Preface to the Princeton Landmarks in Biology Edition vii Preface xi Symbols Used xiii 1. The Importance of Islands 3 2. Area and Number of Speicies 8 3. Further Explanations of the Area-Diversity Pattern 19 4. The Strategy of Colonization 68 5. Invasibility and the Variable Niche 94 6. Stepping Stones and Biotic Exchange 123 7. Evolutionary Changes Following Colonization 145 8. Prospect 181 Glossary 185 References 193 Index 201

The Theory of Island Biogeography

Although ecologists commonly talk about the impacts of nonindigenous species, little formal attention has been given to defining what we mean by impact, or connecting ecological theory with particular measures of impact. The resulting lack of generalizations regarding invasion impacts is more than an academic problem; we need to be able to distinguish invaders with minor effects from those with large effects in order to prioritize management efforts. This paper focuses on defining, evaluating, and comparing a variety of measures of impact drawn from empirical examples and theoretical reasoning. We begin by arguing that the total impact of an invader includes three fundamental dimensions: range, abundance, and the per-capita or per-biomass effect of the invader. Then we summarize previous approaches to measuring impact at different organizational levels, and suggest some new approaches. Reviewing mathematical models of impact, we argue that theoretical studies using community assembly models could act as a basis for better empirical studies and monitoring programs, as well as provide a clearer understanding of the relationship among different types of impact. We then discuss some of the particular challenges that come from the need to prioritize invasive species in a management or policy context. We end with recommendations about how the field of invasion biology might proceed in order to build a general framework for understanding and predicting impacts. In particular, we advocate studies designed to explore the correlations among different measures: Are the results of complex multivariate methods adequately captured by simple composite metrics such as species richness? How well are impacts on native populations correlated with impacts on ecosystem functions? Are there useful bioindicators for invasion impacts? To what extent does the impact of an invasive species depend on the system in which it is measured? Three approaches would provide new insights in this line of inquiry: (1) studies that measure impacts at multiple scales and multiple levels of organization, (2) studies that synthesize currently available data on different response variables, and (3) models designed to guide empirical work and explore generalities.

Impact: Toward a Framework for Understanding the Ecological Effects of Invaders

The use of simple terms to articulate ecological concepts can confuse ideological debates and undermine management efforts. This problem is particularly acute in studies of nonindigenous species, which alternatively have been called ‘exotic’, ‘introduced’, ‘invasive’ and ‘naturalised’, among others. Attempts to redefine commonly used terminology have proven difficult because authors are often partial to particular definitions. In an attempt to form a consensus on invasion terminology, we synthesize an invasional framework based on current models that break the invasion process into a series of consecutive, obligatory stages. Unlike previous efforts, we propose a neutral terminology based on this framework. This ‘stagebased’ terminology can be used to supplement terms with ambiguous meanings (e.g. invasive, introduced, naturalized, weedy, etc.), and thereby improve clarity of future studies. This approach is based on the concept of ‘propagule pressure’ and has the additional benefit of identifying factors affecting the success of species at each stage. Under this framework, invasions can be more objectively understood as biogeographical, rather than taxonomic, phenomena; and author preferences in the use of existing terminology can be addressed. An example of this recommended protocol might be: ‘We examined distribution data to contrast the characteristics of invasive species (stages IVa and V) and noninvasive species (stages III and IVb)’.

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A neutral terminology to define 'invasive' species

The Ecological Society of America has evaluated current U.S. national policies and practices on biological invasions in light of current scientific knowledge. Invasions by harmful nonnative species are increasing in number and area affected; the damages to ecosystems, economic activity, and human welfare are accumulating. Without improved strategies based on recent scientific advances and increased investments to counter invasions, harm from invasive species is likely to accelerate. Federal leadership, with the cooperation of state and local governments, is required to increase the effectiveness of prevention of invasions, detect and respond quickly to new potentially harmful invasions, control and slow the spread of existing invasions, and provide a national center to ensure that these efforts are coordinated and cost effective. Specifically, the Ecological Society of America recommends that the federal government take the following six actions: (1) Use new information and practices to better manage commercial and other pathways to reduce the transport and release of potentially harmful species; (2) Adopt more quantitative procedures for risk analysis and apply them to every species proposed for importation into the country; (3) Use new cost-effective diagnostic technologies to increase active surveillance and sharing of information about invasive species so that responses to new invasions can be more rapid and effective; (4) Create new legal authority and provide emergency funding to support rapid responses to emerging invasions; (5) Provide funding and incentives for cost-effective programs to slow the spread of existing invasive species in order to protect still uninvaded ecosystems, social and industrial infrastructure, and human welfare; and (6) Establish a National Center for Invasive Species Management (under the existing National Invasive Species Council) to coordinate and lead improvements in federal, state, and international policies on invasive species. Recent scientific and technical advances provide a sound basis for more cost-effective national responses to invasive species. Greater investments in improved technology and management practices would be more than repaid by reduced damages from current and future invasive species. The Ecological Society of America is committed to assist all levels of government and provide scientific advice to improve all aspects of invasive-species management.

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Biological invasions: recommendations for U.S. policy and management.

We performed a meta-analysis of published studies and long-term monitoring data sets to evaluate the effects of dreissenid mussels (Dreissena polymorpha and D. rostriformis bugensis), two of the world's most problematic biological invaders, on the biogeochemistry, flora, and fauna of lakes and rivers across North America and Eurasia. Dreissenid effects were structured along two distinct energy pathways. For the pelagic–profundal pathway, large mean reductions in phytoplankton (−35% to −78%) and zooplankton (−40% to −77%) biomass occurred and were dependent on habitat type. The largest effects were found in rivers, followed by littoral and pelagic habitats in lakes. In contrast, benthic energy pathways within littoral habitats of lakes and rivers showed dramatic increases in mean benthic algal and macrophyte biomass (+170% to +180%), sediment-associated bacteria (about +2000%), non-dreissenid zoobenthic biomass (+160% to +210%), and total zoobenthic biomass, which includes dreissenid mussel soft tissues (+...

What a difference a species makes: a meta–analysis of dreissenid mussel impacts on freshwater ecosystems

Limnological features of different lakes may limit the density of, or even completely restrict, populations of the European zebra mussel, Dreissena polymorpha. We developed statistical models to predict the occurrence (presence or absence) and density (number per square metre) of Dreissena in lakes, based on multivariate correlations between the density of Dreissena populations and the limnological characteristics of the lakes they inhabit. We found that both occurrence and average density of Dreissena populations were highly correlated with water chemistry. Dreissena was not found in lakes with average pH values below 7.3 and concentrations of calcium ion below 28.3 mg∙L−1. Above these thresholds, Dreissena density was negatively related to concentration of the algal nutrients PO4 and NO3.

Models to Predict Potential Occurrence and Density of the Zebra Mussel, Dreissena polymorpha

The European zebra mussel, Dreissena polymorpha (Pallas), has recently been introduced to North American freshwaters. Dreissena exhibits a wide range of population dynamics among different European lakes. In some lakes, long-term population size is fairly constant, while populations in other lakes can fluctuate greatly from year to year. Assessments of the potential ecological and economic effects of Dreissena in North America will be affected if populations show large interannual fluctuations in abundance. We develop a multivariate model for predicting population dynamics of Dreissena in novel lake systems. Our model uses the limnological differences between lakes with constant populations of Dreissena and those with variable populations to identify variables that may determine population dynamics. We find that lakes with more constant populations tend to have larger surface areas, lower levels of calcium, and higher levels of phosphate (PO,) compared with lakes with more variable populations. La dreissenie europeenne, Dreissena polymorpha (Pallas), a recemment ete introduite dans les eaux deuces de I’Amerique du Nord. Les Dreissena des lacs europeens presentent une large gamme de dynamiques de population. Dans certains lacs, la taille de la population est passablement constante a long terme tandis que les populations d’autres lacs fluctuent grandement d’une annee a I’autre. Les evaluations des effets ecologiques et economiques eventuels des Dreissena en Amerique du Nord donneront des resultats variables si les populations presentent d’importantes fluctuations annuelles d’abondance. Nous avons elabore un modele a variables multiples pour la prevision de la dynamique des populations des Dreissena dans de nouveaux lacs. Le modele fait appel aux differences limnologiques entre les lacs a populations constantes de Dreissena et ceux ou les populations fluctuent pour la determination des variables pouvant influer sur la dynamique des populations. Nous avons trouve que les lacs a populations plus constantes avaient tendance a presenter une plus grande surface, des teneurs en calcium moins importantes et des teneurs en phosphate (PO,) plus elevees que les lacs a populations plus variables.

Charles W. Ramcharan

Papers

Models to Predict Potential Occurrence and Density of the Zebra Mussel, Dreissena polymorpha

A Multivariate Model for Predicting Population Fluctuations of Dreissena polymorpha in North American Lakes