Turnover dynamics of breeding land birds on islands: is island biogeographic theory ‘true but trivial’ over decadal time-scales?

The theory of island biogeography has revolutionised the study of island biology stimulating considerable debate and leading to the development of new advances in related areas. One criticism of the theory is that it is ‘true but trivial’, i.e., on the basis of analyses of annual turnovers of organisms on islands, it has been posited that stochastic turnover mainly comprises rare species, or repeated immigrations and extinctions thereof, and thereby contribute little to the overall ecological dynamics. Here, both the absolute and relative turnover of breeding land birds are analysed for populations on Skokholm, Wales, over census intervals of 1, 3, 6, 12, and 24 years. As expected, over short census intervals (≤6 years), much of the turnover comprised repeated colonisations and extinctions of rare species. However, at longer intervals (12 and 24 years), a sizeable minority of species (11% of the total recorded) showed evidence of colonisation and/or extinction events despite sizeable populations (some upwards of 50 pairs). These results suggest that a longer-term view is required to take into account turnover involving more common species.