Long-term effects of changes in goose grazing intensity on arrowgrass populations: a spatially explicit model

1 Field studies on effects of geese on arrowgrass (Triglochin palustris) on the Yukon-Kuskokwim Delta (SW Alaska) have demonstrated that Pacific black brant geese (Brant bernicla nigricans) can have both positive and negative effects on arrowgrass populations, but cannot predict unambiguously the ef fects of increased goose numbers on arrowgrass demography. 2 A cellular automata model was used to predict effects of changes in goose grazing intensity on small-scale (within-patch) arrowgrass dynamics. We ex amined effects of making some of the plant competitors edible to geese, of goose faeces increasing arrowgrass reproduction but reducing size of ungraz ed arrowgrass, and of the presence of other species protecting arrowgrass f rom grazing. We also compared the effects of a random vs. patchy distributi on of geese, and of incorporating threshold numbers of arrowgrass below whi ch grazing ceased. 3 The results indicate that arrowgrass populations are likely to be highest at medium to high levels of grazing. Inclusion of edible competitors and p ositive effects of faecal deposition resulted in greater changes in arrowgr ass population dynamics than did inclusion of associative refuges. 4 For a given grazing intensity, models generally resulted in lower arrowgr ass populations with increased aggregation if distributions of geese were p atchy, suggesting that decreased colonization may result from lower dispers al. 5 Inclusion of a feedback effect (grazing only above a certain plant popula tion) caused arrowgrass populations to persist for much longer. Temporal va riability in whether plots were grazed (unrelated to arrowgrass numbers) co uld not account for this result. 6 The model results suggest that knowledge of both small-scale and large-sc ale foraging behaviour is needed to predict the long-term effects of goose grazing on arrowgrass. Small-scale effects on the population may be particu larly important where dispersal distances are short. The ability of plant p opulations to persist locally may be increased if grazing is suspended when herbivory reduces forage plants below a threshold level.