LONG-TERM CHANGES IN MOUNTAIN PASSERINE BIRD COMMUNITIES IN THE SIERRA NEVADA (SOUTHERN SPAIN): A 30-YEAR CASE STUDY

Shifts in composition and abundance of bird communities were studied along an elevational gradient in the Sierra Nevada (S. Spain), comparing censuses made at the same locations in the 1980s and during 2008-2012. Censuses were made in three representative ecosystems: 1) Pyrenean oak woodland, 2) high-mountain juniper scrubland, 3) high-mountain summits. In a global change context, avian-community dynamics are related mainly to changes in two of the following drivers: climate change and land-use change occurring over the past 30 years. Our results show a continuous turnover of the bird community, with an overall 27% change in species composition, in an ecological setting that has changed little, especially in the high-mountain scrubland and summit areas. We detected an increase in species diversity relative to the 1980s, mostly in the Pyrenean oak woodland. Moreover, our results also show a sharp decrease in bird density during the 30-year study period, chiefly affecting the dominant species of the 1980s in the Pyrenean oak forest and in the high-mountain scrubland. In the high-summit ecosystems, the decline of alpine species has paralleled the uphill expansion of some mediterranean ones. The outcome of these processes is a community in continuous flux, both in composition and abundance, where interannual variability is similar to interdecadal variability. We conclude that the bird communities of Sierra Nevada are showing strong spatial and temporal dynamics that are now accelerating, perhaps because of global warming.