Rapid Evolutionary Change in Homogocene


Alien Species and Evolution: the Evolutionary Ecology of Exotic Plants, Animals, Microbes, and Interacting Native Species. Cox, G. W. 2004 . Island Press , Washington , D.C. 389 pp . ( xii + 377 ). $75.00 (hardcover) . ISBN 1-55963-008-6 . $40.00 (paperback) . ISBN 1-55963-009-4 .

The pace of evolution is accelerating as a result of the widespread introduction of species to new geographic regions. This is the central theme of Alien Species and Evolution, a comprehensive text that unites concepts from evolutionary biology and ecology to explain the processes and effects of a largely unrecognized form of global change. Driven by human activities, the global mixing of biota is occurring at unprecedented rates (prompting some ecologists to refer to the current period of Earth's history as “the Homogocene”). Consequently, species are rapidly being exposed to novel biotic and abiotic selection pressures.

In a well-written synthesis, George Cox examines the evolutionary responses of alien species to their new environments and the responses of native species to alien interlopers. There are 20 chapters grouped within four major sections. The first section, “Basic Concepts of Alien Invasion and Evolution,” reviews the fundamental aspects of evolutionary biology applied to alien species, including adaptations for dispersal and establishment, traits conferring invasiveness, founder effects, and the importance of genetic variability. It also briefly summarizes genetic methods for identifying donor regions, invasion routes and invasion rates, all of which yield valuable information for management and policy.

The second section examines processes of evolutionary change and adaptation of alien species in their new environment. Its major focus is on genetic changes in native and alien species through hybridization. Cox relates the story of an alien plant that appeared in salt marshes on the coast of England in the late nineteenth century. The plant spread slowly at first, suddenly became invasive, and eventually colonized estuaries throughout the British Isles and into mainland Europe. It was revealed to be the hybrid of a native cordgrass (Spartina maritima) and a closely related North American cordgrass (Spartina alterniflora). The hybrid was sterile but became invasive after it underwent a genetic mutation that doubled its chromosome number, which restored its fertility and created a new, highly competitive species, Spartina anglica. Because novel combinations of genes can play novel (alien) ecological roles, Cox also devotes several pages to genetically engineered organisms and their potential for genetic exchange with other species. The remaining chapters in this section deal with abiotic and biotic factors (community resistance and enemy release) that stimulate adaptations in alien species.

The third section, “Evolutionary Interaction of Aliens and Natives,” provides examples of adaptation of invaders to new plant and animal hosts, prey, and enemies. An example is the Eurasian codling moth (Cydia pomonella), a worldwide pest of apples, pears, and plums. Within a few decades of invading California, it became a pest of English walnuts. The ubiquity of host shifts by introduced predators and herbivores serves as a warning against predicting an invader's impact based on its behavior in its natural range. Another topic concerns evolutionary shifts by native herbivores to alien plants. In some cases, the alien host is more favorable to the herbivore's development than its natural host. Cox raises the intriguing possibility that the widespread introduction of potential host species may be paving the way for the rapid evolution of new pest insects. He cites the example of the European budworm Choristoneura murinana, which has become exposed to a variety of introduced exotic conifers and thus been given the opportunity to broaden its host repertoire, becoming preadapted to exploiting resources in other regions where it may be introduced.

The last section, “Global Evolutionary Consequences of Alien Invasions,” explores trajectories of global changes wrought by alien species. The context-dependent nature of invasions and their effects is emphasized throughout the book, but nowhere more dramatically than in this section. For example, Cox describes the near extinction of the American chestnut tree by the most destructive forest disease on record, chestnut blight fungus. The pathogen was introduced with Asian chestnut trees planted as ornamentals in New York State during the early twentieth century. Unlike its Asian congener, the American chestnut lacks the immunity gained by evolutionary experience with the fungus. Cox provides several other examples of the role of naiveté in the extirpation of native species, particularly the loss of biodiversity following the introduction of novel predators to islands and lakes. In Lake Victoria, there are myriad novel stressors contributing to rapid change in the once hyperdiverse cichlid community: the introduction of a large predator, Nile perch; the massive proliferation of an introduced weed, water hyacinth; oxygen depletion of bottom waters through eutrophication; and increased turbidity leading to the breakdown of reproductive isolation in cichlids that select mates on the basis of color patterns. Here and elsewhere, Cox is careful to place the effects of alien species in the context of other forms of human-induced selection pressures, especially climate change. The interaction between climate change and adaptive range expansions is highlighted in the final chapter. In a brief discussion of conservation in an era of rapid evolution, Cox provocatively suggests that a distinction be made between invaders moving within continents and between continents. I suspect many invasion biologists would disagree; to the endemic fish fauna of western North America, for example, a largemouth bass from the Mississippi basin is just as alien as an African cichlid. According to Cox, it is impractical to treat all species undergoing intracontinental range shifts (e.g., due to climate change) as aliens.

Despite its broad coverage of technical topics, the book is highly readable. Some topics are treated in a cursory fashion, perhaps because of space constraints. Nevertheless, numerous examples are drawn from a range of taxonomic groups and ecosystem types. Good use is made of the recent scientific literature; there are over 950 references. Data are tabulated in nearly every chapter, but figures are relatively scarce. A glossary of terms is provided.

Studies have only recently begun to integrate evolutionary theory with the dynamics of colonizing populations (reviewed in Lambrinos, J.G. 2004. How interactions between ecology and evolution influence contemporary invasion dynamics. Ecology 85:2061–2070). Alien Species and Evolution should inspire further work in this potentially fertile area. This book is an excellent reference for students and researchers and not just those with a strong interest in alien species. It explores how ecological and evolutionary processes interact over short time scales and thus demonstrates that invasions are valuable in helping us to understand contemporary evolutionary change.