Abstract
Grazing can have implications for ecosystem management, biodiversity conservation, human livelihoods and global biogeochemical cycles. Grazers can either depress or promote plant production, with weak or strong effects on vegetation composition. Such variability is a major challenge for sustaining production while avoiding undesirable vegetation shifts. It is also uncertain how knowledge obtained from native herbivores can be used to manage domestic livestock and vice versa. In addition, grazer effects on production and vegetation composition tend to vary along prominent environmental gradients and are also negatively related to each other. Here, we evaluate these patterns for both livestock and native grazers under comparable grazing intensity and evaluate competing hypotheses that can account for the negative co-variation between these two types of grazer effects. A dataset from a four-year herbivore exclusion experiment in the Trans-Himalayan ecosystem in northern India shows the following: (a) grazer effects on plant production and on vegetation composition were indeed negatively correlated, but the relationship depends on the choice of data metrics; (b) incidental autocorrelation due to an underlying soil moisture gradient does not fully explain this pattern; instead, (c) their relationship is explained by variation in local plant species richness. Vegetation responses after excluding livestock and native grazers were qualitatively similar. But, despite comparable grazing intensity, livestock had quantitatively stronger effects on plant species composition. Production in species-rich communities was more grazing-tolerant and showed greater compositional stability. So, understanding the determinants of variation in species richness and how it is, in turn, influenced by grazing can offer a framework to interpret and manage highly variable impacts of herbivores on grazing ecosystems.
Highlights
Terrestrial grazing ecosystems, featuring both wild and domestic ungulates, represent the most expansive land use, encompassing semi-arid to arid regions of the world (Du Toit et al 2010)
Grazer effect on species richness There were no systematic differences in species richness between the paired grazed and ungrazed plots at the beginning of the study in 2005 (Figure 1a), but richness was higher in the areas used by native grazers (Table 2)
Grazer effect on community evenness There were no systematic differences in plant community evenness (Figure 1c) between the paired grazed and ungrazed plots at the beginning of the study in 2005, but evenness was higher in the areas used by native grazers (Table 2)
Summary
Terrestrial grazing ecosystems, featuring both wild and domestic ungulates, represent the most expansive land use, encompassing semi-arid to arid regions of the world (Du Toit et al 2010). Considerable uncertainties persist over how to utilize information from natural ecosystems to manage human-modified ecosystems and vice versa (Levin 1993; Watkinson and Ormerod 2001) This is further compounded by high variability as the effects of grazing can differ considerably, even idiosyncratically, between ecosystems (Milchunas and Lauenroth 1993; Proulx and Mazumder 1998; Olff and Ritchie 1998; Chase et al 2000; Vesk and Westoby 2001; Gruner et al 2008). Amidst these differences, both within and between ecosystems, an important insight for managers is that grazer effects may vary predictably along prominent natural environmental gradients such as precipitation (Milchunas and Lauenroth 1993; Olff and Ritchie 1998; Anderson et al 2007; May et al 2009)
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