|Hanke, W; Böhner, J; Dreber, N; Jürgens, N; Schmiedel, U; Wesuls, D; Dengler, J: The impact of livestock grazing on plant diversity in drylands: an analysis across biomes and scales in southern Africa, Ecological Applications, 24(5), 1188-1203 (2014), doi:10.1890/13-0377.1|
|Key words: alpha diversity, beta diversity, biodiversity, disturbance, evenness, fence-line contrast, functional diversity, Nama Karoo, rangeland degradation, richness, stability, Succulent Karoo|
A general understanding of grazing effects on plant diversity in drylands is still missing, despite an extensive theoretical background. Cross-biome syntheses are hindered by the fact that the outcomes of disturbance studies are strongly affected by the choice of diversity measures and the spatial and temporal scales of measurements. The aim of this study is to overcome these weaknesses by applying a wide range of diversity measures to a data set derived from identical sampling in three distinct ecosystems. We analyzed three fence-line contrasts (heavier vs. lighter grazing intensity) representing different degrees of aridity (from arid to semi-arid) and precipitation regimes (summer rain, winter rain) in southern Africa. We tested the impact of grazing intensity on multiple aspects of plant diversity (species and functional group level, richness and evenness components, alpha and beta diversity, composition) at two spatial scales and both for 5-yr means and inter-annual variability. Heavier grazing reduced total plant cover and substantially altered the species and functional composition in all sites. However, a significant decrease in species alpha diversity was detected only in one of the three sites. By contrast, alpha diversity of plant functional groups responded consistently across ecosystems and scales with a significant decrease at heavier grazing intensity. The cover-based measures of functional group diversity responded more sensitively and more consistently than functional group richness. Beta diversity of species and functional types increased under heavier grazing, showing that at larger scales the heterogeneity of the community composition and the functional structure were increased. Heavier grazing mostly increased interannual variability of alpha diversity, while effects on beta diversity and cover were inconsistent. Our results suggest that species diversity alone may not adequately reflect the shifts in vegetation structure that occur in response to increased grazing intensity in the dryland biomes of southern Africa. Compositional and structural changes of the vegetation are better reflected by trait-based diversity measures. In particular, measures of plant functional diversity that include evenness represent a promising tool to detect and quantify disturbance effects on ecosystems. Finally, studying the combined effects of climate change and land-use intensification on biodiversity stability is important.