Biology 100
Lecture 4: Populations in the Ecosystem
(Print Version)

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Assignments

Lecture Objectives

Web Resources

Population Definition

Application of
Population Information

Factors that affect
Population Growth

Biotic (Intrinsic)
Factors

Environmental
Resistance Factors

Density Dependent
Factors

Density Independent
Factors

Lecture Syllabus

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Answers to many of these questions can be found on the Text On-Line Learning Center

You may also ask questions and see answers to your classmates' questions in Web Crossing in the "Talk to Beth, Ed and Carrie" discussion.

Objectives:

After studying this material you should be able to:

1. Give an example of a population.
2. Draw a simple graph illustrating a population that is growing at an exponential rate of increase (J-shaped curve).
3. Describe how population growth rate is calculated.
4. Describe how and explain why each of the following factors affects the growth of a population:

birth rate

death rate

immigration

emigration

population growth rate

fecundity

biotic or intrinsic factors

exponential population growth

logistic population growth

density dependent environmental resistance factors

density independent environmental resistance factors

5. Explain the distinction between density dependent and density independent environmental resistance factors that regulate population growth.
6. Explain the relationship between the concepts environmental resistance factors, birth rate, death rate, population growth rate, and carrying capacity.

General Web Resources

• Population Clocks from the U.S. Census Bureau.
• Population and Stabilization from the World Resources Institute.
• POPNET, Population Reference Bureau.

What is a Population?

A population is a group of organisms of the same species in a given geographic location. (Lewis et al., Life, pg. 838)

The Glossary on pg. 938 gives a slightly different wording of the same concept, "A group of interbreeding organisms living in the same area."

Understanding the demography of populations has many critical applications:

Demography is the statistical study of populations, and includes such statistics as population size, density, and distribution.

• Conservation of species: Is this species endangered?

Species Information, from the U.S. Fish and Wildlife Service.

• Sustainability of harvest (eg. timber, fisheries)

No more fish and chips???

Overfishing has slashed stocks--especially of large predator species--to an all-time low worldwide, according to new data. if we don't manage this resource, we will be left with a diet of jellyfish and plankton stew.

• Economic and urban planning
• Health care and epidemiology

Factors that affect the growth of Populations

What is population "growth"?

• What we might talk about as population size is actually population density, the number of individuals per unit area (or unit volume).
• Population growth is based on four fundamental factors: birth rate, death rate, immigration, and emigration.

Population growth rate = (birth rate + immigration) - (death rate + emigration)

"Per capita rates" are calculated as the number of events (births, deaths, or growth) divided by the number of individuals in the population over a specific time period. (Lewis et al., pg. 843)

Biotic or Intrinsic factors that affect population growth

• Population growth is affected by biotic or intrinsic factors that are built into the genetic basis of each species.
• Biotic or Intrinsic factors are specific to each species and include:
• The age of reproductive maturity
• The number of offspring produced per reproductive event
• The number of reproductive events in an individual's lifetime

The three factors above are refered to together as fecundity, or the number of offspring an individual produces in its lifetime.

• Exponential (J-Shaped) Population Growth
• This is the sort of population growth that occurs when only biotic or intrinsic factors affect a population.
• Exponential growth assumes that environmental factors like food and water supply, space and shelter, disease organisms and weather conditions and natural disasters do not affect the birth or death rate.
• Examine the shape of the curve of population growth over time. Graph: Lewis et al., Life, Fig. 42.6, pg. 844
• As long as birth rate exceeds death rate (even slightly) population size will increase exponentially.
• If death rate exceeds birth rate population size will decrease exponentially.
• The human population is growing exponentially. Graph: Lewis et al., Life, Fig. 42.11, pg. 848

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Environmental Resistance Factors that Affect Birth and Death Rates

Obviously, populations cannot realistically grow exponentially. There are environmental limits, called environmental resistance factors, that affect the number of individuals that can survive and reproduce in a given habitat.

Environmental resistance factors fall into two categories: density dependent and density independent.

• A population explosion and crash. Graph: Lewis et al., Life, Fig. 42.7, pg. 844
• Density dependent environmental resistance factors (See Lewis, et. al. Pg 845)
• Density dependent factors include the environmental resources needed by the individuals of a population. Competition for food, water, shelter, etc., results as the population density increases. The survival, health, and reproduction of individuals will be affected if they cannot acquire the basic requirements of life.
• Density dependent factors ALSO include environmental factors, such as predators, infectious disease organisms, and parasites that do not necessarily result in competition for needed resources, but do affect the health, survival, and reproduction of individuals in the population as population density increases. Individuals that are diseased may have a reduced ability to reproduce. Dead individuals cannot reproduce.
• Density dependent factors are referred to as Environmental Resistance Factors that determine the Carrying Capacity of the environment for a population.
• Carrying Capacity - "The theoretical maximum number of individuals that an environment can support for an indefinite time period is its carrying capacity." (Lewis et al. pg. 845)
• Logistic Population Growth
• In the presence of density dependent environmental factors, population growth is constrained at high population densities. This is because the impact of density dependent factors depends on the density of the population.
• At low population densities, density dependent factors exert little influence on population growth, which initially grows rapidly.

This is to say that individuals have an abundance of resources so their health is good. They have a high capacity to reproduce and are less likely to die.

• At high population densities, density dependent factors exert an increasing negative effect on population growth which slows and finally stops at the carrying capacity.

The health of individuals is stressed because of lack of resources, crowding, prevalent diseases, etc. Their reproductive capacity is reduced and their liklihood of dying is greater.

• Density dependent factors therefore produce an S-shaped growth curve.

Graph: Lewis et al., Life, Fig. 42.8, pg. 845

• Density Independent environmental resistance Factors (See Lewis, et. al. Pg 846)
• Density Independent factors are Environmental Resistance Factors that occur or have an effect on a population regardless of the density of the population.
• Density independent factors include weather phenomena and natural disasters that affect the population, but the chance of their occurrence or level of severity is unrelated to the density of the population.
• Density independent factors may affect the availability of resources that are required by the population (density dependent factors), indirectly affecting the carrying capacity of the environment.

A hurricane might destroy trees that are the required nesting site for a population of birds.

General Web Resources

• Population Clocks from the U.S. Census Bureau.