Cattle Grazing and the Management of Invaded Ecosystems
Native, exotic, and domesticated ungulates are important herbivores in many terrestrial regions of the world and commonly influence the structure and dynamics of plant communities and ecosystems. Because of the global nature of biological invasions, these herbivores increasingly inhabit landscapes that are dominated by invasive exotic plant species, and such invaders exert strong influences on resident communities by altering competitive dynamics, fire regimes, and nutrient cycling). How ungulate herbivores impact these invaded landscapes and alter the competitive dynamics among resident plant taxa is of great interest to ecologists and conservation biologists alike, and is likely to vary considerably depending on a range of factors including the ecological characteristics of resident plant taxa.
Plant species from different groups based on life-history and life-form characteristics may vary greatly in their responses to herbivory, as might native and exotic species from within the same group. Such taxa often differ considerably in their responses to disturbance and in their growth, colonization and competitive abilities. Herbivores that have large negative effects on plant species in one life-history/life-form group may also influence their competitive interactions with species from other groups. For example, an herbivore-induced decrease in abundance of members from one group may cause an increase in the abundance of taxa from other competitively subordinate groups. To develop a comprehensive understanding of the impacts of ungulate herbivores on vegetation, and the utility of grazing as a conservation tool, research must address the responses of native and exotic species from different life-history and life-form groups.
Grasslands in California have a long history with both native and domesticated ungulates and are one of the most invaded systems in the world, having undergone a nearly complete transformation from perennial grass and forb-dominated native flora to assemblages dominated by exotic annual grasses. Numerous factors have contributed to this dramatic transformation, including historical droughts, intensive agriculture, and livestock grazing. The increased dominance of exotic annual grasses has adversely affected native bunchgrasses through increased competition and led to the accumulation of tremendous amounts of aboveground dead biomass that can negatively affect germination, recruitment, species richness, and relative dominance of native taxa and even lead to positive feedbacks that further promote exotic annual grasses.
Given the extraordinary degree and spatial scale of these invasions, a major challenge is to understand how domesticated herbivores influence the dominance of exotic plants and the native species that remain. Although implicated in the initial demise of native grasslands, cattle grazing has been shown to reduce the dominance of exotic plant invaders, particularly when the focus is on one or a few target species. However, despite growing use of cattle grazing as a conservation and restoration tool, its effects on invaded grasslands are quite variable and further experimental research is needed to understand the impacts of cattle grazing on plant communities in invaded grasslands.
Using a three-year exclosure experiment, my graduate students and I evaluated the effects of light winter/spring grazing by cattle (an average of 0.75 cow-calf pairs ha-1) on species richness and percent cover of native and exotic plants from different life-history and life-form groups (Skaer, Graydon & Cushman (2013). We found that cattle grazing reduced total vegetative cover by 25% and increased bare ground by 40%. Grazing also caused a 70% reduction in aboveground plant biomass and a 40% reduction in the cover of exotic annual grasses, which were by far the most dominant group in this system. Grazing also reduced the height and spikelet production of the exotic annual grass B. diandrus. In contrast, grazing increased the cover of exotic annual forbs by 15%. Contrary to our expectations, grazing did not affect the cover of native forbs and grasses and we hypothesize that these taxa possess life-history characteristics and/or depleted seed banks that prevented them from responding to grazing during our experiment. Grazing also did not affect the species richness of any native or exotic plant group.
Our three-year exclosure experiment has shown that grazing by cattle caused major changes in a coastal grassland community, with the responses of native and exotic taxa with different life-history and life-form characteristics varying markedly. Understanding the responses of these divergent groups is not only important for basic research evaluating the effects of mammalian herbivores on plant communities, but is also critical for developing science-based approaches for sustainably managing invaded grasslands.
Native, exotic, and domesticated ungulates are important herbivores in many terrestrial regions of the world and commonly influence the structure and dynamics of plant communities and ecosystems. Because of the global nature of biological invasions, these herbivores increasingly inhabit landscapes that are dominated by invasive exotic plant species, and such invaders exert strong influences on resident communities by altering competitive dynamics, fire regimes, and nutrient cycling). How ungulate herbivores impact these invaded landscapes and alter the competitive dynamics among resident plant taxa is of great interest to ecologists and conservation biologists alike, and is likely to vary considerably depending on a range of factors including the ecological characteristics of resident plant taxa.
Plant species from different groups based on life-history and life-form characteristics may vary greatly in their responses to herbivory, as might native and exotic species from within the same group. Such taxa often differ considerably in their responses to disturbance and in their growth, colonization and competitive abilities. Herbivores that have large negative effects on plant species in one life-history/life-form group may also influence their competitive interactions with species from other groups. For example, an herbivore-induced decrease in abundance of members from one group may cause an increase in the abundance of taxa from other competitively subordinate groups. To develop a comprehensive understanding of the impacts of ungulate herbivores on vegetation, and the utility of grazing as a conservation tool, research must address the responses of native and exotic species from different life-history and life-form groups.
Grasslands in California have a long history with both native and domesticated ungulates and are one of the most invaded systems in the world, having undergone a nearly complete transformation from perennial grass and forb-dominated native flora to assemblages dominated by exotic annual grasses. Numerous factors have contributed to this dramatic transformation, including historical droughts, intensive agriculture, and livestock grazing. The increased dominance of exotic annual grasses has adversely affected native bunchgrasses through increased competition and led to the accumulation of tremendous amounts of aboveground dead biomass that can negatively affect germination, recruitment, species richness, and relative dominance of native taxa and even lead to positive feedbacks that further promote exotic annual grasses.
Given the extraordinary degree and spatial scale of these invasions, a major challenge is to understand how domesticated herbivores influence the dominance of exotic plants and the native species that remain. Although implicated in the initial demise of native grasslands, cattle grazing has been shown to reduce the dominance of exotic plant invaders, particularly when the focus is on one or a few target species. However, despite growing use of cattle grazing as a conservation and restoration tool, its effects on invaded grasslands are quite variable and further experimental research is needed to understand the impacts of cattle grazing on plant communities in invaded grasslands.
Using a three-year exclosure experiment, my graduate students and I evaluated the effects of light winter/spring grazing by cattle (an average of 0.75 cow-calf pairs ha-1) on species richness and percent cover of native and exotic plants from different life-history and life-form groups (Skaer, Graydon & Cushman (2013). We found that cattle grazing reduced total vegetative cover by 25% and increased bare ground by 40%. Grazing also caused a 70% reduction in aboveground plant biomass and a 40% reduction in the cover of exotic annual grasses, which were by far the most dominant group in this system. Grazing also reduced the height and spikelet production of the exotic annual grass B. diandrus. In contrast, grazing increased the cover of exotic annual forbs by 15%. Contrary to our expectations, grazing did not affect the cover of native forbs and grasses and we hypothesize that these taxa possess life-history characteristics and/or depleted seed banks that prevented them from responding to grazing during our experiment. Grazing also did not affect the species richness of any native or exotic plant group.
Our three-year exclosure experiment has shown that grazing by cattle caused major changes in a coastal grassland community, with the responses of native and exotic taxa with different life-history and life-form characteristics varying markedly. Understanding the responses of these divergent groups is not only important for basic research evaluating the effects of mammalian herbivores on plant communities, but is also critical for developing science-based approaches for sustainably managing invaded grasslands.