Long-term changes of species richness in a breeding bird community of a small Mediterranean archipelago

We analyzed the pattern of species richness changes in a bird–breeding bird community on a small western Mediterranean archipelago (Columbretes Islands) over a 40–year period (1964–2003). The aim of this study was to qualitatively account for the relative roles of local and regional factors in shaping the community. As expected, we found that regional factors (at the metapopulation spatial scale) increased diversity whereas local factors (i.e. ecological) probably prevented further increases in diversity. We found that the archipelago gained four new species (two seabirds and two falconids) during the study period, whereas no extinctions were recorded. The community seems partially or completely closed to some groups of species (e.g. small–sized birds such as passerines and storm–petrels), probably owing to predatory exclusion by Eleonora falcons (Falco eleonorae). As newly arrived species have breeding calendars that do not fully overlap with those of resident species, competition for space in a rather saturated area is prevented. Preservation of rare species which increase gamma (regional) diversity rather than alpha diversity with common species should be the main local conservation goal.