Herbivores forage in spatially complex habitats, and they must perceive and respond to spatial heterogeneity of food resources when deciding where, when, and for how long to forage. It has been hypothesized that smaller herbivores perceive heterogeneity at finer scales. This has rarely been evaluated experimentally, especially on a range of scales relevant to mammalian grazers. While holding constant the availability by area, we manipulated the spatial distribution of two forages with contrasting quality and measured the short term selectivity of two species of grazing herbivores, sheep and cattle, with similar dietary niche but differing body mass (which varied by one order of magnitude).
Results/Conclusions
Overall, food intake rates scaled allometrically with metabolic weight (mass0.75), but grazing strategies differed among the herbivore species. On the range of scales of selectivity examined, from bites (10-2 m2) to local patches (102 m2), smaller herbivores were more selective and exhibited more plasticity in scales of selectivity than larger herbivores. Nevertheless, both herbivore species were maximally selective when patch sizes were larger. When bite pattern was partitioned into scales ranging from 100 to 103 m2 using Moran’s eigenvector maps, sheep had a stronger component than cattle in the smallest scale (0-102 m2), particularly in the uniform food distribution, where no pattern was imposed by the food on the range of scales considered. When offered exactly the same (random) spatial configuration of forage options, grazing by sheep occurred preferentially at finer-scale patches of preferred forage, whereas, grazing by cattle occurred preferentially at coarser-scale patches. Herbivores did not express selectivity by adjusting the size or frequency of bites within each forage type, but rather by moving to preferred patches or feeding stations. Results demonstrate that the landscape distribution of forage options can be a strong determinant of foraging selectivity, and that scaling laws likely influence patch selection by foragers. These findings have important ramifications for both foraging theory and the management and conservation of grazing animals.
