Unit 8: Population Ecology

What is a population?

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

What does population ecology study?

How biotic and abiotic factors influence population density, distribution, size, and age structure.

What are the three main characteristics of populations?

Density, dispersion, and demographics.

What is population density?

The number of individuals per unit area or volume.

What causes population density to increase?

Births and immigration.

What causes population density to decrease?

Deaths and emigration.

What is dispersion in a population?

The pattern of spacing among individuals within the population boundaries.

What is the most common dispersion pattern?

Clumped dispersion.

Why do organisms often clump together?

To gather around a needed resource.

What is an example of clumped dispersion?

Cottonwood trees along a stream in the arid Southwest.

What causes uniform (evenly spaced) dispersion?

Antagonistic interactions like territoriality or competition.

What is an example of uniform dispersion?

Red-winged blackbirds defending territories during mating season.

When does random dispersion occur?

When there are no strong attractions or repulsions and resources are constant.

What is demography?

The study of birth, death, and migration rates in a population.

What is a survivorship curve?

A graphic way to show birth and death rates in a population.

What does a Type I survivorship curve show?

Low death rates early and midlife; high death rates in older age groups.

What kind of organisms usually show a Type I curve?

Large organisms with long life spans.

What does a Type II survivorship curve show?

A constant death rate over the life span.

What kind of organisms usually show a Type II curve?

Organisms heavily preyed upon, dying steadily over time.

What does a Type III survivorship curve show?

High early death rates, then low death rates for survivors.

What kind of organisms often have a Type III curve?

Many bird species.

What is exponential population growth?

Population growth under ideal, unlimited conditions.

What equation shows exponential growth?

dN/dt = rmaxN

In the equation dN/dt = rmaxN, what does rmax represent?

The maximum per-capita rate of increase.

What is carrying capacity (K)?

The maximum population size that an environment can support without degrading.

What happens to population growth near carrying capacity?

Growth slows down.

What is the logistic growth equation?

dN/dt = rmaxN (K - N)/K

In the logistic model, what happens as N approaches K?

The per-capita growth rate declines.

What are life history traits?

Traits that affect an organism’s schedule of reproduction and survival.

What three questions define an organism’s life history?

1) When reproduction begins, 2) How often it reproduces, 3) How many offspring it produces.

What is K-selection?

Selection for traits that are sensitive to population density and carrying capacity.

What is r-selection?

Selection for traits that maximize reproduction in uncrowded environments.

Which model is associated with K-selection?

Logistic growth model.

Which model is associated with r-selection?

Exponential growth model.

What is a density-independent factor?

A factor where the death rate does not change with population density, like natural disasters.

What is a density-dependent factor?

A factor where death rates rise or birth rates fall as population density increases.

What are examples of density-dependent factors?

Competition for resources, territoriality, disease, predation.

How do density-dependent factors regulate population size?

Through negative feedback.

What are boom-and-bust population cycles?

Regular fluctuations in population size influenced by biotic and abiotic factors.

Is the human population still growing exponentially?

No, but it is still increasing rapidly.

When did the human population growth rate begin to slow?

In the 1960s.

What is demographic transition?

When a population shifts from high birth/death rates to low birth/death rates.

What usually falls first during demographic transition: birth rates or death rates?

Death rates.

What is an age-structure pyramid?

A graph showing the relative number of individuals of each age in a population.

Why is Afghanistan poised for rapid growth?

It has a large proportion of young individuals.

What is the ecological footprint?

The amount of land and water needed to supply the resources a person uses.

What is considered a sustainable ecological footprint per person?

1.7 hectares.

What is the average footprint for a person in the U.S.?

8 hectares.