Plant-herbivore dynamics associated with an erupting ungulate population : a test of hypotheses
Creators: name: Larter, Nicholas C.
Publisher: name: University of British Columbia
Year: 2009

description: This study tests some of the predictions made by two competing hypotheses of plant community and herbivore dynamics: the equilibrium hypothesis, both the original four-stage model proposed by Riney (1964) and Caughley (1970a), and the two-stage model proposed by Sinclair (1979) and Houston (1982), and the facilitation and feedback hypothesis (McNaughton, 1979). This is one of the rare occasions where these predictions have been tested on an erupting indigenous herbivore population subjected to predation; the Mackenzie wood bison (Bison bison athabciscae) population which was reintroduced in 1963. Recently the population split into two subpopulations: the Mackenzie Bison Sanctuary (MB S) which is stabilizing, and the Mink Lake (ML) which is increasing. The aims of this study were to determine 1) if the plant community dynamics were consistent with either of the hypotheses, 2) if herbivore dynamics and demographics were consistent with either of the hypotheses, and 3) the impact of predation on the system. Net primary production of sedges and grasses in areas of willow savannas that were excluded from grazing was similar in MBS and ML, but the standing crop in areas not excluded from grazing was consistently lower in MBS than ML. This difference appears to be a direct result of different grazing pressures. Species composition of willow savannas in MBS had more unpalatable and less preferred species than savannas in ML. These results were consistent with the predictions and assumptions of both models of the equilibrium hypothesis but not the facilitation and feedback hypothesis. Forage quality was not different between MBS and ML with the possible exception of higher levels of some cations found in forages growing in MBS. The two subpopulations appear to be distinct and are at different stages of eruptive oscillation. The demographic characteristics of these subpopulations agree with the predictions of the four-stage model of the equilibrium hypothesis: the MBS subpopulation experienced a negative instantaneous growth rate (r), animals in ML had significantly (p&lt;O.OO3) higher faecal nitrogen levels (an index of diet quality and animal condition), and animals in ML had winter diets containing superior quality items than did animals in MBS. One exception to the predictions was summer diet quality. Animals in MBS had summer diets of superior quality. I argue that this exceptional result is related to predation. The four-stage model of the equilibrium hypothesis assumes that predation will not affect the predictions, but has not previously been tested in systems with a predator. During the past 20 years bison numbers have increased, especially in MBS, while moose numbers appear to have decreased. Moose density in MBS was half that of ML. Bison and moose constituted the majority of the wolf diet, based upon wolf scat analysis. Bison made up a larger proportion of the wolf diet in MBS than in ML, however the occurrence of moose in scats was significantly greater (p&lt;O.O0l) than expected given the availability of prey biomass in both MBS and ML. Wolf activity was greater in MBS than in ML. Given that moose make up a similar proportion of the diet in both areas, and there was a two-fold difference in moose densities between area, wolf predation may be destabilizing and exacerbating the decline in moose numbers.