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nonrandom site selection schemes may be the most practical way to organize
sampling for monitoring programs. However, animal ecologists would do well
to be aware of the serious and lasting potential consequences of nonrandom
site selection. Researchers initiating a survey program are often drawn to sites
with abundant populations, where counts are initiated under the rationale that
visiting low-density or unoccupied sites will be unproductive. If the popula-
tions or habitats under study cycle, however, then initial counts may be made
at cycle peaks. As time progresses, populations at the sites selected will then
tend, on average, to decline. The resulting pattern of decline observed in
counts is an artifact of site selection procedures and does not reflect any real
population trend. This sampling artifact can lead researchers to make erro-
neous conclusions about regional population trends. This problem has com-
promised a regional monitoring program for amphibians (Mossman et al.
1994) and regional game bird surveys (Foote et al. 1958).
These examples highlight why site selection can be an important pitfall in
designing monitoring programs. Unfortunately, few simple recommendations
can be made for guiding the process. A detailed knowledge of habitat associa-
tions of the species under study, as well as the distribution of those habitats in
a region, can provide useful guidance to animal ecologists in selecting a sam-
pling design that is logistically feasible to monitor. Stratifying (or blocking)
sampling effort based on major habitat features such as land cover type will
almost always yield gains in precision of population estimates each sampling
interval (see Thompson 1992). Specifically, researchers would do well to iden-
tify species-habitat associations and generate regional habitat maps before ini-
tiating surveys so that the explicit tradeoffs between alternative sampling
schemes, logistical costs, and sampling bias can be evaluated. One workable
solution to this problem involves two steps. First, populations at selected sites
that are presumably representative of particular habitat strata in a region are
rigorously monitored. Second, an independent program is established that
explicitly monitors changes in the distribution and abundance of habitats in
the region. Trends in habitats can then be linked to trends in populations at
specific sites to extrapolate regional population trends.
Monitoring Indices Over Time
j
Once animal ecologists attempting to monitor populations have addressed
issues of index validity and sampling schemes for selecting survey sites, another
set of issues related to the intensity of monitoring over time must be consid-
ered. These issues include how many plots to monitor, how often to survey plots
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