A CENTURY OF TURNOVER - COMMUNITY DYNAMICS AT 3 TIMESCALES

1. We calculated the observed species turnover of the bird communities on 13 small islands off the coast of the British isles and the Republ ic of lreland for different census intervals. For seven of these islan ds, the maximum intervals exceeded 80 years. 2. We developed a non-lin ear, asymptotic model to describe how observed turnover should change with census interval. Our assumptions were traditional ones, based upo n the assumption of dynamic equilibrium in familiar island biogeograph y theory, even though we knew that such equilibrium was rare in these islands. Furthermore, the model considered the average dynamics of the species present and not the dynamics of the individual species. 3. Th is model showed that neither the observed turnover calculated over dif ferent intervals, nor the turnover rate obtained by dividing this obse rved turnover by the interval, are statistics that permit comparison b etween islands. Observed turnover increased over time, thus 1-year tur nover underestimated the turnovers over a decade or a century. The inc rease was less than linear, however, so dividing observed turnover by the number of years in its calculation produced a statistic that decli ned progressively with that number. 4. The model provided a significan t overall fit to the data, but underestimated turnover at both the sho rtest and longest census intervals. We modified the model to reduce th e amount of underestimation, by incorporating the long-term changes in the number of species on each island and hence removing the assumptio n of equilibrium. This non-equilibrium model provided a much improved fit to the data, but it still failed to describe turnover at the very shortest intervals. These, however, are known from other studies to be inflated by individuals-floaters-that nest only once or twice on the islands. 5. The improved, non-equilibrium model made good predictions of the observed turnover over a 4-year interval. These predictions may be used to compare islands, including those for which empirical data on 4-year turnovers are sparse, 6. We divided the non-equilibrium mode l into an intrinsic and an extrinsic component, representing the influ ence of within-community and external factors, respectively. 7. Even t he intrinsic components of turnover are large, involving differences i n species composition of 6-36% between widely separated censuses. How these intrinsic components of turnover vary from island to island is n ot clear because previous studies have been unable to compare turnover at different time scales. The number of islands in this study was too few for this purpose and we leave a more broadly based comparison for a future paper.