Geoscience Reference
In-Depth Information
INDEX-ABUNDANCE FUNCTIONS
An index to population size (or abundance) is simply any “measurable correl-
ative of density” (Caughley 1977) and is therefore presumably related in some
manner to actual abundance. Most animal ecologists assume that the index
and actual abundance are related via a positive, linear relationship with slope
constant across habitats and over time. In some situations, these relationships
hold true (figure 7.1a, b, c). However, the relationship often takes other forms
in which changes in the index may not adequately reflect changes in the actual
population (figure 7.1d, e, f ).
A nonlinear (asymptotic) relationship may be common in situations where
the index effectively becomes saturated at high population densities. Such may
be the case for anurans monitored using an index of calling intensity (Moss-
man et al. 1994). The index is sensitive to changes at low densities of calling
male frogs in breeding choruses because calls of individuals can be discrimi-
nated by frog counters. At higher densities, however, calls of individual frogs
overlap to an extent that size variation of choruses cannot be discriminated by
observers. In other words, the index increases linearly and positively with
abundance to a threshold population density, and then becomes asymptotic.
Another example of a nonlinear index-abundance relationship concerns
use of presence/absence as a response such that the proportion of plots occu-
pied by a given species is the index of abundance. At low population densities,
changes in population size can be reflected in changes in degree of plot occu-
pancy. Once all plots are occupied, however, further population increases are
not reflected by the index because the index becomes saturated at 100 percent
occupancy. A final example involves bait stations for mammals (Conroy
1996), which may be frequented by subdominant animals more at low popu-
lation densities than at high densities because of behavioral inhibition. The
main implication of this type of nonlinear index-abundance relationship is
that it prevents detection of population change (in any direction) above the
saturation point of the index.
A threshold relationship also may occur in index-abundance relationships
if the index effectively bottoms out at low population densities. For example,
if sample plots are too small, listening intervals too short, or sample numbers
too few, observers may simply fail to register individuals even though they are
present at low densities (Taylor and Gerrodette 1993). Consequently, detec-
tion of population change below the threshold of the index is precluded. This
situation probably occurs in surveys for many rare, endangered, or uncommon
species (Zielinski and Stauffer 1996). The threshold and saturation phenom-
ena can combine in some situations. For example, because calling behavior
