Geoscience Reference
In-Depth Information
Chapter 7
Monitoring Populations
JAMES P. GIBBS
Assessing changes in local populations is the key to understanding the tempo-
ral dynamics of animal populations, evaluating management effectiveness for
harvested or endangered species, documenting compliance with regulatory
requirements, and detecting incipient change. For these reasons, population
monitoring plays a critical role in animal ecology and wildlife conservation.
Changes in abundance are the typical focus, although changes in reproductive
or survival rates that are the characteristics of individuals, or other population
parameters, also are monitored. Consequently, many researchers and managers
devote considerable effort and resources to population monitoring. In doing
so, they generally assume that systematic surveys in different years will detect
the same proportion of a population in every year and changes in the survey
numbers will reflect changes in population size.
Unfortunately, these assumptions are often violated. In particular, the fol-
lowing two questions are pertinent to any animal ecologist involved in popu-
lation monitoring. First, is the index of population abundance used valid?
That is, does variation in, for example, track densities of mammals, amphibian
captures in sweep nets, or counts of singing birds reliably reflect changes in
local populations of these organisms? Second, does the design of a monitoring
program permit a reasonable statistical probability of detecting trends that
might occur in the population index? In other words, are estimates of popula-
tion indices obtained across a representative sampling of habitats and with suf-
ficient intensity over time to capture the trends that might occur in the popu-
lation being monitored? Failure to address these questions often results in
costly monitoring programs that lack sufficient power to detect population
trends (Gibbs et al. 1998).
