Geoscience Reference
In-Depth Information
Table 4.1
Effect of Habitat Availability on Perceived Selection
Manly-Chesson
Selectivity
Index
a
Manly-Chesson
Standardized
Index
b
Comparison
Habitat
% Available
% Used
A vs B
A
50
25
0.5
0.25
B
50
75
1.5
0.75
C vs B
C
80
40
0.5
0.14
B
20
60
3.0
0.86
Chesson (1978) used this comparison (with foods instead of habitats) to demonstrate the advantages
of the Manly-Chesson index, but the lower standardized index for C than for A, despite C's greater
use, is not intuitive.
a
% Used/% available.
b
Selectivity indices standardized so that they sum to 1 (selectivity index divided by sum of selectivity
indices).
at all, may be true for only some resources, or may hold only within a narrow
range of availabilities. Manly et al. (1993) made the explicit assumption, appli-
cable for all models (except the previously discussed adaptation of Arthur et al.
1996) that availability remains constant for the period of study (if availability
changes seasonally, data can be analyzed by season). This may seem like a
benign assumption, but in reality it masks a fundamental weakness of the
process. Of what value are measures of selection if they are specific to a single
array of habitats? Measures of selection are supposed to be reflections of inher-
ent preference—expected choices when availabilities of all habitat types are
equal—so if selection appears to change as availability changes, then prefer-
ence cannot be inferred from perceived selection when availabilities of habitats
are unequal. In other words, if the goal is to assess habitat preferences for a
population of animals based on habitat selection observed among a collection
of individuals in that population, then something is amiss if selectivity appears
to differ among these individuals simply because they have different habitat
compositions available to them.
Consider a human analogy that demonstrates the effects of changes in
availability on perceived selection. While at home a person spends 50 percent
of the time sleeping and 20 percent preparing food and eating meals in the
kitchen; the bedroom occupies 20 percent of the area of the house, and the
