Contrasting Effects of Different Mammalian Herbivores on Sagebrush Plant Communities

Abstract

Herbivory by both grazing and browsing ungulates shapes the structure and functioning of terrestrial ecosystems worldwide, and both types of herbivory have been implicated in major ecosystem state changes. Despite the ecological consequences of differences in diets and feeding habits among herbivores, studies that experimentally distinguish effects of grazing from spatially co-occurring, but temporally segregated browsing are extremely rare. Here we use a set of long-term exclosures in northern Utah, USA, to determine how domestic grazers vs. wild ungulate herbivores (including browsers and mixed feeders) affect sagebrush-dominated plant communities that historically covered ~62 million ha in North America. We sampled plant community properties and found that after 22 years grazing and browsing elicited perceptible changes in overall plant community composition and distinct responses by individual plant species. In the woody layer of the plant community, release from winter and spring wild ungulate herbivory increased densities of larger Wyoming big sagebrush (Artemisia tridentata, ssp. wyomingensis) at the expense of small sagebrush, while disturbance associated with either cattle or wild ungulate activity alone was sufficient to increase bare ground and reduce cover of biological soil crusts. The perennial bunchgrass, bottlebrush squirretail (Elymus elymoides), responded positively to release from summer cattle grazing, and in turn appeared to competitively suppress another more grazing tolerant perennial grass, Sandberg’s blue grass (Poa secunda). Grazing by domestic cattle also was associated with increased non-native species biomass. Together, these results illustrate that ungulate herbivory has not caused sagebrush plant communities to undergo dramatic state shifts; however clear, herbivore-driven shifts are evident. In a dry, perennial-dominated system where plant community changes can occur very slowly, our results provide insights into potential long-term trajectories of these plant communities under different large herbivore regimes. Our results can be used to guide long-term management strategies for sagebrush systems and improve habitat for endemic wildlife species such as sage-grouse (Centrocercus spp.).

Are cattle surrogate wildlife? Savanna plant community composition explained by total herbivory more than herbivore type

“…In particular, presence or absence of certain herbivore species or guilds is not necessarily associated with pervasive plant community changes or plant species extinctions (Veblen et al. ). Rather, our analyses suggest that both heavy grazing (by any type of herbivore) and the total absence of herbivory are the conditions mostly likely to effect major plant community changes.…”

“…While in some areas livestock have completely replaced native ungulate herbivores, in others livestock have replaced native herbivores only incompletely and at moderate densities, and in some cases this type of mixed use appears to be sustainable (Reid , Veblen et al. ). The process of wildlife replacement can be broken into two distinct, but interrelated components: (1) loss or reduction in numbers of individual wildlife species or guilds and (2) addition of livestock.…”

“…), and even fewer have investigated responses of overall plant community composition (but see Veblen et al. ).…”

Effects of livestock grazing on rangeland biodiversity: A meta‐analysis of grouse populations

“…Livestock grazing systems are a complex combination of factors that include animal type, stocking rate, animal distribution, timing, duration, frequency, and many more (Briske et al., ; Heitschmidt & Walker, ; Teague et al., ; Veblen, Nehring, McGlone, & Ritchie, ; Veblen & Young, ). Livestock grazing may not be invariably “good” or “bad” for wildlife—rather, there can be positive, negative, or benign effects dependent on aforementioned factors in combination with soil conditions, precipitation, plant community, and the organism of concern (Krausman et al., ).…”

“…The role of human dimensions in grazing systems can indirectly contribute to the ecological outcome of grazing systems (Briske et al., ). The manner in which livestock grazing is managed affects the structure of rangeland ecosystems, which in turn influences the flows of other ecosystem goods and services from rangelands and ultimately affects wildlife populations (Dahlgren et al., ; Heitschmidt & Walker, ; Veblen et al., ). While grazing has been a part of many researched systems, its effects on wildlife populations are rarely investigated in an explicit and rigorous scientific manner.…”