Dynamic biological processes influence population density, dispersion, and demographicsChapter 53, Section 1

Overview

Population ecology explores how biotic and abiotic factors influence the density, distribution, and size of populations.

A population is a group of individuals of a single species living in the same general area. Members of a population rely on the same resources, are influenced

by similar environmental factors, and are likely to interact with one another.

Populations evolve.

Populations are often described by their boundaries and size.

Density: A Dynamic Perspective

Density: the number of individuals per unit area of volume. In rare cases, population size and density can be

determined by counting all individuals within the boundaries of the population.

In most cases, it is impractical or impossible to count all individuals in a population.

Ecologists use a variety of sampling techniques to estimate density and total population size.

Density is not a static property. Additions to population due to birth and

immigration.

Removal of individuals from a population occurs through death and emigration.

Patterns of Dispersion

Dispersion: the pattern of spacing among individuals within the boundaries of the population.

Within a population’s geographic range, local densities may differ.

Differences in local density provide insight into the environmental associations and social interactions of individuals in the population.

Clumped: individuals are aggregated in clumps.

Uniform: individuals are evenly spaced.

Random: the position of each individual is independent of other individuals.

Demographics: Life Tables

Demography: the study of the vital statistics of populations and how they change over time.

Life tables: age-specific summaries of the survival pattern of a population.

How life tables are made… The best way to construct a life table is to

follow the fate of a cohort, a group of individuals of the same age, from birth until death.

Scientists then determine the number of individuals that die in each age group and calculate the proportion of the cohort surviving from one age class to the next.

Demographics: Survivorship Curves

A graphical method of representing some of the data in a life table is a survivorship curve, a plot of the proportion or numbers in a cohort still alive at each age group.

Type I: curve is flat at the start, reflecting low death rates during early and middle life, and then drops steeply as death rates increase among older age groups.

Type II: curves are intermediate, with a constant death rate over the organism’s life span.

Type III: curve drops sharply at the state, but flattens out as death rates decline for those individuals that survive the early period of die-off.

Demographics: Reproductive Rates

Demographers who study sexually reproducing species generally ignore the males and concentrate of the females in a population because only females produce offspring.

Demographers view populations in terms of females giving rise to new females.

A reproductive table, or fertility schedule, is an age specific summary of the reproductive rates in a population.

It is constructed by measuring the reproductive output of a cohort from birth until death.

Vary considerably by species.