Species richness in Madeiran land snails, and its causes

Aim To unravel the reasons why a group of islands of 800 km(2) area should support over 200 extant land snail species. Location The temperate Atlantic Madeiran archipelago. Methods Distributional surveys and model testing. Results Repeated volcanic events have changed the topography of the islands and created periodic further isolates. There has been climatic fluctuation and varying sea level. Individual locations are not species-rich, but there is replacement from one subregion to another. Evidence of competitive interactions is lacking. Main conclusions High diversity results from the fortuitous coincidence of rates of geological and climatic change and isolation on the one hand, and migration and genetic divergence on the other. Hubbell's neutral model of biodiversity explains species richness if a factor is added describing structural instability or periodicity, here called the geodetic rate. This geodetic, biological interaction can explain why some archipelagos are species-rich.