Context‐dependent effects of shifting large herbivore assemblages on plant structure and diversity

Abstract

Despite wide recognition of the importance of anthropogenically driven changes in large herbivore communities—including both declines in wildlife and increases in livestock—there remain large gaps in our knowledge about the impacts of these changes on plant communities, particularly when combined with concurrent changes in climate. Considering these prominent forms of global change in tandem enables us to better understand controls on savanna vegetation structure and diversity under real‐world conditions. We conducted a field experiment using complete and semi‐permeable herbivore exclosures to explore the difference in plant communities among sites with wild herbivores only, with cattle in addition to wild herbivores, and with no large herbivores. To understand variation in effects across climatic contexts, the experiment was replicated at three locations along a topoclimatic gradient in California. Critically, this is the first such experiment to compare cattle and wildlife impacts along an environmental gradient within a single controlled experiment. Vegetation structure responded strongly to herbivore treatment regardless of climate. Relative to the isolated effects of wildlife, exclusion of all large herbivores generally increased structural components related to cover and above‐ground biomass while the addition of cattle led to reductions in vegetation cover, litter, shading and standing biomass. Furthermore, wildlife had a consistent neutral or positive effect on plant diversity, while the effect of livestock addition was context dependent. Cattle had a neutral to strongly negative effect at low aridity, but a positive effect at high aridity. These results suggest that (a) herbivore effects can override climate effects on vegetation structure, (b) cattle addition can drive different effects on diversity and (c) herbivore effects on diversity are modulated by climate. Synthesis. Our results illustrate very distinctive shifts in plant communities between two realistic forms of change in ungulate herbivore assemblages—livestock addition and large herbivore losses—particularly for plant diversity responses, and that these responses vary across climatic contexts. This finding has important implications for the management and protection of plant biodiversity given that over a quarter of the Earth's land area is managed for livestock and climate regimes are changing globally.

Grazing Intensity Effects on Fire Ignition Risk and Spread in Sagebrush Steppe

“…Similar results have been documented in short- (Blackmore and Vitousek 2000; Davies et al 2010; 2015; 2017; Bates and Davies 2014) and long-term (Davies et al 2016) grazing studies with moderate to high utilization in intact sagebrush steppe. Elsewhere, grazing has been found to reduce herbaceous cover and biomass across a wide range of systems, from California (Orr et al 2022) to Kenya (Young et al 2013), and further, has been demonstrated to directly decrease fire severity in these and other arid and semiarid ecosystems globally (Van Langevelde et al 2003; Kimuyu et al 2014).…”

Native annual forbs decline in California coastal prairies over 15 years despite grazing

“…Importantly, grazing and precipitation often have interactive effects on native species richness and cover [ 40 , 62 , 68 , 69 ]. Species richness generally increases under grazing in mesic and highly productive environments, but richness often declines under grazing in arid environments [ 10 , 19 , 70 , 71 ] [but see 40 ].…”

“…Importantly, grazing and precipitation often have interactive effects on native species richness and cover [ 40 , 62 , 68 , 69 ]. Species richness generally increases under grazing in mesic and highly productive environments, but richness often declines under grazing in arid environments [ 10 , 19 , 70 , 71 ] [but see 40 ]. Grazing during the extreme drought of 2012–2014 may have directly reduced native annual forb species richness, consistent with prior studies showing that grazing can reduce biodiversity during drought conditions [ 68 , 69 ].…”

“…Deer primarily eat broadleaved plants and cattle primarily graminoids [ 39 ]. Moreover, a recent study suggests that wild ungulate grazing in California has minor impacts on plant communities relative to cattle grazing [ 40 ].…”

Positive indirect effects more than balance negative direct effects of ungulate grazers on population growth of a grassland herb

“…In contrast, indirect effects of herbivores can affect the fitness of focal species both positively and negatively. Trampling can create bare patches that facilitate seedling establishment (Collins 1987; Belsky 1992; Bullock et al 1994; Jutila & Grace 2002), and selective grazing may reduce competition for light and nutrients for plants that escape damage (McNaughton et al 1997; Veen et al 2008; Borer et al 2015; Koerner et al 2018; Orr et al 2022). Reduced vegetation height as a result of grazing may in turn affect several other biotic interactions.…”