LAUSR

The altitudinal distribution of species richness is affected by climate and habitat changes, and the balance between these drivers will produce an idiosyncratic pattern in each mountain range. However, the potential effect of habitat change has rarely been included in studies designed to detect the effect of climate change on mountain biodiversity. This paper explores the changes in the altitudinal distribution of forest bird richness in the Guadarrama Mountains (600–2400 m a.s.l., Central Spain) over the last several decades. These mountains are affected by global warming and increased tree density resulting from rural abandonment. Nothing is known, however, about the way these changes have affected the altitudinal distribution of bird richness. Bird counts carried out in 1976–1980 were repeated in 2014–2015 along an altitudinal succession of forest belts. The results show that the relationship of bird richness to elevation shifted from a hump-shaped model typical of dry mountains to a monotonic negative model characteristic of humid mountains. These trends diverged from predictions on the effects of climate warming and were related to a loss of bird richness in endemic Scots pinewoods in the upper parts of the mountains. Variations in the composition of bird assemblages suggested that these pinewoods have experienced a process of tree densification and understory loss. As a result, the reshuffling of the altitudinal distribution of bird richness produced by habitat changes has eclipsed any effects of climate warming. These results emphasize the importance of considering habitat and climate interactions when exploring the altitudinal shift of species richness in the context of global change.