Consider an ecologist who surveys ponds in a city for frogs. On her first visit to a pond, she searches the edge and listens for frog calls over a 20-minute period. The southern brown tree frog (Litoria ewingii) is the most common species in her study area, but it is not found on this particular visit (Fig. 1.2). However, the researcher would not be particularly surprised that the species was not detected because she knows from experience that when surveying ponds, southern brown tree frogs are detected on only 80% of visits when they are in fact present. Given this information, what can she conclude about whether the southern brown tree frog is present at the site or not?
The question about the presence of a species is a simple example of those asked by ecologists. We assume that there is a particular true state of nature and we hope to use scientific methods to determine a reasonable approximation of the truth. However, the probability that a species is
present at a site is rarely calculated by ecologists, although it should be a fundamental part of any field study that depends on knowing where a species does and does not occur. This probability is not calculated partly because the statistical methods used by most ecologists are not well-suited to this question. I will examine three different approaches to answering this question and demonstrate that a satisfactory answer requires Bayesian methods.
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