55Population Ecology

• Population dynamics• The patterns and processes of change in

populations

• A population consists of individuals of a species that interact with one another within a given area at a particular time

• The study of population processes is called demography.

• Population density• The number of individuals per unit area or

volume

• Addition: births, immigration• Reduction: death, emigration

• Age structure• distribution of individuals across age

categories.

• Dispersion pattern• spatial distribution of individuals in the

environment.

• These characteristics are constantly changing and affect the ways in which populations interact.

• Studying populations• Count individuals• Determine ages• Calculate rates individuals leave

and enter population

• Individuals are often tagged or marked in some way to facilitate counting.

• If the organisms are large and the population size is small, a full census is possible—counting every individual.

• 760 elephants in national reserves in Kenya could be identified based on ear markings.

• But for most species, artificial marks or tracking devices are necessary

• Population size is then estimated using statistical methods

• For sedentary organisms, individuals in representative habitats can be counted, and the numbers extrapolated to the whole ecosystem.

• Individuals may be counted within measured areas called quadrats.

• Plants are often counted along a linear transect: A line drawn across the population’s range.

• Counting mobile organisms is more difficult.• The mark-recapture method involves

capture, marking, and releasing some individuals, then later capturing another sample of individuals.

• The proportion of marked individuals in the new sample is used to estimate population size

• The age structure of a population is the distribution of individuals across all age groups.

• It affects population growth because reproductive capacity varies with age.

• Populations with a large proportion of young individuals have a greater potential to grow than populations dominated by post-reproductive individuals.

• Dispersion—distribution of individuals in space.

• Dispersion determines patterns of interaction among individuals.

• Ecologists must know dispersion patterns of a species to choose appropriate sampling areas and methods for estimating population sizes.

Dispersion patternsThree basic dispersion patterns: Clumped—presence of one individual

at any point increases probability of others being near that point

Regular—presence of one individual at any point reduces probability of others being near that point

Random—there is an equal probability of an individual occupying any point in space

• Multiple estimates of population densities over time can be used to estimate the rate of population growth or decrease.

• Populations grow when individuals are added by births or immigration

• Populations decrease by the number of individuals lost by death and emigration

Life histories• An organism’s life history strategy

describes how it allocates time and energy among the various activities throughout its life.

• Life history strategies can vary dramatically. Variations determine how fast populations can grow.

• Life table data can be used to calculate per capita growth rate—the intrinsic rate of increase (r).

• If birth rate (b) exceeds death rate (d), r > 0, and the population is growing; and vice versa. If r = 0, population is stable.

dbr

• Some species reproduce continuously, others are limited to certain times or locales.

• Species that reproduce multiple times during their lives are iteroparous.

• Species that reproduce only once are semelparous. They may produce many offspring at once.

• All populations have the potential for explosive growth

• Nutrients, space are plentiful

• Exponential growth• This growth will lead to

population crash• When there is depletion of

nutrients• When disease spreads• Etc.

Factors limiting population densities

• Real populations cannot maintain exponential growth for long

• Environmental carrying capacity (K) • #of individuals of any

particular species that can be supported in an environment indefinitely

• When population reaches that number, shows logistic growth

• S-shaped curve

• Predators may be attracted to high densities of prey, increasing death rate

• Pathogens can spread more easily in a high-density population

• Other factors are density-independent, such as intense storms or cold periods, etc. They are usually abiotic factors.

• Does this sound like it might apply to human exponential growth at the moment?

• r-strategists—life history strategies that allow for high intrinsic rate of increase.

• K-strategists—life history strategies allow them to persist at or near the carrying capacity.

Figure 55.8 Two Life History Strategies

4 factors influencing population densities

• Species that achieve high densities usually:

• use abundant resources rather than scarce• Have small body size

• Need less energy

• Have complex social interaction• Ants• Termites• Even humans (social interaction is allowing us

to over-ride some other factors)

• Newly introduced species to an environment

• May not have predators, etc.

• Understanding life history strategies can be useful in managing species of commercial value

• Example: knowing size fish species reach reproductive rates

• This leads to minimum size requirements for catch

• Want to make sure every individual has opportunity to reproduce at least once

• Birth rate tends to be highest when a population is well below its carrying capacity

• Want to manage population so that it is just below its carrying capacity

• Many of our resources have been exploited and populations have decreased

Figure 55.13 Overharvesting Can Reduce Fish Populations

• Rapidly reproducing species can often recover if overharvesting is stopped, but recovery is more difficult for slowly reproducing species.

• 20th-century whalers hunted the blue whale, Earth’s largest animal, nearly to extinction.

• These whales reproduce very slowly, and not surprisingly, the population has failed to recover.

• Humans also wish to reduce populations of pest species.

• Killing part of a population will only reduce it to a density at which it reproduces at a higher rate.

• More effective: Remove its resources, lowering the carrying capacity of the environment.

• Biological control is the use of natural enemies (predators, parasites, pathogens) to control pests

• The pest is often a newly introduced species, and control species are brought in from the native region of the pest

• This, however, can lead to problems• Can itself become a pest or can have an

effect on another organism located in that environment

• Cane toads were introduced into Australia to control cane beetles in sugar cane fields.

• The toads couldn’t reach the beetles high up on the sugar cane plants, and had no effect on them.

• But they have been an ecological disaster for other species

• Some mammals in area die from eating them

• Deplete resources for other animals

• The size of the human population now contributes to most environmental problems.

• For thousands of years, Earth’s carrying capacity for humans was low due to the relative inefficiency with which humans could obtain food and water.

• Contributions to human population growth:

• Development of social systems and communication

• domestication of plants and animals• ever-increasing crop and livestock yields

through ongoing technological advances• medical advances

• A few European countries are experiencing population declines.

• Developing countries are experiencing exponential growth rates and have populations highly skewed toward younger age classes, portending high rates of population growth in the future.

• Earth’s current carrying capacity for humans is set in part by the biosphere’s ability to absorb our by-products, especially CO2 from fossil fuels

• Also by water availability and our willingness to cause extinction of other species to accommodate our increasing use of Earth’s resources