Biotic homogenization and changes in species diversity across human-modified ecosystems

Changing land use and the spread of ‘winning’ native or exotic plants are expected to lead to biotic homogenization (BH), in which previously distinct plant communities become progressively more similar. In parallel, many ecosystems have recently seen increases in local species (α-) diversity, yet γ-diversity has continued to decline at larger scales. Using national ecological surveillance data for Great Britain, we quantify relationships between change in α-diversity and between-habitat homogenizations at two levels of organization: species composition and plant functional traits. Across Britain both increases and decreases in α-diversity were observed in small random sampling plots (10–200 m2) located within a national random sample of 1 km square regions. As α-diversity declined (spatially in 1978 or temporally between 1978 and 1998), plant communities became functionally more similar, but species-compositional similarity declined. Thus, different communities converged on a narrower range of winning trait syndromes, but species identities remained historically contingent, differentiating a mosaic of residual species-poor habitat patches within each 1 km square. The reverse trends in β-diversity occurred where α-diversity increased. When impacted by the same type and intensity of environmental change, directions of change in α-diversity are likely to depend upon differences in starting productivity and disturbance. This is one reason why local diversity change and BH across habitats are not likely to be consistently coupled.