| Summary: | 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<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<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.
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