Invasion Biology

  • Mark A. Davis
Oxford University Press: 2009. 288 pp. $55 9780199218769 | ISBN: 978-0-1992-1876-9

Ascension Island in the South Atlantic Ocean is a good example of the changes that invasive species can wreak. Its volcanic mountain tops once hosted a monotonous carpet of ferns. But in 1843, botanist Joseph Hooker recommended that the bleak island be wooded by importing many new plants — what modern ecologists would see as a massive, human-mediated biological invasion. Surprisingly, this resulted not in ecological meltdown, but in the creation of a lush cloud forest. The forest traps mists, cycles nutrients and survives, generation after generation, without its species having evolved together. A study of this anomalous system is cited in Mark Davis's new book Invasion Biology. Why? Maybe because it is not so anomalous.

Ascension Island: not all imported species are destructive. Credit: K. SCHAFER/ALAMY

Invasion Biology starts out as a graduate-level text on how organisms brought far from their homes by humans can flourish, often at the expense of native species in the places they 'invade'. But on turning the pages, the book reveals itself to be an iconoclastic argument that much of the field's conventional wisdom is wrong, that biologists are more swayed by their emotions about invasive species than they care to admit, and that invasion biology as a field should be disbanded. Davis writes, “This may be the first time that an author has concluded a book, the title of which is the same as the discipline being reviewed, by recommending that participants consider abolishing their discipline.”

Davis is not on the fringe. His arguments crystallize a rumbling of dissent recently heard among those who study invasive species. As he puts it, “There is little about biological invasions that make them so unique that a specialized sub-discipline need be sustained to study them.”

Invasion biology began in earnest in 1958 when ecologist Charles Elton published his pioneering book, The Ecology of Invasions by Animals and Plants (see Nature 452, 34; 2008). Elton saw species 'invasions' in the context of niches. In an intact, co-evolved ecosystem, every species will have a slightly different role, or niche, and often every niche will be filled. For example, predators eat herbivores; herbivores eat plants; some plants grow on wet soil and some grow on dry. When new species are introduced, the theory goes, they can get a foothold only by finding a vacant niche or by throwing out another species.

Niche theory gives rise to the diversity-invasibility hypothesis, which posits that the more species there are in an ecosystem, the more niches will be filled and the harder it will be for a new species to become established.

But the evidence does not bear this out. Many studies have failed to find any strong relationship between how diverse a place is and how easy it is to invade. Davis concludes that, despite its appeal and its “implicit affirmation of the value of diversity”, the hypothesis is not true. In fact, the opposite may hold. In any ecosystem, each individual plant or animal has to get a foothold, irrespective of its origin. A seed does not care whether it is exotic or native when it lands on the ground, and neither do the surrounding species. The key insight is that there is nothing fundamentally different about exotics other than where they came from.

Davis challenges other received wisdom, such as the idea that newcomers are more likely to compete with or predate on natives than help them flourish, and that introduced populations are unlikely to be genetically diverse. He refuses to exaggerate the differences between natives and exotics, or to see exotics as the enemy.

Elton's 1958 book was an expansion of a series of radio broadcasts aimed at the public. Davis speculates that this audience was the reason behind Elton's colourful, militaristic comparisons of “ecological explosions” with bombs. This may have sown the seeds of the current 'good-versus-evil' rhetoric of species invasion, with its talk of biological pollution, killer weeds and battling garlic mustard.

Davis is not a fan of such heated rhetoric. He feels that the dichotomous approach is not ecologically enlightening. Life is much messier, more dynamic and more complex, he says. He stuffs the book with examples of exotic species that play nicely with their new neighbours. For every pest, there are many more unobtrusive immigrants that live quietly in their new haunts, even helping the growth and development of native species. This does not mean that invading species are never a problem, but Davis argues that they are not always troublesome.

Davis writes well, and clearly. But his big contribution is to the sceptical re-examination of the field as a whole. This book will not kill it off. But if, over time, invasion biology were to become absorbed into broader ecological fields that focus on the movement of species, future historians of science might see Invasion Biology as the beginning of the end.