Generalities in grazing and browsing ecology: using across-guild comparisons to control contingencies.

In community ecology, broad-scale spatial replication can accommodate contingencies in patterns within species groups, but contingencies in processes across species groups remain problematic. Here, based on a focused review of grazing and browsing by large mammals, we use one trophic guild as a "control" for the other to identify generalities that are not contingent upon specific consumer-resource interactions. An example of such a generality is the Jarman-Bell principle, which explains how allometries of metabolism and digestion influence dietary tolerance and thereby enable resource partitioning within both guilds at multiple scales. By comparing the grazing succession with browsing stratification we show how competition from smaller herbivores, rather than facilitation from larger ones, is the underlying process structuring ungulate assemblages when shared resources become limiting. Also, grazing lawns and browsing hedges are functionally similar. In each case, plants expressing tolerance traits can withstand chronic grazing or browsing in sites where the nutritive value of the local food resource is enhanced in positive feedback to the actions of its consumers. The debate over whether ungulates accelerate or decelerate nutrient cycling can be resolved by comparing grazing and browsing effects in the same ecosystem type. Evidence from African savannas points to the rate of nutrient cycling being controlled by the mix of tolerance and resistance traits in plants; not the relative dominance of grazing or browsing by local herbivores. We recommend this across-guild comparative approach as a novel solution with widespread utility for resolving contingencies in community processes.