Figure 11.8 Estimating population abundance, density, and distribution. (a) Area-based studies count the number of individuals within a fixed area, such as this researcher working on the Dendles National Natural Reserve in England. (b) Linear-transect studies, including this one at the Great Barrier Reef in Australia, count the number of individuals that are observed along a predefined line. (c) Mark-recapture studies collect a sample of the population, mark them, and then return them. For example, horseshoe crabs (Limulus polyphemus) in Delaware are marked with small circular tags. A short time later, a second sample is collected to determine the proportion of marked animals in the population, which can be used to estimate the size of the total population.

Photos by (a) Paul Glendell/Alamy; (b) Suzanne Long/Alamy; (c) Patrick W. Grace/Photo Researchers, Inc.