Grazer exclusion alters plant spatial organization at multiple scales, increasing diversity

Abstract

Grazing is one of the most important factors influencing community structure and productivity in natural grasslands. Understanding why and how grazing pressure changes species diversity is essential for the preservation and restoration of biodiversity in grasslands. We use heavily grazed subalpine meadows in the Qinghai-Tibetan Plateau to test the hypothesis that grazer exclusion alters plant diversity by changing inter- and intraspecific species distributions. Using recently developed spatial analyses combined with detailed ramet mapping of entire plant communities (91 species), we show striking differences between grazed and fenced areas that emerged at scales of just one meter. Species richness was similar at very small scales (0.0625 m2), but at larger scales diversity in grazed areas fell below 75% of corresponding fenced areas. These differences were explained by differences in spatial distributions; intra- and interspecific associations changed from aggregated at small scales to overdispersed in the fenced plots, but were consistently aggregated in the grazed ones. We conclude that grazing enhanced inter- and intraspecific aggregations and maintained high diversity at small scales, but caused decreased turnover in species at larger scales, resulting in lower species richness. Our study provides strong support to the theoretical prediction that inter- and intraspecific aggregation produces local spatial patterns that scale-up to affect species diversity in a community. It also demonstrates that the impacts of grazing can manifest through this mechanism, lowering diversity by reducing spatial turnover in species. Finally, it highlights the ecological and physiological plant processes that are likely responding to grazing and thereby altering aggregation patterns, providing new insights for monitoring, and mediating the impacts of grazing.

Grazing exclusion unleashes competitive plant responses in Iberian Atlantic mountain grasslands

“…As observed in our study, large aggregations of overgrazing‐tolerant species also displaced grazing‐intolerant species in grasslands of the Tibetan Plateau (Zhang et al. ) and in North Dakota (Meyers et al. ).…”

“…The removal of grazing disturbance led to aggregation of tall competitive species and loss of diversity in our study, whereas the high intensity of grazing disturbance led to aggregation of overgrazing‐tolerant species and loss of diversity in grasslands of the Tibetan Plateau (Zhang et al. ) and North Dakota (Meyers et al. ).…”

“…() and Zhang et al. (), did not explicitly account for the effects of niches by means of environmental models. For this reason, the underlying environmental heterogeneity may have masked patterns generated by biotic processes after grazer exclusion.…”

“…; Zhang et al. ; Meyers et al. ) and, in turn, patterns generated by species interactions are directly related to diversity (Chesson ).…”

Functional traits associated with plant colonizing and competitive ability influence species abundance during secondary succession: Evidence from subalpine meadows of the Qinghai–Tibetan Plateau

“…However, there were no observed significant changes in leaf turgor loss point–species abundance relationships along the successional gradient. Although the total annual precipitation in this region is low (occurring primarily during the growing season, from July to August), the soil appears to be water saturated (Zhang, Gilbert, Wang, Liu, & Zhou, 2013). This may explain the nonsignificant correlations between leaf turgor loss point and species abundance, suggesting that moisture limitation may not be critical for plant growth and survival during the growing season in our study system.…”