Geoscience Reference
In-Depth Information
may be stimulated by group size in frogs, individuals may not call (or may do
so infrequently) when choruses are small and may be overlooked by frog coun-
ters, but increasing numbers of calling frogs above a certain threshold may also
be indistinguishable to frog counters.
Occasionally indices used have no relationship to abundance (figure 7.1f ),
although sometimes an apparent lack of an index-abundance relationship
may simply be a result of sampling error or too few samples taken to verify
the relationship (Fuller 1992; White 1992). Nevertheless, the possibility that
a seemingly reasonable, readily measured index has no relationship to the
actual population must always be considered by animal ecologists using an
unverified index, and preferably be examined as a null hypothesis during a
pilot study.
VARIABILITY OF INDEX-ABUNDANCE FUNCTIONS
Independent of the specific form of the index-abundance relationship, most
researchers assume it to be constant among habitats and over time. However,
in perhaps the most comprehensive validation study of an indirect index, a
study by Reid et al. (1966) on mountain pocket gophers (
Thomomys talpoides
),
the index used (numbers of mounds and earth plugs) consistently displayed a
positive, linear relationship to actual gopher numbers, whereas the intercept
and slope varied substantially between habitats (figure 7.2a, b). Other situa-
tions, such as electroshocking freshwater fishes, apparently yield comparable
index-abundance relationships between habitats despite large differences in
densities between habitats (figure 7.2c, d). In contrast, index-abundance rela-
tionships in different habitats can be reversed (figure 7.2e, f ) although these
examples may be compromised by sampling error. Finally, the slope, intercept,
and precision of the relationship may vary among years within the same habi-
tats (figure 7.3a, b, c).
Inferences about population change drawn from indices are also often
hampered by sampling error. Whatever the form of the index-abundance rela-
tionship between habitats and over time, the precision of the relationship can
be quite low (figure 7.1d, e). This is particularly true for indirect indices, in
which variation is strongly influenced by environmental factors such as
weather and time of day, as well as by observers (Gibbs and Melvin 1993).
Such index variation can substantially reduce the power of statistical tests
examining changes in index values between sites or over time (Steidl et al.
1997).
