Biodiversity gains and losses: Evidence for homogenisation of Scottish alpine vegetation

Alpine areas are important biodiversity reservoirs, but are subject to anthropogenic drivers including climate change, nitrogen deposition and changing land use. Alpine vegetation has been proposed as an indicator of climate change impacts, but this requires long-term data since these communities have high inertia. Most studies have focussed on climate impacts in open, high-alpine summit communities; we investigated responses of closed low- and middle-alpine communities to multiple drivers. Scottish alpine vegetation data collected 1963–1987 was used as a baseline to assess biodiversity change across a range of habitats and a wide geographic spread. Change was assessed over a 20–40 years period using a variety of metrics including α- and β-diversity indices and biodiversity changes were contrasted between habitats and areas. We also examined changes in key species′ distribution and cover. Species richness increased in most habitats, while diversity at the plot scale and β-diversity declined, resulting in increased homogeneity of vegetation. This occurred in closed alpine communities over a 20–40 years period, implying that these communities are considerably more dynamic than previously thought. Key northern and alpine species declined while lowland generalist species increased. This change was consistent with predicted impacts of climate change, but other elements of spatial pattern (decline in lichen richness in high deposition areas) were consistent with effects of nitrogen pollution. Assessment of biodiversity change differed according to the metrics used and we argue that biodiversity targets for conservation management need accurate definition and that multiple measures of biodiversity are required to accurately assess long-term change.