All the many ways of counting or estimating animal populations fall into one or more of the following categories. These categories differ in the type of information they generate, their uses, and their limitations.

• Species list
• Species map
• Index to abundance
• Estimate of Percent Area Occupied
• Estimate of Population Size

Traditionally, most land managers have used an index of relative abundance to estimate changes in their populations. An index is some count of an animal population that is thought to be representative of the total number of animals in the area (e.g., number of birds heard calling at a point, number of toads found on a stretch of road at night, number of hawks seen flying past an observation tower). The use of indices as a means of tracking changes in populations has become controversial recently and so it is worth taking some time to explore the risks, tradeoffs, and rewards of indexing populations using counts of animals and the alternative approaches that estimate population sizes explicitly.

The choice of which approaches to take is an important one as each has specific costs, risks, and benefits impact the types of statements and conclusions that you can make once you analyze your data. We strongly suggest that you read over the above summaries to each approach before looking through the available techniques.