Independent of the specific form of the index—abundance relationship, most researchers assume it to be constant among habitats and over time. However, in perhaps the most comprehensive validation study of an indirect index, a study by Reid et al. (1966) on mountain pocket gophers (Thomomys talpoides), the index used (numbers of mounds and earth plugs) consistently displayed a positive, linear relationship to actual gopher numbers, whereas the intercept and slope varied substantially between habitats (figure 7.2a, b). Other situations, such as electroshocking freshwater fishes, apparently yield comparable index—abundance relationships between habitats despite large differences in densities between habitats (figure 7.2c, d). In contrast, index—abundance relationships in different habitats can be reversed (figure 7.2e, f) although these examples may be compromised by sampling error. Finally, the slope, intercept, and precision of the relationship may vary among years within the same habitats (figure 7.3 a, b, c).
Inferences about population change drawn from indices are also often hampered by sampling error. Whatever the form of the index—abundance relationship between habitats and over time, the precision of the relationship can be quite low (figure 7.1d, e). This is particularly true for indirect indices, in which variation is strongly influenced by environmental factors such as weather and time of day, as well as by observers (Gibbs and Melvin 1993). Such index variation can substantially reduce the power of statistical tests examining changes in index values between sites or over time (Steidl et al.
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