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, mark-recapture methods can be very effective. The first step is to capture and mark a sample of individuals. Marking methods depend on the species: birds can be banded with a small aluminum ankle bracelet, snails can be marked with waterproof paint on their shells, butterflies can have labels taped to their wings, large mammals can be fitted with collars, fish fins can be notched, and amphibians can have nontoxic dyes injected under the skin. Marked animals are immediately released as close as possible to the collection site. After giving the animals time to recover and to mix randomly with the whole population, the ecologist goes out on a second collecting trip and gathers a second sample of the organisms. The size of the population can then be estimated from the number of marked individuals recaptured on the second day. The assumption behind mark-recapture methods is that the proportion of marked individuals recaptured in the second sample represents the proportion of marked individuals in the population as a whole. In algebraic terms, R = M S N
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, mark-recapture methods can be very effective. The first step is to capture and mark a sample of individuals. Marking methods depend on the species: birds can be banded with a small aluminum ankle bracelet, snails can be marked with waterproof paint on their shells, butterflies can have labels taped to their wings, large mammals can be fitted with collars, fish fins can be notched, and amphibians can have nontoxic dyes injected under the skin. Marked animals are immediately released as close as possible to the collection site. After giving the animals time to recover and to mix randomly with the whole population, the ecologist goes out on a second collecting trip and gathers a second sample of the organisms. The size of the population can then be estimated from the number of marked individuals recaptured on the second day. The assumption behind mark-recapture methods is that the proportion of marked individuals recaptured in the second sample represents the proportion of marked individuals in the population as a whole. In algebraic terms, R = M S N
Population_Ecology_draft.docx