisms is when it starts to reproduce, how often it reproduces, how many offspring it
produces each time, and how well they are able to compete for resources) are collectively
referred to as Life History Traits. The following are some life history traits displayed by
different types of organisms:
1. Semelparity vs. Iteroparity: Semelparity is where an organism mates once, and
produces a large number of offspring from the single mating. Examples are fish
(spawning Salmon) and social insects (bees and ants etc.). With Iteroparity, mating
occurs regularly (at periodic intervals like seasons, for example), and only a few
offspring are produced (most mammals display Iteroparity in reproduction).
2. K-Selection vs. r-Selection: Recall that the maximum carrying capacity of an
environment is symbolized by the letter K, and the maximum growth rate of a
population is symbolized by the letter r. K-selection or ‘K-selected traits’ are
reproductive and competitive characteristics that are designed for survival in an
environment where the population is close to the carrying capacity (K) for that
environment, and organisms face strong competition for resources from other
organisms of the same species. Highly sophisticated hunters that produce only a few
offspring, like lions, are examples of K-selected organisms, where they have a highly
developed ability to compete for food. By contrast, ‘r-selected’ traits are traits that
favor survival in an environment that is well below carrying capacity. Such organisms
produce lots of offspring which do not have an advanced ability to compete for
resources. Mollusks and Rodents are examples of this.
3. Density-Dependent vs. Density-Independent Regulators of Population
Density: The number of offspring that a female organism can have is not dependent
on the number of individuals in the population. It is fixed, and is thus an example of a
Density-Independent regulator of population density. By contrast, the mating rate
between organisms increases as population increases, because it is easier to find a
mate. The mating rate is therefore a Density-Dependent regulator of population
density. The amount of food available is also an example of a Density-Dependent
regulator. Disease is also considered to be a Density-Dependent regulator because
diseases spread more rapidly in dense populations.
PRACTICE QUESTIONS:
Essay Type Questions:
1.
2.
3.
4.
5.
6.
What is the Mark-Recapture method of estimating a population of mobile animals, and what
mathematical formula does it use to estimate the total size of a population? (10 marks)
Describe the three general types of population dispersion, and indicate which are related to the
behavior of the animals in question, and can therefore be predicted. (10 marks)
What are ‘vital statistics’ in the context of Population Ecology? Give three examples of vital
statistics. (10 marks)
What is a cohort, and what is a cohort study (also known as a longitudinal study)? What is the
difference between a longitudinal (cohort) study and a cross-sectional study? (20 points)
Explain the difference between a Type I, Type II and Type III Survivorship Curve. Which type of
Survivorship Curve do Humans usually exhibit? Which type of Survivorship Curve do Mollusks
usually exhibit? Which type of curve do Rodents usually exhibit? Which type of curve do Bears
usually exhibit? (20 points)
What is the difference between the Logistic and Exponential Models of Population Growth, and
how do they relate to the Carrying Capacity of the Environment? (20 points)
MATCHING QUESTIONS:
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
K.
Carrying Capacity
Cohort
Cross-Sectional
Demographics
Epidemiology
Exponential
Iteroparity
K-Selected
Life History Traits
Life Tables
Logistic
L.
M.
N.
O.
P.
Q.
R.
S.
T.
U.
V.
Longitudinal
Molt
Mortality
Per Capita
r-Selected
Semelparity
Survivorship Curves
Type I Survivorship Curve
Type II Survivorship Curve
Type III Survivorship Curve
Vital
1. Name for a Population Growth Model that only takes the maximum rate of reproduction into
account?
2. A type of study where several individuals of different ages and sexes are sampled, once, from a
population.
3. The study of populations, including birth and death rates, sex ratios etc.
4. Reproducing or mating only once, but producing large numbers of offspring as a result.
5. Another name for a long-term cohort study.
6. Survival and reproduction strategy where only a few offspring are produced that are able to
compete for limited resources.
7. Type of survivorship curve that remains at or close to 100% survival for most of the natural life
span.
8. Maximum number of individuals an environment can support.
9. Collective term describing reproductive and behavioral survival strategies.
10. Fancy word meaning ‘life.’ (ie- “_____ Statistics”)
11. Type of survivorship curve marked by a rapid mortality rate at the beginning, and then long term
survival of just a few individuals.
12. Survival and reproduction strategy where many offspring are produced, who do not necessarily
have a strong ability to compete for limited resources.
13. Type of survivorship curve marked by a more or less constant mortality rate.
14. A number normalized to the size of a population. Calculated by dividing the total number of
things you are counting by the total number of individuals.
15. Reproducing repeatedly, often at regular intervals, but producing only a few offspring each time.
16. A group of equivalent individuals that are all the same age, and all in the same situation, who
are watched over time by a researcher.
17. A graphic representation of how many individuals are still alive over time.
18. Another term for ‘death rate.’ (ie-the “_____ Rate” of a population.)
19. When a mollusk or arthropod loses its shell in order to grow, and must produce a new, larger
one.
20. Name for a Population Growth Model that takes both the maximum rate of reproduction and
the carrying capacity (K) of the environment into account.