Transcript Populations

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