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as duration and interval of monitoring, count means and variances, and num-
ber of sites and counts made per season. Several other, somewhat arbitrary fac-
tors 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 estab-
lish 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
b
b
error =
). An initial statement of objectives is important
because subsequent efforts to judge the success or failure of a monitoring pro-
gram are made in terms of those objectives.
, with power = 1 -
Conclusions
j
Identifying change in local populations is fraught with difficulties. Dubious
population indices, bias in selection of survey sites, and weak design of moni-
toring programs can undermine trend detection. The practice of assessing
population change in animal ecology could therefore be improved substan-
tially. First, one should not blindly assume that any readily measured popula-
tion 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 sta-
tistically 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, sta-
tistical power considerations (trend strength, sample size, index variability,
a
,
