Geoscience Reference
In-Depth Information
needed. Hand-reared animals can be equipped with radiotransmitters and
released into the wild several months before data collection. Lead animals are
often visited by their handler to remain acclimated to humans, but should no
longer be naive about natural foods (Heim 1988). Use and availability can be
accessed by walking with the animal as it feeds, recording use, and returning to
measure availability along a marked trail (Heim 1988).
USE OF ISOTOPE RATIOS
Biogeochemists have demonstrated the utility of comparing the relative con-
centration of various isotopes of carbon, nitrogen, and sulfur to reveal ele-
mental cycles (Petersen and Fry 1987). This approach has been applied to
examine current (Hobson and Clark 1992; Hilderbrand et al. 1996) and his-
toric (Chisholm and Schwarcz 1982; Tieszen et al. 1989) food use patterns of
some vertebrates. Essentially, the analysis of carbon isotopes can be used to
determine the relative contributions of marine and terrestrial sources to an
individual's carbon pool (Ramsey and Hobson 1991; Hobson and Welch
1992). In marine ecosystems, carbon enters as a bicarbonate and
13
C is
enriched relative to terrestrial ecosystems (Petersen and Fry 1987; figure 5.2).
This information can then be used to compare the importance of marine and
terrestrial sources of forage. For example, in the northwestern United States,
diets of historic populations of brown bears (
Ursus arctos
) were assessed by
examining isotopic signatures from hair and bone collagen from specimens
collected 140 years ago (Hilderbrand et al. 1996). The relative abundance
13
C
also can be used to examine consumption of C
4
(many grasses) versus C
3
(trees
and shrubs) plants (Tieszen et al. 1989). Nitrogen-15 is enriched at each step
of the food chain (DeNiro and Epstein 1980), probably because the preferen-
tial excretion for lighter
14
N in urine (Peterson and Fry 1987; figure 5.3).
EXPERIMENTAL MANIPULATIONS
Although it is widely accepted that populations of vertebrates are ultimately
limited by the abundance of food, there has been substantial controversy
among ecologists on how food abundance affects population dynamics and
community structure (Lack 1954; Hairston et al. 1960; Wiens 1977; White
1978). This has led a number of investigators to experimentally manipulate
food availability. Most of these studies were concerned with understanding
how food supply affected such characteristics as reproduction and density, not
with food selection or preference. In his review of more than 130 studies,
