The U.S. Endangered Species Act (ESA) is one of the most important and controversial legislative acts in recent years. This law requires the use of specific measures to protect certain species of plants or animals that are listed as threatened species or endangered species. Before a species is listed as threatened or endangered, biologists must determine if a viable population of the plant or animal in question exists in the wild. This usually means determining the number of existing individuals, the sex of each, the number within breeding age, the breeding success rate, mortality rates, birth rates, whether sufficient genetic diversity exists, and many other factors. To answer these questions, the number of plants or animals must be counted.

Study Methods Used to Estimate Population Size

Study methods include observation and photography, live trapping, and transect sampling. All of the methods result in...