Biological Conveyor Belts


Invasive Species: Vectors and Management Strategies. Ruiz, G. M., and J. T. Carlton, editors. 2003. Island Press, Washington, D.C. 531pp. (518 + xiii). $40.00 (paperback). ISBN 1–55963–903–2.

Biological invasions, the spread and establishment of species into new regions, have occurred throughout history. Under human influence, however, the rate and spatial scale of invasions have increased to unprecedented levels; they constitute a dominant form of global change. Most invasions cause no discernible impact on the recipient ecosystem, but a small percentage of them (numbering in the thousands and growing) exert significant effects on native biodiversity, ecosystem function, regional economies, and human health. Once introduced into a new region, these high-impact species often show an ability to spread rapidly (i.e., they are invasive).

Invasive Species: Vectors and Management Strategies is the first book to focus on the proximate causes of invasions: vectors (modes of transport) and pathways (routes of transport) that transfer organisms. It was developed through contributions from a conference organized by the Global Invasive Species Program—an international, scientifically based effort to understand, anticipate, prevent, and manage invasions. The volume consists of 18 chapters written by 40 authors from seven countries. Despite the diversity of perspectives and regional experiences, the chapters are united by a common premise: if vectors can be controlled, then invasions and their impacts will be reduced. Yet, this is an enormous task. New vectors and pathways are proliferating under the force of international trade, which creates opportunities for species to disperse thousands of kilometers beyond their historical ranges. Owing to these great biological conveyor belts, geographic barriers to species dispersal are becoming increasingly negligible. An ocean is no longer a protective buffer against pests and pathogens residing “far away.”

In contrast to the classical concept of biological invasions as being governed by competition and disturbance in the recipient community (Elton 1958), modern concepts emphasize the role of human vectors (MacIsaac et al. 2001). Human vectors not only play essential roles in the large-scale dispersal and (through repeated introductions of colonists) the establishment of most nonindigenous species, they also act as selective filters that ultimately determine what invades the recipient community. These concepts are addressed throughout the book, which is organized into three parts.

Part I (nine chapters) is a data-rich compendium of information on vectors and pathways. The first chapter uses a military analogy to argue a complete defense of all points of entry against foreign invaders is impossible; therefore, limited resources must be allocated based on advance knowledge of the most likely vectors (how they will arrive) and pathways (from which direction they will come) and of the nature of the invader itself in order to prioritize impending threats. This is a prelude to later chapters that deal with risk assessment. The author (R. N. Mack) offers many fascinating examples to illustrate the historic role of humans in the inadvertent and deliberate dispersal of plants worldwide. Often transported with plants are disease-causing fungi, many of which are undescribed and become conspicuous only after they cause severe mortality in plant species that have no evolutionary experience with them. This problem is the topic of chapter 2. Other chapters in part I deal with insects, gastropods, freshwater and terrestrial vertebrates, and marine plants. Comprehensive analyses of taxa and geographic ranges are not attempted here. But some common temporal and spatial patterns emerge nonetheless: rates of invasion—regardless of taxon or region—are on the rise and are often exponential; these rates are strongly dependent on human vectors; vectors and pathways determine the composition of exotic species arriving at a given site; and, as the number of sites invaded by a species increases, so do the opportunities that can lead to its further spread.

Faced with the myriad invasion opportunities discussed in the first section, one might be discouraged by the enormity of the problem. However, the book offers plans of action. Part II (8 chapters) reviews current invasion management and policies for various taxa and geographic regions (Australia, New Zealand, South Africa, and the United States). Risk-assessment strategies are discussed in three chapters, and probabilistic approaches are illustrated with simple flowcharts and real-world examples (e.g., involving ballast water). Recognizing that invasive species do not respect political boundaries and that international cooperation is therefore required to reduce their spread, there is even a chapter on “environmental diplomacy.” Written from a U.S. perspective, it discusses legal instruments and the emerging social, economic, and environmental trends that will influence future policies.

Part III is a single, 46-page chapter that offers an integrative framework for studying and managing invasion vectors. Written by the book's editors (Ruiz and Carlton), this chapter eloquently unites the science and management themes of the book. Here, the authors present a conceptual model that links vector characteristics (e.g., frequency, volume, and speed of transfer) to the probability of species establishment. They discuss factors affecting the “dose-response relationship” between the number of organisms transferred and invasion success. Finally, they propose an adaptive management process aimed at reducing the opportunities for organismal transfer. This process involves an analysis of the invasion history of a given region to identify active vectors and determine their relative contribution to invasions in the region. The results of this retrospective analysis can be used to target the most important vectors, with the aim of disrupting or reducing the flow of organisms in the most efficient manner possible. The efficacy of such management practices can be evaluated by analyses of subsequent invasion patterns.

Some minor criticisms can be raised. The book's title, Invasive Species, is misleading because it is used here as a synonym for nonindigenous or exotic (i.e., all species introduced to regions outside of their historic range). The same term is used elsewhere to describe highly successful (widespread) invaders or invaders that have adverse effects on the recipient ecosystem (Colautti & MacIsaac 2004). Data in the book are of variable quality; the time series are not all up to date. On the other hand, there are few redundancies and factual inconsistencies, despite the chapters having been written separately by a diverse group of authors.

To its credit, the book uses quantitative rather than anecdotal evidence to inform its audience. There are abundant graphics and tables but no color plates or photographs. Invasive Species is clearly targeted to specialists (graduate student level and upward). With the exception of the final chapter, the book contains little theory, so it would not serve as a course text per se, although it provides a plethora of case studies that can augment lecture material. Nevertheless, this book examines the most important focal points for predicting and controlling invasion threats. It is a rich source of information that will be a valuable reference for researchers, environmental managers, and policy makers.