Grazers increase β-diversity of vascular plants and bryophytes in wood-pastures

Questions: How does the presence of grazers impact plant diversity at various spatial scales? What are the effects on plant beta-diversity and its two components, species turnover and nestedness? Are the effects caused by defoliation, trampling or defecating? Location: Twenty-four currently grazed and 24 abandoned wood-pasture sites in Central Finland. Methods: The species richness of vascular plants and bryophytes was studied at four spatial scales: within 4-m(2) subplots (alpha(1)), within 100-m(2) plots (alpha(2)), within sites (alpha(3)) and within the landscape (gamma). beta-Diversity was studied between subplots within plots (beta(1)), between plots within sites (beta(2)) and between sites within the landscape (beta(3)). Results: Currently grazed sites had higher species richness at all of the spatial scales (alpha(1), alpha(2), alpha(3), gamma) and higher within-plot and within-site beta -diversities (beta(1) and beta(2)). At the within-plot scale (beta(1)) plant beta-diversity was increased primarily by variation in defoliation. At this scale both turnover and nestedness were increased by grazers. At the larger within-site scale (beta(2)) only turnover was increased, and it was mostly caused by variation in trampling intensities. Within the landscape (beta(3)) currently grazed sites had higher between-site turnover and lower nestedness than abandoned sites. Conclusions: Grazing creates and maintains high species richness and beta-diversity of vascular plants and bryophytes, mostly due to high species turnover between patches. Defoliation, dunging and trampling are all important turnover agents, although their relative importance varies with spatial scale. At the landscape scale the currently grazed sites had high species turnover between them, and thus the conservation of all grazing-dependent species requires continued management at various sites, while high nestedness between abandoned sites implies that their biodiversity can be efficiently conserved through protecting the most species-rich sites.