Prehistoric species richness of birds on oceanic islands

Using data on prehistoric and modern birds from seven islands in the Kingdom of Tonga, we demonstrate that there is no positive relationship between species richness (S) and island area (A) over the observed range of A (1.8-259 km²). The uniform S-values occur across more than three orders of magnitude of A when prehistoric data are included, and the strongest predictor of S on any island is the level of fossil sampling (number of identified bones). Below a minimum value for A (in Tonga < 1.8 km²), S declines to zero as A does the same. Within the ranges of island elevation (E) and inter-island isolation (I) among the seven islands, neither E (11-312 m) nor I (0.6-38 km) has much if any effect on S. Under natural (pre-human) conditions, a positive species-area relationship may not be a valid generalization for birds on oceanic islands.