MODIFICATION OF ECOSYSTEMS BY UNGULATES

Ecosystem ecologists traditionally have focused their attention on dir ect interactions among species, particularly those interactions that c ontrol flows of energy and materials among trophic levels. Emerging ev idence suggests that indirect interactions may be more important than direct ones in determining ecosystem patterns and processes. Here I re view indirect effects of ungulates on nutrient cycling, net primary pr oduction, 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 avail able 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 di et, 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 incr ease or decline in response to ungulate grazing. The direction of this response depends on the intensity of grazing or browsing, the evoluti onary history of the ecosystem, and the opportunity for regrowth. Oppo rtunity for regrowth is determined by physiological and morphological characteristics of the plant as well as environmental conditions, part icularly the extent and timing of moisture availability. Ungulates inf luence fire regimes by altering the quality and quantity of fuels avai lable 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 reduci ng the likelihood of surface fires. I develop the case that the way th at ungulates influence ecosystem process is contingent on historical c ontext, in particular the long-term context provided by plant-animal c oevolution and soil development and the short-term context created by climate and weather. I show that ungulates are important agents of cha nge in ecosystems, acting to create spatial heterogeneity, modulate su ccessional processes, and control the switching of ecosystems between alternative states.