Plant diversity partitioning in grazed Mediterranean grassland at multiple spatial and temporal scales

摘要

Grazing by large ungulates may affect plant species richness and diversity at multiple spatial and/or temporal scales, because grazing affects small-scale resource heterogeneity and plant interactions at the local scale, while effects at the landscape scale are related to grazing intensity and timing. We used diversity partitioning to analyse long- and short-term effects of cattle grazing on plant species richness and diversity in an experimental spatial hierarchy in Mediterranean annual grassland. Short-term changes during secondary succession in grazed plots (2003-2005) at two grazing intensities (heavy and moderate) were analysed and compared with long-term protected vegetation. We applied Hill's q-diversity metrics at q=0 (species richness) and q=2 (reciprocal Simpson diversity) to examine the partitioning of species richness and diversity between their alpha (α) and beta (β) components in the different treatments at four spatial scales: quadrats, within exclosures, within plots, within treatments. At q=0, α-diversity was always significantly lower, and β-diversity significantly higher, than predicted by the randomised null model. Diversity partitioning at q=2 showed a similar trend at the quadrat scale. At the exclosure scale, partitioning exhibited a similar trend during the first 2years of secondary succession but did not deviate from the null model in the third year, as observed in protected vegetation in all years. At q=0, diversity decreased across all treatments in the short term. At q=2, diversity was initially higher in grazed plots than in protected vegetation; α and β components both decreased during secondary succession, to the levels observed in the protected vegetation. Synthesis and applications. Lower dominance in grazed vegetation indicates that grazing affects competitive exclusion at the local, small scale and accentuates natural heterogeneity (e.g. patchiness of soil resources, presence of rocks in the landscape) at a larger scale. The results of this study emphasise the importance of grazing as a management tool for maintaining plant diversity at multiple scales. This is a major concern worldwide, as the area covered by natural ecosystems continues to dwindle, necessitating management of grasslands for multiple functions such as animal production, resource protection and wildlife enhancement.