Geoscience Reference
In-Depth Information
studies of birds, moths, and butterflies failed to present raw counts that could
be detrended and standardized, so the means and error terms as presented in
these studies were used. The index variabilities for these groups are therefore
potentially biased high in relation to those estimates for other taxa). CVs were
subsequently averaged within groups of taxonomically and ecologically related
species.
A total of 512 time series for local animal and plant populations were ana-
lyzed (appendix 7.1), which provided estimates to calculate average index vari-
abilities for each of 24 separate taxonomic and ecological groups (table 7.2).
Few groups had low variability indices (CV below 25 percent), including large
mammals, grasses and sedges, and herbs. A larger number had intermediate
variability indices (CV 25-50 percent), including turtles, terrestrial salaman-
ders, large birds, lizards, salmonid fishes, and caddis flies. Most groups had
indices with CVs between 50-100 percent, including snakes, dragonflies,
small-bodied birds, beetles, small mammals, spiders, medium-sized mammals,
nonsalmonid fishes, pond-breeding salamanders, moths, frogs and toads, and
bats. Finally, only butterflies and drosophilid flies had average indices with
CVs above 100 percent. Although a pilot study is clearly preferable, lacking
one of their own animal ecologists can refer to the specific studies (appendix
7.1) or to the summary (table 7.2) for information useful for designing moni-
toring programs for a particular species.
It is important to note that index variabilities (table 7.2) reflect temporal
variation inherent in populations as well as sampling error associated with the
counting methods. For example, direct count methods were used most often
for those groups with the lowest index variability, including large mammals, all
plants, terrestrial salamanders, and large-bodied birds. An exception was but-
terflies, which typically were counted with time-constrained visual searches.
Nets and traps were used to capture individuals in most remaining groups.
Trapping methods that sampled only a segment of a population (e.g., frogs,
toads, and pond-breeding salamanders on breeding migrations) or that relied
on attractants (e.g., most small- and medium-sized mammals at bait stations,
moths and caddis flies at light traps, and drosophilid flies at fruit baits) were
associated with high index variabilities. Similarly, most studies of small-bodied
birds were based on counts of singing individuals and also displayed high vari-
ability. Both method-associated sampling error and inherent population vari-
ability clearly make important contributions to overall index variability, and
the recommendations that follow assume that researchers will use the same
standardized counting methods used by the researchers who generated the
count series analyzed here (appendix 7.1).
