Species Invasions. Insights into Ecology, Evolution, and Biogeography . Sax, D. F., J. J.Stachowicz, and S. D.Gaines , editors . 2005 . Sinauer Associates , Sunderland , Massachusetts . 508 (xiii + 495) pp . $51.95 (paperback) . ISBN 0-87893-811-1 .
Experimenting with ecosystems on a large scale to test ecological processes is rarely acceptable. Yet experiments in nature where non-native species invade new environments have been occurring for centuries, offering insights into species relationships over varying scales of time and space. Capitalizing on studies examining community responses to invasions, the authors of this volume delve into the literature to explore long-held views on ecology, evolution, and biogeography. The breadth of the topics and the complexity of issues promise to stimulate and challenge readers who may not agree with all the conclusions. Although the issues are complex, the format of the book with its introduction and introductory essays to each section (ecology, evolution, and biogeography) is a useful roadmap to subsequent sections and chapters.
Successful invasions often are attributed to non-native species' ability to be more effective as competitors, predators, and facilitators. Bruno et al. examine the prevailing view that competition is a dominant force in structuring communities and conclude that it alone rarely causes local extinction of species. As one example, they refer to studies of an exotic wetland plant, purple loosestrife (Lythrum salicaria), to illustrate that the evidence does not support its role in competitively excluding native plants, as is generally believed. Adding another dimension to the discussions, Callaway et al. discuss a neglected aspect of invasion science, allelopathy, and the role of microorganisms in plant community structure and function. Two species of knapweed (Centaurea maculosa and C. diffusa) effectively exclude native American plants and form large monocultures by biochemical releases from rhizomes, but are less effective against native Eurasian plants. Centaurea maculosa produces catechin, which is phototoxic and antimicrobial; C. diffusa produces 8-hydroxyqinlone, a metal chelator, fungicide, and antiseptic. All these compounds are effective against other plant species, but they are not autotoxic. These two examples appear to be contradictory; thus, a discussion of scale and experimental versus observational data would be useful here. Nonetheless, the conclusion that biotic interactions of competition, predation, and facilitation are equally important in structuring communities and that these factors act comparably on native and non-native species is a consistent theme throughout the book.
Other authors (Stachowicz, D'Antonio, Hobbie, Blackburn, and their respective coauthors) use plant and bird communities and examples from aquatic and marine ecosystems to examine the relationships among species diversity, community saturation, and other ecosystem processes and functions. Although predicting which species will be successful invaders remains elusive, it is known that several attributes of exotic species help them thrive in new habitats. Some species modify their environments, for example, by fixing nitrogen or creating conditions that promote fire and thereby facilitate their own success and that of other species, both native and non-native. Lafferty et al. examine data on non-native species introductions to gain insights into infectious diseases. Some exotic species may arrive with fewer pathogens and greater resistance to native ones, may reach larger sizes, and may exhibit greater abundance, any of these factors may impart a competitive advantage on life-history characteristics and population growth. A different scenario may result when pathogens affect keystone species or where native pathogens affect non-native species.
The perennial question of whether highly diverse communities are more likely to resist new invaders still cannot be answered definitively. It is expected that increased introductions will lead to a homogenization of communities—either by decreasing diversity or increasing abundance; however, data suggest the opposite. McKinney and Lockwood identify parallel patterns of underlying factors (e.g., distance and latitudinal separation) operating similarly on native and non-native species that may explain the observed increases in diversity and abundance coincident with new invasions. Vermeij reviews fossil records to document historical biotic exchanges and concludes that these are natural and widespread phenomena that result in greater diversity.
Ricklefs, Novak, Wares, Holt, Rice, and their respective coauthors explore facets of evolutionary processes. A review Ricklefs' taxon cycles examines invader success, propagule pressure, diversity, release from parasites, and community resistance. Although species that have invaded naturally over time decline and become extinct, population declines of introduced species are rarely documented, barring human intervention. One would expect the opposite because small propagule size is characteristic of invading species. Conventional theory would suggest that genetic diversity would be reduced lowering competitiveness of non-native species. Species with inbreeding do exhibit low genetic diversity. Others use outcrossing or large numbers of progeny as mechanisms for increasing diversity. Hybrid vigor (hybrids are more competitive than either parent) is illustrated with cordgrass (Spartina spp.).
Rates of evolution appear to be quite rapid, based on several studies discussed by Huey et al. An introduced fruit fly, Drosophila subobscura, has exhibited changes in gene frequency; their wing size and other traits have evolved measurably within the last 25 years. Other examples in marine ecosystems also document rapid change, suggesting that relatively rapid change may be a common occurrence. In one study of the introduction of a non-native crab, gastropods exhibited a rapid change in the phenotypic distribution of shell shapes.
The chapters on biogeography cover a broader perspective of species distribution in space and time. Human intervention has accelerated this process and changed the dynamics. Callaway, McKinney, Kinlan, Labra, Sax, and colleagues touch on issues of homogenization, rates of spread, range expansion, distribution and abundance, and dynamic processes. Rates of spread vary by species and life-history characteristics and dispersal mechanisms differ between marine and terrestrial habitats. Two marine species, the compound sea squirt (Botrylloides violaceus) and the barnacle (Balanus improvisus) differ only approximately twofold in rate of spread (about 16 km/year and 30 km/year, respectively) but use multiple dispersal mechanisms. Botrylloides reproduces sexually, releasing larvae that settle within hours of release, and asexually, in which broken pieces of colonies drift and float to new habitats at some distance and re-establish colonies. Balanus improvisus only reproduces sexually and expands to new areas by dispersion of larvae that can spend 10–15 d in the water column. Conceptual models of species dynamics and scaling patterns of range expansion are examined with a variety of species in different environments. Yet on a larger scale, questions of species capacity, the number of species that have become extinct, and rates of establishment and extinction remain to be explored in future studies.
There is much to recommend this book. Using studies of species introductions, the authors revisit long-held concepts and theories in ecology, evolution, and biogeography and offers new insights and theories. One of its major limitations is the paucity of experimental data-based-invasions studies that are used to reach conclusions. The importance of experimental work is noted by Bruno et al. in the first chapter, but even they use nonexperimental papers in reaching some of their conclusions. In the concluding chapter Sax et al. call for developing a conceptual framework for invasion biology in the context of ecology and ecosystems. Whether this is possible or not with broader reviews of currently available data, this challenge should generate new research in pursuit of a unifying paradigm. As a resource for connecting invasions and theoretical concepts in ecology, evolution, and biogeography, Species Invasions is a welcome addition to one's library.