Published online by Cambridge University Press: 05 September 2012
Introduction
In the previous chapter we saw how to derive conclusions about a whole population from information from sample areas. In this chapter, we are concerned with how to get the information from the individual sample areas (or from the whole population if it can be completely covered). To be clear, we shall refer to those areas as ‘sample areas’ or ‘study areas’; if we need to refer to the whole population of interest, we shall use that explicit phrase.
There are three approaches to measuring population within a study area. Most obviously, one may be able to carry out a complete count. Unfortunately, this is often impossible because one cannot be sure that one has detected all the individuals in the population. This has led to the development of various methods that involve counting just some of the individuals that are present and estimating (either explicitly or implicitly) the detectability of individuals, so that the total population of the sample area can be estimated from the sample of animals or plants actually observed by allowing for the detectability being less than perfect. However, it is often not easy, indeed it may even be impossible, to make such estimates. In such cases, one may settle for the third approach, namely obtaining an index of the population. That is, a measure that is related to population size but is not an estimate of the actual population size.
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