Estimating Population Size:
Parts of this lab adapted from
General Ecology Labs
, Dr. Chris Brown, Tennessee Technological University and
, Dr. Robert Kingsolver, Bellarmine University.
One of the goals of population ecologists is to explain patterns of species distribution and
abundance. In today’s lab we will learn some methods for estimating population size and for
determining the distribution of organisms.
Measuring Abundance: Mark-Recapture
Mobile animals are usually simpler to define as individuals, but harder to count, because they tend
to move around, mix together, and hide from ecologists. Quadrats are not a good approach with
mobile animals because immigration and emigration in and out of the study site make it hard to
know what area the entire population occupies. For largemouth bass in a farm pond, you could
easily draw a line around a map of the population, but how would you define the edges of a
population of house sparrows in your community? Although house sparrows tend to be more
concentrated in towns and urban areas, they do not stop and turn back at the city limit sign. For
zoologists, a fuzzy definition of the space occupied by the population often forces an arbitrary
designation of the survey group, such as the "population" of robins nesting on your campus in the
spring. Knowing the number of animals in a designated study area is interesting, but we must bear
in mind that the ecological population is defined in terms of interactions among organisms of the
same species, and not by the ecologist's convenience.
After defining the individual and establishing the limits of the population you wish to count, your
next task is to choose a counting method. Arctic and prairie habitats lend themselves to accurate
survey by aerial reconnaissance. This approach works poorly in forests, at night, underwater, or in
soil habitats. If animals can be collected or observed in a standard time or collecting effort, you
can get an idea of relative abundance, but not absolute numbers. For example, the number of
grasshoppers collected in 50 swings with an insect net through an old field community produces
data that could be used to compare relative abundance in different fields, but would not tell you
how many grasshoppers were in the population.
For estimates of absolute numbers,
can be very effective. The first step
is to capture and mark a sample of individuals. Marking methods depend on the species: birds can