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Plant-herbivore dynamics associated with an erupting ungulate population : a test of hypotheses Larter, Nicholas C.

Abstract

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

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