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.