Aim We consider three hypotheses – MacArthur and Wilson’s island biogeography theory (IBT), Lack’s habitat diversity idea and the ‘target effect’– that explain the pattern of decreased species richness on small and distant islands.
Location We evaluate these hypotheses using a detailed dataset on the occurrence and abundance of terrestrial birds on nine islands off the coast of Britain and the Republic of Ireland.
Methods Unlike previous studies, we compile data on species that visit the islands, rather than just those that breed on them. We divided the species into five mutually exclusive categories based upon their migratory status and where they regularly breed: British residents, summer visitors to Britain, winter visitors to Britain, and vagrants from Europe or beyond Europe. For each species group on each island we calculated the average number of species visiting each year. We then regressed the average number of species against island area and distance to the mainland (all variables were log-transformed). We also compared the average number of species visiting each island with the average number of species breeding on each island.
Results Average number of visiting British residents decreased significantly with increasing island distance, but showed no relationship with island area. There was no significant relationship between island area or island distance and average number of summer or winter visitors. European and non-European vagrants likewise showed no relationship between numbers of species visiting and island distance. However, the relationship between island area and number of visiting species was significant for both these categories; as island area increases so too does the number of visiting species.
Main conclusions As predicted by IBT, there were fewer visiting species on more distant islands. There were substantially more visitors to each island than breeding species, supporting Lack’s argument that lower bird richness is not a result of varying immigration rates (as predicted by IBT) but rather a result of some other island property, e.g. fewer resources. Birds make a decision to either leave an island or stay and breed. The target effect was also clearly demonstrated by the increase in European and non-European breeders with increasing island size.