A cellular automata model was used to predict effects of changes in goose grazing intensity on small-scale (within-patch) arrowgrass (Triglochin palustris) dynamics. Mulder and Ruess examined the effects of making some of the plant competitors edible to geese, of goose feces increasing arrowgrass reproduction but reducing size of ungrazed arrowgrass, and of the presence of other species protecting arrowgrass from grazing. The results indicated that arrowgrass populations are likely to be highest at medium to high levels of grazing. For a given grazing intensity, models generally resulted in lower arrowgrass populations with increased aggregation if distributions of geese were patchy, suggesting that decreased colonization may result from lower dispersal. Inclusion of a feedback effect caused arrowgrass populations to persist for much longer. The model suggests that knowledge of both small-scale and large-scale foraging behavior is needed to predict the long-term effects of goose grazing on arrowgrass. Small-scale effects on the population may be particularly important where dispersal distances are short. The ability of plant populations to persist locally may be increased if grazing is suspended, when herbivory reduces forage plants below a threshold level.
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