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Figure 5.2 Relationship between 13 C signatures of the diet of equilibrated plasma in black bears
(Ursus americanus, (•) and polar bears (U. maritimus, (
m
). Copyright 1996 Hilderbrand et al.
Reprinted by permission of the Canadian Journal of Zoology.
Boutin (1990) found that most studies increased food abundance. Among
individuals with access to supplemental foods, there were some clear tenden-
cies: reduced home ranges, increased body weights, and advanced age and tim-
ing of reproduction. Populations typically doubled to tripled in density where
food was essentially unlimited. However, food addition did not prevent
declines in fluctuating populations (Boutin 1990).
The ability of herbivores to shape the composition and structure of vegeta-
tion via their foraging patterns has generated considerable interest in a variety
of ecosystems (Bazley and Jefferies 1986; Brown and Heske 1990; Huntly
1991; Jones et al. 1994, Johnston 1995). This has also been the subject of sev-
eral experimental manipulations. For instance, Ostfeld and Canham (1993)
planted seedlings and monitored herbivory by meadow voles ( Microtus penn-
sylvanicus ) to reveal that this small mammal can effectively delay the invasion
of trees into old-field communities. Spatial variation in foraging by voles also
may create a mosaic of woody and nonwoody vegetation in these sites (Ostfeld
and Canham 1993).
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