No abstract

Balsam fir (Abies balsamea) population parameters were analyzed in Isle Royale National Park, Michigan, to asses moose (Alces alces andersoni) herbivory in relation to the population density of both fir and moose. Fir population parameter were determined at nine study sites, each representing a different combination of relative fir and moose density (high, medium, and low levels, respectively). In general, moose suppression of fir height growth and recruitment to the canopy increased with increasing moose density and decreasing fir density. Where fir densities were low, sapling heights were restricted to <1 m regardless of moose density. In such sites, sapling growth suppression has been continuous since a peak in the moose population in the 1920s. Canopy fir trees at most sites established prior to arrival of moose on Isle Royale around 1910. At high fir density release from growth inhibition was common following a period of low moose numbers from the mid—1970s to early 1980s. Here this release should facilitate recruitment of fir and effect replacement of canopy losses due to senescene. In other areas, however, continuing canopy looses and lack of recruitment due to intensive herbivory by moose will greatly reduce the fir component of the canopy, effectively preventing any return to the fir—dominated forests of the past.

The long-term influence of moose (Alcesalces) on the structure and species composition of three forest types in Isle Royale National Park, Michigan was evaluated using four moose-proof exclosures established in 1949–1950. Results obtained during the present study were compared with the original vegetation at each study site and with data collected by Krefting (1974) in order to discriminate between the effects of moose browsing and forest community succession. In the spruce–birch–fir climax and aspen–birch–conifer forest study sites, self-thinning and reduced light availability reduced the density of woody plant stems in exclosure areas. In the adjacent control areas, moose browsing has slowed the rate of succession. Repeated browsing by moose on palatable species has retarded vertical growth and has prevented stems from growing beyond reach. Effects of moose browsing on succession in the aspen–birch study sites in the 1936 burn were less clear. In all three forest types, stem densities of most species were higher in control areas than in exclosures. Species diversity was also greater in control areas. American yew (Taxuscanadensis) and red-osier dogwood (Cornusstolonifera) were the only species that declined in abundance as a result of browsing by moose.

Foraging theory predicts that animals should feed more selectively in response to an increased abundance of high-quality foods. We tested these predictions by examining changes in patterns of diet selection by white-tailed deer (Odocoileus virginianus) and Angora goats (Capri hircus) in response to habitat enrichment. When a high-quality supplement was provided ad libitum, both species responded by increasing foraging effort and by feeding more selectively on natural forages. Supplemented animals consumed a greater proportion of plants containing high concentrations of crude protein (CP) and digestible energy (DE), which resulted in an increase in average diet quality. Responses of individual deer and goats varied and appeared to be related to differences in habitat heterogeneity between trial enclosures. Our results suggest that supplementation may be disruptive to normal behavioral processes affecting the distribution of free-ranging herbivores on the landscape. These processes may be important in reducing the probability of excessive utilization of palatable forage species, and to the competitive relationships between range plants.