Geoscience Reference
In-Depth Information
occurred. If more than one species is known to forage in the area, than there
must be a way to differentiate consumers (e.g., foraging by lagomorphs versus
foraging by ungulates). Among herbivores, grazed grasses and forbs or browsed
twigs are counted and converted into percentages of the total forage con-
sumed. For woody plants, it is possible to estimate the biomass consumed by
measuring the diameter of the twig at the point it was clipped and using regres-
sion tables specific for each species (Basile and Hutchings 1966; Telfer 1969).
Such surveys may be biased by “invisible consumption” when an entire plant
is consumed and there is no residual indicating the occurrence of that plant
(McInnis et al. 1983). For carnivores, this approach has been used to identify
the age, sex, and physical condition of large prey (Mech 1966; Sinclair and
Arcese 1995), but provides no information on prey that are eaten whole or
cached. A major disadvantage of this approach is the lack of opportunity to
investigate sex or age differences in food use (table 5.1).
Difference comparisons
This approach is restricted to herbivores and relies on comparisons between
used and unused plots or sites surveyed before and after foragers have passed
through an area. Species consumed (based on inventory) or biomass removed
can be estimated by clipping used and unused plots (Bobek et al. 1975; Cook
and Stubbendieck 1986). An obvious limitation of used and unused
sites is the ability of the investigator to locate comparable sites. Also, unless the
investigator is able to verify nearly exclusive use of a site by the herbivore under
investigation, before and after comparisons may be clouded by a variety of her-
bivores foraging at the same site (table 5.1). In general, difference methods do
not detect small differences in use. Therefore, they should not be considered
unless use is expected to be more than 50 percent on the forage species (Coop-
errider 1986).
A more accurate but labor-intensive technique for assessing use has been
applied in the western United States (Nelson 1930; Smith and Urness 1962).
Stems of potential browse species are tagged and total length of annual twig
growth is measured during autumn. In spring, the investigator returns and
measures the length of annual growth that remains. Although the technique
does not directly measure biomass removed, the length of the twig removed is
highly correlated with the amount of forage removed (Smith and Urness
1962). A limitation of this approach is that it requires some knowledge of the
plants that are likely to be consumed (table 5.1).
