Geoscience Reference
In-Depth Information
The purpose of this chapter is to assess key assumptions made by animal
ecologists attempting to identify population change and to make practical
suggestions for improving the practice of population monitoring. This is done
within a framework of statistical power analysis, which incorporates the
explicit tradeoffs animal ecologists make when attempting to obtain statisti-
cally reliable information on population trends in a cost-effective manner
(Peterman and Bradford 1987). The chapter covers five topics. First, the use
and misuse of population indices are reviewed. Second, sampling issues related
to the initial selection of sites for monitoring are discussed. Third, a numerical
method is described for assessing the balance between monitoring effort and
power to detect trends. Fourth, a review of the most critical influence on
power to detect trends in local populations, the temporal variability inherent
in populations, is presented, based on an analysis of over 500 published, long-
term counts of local populations. Fifth, the numerical method and variability
estimates are integrated to generate practical recommendations to animal
ecologists for improving the practice of monitoring local populations.
Index-Abundance Relationships
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TYPES OF INDICES
Making accurate estimates of absolute population size is difficult. Animals
often are difficult to capture or observe, they are harmed in the process, or the
associated costs and effort of making absolute counts or censuses are prohibi-
tive. Therefore, animal ecologists often rely on indices of population size and
monitor these indices over time as a proxy for monitoring changes in actual
population size. Indices may be derived from sampling a small fraction of a
population using a standardized methodology, with index values expressed as
individuals counted per sampling unit (e.g., fish electroshocked per kilometer
of shoreline, tadpoles caught per net sweep, salamanders captured per pitfall
trap, birds intercepted per mist net, or carcasses per kilometer of road). These
examples involve direct counts of individuals. When individuals of a species
under study are difficult to capture or observe, another class of indices makes
use of indirect evidence to infer animal presence. Auditory cues are often used
as indirect indices (e.g., singing birds per standard listening interval, overall
sound volume produced by insect aggregations, howling frequency by packs of
wild canids, or calling intensity in frog choruses). Other indirect indices are
based only on evidence of animal activity (e.g., droppings per unit area, tracks
per unit transect length or per bait station, or quantity of food stored per den).
