Decline of dry grassland specialists in Mediterranean high-mountain communities influenced by recent climate warming

QuestionsWhat changes have occurred in Mediterranean high-mountain communities during the last 50yr? Do these changes reflect upward shifts and decline in dry grassland specialists due to climate warming? LocationUpper belt (1900-2400m a.s.l.) in Sierra de Guadarrama, Spain. MethodsWe re-surveyed dry grassland communities in 50 summits that were previously sampled between the 1960s and 1980s. New plots were placed in the same localities and the same conditions as the original surveys. Changes at the species level were evaluated by comparing, between the two survey periods, species frequencies and optimal altitude estimated by local distribution models done with Maxent. Changes at the community level were assessed by comparing species richness and composition of the two surveys in relation to altitude, plot size and time between surveys. We evaluated observed changes in the whole data set and species groups by structuring the species into dry grassland specialists, other alpine species and generalists occurring at wider altitudinal ranges. ResultsThere was a general decline in the frequency of dry grassland specialists, an increase in frequency of generalists and no clear trends in the other species. Upward shifts were mainly detected in generalists, with significant increases in frequency. At the community level, we found a general increase in species richness that was mainly determined by the increase in generalists; coupled with a decline in number of dry grassland specialists. The structure of the communities evidenced changes between the historical surveys and the re-survey, which were mainly correlated with the number of years and the increasing dominance of generalists. ConclusionsIn contrast with trends observed in other Mediterranean high-mountain communities, our study reports a general increase in species richness, although this effect is concurrent with a decline in dry grassland specialists. The observed trends support on-going replacement of endemic species by low-altitude species, and associated changes in community assemblages. Despite the limitations of re-visitation studies for assessing climate-driven changes, our results highlight the necessity of developing more studies for assessing the effects of drought in Mediterranean high-mountain communities.