Grazing and temporal turnover in herbaceous communities in a Mediterranean landscape

Question: What are the direct and indirect effects of cattle grazing on temporal turnover in Mediterranean herbaceous communities, and are there differences between species (taxonomic) and functional turnover rates? Are turnover rates related to changes in precipitation patterns?. Location: The long term ecological research (LTER) station, Ramat Hanadiv Nature Park, Israel. Methods: We used data from a Mediterranean LTER site to analyse temporal turnover in herbaceous communities under cattle grazing and control conditions, while accounting for the potential effects of precipitation and woody cover. We analysed two types of temporal turnover: taxonomic turnover (changes in species composition) and turnover in functional traits (changes in the relative abundance of species with different functional traits). Results: We found that communities exhibited rapid taxonomic turnover between sampling years, which was mostly explained by changes in precipitation patterns. Furthermore, there was a strong and consistent directional trend in taxonomic turnover (21% per decade) throughout the study period, which was correlated with a long-term trend of decreasing precipitation. This trend, however, was not significantly explained by grazing, woody cover change or their interaction. Grazing did have a significant effect on short-term turnover rates, but its effects varied in both direction and magnitude among sites located in different habitats. Temporal turnover in functional traits was much slower than taxonomic turnover, and was also inconsistent among traits and sites. Grazing reduced the bi-annual turnover rates in the composition of different life forms, and reduced the decadal turnover rate in the composition of different height groups in the community. Conclusions: The results of this analysis suggest that the effect of cattle grazing on the temporal dynamics of herbaceous communities in Mediterranean ecosystems is habitat- and trait-specific, and unrelated to changes in woody cover. In contrast, inter-annual changes in precipitation patterns have a much stronger effect on turnover rates. The relationship between temporal species turnover and precipitation in these communities is therefore in line with modern co-existence theory, by which fluctuating environmental conditions facilitate long-term species co-existence.