More Species, Fewer Specialists: 100 Years of Changes in Community Composition in an Island Biogeographical Study

Aim We measured the changes in an island avifauna over more than 100 years (1898-2006), using community indices accounting for difference in expected species sensitivity to land-use and climate changes. Location Ouessant Island, France, Great Britain. Methods We assessed the temporal trend of the relative proportion of generalist species breeding on Ouessant island and whether high-temperature tolerant species have replaced less tolerant species over this time period. We further tested the relationship between the observed change in the avifauna composition, and long-term population species' trends measured independently in potential source regions of colonist species (France and Great Britain). Results During the whole study period, Ouessant island has experienced a strong increase in species richness (+41%), but a severe decline in specialist species. In contrast, we found no change in species composition in terms of their temperature-tolerance. The observed trend was highly correlated with species trends measured in the Great Britain. Main conclusions Our results revealed an ongoing biotic homogenization process towards more generalist species, coupled with a strong local increase in species richness. The observed trend was most likely driven by a strong habitat change in the island occurring during the period considered, favouring the colonization of generalist species. Our results show that an increase in species richness can be misinterpreted as a sign of conservation improvement and that assessing change in community composition using species-specific ecological traits provides more accurate insights for conservation planning purposes.

Biotic homogenization | bird community | indicators | long-term trend | protected area | specialist-generalist | Species | Protected areas | Breeding | Conservation biology | Habitat conservation | Biodiversity conservation | Aviculture | Wildlife conservation | Homogenization | Extinct species