Ingrid M. Parker

Impact: toward a framework for understanding the ecological effects of invaders

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.

Founding events in species invasions: genetic variation, adaptive evolution, and the role of multiple introductions

Invasive species are predicted to suffer from reductions in genetic diversity during founding events, reducing adaptive potential. Integrating evidence from two literature reviews and two case studies, we address the following questions: How much genetic diversity is lost in invasions? Do multiple introductions ameliorate this loss? Is there evidence for loss of diversity in quantitative traits? Do invaders that have experienced strong bottlenecks show adaptive evolution? How do multiple introductions influence adaptation on a landscape scale? We reviewed studies of 80 species of animals, plants, and fungi that quantified nuclear molecular diversity within introduced and source populations. Overall, there were significant losses of both allelic richness and heterozygosity in introduced populations, and large gains in diversity were rare. Evidence for multiple introductions was associated with increased diversity, and allelic variation appeared to increase over long timescales (~100 years), suggesting a role for gene flow in augmenting diversity over the long‐term. We then reviewed the literature on quantitative trait diversity and found that broad‐sense variation rarely declines in introductions, but direct comparisons of additive variance were lacking. Our studies of Hypericum canariense invasions illustrate how populations with diminished diversity may still evolve rapidly. Given the prevalence of genetic bottlenecks in successful invading populations and the potential for adaptive evolution in quantitative traits, we suggest that the disadvantages associated with founding events may have been overstated. However, our work on the successful invader Verbascum thapsus illustrates how multiple introductions may take time to commingle, instead persisting as a ‘mosaic of maladaptation’ where traits are not distributed in a pattern consistent with adaptation. We conclude that management limiting gene flow among introduced populations may reduce adaptive potential but is unlikely to prevent expansion or the evolution of novel invasive behaviour.

Evaluating approaches to the conservation of rare and endangered plants

Schemske, D.W., B. C. Husband, M.H. Ruckelshaus, C. Goodwillie, I.M. Parker, and J.G. Bishop. 1994. Evaluating approaches to the conservation of rare and endangered plants. Ecology 75: 584-606.

Introduced species policy, management, and future research needs

Introduced species represent an accelerated global change, and current efforts to manage them, though effective in particular situations, are not controlling the general problem. In the US, this failure is the result of insufficient policy, inadequate research and management funding, and gaps in scientific knowledge. Comparative policy analysis is urgently needed; the main US shortcoming is the absence of a coherent set of policies to address the entire issue, rather than individual invaders. Deliberate introductions should be more stringently regulated and risk assessments must become more predictive. Monitoring and attempts to identify new invasions (both deliberate and inadvertent) are technically feasible but not sufficiently funded and coordinated. Techniques to manage established invaders have often succeeded, but have been hamstrung by inconsistent funding. All of these problems could be improved by more fundamental research, ranging from basic natural history and simple advances in control technol...

INVASION DYNAMICS OF CYTISUS SCOPARIUS: A MATRIX MODEL APPROACH

It is at the level of population dynamics that an invasion either fails or succeeds. By elucidating patterns of variation in population growth rates or demographic rates, it is possible to forge a connection between quantitative field data and theoretical ideas about invasiveness, invasibility, and rates of spread. Demographic models also provide a tool to guide control strategies for invasive pests. Here I report the results of a demographic study of Cytisus scoparius, an exotic shrub on the west coast of North America. I used matrix population models to describe demographic patterns in six populations (three in prairies and three in urban fields) and across advancing stages of invasion. At the edge of the invading front, all populations showed finite rates of increase (λ) >1; however, prairie populations were increasing much more rapidly than urban ones. While many individual vital rates differed between prairie and urban populations, Life Table Response Analysis revealed that seedling establishment mad...

POLLINATOR LIMITATION OF CYTISUS SCOPARIUS (SCOTCH BROOM), AN INVASIVE EXOTIC SHRUB

Introductions of exotic species provide unique opportunities to study the demographic significance of species interactions, but as yet there is little information on how mutualistic interactions affect the invasion process. A shortage of mutualists could potentially limit the rate of population growth for an invading species. The introduced shrub Cytisus scoparius (Scotch broom, Leguminosae) is a pest plant on the west coast of North America. It produces flowers that are “tripped” open when pollinated and has a nearly obligatory relationship with resident bumble bees and honey bees. Experiments in the state of Washington showed that <1% of untripped flowers produced fruits and that outcross-pollinated flowers yielded fourfold more fruit than self-pollinated flowers, revealing apparent inbreeding depression. Mean pollinator visitation rate, as determined by the proportion of flowers tripped, varied among three years and among four populations but was low (3–30%) in every case. Two urban populations (Magnus...

Direct and Interactive Effects of Enemies and Mutualists on Plant Performance: a Meta-Analysis

Plants engage in multiple, simultaneous interactions with other species; some (enemies) reduce and others (mutualists) enhance plant performance. Moreover, effects of different species may not be independent of one another; for example, enemies may compete, reducing their negative impact on a plant. The magnitudes of positive and negative effects, as well as the frequency of interactive effects and whether they tend to enhance or depress plant performance, have never been comprehensively assessed across the many published studies on plant-enemy and plant-mutualist interactions. We performed a meta-analysis of experiments in which two enemies, two mutualists, or an enemy and a mutualist were manipulated factorially. Specifically, we performed a factorial meta-analysis using the log response ratio. We found that the magnitude of (negative) enemy effects was greater than that of (positive) mutualist effects in isolation, but in the presence of other species, the two effects were of comparable magnitude. Hence studies evaluating single-species effects of mutualists may underestimate the true effects found in natural settings, where multiple interactions are the norm and indirect effects are possible. Enemies did not on average influence the effects on plant performance of other enemies, nor did mutualists influence the effects of mutualists. However, these averages mask significant and large, but positive or negative, interactions in individual studies. In contrast, mutualists ameliorated the negative effects of enemies in a manner that benefited plants; this overall effect was driven by interactions between pathogens and belowground mutualists (bacteria and mycorrhizal fungi). The high frequency of significant interactive effects suggests a widespread potential for diffuse rather than pairwise coevolutionary interactions between plants and their enemies and mutualists. Pollinators and mycorrhizal fungi enhanced plant performance more than did bacterial mutualists. In the greenhouse (but not the field), pathogens reduced plant performance more than did herbivores, pathogens were more damaging to herbaceous than to woody plants, and herbivores were more damaging to crop than to non-crop plants (suggesting evolutionary change in plants or herbivores following crop domestication). We discuss how observed differences in effect size might be confounded with methodological differences among studies.

Invading populations of an ornamental shrub show rapid life history evolution despite genetic bottlenecks.

Human-mediated species introductions offer opportunities to investigate when and how non-native species to adapt to novel environments, and whether evolution has the potential to contribute to colonization success. Many long-established introductions harbour high genetic diversity, raising the possibility that multiple introductions of genetic material catalyze adaptation and/or the evolution of invasiveness. Studies of nascent invasions are rare but crucial for understanding whether genetic diversity facilitates population expansion. We explore variation and evolution in founder populations of the invasive shrub Hypericum canariense. We find that these introductions have experienced large reductions in genetic diversity, but that increased growth and a latitudinal cline in flowering phenology have nevertheless evolved. These life history changes are consistent with predictions for invasive plants. Our results highlight the potential for even genetically depauperate founding populations to adapt and evolve invasive patters of spread.

Reducing the risks of nonindigenous species introductions.

A though plants and animals have always spread across the globe, expanding their ranges and invading new territories, human civilization has accelerated these biological invasions to an extraordinary degree. Nonindigenous species represent a major threat to the integrity of natural systems, because they can alter nutrient regimes, develop into monocultures, and drive native species to extinction (e.g., the nitrogen-fixing tree Myrica faya [Vitousek and Walker 1989], the zebra mussel Dreissena polymorpha [Mills et al. 1994, Roberts 1990], the purple loosestrife Lythrum salicaria [Mercer 1990], and the brown tree snake Boiga irregularis [Burdick 1994]). Although the US government recognizes the risks posed by nonindigenous species and for nearly a century has had legislation in place aimed at reducing harm-

Invasiveness of Ammophila arenaria: Release from soil-borne pathogens?

The Natural Enemies Hypothesis (i.e., introduced species experience release from their natural enemies) is a common explanation for why invasive species are so successful. We tested this hypothesis for Ammophila arenaria (Poaceae: European beachgrass), an aggressive plant invading the coastal dunes of California, USA, by comparing the demographic effects of belowground pathogens on A. arenaria in its introduced range to those reported in its native range. European research on A. arenaria in its native range has established that soil-borne pathogens, primarily nematodes and fungi, reduce A. arenarias growth. In a greenhouse experiment designed to parallel European studies, seeds and 2-wk-old seedlings were planted in sterilized and nonsterilized soil collected from the A. arenaria root zone in its introduced range of California. We assessed the effects of pathogens via soil sterilization on three early performance traits: seed germination, seedling survival, and plant growth. We found that seed germinatio...