Geoscience Reference
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(Keith et al. 1984). Snow (Halpin and Bissonette 1988; Fuller 1991; Brown
and Litvaitis 1995) and vegetation structure (Beier and Drennan 1997) also
may influence the availability or vulnerability of prey to carnivores. Likewise,
the presence of predators can affect the distribution of foraging activity by
herbivores (Lima and Dill 1990; Barbour and Litvaitis 1993). As a result,
many investigators have relied on an estimate of food abundance as a surrogate
to availability. Is this an appropriate compromise? Estimates of relative abun-
dance are often in units that do not necessarily correlate with density or bio-
mass, such as captures per 100 trapnights or individuals observed per kilome-
ter of transect surveyed (Windberg and Mitchell 1990). Obviously, large
differences between consumption and relative abundance are pertinent to
understanding forage or prey selection. However, studies that incorporate esti-
mates of food abundance without considering the limitations of estimates of
availability should be viewed as inferential.
CAFETERIA EXPERIMENTS
The ability of an investigator to provide equal access to all foods as a method of
identifying preference is obviously limited to captive situations. Rather than
simply identifying preferred foods, most researchers who have used cafeteria
experiments have attempted to identify the components that affect diet com-
position (Rodgers 1990) and therefore complement field observations (Top-
ping and Kruuk 1996). For example, Klein (1977) observed that snowshoe
hares (
Lepus americanus
) consumed twigs from sprout growth disproportion-
ately less than twigs from older trees. Nutritional analysis (including protein
content) did not explain this differential consumption. Klein speculated that
hares might have been responding to antiherbivore chemicals in juvenile plants.
Bryant and colleagues (Bryant 1981; Bryant et al. 1994) later examined this
relationship by extracting resins from juvenile plants and then compared con-
sumption rates of resin-coated and uncoated twigs by captive hares, providing
experimental evidence that these compounds do exist and may indeed explain
foraging preferences by snowshoe hares (but see Sinclair and Smith 1984).
Innovations
j
IMPROVEMENTS ON LEAD ANIMAL STUDIES
Recent improvements in the use of lead animals may cause this approach to
become more commonly used if very detailed information on food use is
