Pheidole Megacephala and Iridomyrmex Humilis in Bermuda‐Equilibrium or Slow Replacement?

Ants which actively extent their ranges and displace long—established populations of indigenous species on a world scale are limited to relatively few species. Two of the most striking are Pheidole megacephala and Iridomyrmex humilis. The former expanded its range from its original supposed home in middle Africa during the last century, displacing native species on a vast scale from Australia and Japan to the West Indies and Florida. Beginning somewhat later, Iridomyrmex humilis spread from western South America in a similar fashion and likewise attained a worldwide distribution. Its habitat preferences bring it into direct competition with Pheidole. During the second half of the nineteenth century its is known to have replaced Pheidole in Madeira, where Pheidole itself had earlier replaced the native ant fauna. Nothing is known in detail however, of the pattern of this replacement. Pheidole megacephala is known to have occupied Bermuda by the beginning of the present century, and largely replaced the native ant fauna. In 1957 it was observed by the authors that Iridomyrmex humilis had been introduced into Bermuda. This introduction, paralleling that in Madeira almost a century earlier, offers an excellent opportunity for a detailed analysis of the pattern by which one aggressive species of worldwide distribution may replace or coexist with another equally aggressive in an environment eminently suited to both, and in which there is evidently considerable competition for identical microhabitats. The present paper summarizes the result of two detailed surveys of the distribution of colonies of P. megacephala and I. humilis in Bermuda, made in 1959 and 1963. They evidence a gradual extension of the range of I. humilis at the expense of P. megacephala in a characteristic pattern. It is hoped that other workers will be encouraged to undertake further surveys of this change as it proceeds.