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.