Livestock are thought to be one of the main drivers of grassland diversity. Their effect seems to depend on productivity. Three main hypotheses have been put forward in this respect: (1) Livestock increase plant diversity through competitive release, especially in productive environments. (2) Livestock causes the extinction of plant species that cannot compensate for biomass removal in unproductive sites, thus reducing species richness. (3) Livestock has no effect on diversity in unproductive sites because plants that endure stress also tolerate grazing. Most studies regarding these ideas have been conducted on large spatial scales. Here we test these hypotheses on a local scale. The study site is a semi-arid grassland in southern Mexico. We performed a long-term exclosure experiment, recording plant diversity annually over 17 years. Because the shape of the relationship between grazing intensity and diversity may shed light on the underlying mechanisms, in 2012 we measured species richness over a grazing-intensity gradient. A productivity gradient highly correlated with soil depth was also considered in both datasets.
Results/Conclusions
Most of our results support hypothesis 1. Plant diversity increased with grazing intensity, especially in deep soil. However, based on available evidence it is unclear whether there was an interaction between productivity and grazing intensity. Diversity decreased inside of exclosures in comparison to that outside of them in high productivity conditions. This effect was reversed in low productivity. Using published data on species competitivity, we found that, as grazing became more intensive, species richness increased due to a progressive incorporation of increasingly poor competitors to the community. Our evidence suggests that this pattern was stronger in higher productivity, but this result was inconclusive. Thus, under high productivity conditions, the effects of livestock on local diversity and species composition were consistent with competitive release, with herbivores limiting more competitive species. This prevents competitive exclusion and favors coexistence. Interestingly, we observed at a small scale the same effects of productivity that have been reported for large areas. These results imply that excluding livestock can be detrimental to diversity conservation efforts when plants are in productive sites, but on limiting environments livestock may need to be kept at low densities to preserve diversity.