DEVASTATION OF PREY DIVERSITY BY EXPERIMENTALLY INTRODUCED PREDATORS IN THE FIELD

HISTORICAL ecology contains various examples of how predators introduc ed onto islands by man have apparently exterminated native prey specie s(1-6). Conversely, a pioneering experiment(7) showed an increase in n umber of species with predator presence. Subsequent experiments have s hown both increases and decreases in prey diversity(8-10). Here we inv estigate how predator introduction affects one aspect of prey diversit y (number of species or species richness), and prey abundance, We ran a seven-year experiment on an entirely natural system of small islands , using the commonest local lizard as the predator and web spiders as prey. Lizard introduction caused rapid and devastating effects on spid er diversity and abundance: within two years, islands onto which lizar ds had been introduced became almost identical to islands with natural lizard populations. The proportion of species becoming extinct was 12 .6 times higher on 'lizard-introduction' islands than on islands witho ut lizards. Locally common and rare species were both reduced by the i ntroduction of lizards, but nearly all of the latter became permanentl y extinct.