Modification of Ecosystems by Ungulates

Ecosystem ecologists traditionally have focused their attention on direct interactions among species, particularly those interactions that control flows of energy and materials among trophic levels. Emerging evidence suggests that indirect interactions may be more important than direct ones in determining ecosystem patterns and processes. Here I review indirect effects of ungulates on nutrient cycling, net primary production, and disturbance regimes in terrestrial ecosystems. Ungulates influence the nitrogen (N) cycle by changing litter quality, thereby affecting conditions for N mineralization, and by adding readily available N to upper levels of the soil in urine and feces. As a result of these additions, natural heterogeneity in the spatial distribution of N within landscapes is amplified by ungulate selection of habitats and patches. The magnitude of returns of plant N to the soil in urine and feces is a function of animal body mass and characteristics of the diet, particularly N content and levels of tannin. Effects on N cycling can cascade throughout the ecosystem, and can stabilize or destabilize the composition of plant communities. Net primary production can increase or decline in response to ungulate grazing. The direction of this response depends on the intensity of grazing or browsing, the evolutionary history of the ecosystem, and the opportunity for regrowth. Opportunity for regrowth is determined by physiological and morphological characteristics of the plant as well as environmental conditions, particularly the extent and timing of moisture availability. Ungulates influence fire regimes by altering the quality and quantity of fuels available for combustion. In grasslands, ungulates often reduce the extent, frequency, and intensity of fires, while in shrublands and forests, their effects can increase the likelihood of crown fires, while reducing the likelihood of surface fires. I develop the case that the way that ungulates influence ecosystem process is contingent on historical context, in particular the long-term context provided by plant-animal coevolution and soil development and the short-term context created by climate and weather. I show that ungulates are important agents of change in ecosystems, acting to create spatial heterogeneity, modulate successional processes, and control the switching of ecosystems between alternative states. J. WILDL. MANAGE. 60(4):695-713