Alien Species and Evolution: The Evolutionary Ecology of Exotic Plants, Animals, Microbes, and Interacting Native Species

  • George W. Cox
Island Press: 2004. 377 pp. $75 (hbk), $40 (pbk) 1559630094 | ISBN: 1-559-63009-4
Alien invader: the Surinam toad was introduced to Australia from its native South America. Credit: K. SCHAFER/NHPA

While some governments are preoccupied with preventing border crossings by terrorists, thousands of alien species (those from other regions or continents) continue to be allowed free entry into most countries of the world. Some of these alien species are certain to cause great harm to the environment, native species, national economies and human health, as other species have done in the past. In this era of supposedly great attention to border security, how do the aliens keep on getting through?

Commercial markets in live food, pets, horticulture and aquaculture intentionally import a wide range of alien species into many countries every year, with little government supervision and often no analysis of the attendant environmental, health and financial risks. Thousands of other species hitch-hike on legitimate cargo or the ships, planes and other vehicles that carry them.

Most of these aliens will do little or no harm, but some will cause irreversible damage. Recent examples include the North American grey squirrel in Europe; the Asian longhorned beetle in North America; the Northern Pacific seastar and the Surinam toad in Australia; the European red deer in South America; the South American water hyacinth in Africa; and the Australian brown tree snake on Pacific islands. Changing patterns of trade mean that increasing numbers of alien species come from previously isolated regions.

The environmental and economic damage wrought by alien species includes the extinction of native species, and large alterations in ecosystem characteristics, such as nutrient fluxes and fire frequency. Charles Elton anticipated many of these environmental effects in his 1958 book The Ecology of Invasions by Animals and Plants. Indeed, it often seems that invasion biology (a recent addition to the list of biological sub-specialisms) is little more than Elton redux. However, in Alien Species and Evolution, George Cox extends traditional concerns about alien species beyond the ecological theatre, and puts the evolutionary play on centre stage. His main concern is genetic change, both in alien species, which are subject to founder effects and new selection pressures, and in native species, as they experience new selection pressures imposed by the aliens.

This extremely readable book is aimed primarily at students and researchers. Cox provides comprehensive coverage of alien species in different taxonomic groups and in different habitats: terrestrial, freshwater and marine. Replete with examples and abundantly referenced, the book provides an excellent evolutionary synthesis. Cox occasionally makes extended forays beyond alien species, but only to illustrate the broader context in which adaptation and counteradaptation occur. The book is therefore also a good introduction to the broader intellectual landscape of evolution and global environmental change.

The coverage of hybridization between alien and native species may be particularly useful, as many readers might not have encountered it before. The interaction between hybridization and polyploidy has already produced a number of new terrestrial plant species from ancestral species, for instance when European salsifies (Tragopogon) were introduced into North America. Hybridization and introgression have also been common in freshwater fishes, crustaceans and molluscs, as human interventions have brought closely related species together. This often results in the loss of native species as an evolutionary and ecological entity, as well as a chance to study evolution in action.

Cox provides a guide to other research topics where an understanding of evolution is essential, including the development of invasion resistance by native communities. He considers evolution during lag times of invasion, which arise because different life-history characteristics are often required for dispersal and persistence in a particular environment. Cox also provides an introduction to coevolution between alien and native species, and to the effect of invasion on geographic speciation — although he makes it clear that this result is relatively minor compared with extinctions caused by invasions.

Little explicit attention is given to policy responses to species introductions, but Cox illustrates the need for greater consideration of evolutionary processes in risk analyses for alien species. In particular, he gives several examples of hybridization between wild species and related crops that have been genetically engineered for resistance to herbicides or insect pests. Transgenes have already flowed (or almost certainly will if they haven't already) from cultivated sorghum into Johnson grass, from oilseed rape into field mustard, from sunflower crops into wild sunflowers, and from wheat into jointed goatgrass. Wild species do not seem to suffer any reduction in fitness from incorporating some transgenes, contrary to the claims of many proponents of genetic engineering, so there are likely to be detrimental effects on native insects. Cox convincingly makes the case that evolution is central to any understanding of invasions, and that the analysis of risk is incomplete without a consideration of evolution.