Geoscience Reference
In-Depth Information
approach include the efforts to reduce large carnivores in North America.
Detailed investigations have revealed that the actual nuisance individuals may
be only a portion of the population and depredation may be restricted to a lim-
ited time period (Till and Knowlton 1983). As a result, control efforts can be
more exact and cause less ecological damage.
A recent motivation to examine animal food is the emergence of environ-
mental assessments during large-scale habitat alterations, such as road con-
struction and commercial timber harvesting. Biologists must identify the
important food and cover resources in an affected area if they are to mitigate
the effects of these activities.
Regardless of the specific question being addressed, nearly all investigations
of food habits can be distilled to two basic questions. What is the importance
of a specific food to the fitness of an organism (especially survival and repro-
ductive success)? How does the feeding niche of an organism affect commu-
nity composition? In this chapter I compare the relative effectiveness of meth-
ods commonly used to investigate food habits and how appropriate each
method is at answering these questions. I also consider several recent innova-
tions and how potential improvements may improve our ability to understand
the significance of food usage. Readers interested in learning more about the
actual procedures should consult references by Korschegen (1980), Cooper-
rider (1986), Reynolds and Aebischer (1991), and Litvaitis et al. (1994).
Conventional Approaches and Their Limitations
j
DIRECT OBSERVATION
Direct observations have been widely applied to document the forage or prey
used by a variety of species. Individual animals or groups are observed through
binoculars as they graze or feed on an animal carcass. Observations at bird
nests also have provided information on foods brought to juveniles (Errington
1932; Marti 1987; Bielefedt et al. 1992). The basic approach is simple and
relies on limited equipment. For researchers studying herbivores, bite counts
or feeding minutes by plant species are recorded. These values can then be
translated into relative occurrence in the diet by comparing total bites or min-
utes of foraging and the contribution of each species to the total observed. Bio-
mass consumed can be approximated by estimating the average mass per bite
for each species incorporated in the diet (Smith and Hubbard 1954). Addi-
tionally, direct observations are useful in identifying differences in foraging
among sexes or age classes (Illius and Gordon 1987). Unfortunately, this tech-
nique is hampered by several limitations (table 5.1). Observations are usually
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