The success of animal invaders.

Z ebra mussel, kudzu, fire ant, gypsy moth, common carp— whether you are a farmer, outdoors enthusiast, cottage owner, or ecologist, you have undoubtedly witnessed the spread and adverse impacts of invasive species. As long as humans have migrated, we have carried species from their homeland to provide some familiar comforts of home in new lands. This trend was exemplified by the acclimatization societies of North America in the 19th century, which systematically imported and introduced Old World species. The expansion of global trade and tourism during the past century has fostered an explosion of unintentional (mostly invertebrate) species introductions, further accelerating the spread of species beyond their native range, as has been documented for the Laurentian Great Lakes (Fig. 1). The article in this issue of PNAS by Jeschke and Strayer (1) provides a much needed assessment of invader success patterns for vertebrate species exchange between the continents of Europe and North America. The introduction and spread of invasive species, both intentional and unintentional, is emerging as among the most dramatic ways that humans are transforming the planet (2), and nonnatives are now recognized as a central agent of human-caused global change (3). What’s more, the combined effect of the spread of nonnatives and the extinction of rare species is causing ecological communities worldwide to become increasingly similar (4). In other words, the global biota is being homogenized. In light of these trends, there is an urgent need to better understand invasive species spread and success. Although efforts have been made to describe the attributes of successful invaders and invasible ecological communities (5), ecologists have lamented the difficulty in making quantitative predictions about invaders (6), and broad-scale statistical patterns of invader success remain poorly known. The article by Jeschke and Strayer (1) makes several important contributions to our understanding of invasive species. It is now widely accepted that a small proportion of introduced species establish, and a small proportion of established species spread or become pests (6). This belief has been presented more formally as the oft cited ‘‘tens rule’’ (7), which holds that 1 of 10 imported species ‘‘escapes’’ to the wild, 1 of 10 of these introduced species becomes established in the wild, and 1 of 10 established species spreads and becomes a pest. Although this rule of thumb was developed by Williamson and Fitter (7) on the basis of examination of plant invaders in the United Kingdom, this rule has been broadly applied (probably beyond the original intent of the authors)