Tall swards and small grazers : competition, facilitation and coexistence of different-sized grazers

Differences in body mass are assumed to be a major factor leading to resource partitioning and the reduction of competition between species within a guild. To study the effects of body mass on foraging behaviour of grazers independently of morphological adaptations we used intra-specific size differences between subspecies of the Canada goose Branta canadensis and between breeds of domestic rabbits Oryctolagus cuniculus. First we measured instantaneous intake rate and daily intake on small, monospecific Lolium perenne plots. The different goose sizes showed very similar dome-shaped functional responses, with an optimum at low grass biomass. On a daily time scale food intake in rabbits was little affected by sward characteristics and scaled with body mass to the power of 0.75, just as metabolic requirements. We did not find differences in food digestibility between breeds. When comparing food selection on the plant/leaf level, there were no differences in the choice of leaves between goose size classes; however, the large geese removed a larger fraction of each individual leaf. Consequently, patch use in an allopatric situation did not reveal large differences between the size classes; all geese preferred short swards. The largest geese used taller swards more than the two smaller subspecies. When all three size classes were grazing in the same enclosure patch use did not differ from the allopatric situation: the small and intermediate-sized geese were very similar and the largest size class again used taller patches more than the other two, but all showed a preference for the shortest patches. Patch depletion negatively affected foraging efficiency in the short term as daily foraging time increased with increasing depletion and the geese showed a preference for ungrazed patches. Furthermore our results indicate that patch depletion affected the larger geese more than the smaller ones. Therefore competitive exclusion of the larger geese by the smaller geese will occur on very short swards.