Setting Objectives For A Monitoring Program

It is important to emphasize that conclusions drawn from these analyses are contingent on the initial statement of a monitoring program's objectives. Power estimates are influenced by many factors controlled by researchers, such as duration and interval of monitoring, count means and variances, and number of sites and counts made per season. Several other, somewhat arbitrary factors also exert an important influence on power estimates. These include trend strength (effect size), significance level (type I error rate), and the number of tails to use in statistical tests. It is therefore critical that animal ecologists establish explicit and well-reasoned monitoring objectives before the initiation of any monitoring program (Steidl et al. 1997; Thomas 1997). These goals should address what magnitude of change in the population index is sought for detection, what probability of false detections will be tolerated (a type I error = a), and what frequency of true declines can go undetected (a type II error = P, with power = 1 - P). An initial statement of objectives is important because subsequent efforts to judge the success or failure of a monitoring program are made in terms of those objectives.

■ Conclusions

Identifying change in local populations is fraught with difficulties. Dubious population indices, bias in selection of survey sites, and weak design of monitoring programs can undermine trend detection. The practice of assessing population change in animal ecology could therefore be improved substantially. First, one should not blindly assume that any readily measured population index can serve as a valid proxy for estimating actual abundance. As an alternative, performing simple pilot studies to ascertain the basic relationship between the index used and actual abundance will give animal ecologists much insight. Such pilot studies can indicate whether the index used might yield misleading results, how it might be modified, and how it could potentially compromise trend detection. Second, animal ecologists must be aware of the potential pitfalls of nonrandom schemes for selecting sites for monitoring. A major challenge is to devise sampling methods that permit unbiased and statistically powerful surveys to be made in a logistically feasible manner. Finally, conducting power analyses during the pilot phase of a monitoring program is critical because it permits an assessment of a program's potential for meeting its stated goals while the opportunity for altering the program's structure is still available. The simulation method outlined and the summary of taxon-specific index variabilities can provide animal ecologists just such an option.

Successful monitoring of populations is based on making the best choices among sampling designs that yield precise estimates of a population index, statistical power considerations (trend strength, sample size, index variability, a,

Table 7.3 Sampling Intensities Needed to Detect Overall Population Changes of 50%, 25%, and 10% over 10 Years of Annual Monitoring of Animal Populations

Table 7.3 Sampling Intensities Needed to Detect Overall Population Changes of 50%, 25%, and 10% over 10 Years of Annual Monitoring of Animal Populations


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