Effect of island geological age on the arthropod species richness of Azorean pastures

Species richness of six pasture arthropod assemblages (total arthropod spec ies, total herbivore species, sucking and chewing herbivores, total predato ry species and spiders) were regressed against several geographical variabl es (area, distance fern thc nearest mainland, maximum elevation and geologi cal age of the islands) of three Azorean islands (S. Maria, Terceira and Pi co). The species were sampled by the fixed-quadrat size sampling method and the results obtained are consistent with the geological age hypothesis, i. e. the species richness of the six indigenous arthropod assemblages increas es with the geological age of the islands, both at local and regional scale s. Higher values of indigenous and endemic species richness were consistent ly found on the older island (S. Maria), and the lowest values on die most recent island (Pico). Moreover, when considering the age of Faial (an older island probably once connected with Pico) as a estimate of the age of Pico , correlations between species richness and island age were improved: there by strengthening the relationship. The older island (S. Maria) has more spe cialized herbivores and a greater proportion of herbivores in relation to p redatory arthropods. Ecological and biogeographical studies in the Azores s hould take into account the effects of the time each island has been availa ble for colonization and evolution. (C) 1999 The Linnean Society of London.