Populations

• A population is made up of the individuals of a species within a particular area:– each population lives in patches of suitable

habitat

• Habitats naturally exist as a mosaic of different patches:– many populations are thus broken into

somewhat isolated subpopulations

Population Structure & Dynamics

• Population structure refers to:– the density and spacing of individuals within suitable

habitat

– the proportions of individuals in various age classes

– mating system

– genetic structure

• Populations exhibit dynamic behavior, changing through time because of births, deaths, and movements of individuals.

Figure 13.3

Figure 5.4

Barriers to long-range dispersal limit geographic distribution.

• Introduced species often expand successfully into new regions:– 160 European starlings were introduced near New York

City in 1890 and 1891; within 60 years, the North American population of starlings covered more than 3 million square miles

• Other examples of successful introductions:– dogs in Australia, pigs and rats in Pacific islands

– fast-growing pines and eucalyptus trees worldwide

Dispersion of Individuals within Populations

• Dispersion of individuals within a population describes their spacing with respect to one another.

• A variety of patterns is possible:– clumped (individuals in discrete groups)

– evenly spaced (each individual maintains a minimum distance from other individuals)

– random (individuals distributed independently of others within a homogeneous area)

Figure 13.6

Figure 13.7

Causes of Dispersion• Even spacing may arise from direct interactions

among individuals:– maintenance of minimum distance between individuals

or direct competition for limited resources may cause this pattern

• Clumped distribution may arise from:– social predisposition to form groups

– clumped distribution of resources

– tendency of progeny to remain near parent

• Spatial pattern is scale-dependent

Figure 13.3

Populations exist in heterogeneous landscapes.

• Uniform habitats are the exception rather than the rule:– most populations are divided into subpopulations

living in suitable habitat patches

• Degree to which members of subpopulations are isolated from one another depends on:– distances between subpopulations

– nature of intervening environment

– mobility of the species

Metapopulation Model

• The metapopulation model views a population as a set of subpopulations occupying patches of a particular habitat:– intervening habitat is referred to as the habitat

matrix:• the matrix is viewed only as a barrier to movement

of individuals between subpopulations

Source-Sink Model & Mass effect Model

• The source-sink model recognizes differences in quality of suitable habitat patches:– in source patches, where resources are abundant:

• individuals produce more offspring than needed to replace themselves

• surplus offspring disperse to other patches

– in sink patches, where resources are scarce:• populations are maintained by immigration of individuals

from elsewhere

Landscape Model• The landscape model considers effects of

differences in habitat quality within the habitat matrix:– the quality of a habitat patch can be affected by the

nature of the surrounding matrix• quality is enhanced by presence of resources, such as nesting

materials or pollinators• quality is reduced by presence of predators or disease

organisms

– some matrix habitats are more easily traversed than others

Figure 13.10

Variation in Populations over Space and Time

• Populations tend to vary in size over time.

• Long-term records often reveal fluctuations that might be overlooked in shorter term:– infestation by chinch bugs in Illinois monitored

over decades reveals populations fluctuations:• in some years, populations averaged 1000/m2 over an

area of 300,000 km2

• in other years farmers reported little damage