Geoscience Reference
In-Depth Information
% Availability
Figure 4.4
The assumed linear relationship between use and availability of habitats can be tested,
to an extent, by plotting use and area of each habitat in individual home ranges. The figure depicts
such a plot for 20 hypothetical home ranges (each point represents habitat use and availability in
one home range) and two (of several) types of habitat (sums for both use and availability for all habi-
tat types for each individual would total 100%). The data suggest that increased availability of habi-
tat 1 prompted a corresponding increase in use. In habitat 2, however, the animals showed an
asymptotic relationship between use and availability (as in figure 4.2a); 10% availability seemed to
represent an approximate threshold, above which use no longer increased. That is, if at least 10% of
their home range was composed of habitat 2, the animals could obtain whatever resources they
needed from this habitat by spending 10-20% of their time there. A similar threshold may exist for
habitat 1 at availabilities >50%; assumptions should not be made beyond the data.
tions. These should be given greater emphasis. Oddly, Hansson (1996) sug-
gested that habitat-specific survival might confuse perceptions of habitat selec-
tion. In his small mammal study, perceived habitat selection appeared to be a
consequence rather than a cause of differential survival. It seems to me that
such knowledge of habitat-specific survival is exactly the desired objective;
habitat selection studies are just an indirect approach toward this end.
For many species, habitat-specific densities may be easier to measure than
habitat-specific reproduction or survival, but density studies may yield uncer-
tain or misleading results because density is the end result of various processes,
