1 | P a g e

Population Ecology

Ecology is the scientific study of the interactions between organisms and the environment.

Ecological studies of species growth must take into account abiotic (non-living) and biotic

(living) factors. Biotic factors may include behaviors as well as interactions with other

species. Abiotic factors include the chemical and physical components of the ecosystem

such as light, salinity, temperature and soil pH.

A population is a group of individuals of a single species living in the same area.

Population ecology explores how biotic and abiotic factors influence the density,

distribution, size and age structure of populations.

A. Fundamental Characteristics of Organisms in a Population

1. Density

2. Dispersion

2 | P a g e

3. Demography is a study of the vital

statistics in a population, especially birth

and death rates. A graphic way to

represent this is in survivorship curves.

Type I: low death rates during early and

midlife; then the death rates increase

sharply in older age groups.

Type II: shows a constant death rate

over the organism’s life span

Type III: shows a high death rate early,

then a flat rate for the few surviving to

older age groups

Label the survivorship curves above as type I, II, or III

B. Biotic Potential is the maximum growth rate of a population under ideal conditions,

with unlimited resources and without any growth restriction. How would the following

factors contribute to the biotic potential of a species? Increase or decrease

Age of reproductive maturity

Clutch size (# of offspring produced at a single reproductive event)

Frequency of reproduction

Reproductive lifetime

Survivorship of offspring to reproductive maturity

3 | P a g e

C. Exponential Growth vs. Logistic Growth Curves

1. Exponential Growth

2. Logistical Growth

a. Limiting factors

Density dependent

Density independent

b. Carrying Capacity (K)

4 | P a g e

c. Describe the shape of the exponential growth curve.

d. Predict why a population would be experiencing exponential growth.

e. Could a population continue growing exponentially? Explain.

f. Describe the shape of the logistic growth curve. Why does this curve flatten out

when the population (N) reached 1500 individuals?

5 | P a g e

Population Models and Growth of Populations

Population growth models: observations, experiments, and mathematical modeling are used to

determine rates of population growth. These models are used to study variables affecting

growth and to predict future population growth.

A. Exponential Model describes an idealized in an unlimited environment.

1. Unlimited resources

2. Excluding immigration and emigration, change in population size would be equal

to the number of births (B) minus the number of deaths (D) in a specific time

(ΔN/ Δ t).

3. Annual per capita birth rate refers to the average number of offspring produced

by an individual within a population. For example, if I tell you that there were five

births in a population in a year, that doesn't tell you much about what's really going

on in that population. Is five a lot or a little? The answer depends on the number

already in that population.

To determine per capita birth or death rates, you simply divide the absolute number

of births ("B") or deaths ("D") by the number in the population ("N") at the midpoint

of the time interval (usually year). By convention, for human demographics, we use

the total number ("N") of people, regardless of age or sex. Knowing the per capita

birth and death rates, population ecologists can calculate the expected number of

births and deaths in a given year.

We will use lower case "b" and "d" to indicate per capita birth and death rates,

respectively.

b = B/N and d = D/N.

Check yourself #1: Calculate "b" and "d" for the U.S. in 2012.

US Population size in 2012 (N) = approximately 314 million Number of births in the US in 2012 (B) = approximately 4.1 million Number of deaths in the US in 2012 (D) = approximately 2.5 million

dN/ dt = B –D

ΔN = population change, ΔT = time interval, B =births, D = deaths

6 | P a g e

What is the per capita birth rate ("b") for the US in 2012? _______________

What is the per capita death rate ("d") for the US in 2012? ________________

4. Population ecologists use (r) to identify the difference in per capita birth and death rates.

r = b – d What would a positive r and a negative r value indicate?

If r = 0, what does this tell you about population growth rates?

Check yourself # 2: A population of 265 swans is introduced to Mill Pond. The

population birth rate is 0.0341 swans/year, and the death rate is 0.296 swans/years.

What is the rate of population growth and is it increasing or decreasing?

5. Intrinsic Rate (rmax) is the fastest growth rate possible for a population

reproducing under ideal conditions.

Population Factors that influencing intrinsic growth rates.

a. Age at the beginning of the population

b. Number of young reproduced

c. How well the young survive

6. Exponential Growth Rate dN/dt = rmax N

a. The size of the population is rapidly increasing.

b. J shaped growth curve when population is plotted over time

c. Characteristic of some populations that are either:

- Introduced into a new or unfilled environment

- Or populations rebounding in numbers after a catastrophe

Do you think that a population can continue to grow exponentially forever? Why or why

not?

Identify 2 biotic and 2 abiotic factors that might prevent unlimited population growth?

7 | P a g e

B. Logistic Models incorporate the notion of carrying capacity (K). Carrying capacity is the

maximum number of organisms that can be supported by an ecosystem. Logistic growth

curves model the "S-shaped" behavior (abbreviated S-curve) of growth of some

population. The initial stage of growth is approximately exponential; then, as saturation

begins, the growth slows, and at maturity, growth stops.

What happens to N as the population approached K?

Why does this occur?

Logistic growth can be represented by the following equation:

How does the logistic growth formula differ from the exponential growth formula?

8 | P a g e

C. Practice: Using the equations above, solve the following population problems.

1. There are 190 grey tree frogs in a swamp. If r = -0.093 frogs/year, predict the population

size next year.

2. A population of 1,492 Baltimore Orioles is introduced to an area of Nerstrand

woods. Over the next year, the Orioles show a death rate of 0.395 while the population

drops to 1,134. What’s the birth rate for this population? Is this proving to be a suitable

habitat?

3. One dandelion plant can produce many seeds, leading to a high growth rate for

dandelion populations. If a population of dandelions is currently 40 individuals, and

rmax= 80 dandelions/month, predict dN/dt if these dandelions would grow

exponentially.

4. Imagine the dandelions mentioned in #5 cannot grow exponentially, due to lack of

space. The carrying capacity for their patch of lawn is 70 dandelions. What is their

dN/dt in this logistic growth situation?

9 | P a g e

D. Interpreting Age Structured Pyramids

1. How can you explain the rapid population growth in Afghanistan?

2. What percentage of the US population is between 40 and 49 years of age?

3. How would you describe population growth in the United States?

4. Is Italy experiencing positive or negative population growth? Why?

10 | P a g e

E. Population Cycles

Use the graph above to answer the following questions

1. Based on the graph, which organism is the predator and which is the prey. Use

numerical evidence to support your claim.

2. What happens to the lynx population when the hare population increases? Why

does this occur?

3. What happens to the hare population as the lynx population grows?

4. Notice that the maximum population of the predator and prey are separated by

periods of time. The same can be said for the minimum population numbers. This is

commonly referred to as the population lag time. Using the graph, calculate the

approximate population lag time between the lynx and hare.

11 | P a g e

Community Ecology

Community ecology is concerned with the interaction of all the living organisms in a particular

area. The concept of competition different species is known as interspecies competition. G.F.

Gause studied the idea of the competitive exclusion principle. When two species compete for

the same resources, one is likely to be more successful than the other.

1a. How do these two graphs

represent the competitive exclusion

principle?

1b. Predict why P. caudatum declined

in the mixed culture?

2. Often times, populations will coexist in spite of the apparent competition for the same food.

Based on the diagram, how would you define the term resource partitioning?

12 | P a g e

3. Fundamental vs. Realized Niche

The ________________________________ niche is the niche that an organism

occupies in the absence of competition. The _______________________ niche is the

result of competing species and the fact that their existence results when niche

overlap is absent.

Under experimental conditions, one species of barnacle (Chthalamus) can live on

rocks that are exposed to the full range of tides. In the natural environment, a

second species, Balanus, outcompetes Chthalamus, but only at low tide.

What is the fundamental niche of Chthalamus?

Explain why the introduction of Balanus forces each species of barnacles to

live within their realized niche. How does this ensure survival of both

species?

13 | P a g e

4. Community Interactions

Complete the second column using the following terms: mutualism, commensalism, and

parasitism. Complete the last column using the following relationships; (+,+) (+,-) (+,0)

Barnacles live on whales. One organism is helped and the other is unaffected.

Protozoa live in the digestive tracts of termites and both benefit.

Wasps lay their eggs in the body cavity of caterpillars. The larvae survive while the caterpillars do not.

Nitrogen fixing bacteria live in the roots of legumes (bean sprouts) These bacteria convert N2 gas into useable nitrates (NO3

-).

5. Protective Measures between Predator and Prey

a. Secondary compounds (toxins)

b. Camouflage (cryptic coloration)

c. Aposematic (warning coloration)

d. Mimicry

1. Mullerian mimicry

2. Batesian mimicry

14 | P a g e

Ecological Succession

Ecosystems constantly change. A tree falling in a forest affects the forest ecosystem. A fire might

alter the forest habitat so much that some species cannot survive and others can thrive. The process of

one community replacing another as a result of changing abiotic and biotic factors is called ecological

succession.

How does soil form in primary succession?

There are two types of ecological succession – primary and secondary succession. Primary

succession is the establishment of a community in an area of bare rock that does not have topsoil. For

example, suppose a lava flow alters an ecosystem. The lava hardens to form bare rock. Usually, lichens

begin to grow on the rock first. Because lichens and some mosses are among the first organisms to

appear, they are called pioneer species.

Pioneer species secrete acids that help break down rocks. As pioneer species die, their decaying

organic materials mix with small pieces of rock. This is the first stage of soil development. Small weedy

plants begin to grow in the soil. These organisms die, adding to the soil. Seeds brought by animals,

water, and wind begin to grow. Eventually, enough soil forms to support trees and shrubs.

It might take hundreds of years for the ecosystem to become balanced and achieve equilibrium.

When an ecosystem is in equilibrium, there is no net change in the number of species. New species

come into the community at about the same rate that others leave the community. This is a climax

community – a stable, mature community in which there is little change in the number of species.

How does secondary succession occur?

Disturbances such as fire or flood can disrupt a community. After a disturbance, new species of

plants and animals might occupy the habitat. Over time, the species belonging to the climax community

are likely to return. Secondary succession is the orderly and predictable change that takes place after a

community of organisms has been removed but the soil remains. Pioneer species begin the process of

restoring a habitat after a disruption. The figure below shows how the community changes after a forest

fire, leading again to a mature climax community.

15 | P a g e

1. What are two things mentioned in the reading that can happen that might affect an ecosystem?

2. Define ecological succession.

3. What is primary succession?

4. Describe an example of primary succession.

5. What is a pioneer species?

6. How long might it take for an ecosystem to become balanced?

7. What happens when an ecosystem is in equilibrium?

8. Define climax community.

9. Name two things that can disrupt a community.

10. What is secondary succession?