Understanding Ungulate Herbivory–Episodic Disturbance Effects on Vegetation Dynamics: Knowledge Gaps and Management Needs

Abstract

Herbivory by wild and domestic ungulates is a chronic disturbance that can have dramatic effects on vegetation dynamics. Herbivory effects, however, are not easily predicted under different combinations of episodic disturbance such as fire, timber harvest, drought, and insect defoliation. This lack of predictability poses a substantial obstacle to effective management of ungulate herbivory. Traditional models of vegetation transition in forested ecosystems have ignored the influences of ungulate herbivory, while research on effects of herbivory have typically excluded other disturbances. Of the 82 contemporary studies on ungulate herbivory we examined, only 15 (18%) considered the interactions of herbivory with episodic disturbances. Moreover, only 26 (32%) evaluated vegetation response to ungulate herbivory beyond the simplistic treatment levels of herbivory versus no herbivory. Only 31 (38%) used a repeated‐measures design of sampling responses over 3 or more time periods. Finally, just 7 (9%) explicitly made inferences to large landscapes such as watersheds, which are often used for management planning. We contend that useful landscape research on herbivory must examine the interactions of ungulate grazing with other disturbance regimes at spatial extents of interest to forest and rangeland managers and under varying ungulate densities and species. We identify herbivory models that could accommodate such information for forested landscapes in western North America. Such models are essential for identifying knowledge gaps, designing future studies, and validating relations of ungulate herbivory on landscapes where episodic disturbances are common, such as those of western North America.

Changes in vegetative cover on Western Arctic Herd winter range from 1981 to 2005: potential effects of grazing and climate change

“…Grazing is an important ecosystem driver (Hobbs, 1996;Augustine & McNaughton, 1998;Mysterud, 2006;Wisdom et al, 2006). Our results are in agreement with the assertion that grazing by caribou can be an important factor in the depletion of lichen of large areas of continental ranges (Moser et al, 1979;Messier et al, 1988, Arseneault et al, 1997 and the rapid transition from lichen-dominated to graminoid-dominated tundra communities (van der Wal, 2006).…”

The differential influences of human‐induced disturbances on tree regeneration community: a landscape approach

“…The impacts of human-induced disturbances on forest ecosystems have been extensively reported throughout the world (Belsky and Blumenthal 1997, Augustine and McNaughton 1998, Wisdom et al 2006, Baraloto et al 2012, Clark and Covey 2012. Forest degradation by both intense, episodic disturbances (e.g., extensive logging or forest conversion to other land uses) and low intensity, chronic disturbances (e.g., grazing, selective logging or invasion of exotic species and fires) have been studied in several forests (Belsky and Blumenthal 1997, Ramírez-Marcial et al 2001, Stern et al 2002, Timmins 2002, Fisher et al 2009, Baraloto et al 2012, Clark and Covey 2012.…”

Reintroducing fire into a ponderosa pine forest with and without cattle grazing: understory vegetation response

“…Exclusion of livestock allows a researcher to isolate fire or other treatment effects, but application of results from such studies to realistic management situations where livestock grazing is a factor may be misleading. Ungulates can exert important influences on ecosystem processes and have profound direct and indirect effects on vegetation development and species composition (Hobbs 1996, Weisburg and Bugmann 2003, Wisdom et al 2006, Bakker and Moore 2007. Livestock consume and reduce the abundance of grasses and the herbaceous understory, and can shift dominance from palatable perennial bunchgrasses to more unpalatable grasses and herbaceous species, and aid shrub and tree encroachment (Madany and West 1983, Belsky and Blumenthal 1997, Augustine and McNaughton 1998, Jones 2000.…”