Geoscience Reference
In-Depth Information
individuals (e.g., radiocollared animals) but habitat availability is considered
the same for all individuals (so individuals are typically pooled for analysis),
and one in which use and availability are measured and compared for each
individual. They also reviewed papers published in the
Journal of Wildlife
Management
(1985-1988) and found that nearly twice as many studies col-
lected data on individuals but pooled them for analysis than either of the other
two approaches.
Studies that pooled animals for analysis have commonly compared fre-
quencies of use and availability for an array of habitats using a chi-square test.
Two-thirds of the use-availability studies that I reviewed (61 of 90) did this.
Determination of which habitat types were used more or less than expected is
generally made by comparing availability of each habitat type to Bonferroni
confidence intervals around the percentage use of each type. This procedure
was described initially by Neu et al. (1974) and clarified by Byers et al. (1984),
although a more accurate method of constructing such confidence intervals
was recently proposed by Cherry (1996). If the areas of available habitats are
estimated (e.g., from sampling) rather than measured (e.g., from a map), use
and availability should be compared with the chi-square test for homogeneity
rather than goodness-of-fit (Marcum and Loftsgaarden 1980). A chi-square
goodness-of-fit test assumes that the availabilities are known constants against
which use is compared, so if availabilities are actually estimated, with some
sampling error, this test is more prone to indicate selection when there is none
(type I error) (Thomas and Taylor 1990).
Various other methods of comparing use and availability have been
advanced but less often used in wildlife habitat studies. Ivlev (1961) proposed
an electivity index to measure relative selection of food items on a scale from
-1 to 1; this has since been adopted for some habitat selection studies. How-
ever, Chesson (1978, 1983) noted that Ivlev's index may yield misleading
results because it varies with availability even if preference is unchanged, and
advocated use of a 0 to 1 index originally proposed by Manly et al. (1972), also
for feeding preference studies. This Manly-Chesson index is simply the pro-
portional use divided by the proportional availability of each habitat, stan-
dardized so the values for all habitats sum to 1. As adapted to habitat studies,
it is interpretable as the relative expected use of a habitat had all types been
equally available (i.e., preference). Thus in an area with four habitats, an index
of 0.25 for each habitat would indicate no preference, whereas deviations from
this would indicate relative preference for or against certain habitat types.
Heisey (1985) and Manly et al. (1993) extended this method to test for differ-
ences in habitat preference among individuals or sex-age groups, and also
showed how to test for statistically significant differences among preferences
