Invasive species pose a major, yet poorly addressed, threat to sustainable forestry. Here we set forth an interdisciplinary science strategy of research, development, and applications to reduce this threat. To spur action by public and private entities that too often are slow, reluctant, or unable to act, we recommend (a) better integrating invasive species into sustainable forestry frameworks such as the Montréal Process and forest certification programs; (b) developing improved cost estimates to inform choices about international trade and pest suppression efforts; and (c) building distributed information systems that deliver information on risks, identification, and response strategies. To enhance the success of prevention and management actions, we recommend (a) advancing technologies for molecular identification, expert systems, and remote sensing; (b) evolving approaches for ecosystem and landscape management; and (c) better anticipating interactions between species invasions and other global change processes.
Worldwide interest in sustainability—an approach to managing natural resources that meets present human needs while maintaining the earth's capacity to meet the needs of future generations—has burgeoned over the past two decades (NRC 1999). Forest management has been a focal part of this transition, because forests occur on about a third of the earth's land surface, support the lives and livelihoods of hundreds of millions of people, and provide habitat for much of the world's biological diversity (FAO 2001). More than 150 nations, the world's largest forest and paper companies, environmental and conservation organizations, scientists, and other stakeholders are now developing approaches for implementing sustainable forestry. Underlying these efforts is the premise that sustainable forest management must be economically feasible, socially acceptable, and environmentally sustainable.
Escalating rates of biological invasions pose a major, but as yet poorly addressed, threat to sustainable forestry in the United States and worldwide. Here we identify priorities for science and technology that could help reduce this threat. As in recent policy applications, by “invasive species” we mean “a species that is not native to an ecosystem and whose introduction does or is likely to cause economic or environmental harm or harm to human health” (Exec. Order No. 13112, 64 Fed. Reg. 6183–6186 ). Our focus on US forests and institutions provides a tractable scale for exploring the interplay among the economic, social, and environmental objectives of sustainable forestry. Nevertheless, advances identified here should be applicable to other sustainability contexts and should yield benefits worldwide.
Invasive species impacts on US forests
Over the past century, invasive species in the United States have detrimentally affected all of the attributes of forest ecosystems that sustainable forestry seeks to retain, including biological diversity, forest health and productivity, water and soil quality, contribution to the carbon cycle, and socioeconomic values. By some estimates, the US annual monetary losses of forest products caused by invasive species are more than $2 billion (Pimentel et al. 2000). Further, the effects of forest invaders have proved to be lasting and cumulative, and consequently threaten to undermine the obligation to future generations that forms the foundation of sustainability.
By damaging and killing dominant tree species, invasive pathogens and insects have caused cascading changes in the ecology, function, and value of diverse forest ecosystems (table 1, figure 1). Invasive plants have also significantly modified forest ecosystems by altering fire and hydrological regimes, food webs, and the recruitment of dominant tree species (table 1). New invasions continue with recent invaders, such as the sudden oak death pathogen (Phytophthora ramorum; Rizzo and Garbelotto 2003) and the emerald ash borer (Agrilus planipennis; 69 Fed. Reg. 55719–55733 ), which could have similarly profound impacts (table 1).
Moreover, US forests have not yet experienced the full number of possible invasions or the full effects of already established nonnative species. Despite improvements such as seed purity requirements and impending sanitation regulations for imported wood packing materials (69 Fed. Reg. 55719–55733 ), key pathways for the entry or spread of forest invaders remain open or partially regulated (Campbell 2001). Rates of species movement are on the rise because of increasing international and interstate commerce (Campbell 2001). The pool of potential invading species is expanding as the diversity of woody materials and horticultural products imported to the United States grows and as trade with new partners enables species transfers between previously unlinked regions (USDAAPHIS/FS 2000, Campbell 2001, Haack 2001). Continued increases in human access, fragmentation, and disturbance are escalating opportunities for invaders to penetrate and become established in US forests. Climate change will amplify these processes by altering disturbance regimes and the geographic ranges of forest and pest species (Dale et al. 2001, Williams and Liebhold 2002, Logan et al. 2003). As in the past, future invasions are likely to have enormous social, economic, and ecological consequences (table 1).
Prevention and response strategies
Three complementary strategies are essential to counter invasive species in US forests: (1) prevention of harmful new introductions by identifying and impeding pathways for invasive species introduction and spread, (2) detection and eradication of invaders that elude prevention, and (3) long-term management of well-established invasive species (figure 2). The effectiveness of each activity depends on the capacity and will of institutions and people to act and on the availability of and access to adequate knowledge and effective tools. Consequently, here we identify science advances that would better integrate invasive species into decisions affecting sustainable forestry and that would deliver crucial new tools and concepts. This is an outcome-based science plan rather than a conventional research agenda; it focuses on research, development, and applications to enable effective actions. Two themes permeate our analysis. First, reducing the invasive species threat poses problems that are inherently interdisciplinary (Leung et al. 2002), and the priorities presented here incorporate approaches from the social and natural sciences (figure 3). Second, invasion processes occur at geographic scales ranging from site to regional to continental, and can be strategically countered by prevention and management at all of these scales (figure 2). Each science priority we identify is relevant at several scales and would improve the effectiveness of several actions shown in figure 2.
Integrating invasive species into decisions that affect sustainable forestry
Addressing invasive species in sustainable forestry involves many individuals and organizations and, frequently, balancing their varied and sometimes conflicting objectives (table 2). In addition, some level of uncertainty will always characterize species invasions, because predictive systems are imperfect, invasion opportunities and impacts can change, and prevention is unlikely to be 100% effective (NRC 2002). Effective action, under these conditions, requires flexible and adaptive approaches in which forest managers and public agencies share a common understanding of the invasive species threat and have ready access to new information so that they can rapidly respond to change and to the unexpected. Here we discuss how scientific knowledge about invasive species could be made available, better integrated, and used to improve decisions that affect sustainable forestry.
Build effective metrics into emerging sustainable forestry frameworks
Since 1993, public- and private-sector working groups have been developing several frameworks for sustainable forestry that consist of agreed-upon goals and approaches (SFI 2002, FSC 2004, USDA FS 2004). Participating nations and forest managers agree to report on specific metrics (variously termed “indicators”and “criteria”) intended to assess their progress in achieving sustainable forestry. These frameworks hold particular promise for addressing invasive species, because they will encourage collective action across boundaries between nations, states, and land ownerships. Certain metrics and the methods for quantifying them are straightforward; for example, some measures related to forest productive capacity rely on existing forest inventory systems. In contrast, present approaches for quantifying how invasive species affect sustainable forestry's goals are immature and require further development.
One of the most important sustainability frameworks for North American forests is the Montréal Process, a joint initiative of 12 nations, including the United States, that collectively contain 90% of the world's temperate and boreal forests (USDA FS 2004). Participants have identified 67 indicators for measuring performance within seven general categories. Of these 67, only 2 refer to exotic species. Indicator 12, developed to help assess forest productive capacity, measures the “area and growing stock of plantations of native and exotic species.” Indicator 15, developed to help assess the maintenance of forest health and vitality, measures the “area and percent of forest affected by processes or agents beyond the range of historic variation.”However, these two indicators together do not adequately assess the threat posed by invasive species to sustainable forestry. For example, the recent National Report on Sustainable Forests (USDA FS 2004), in reporting US implementation of the Montréal Process, does not provide quantitative data on the overall economic and ecological impacts of invasive species on US forests—information that would provide a crucial baseline for weighing invasive species relative to other threats or for evaluating the effectiveness of related US policies and management actions.
Forest certification frameworks are a nongovernmental approach intended to have powerful effects on the policy and management choices of government agencies, timber and paper product companies, and other forest owners and managers. Owners and managers obtain certification that their forests are under sustainable forest management by demonstrating progress in fulfilling specific requirements to third-party auditors. Metrics relevant to invasive species in two of the more widely adopted certifications in the United States, the Forest Stewardship Council and the Sustainable Forestry Initiative, generally assess the extent to which participants minimize risks related to exotic tree plantings and monitor and manage forests to prevent and minimize outbreaks of pests, diseases, and invasive plants and animals (SFI 2002, FSC 2004). While these and other certification systems are creating institutional structures suitable for addressing invasive species, their present metrics do not reflect current scientific understanding of the magnitude of this threat to all of sustainable forestry's goals or of potential solutions.
A need now exists to carefully evaluate whether the Montréal Process and the certification programs identify the correct metrics for fully assessing the invasive species threat and for measuring participants' implementation of appropriate prevention and response strategies. This evaluation should occur through the frameworks' various internal review mechanisms and should integrate external scientists and technical experts. An improved system of invasive-species metrics for sustainable forestry would provide sufficient specificity to track whether participants (a) eliminate pathways of invasive species spread; (b) manage forests in ways that reduce the likelihood of new invasions; (c) monitor with enough sensitivity to detect new invasions early and implement rapid response measures; (d) suppress established invaders, including by participating in area-wide monitoring and management efforts; and (e) consider how eradication strategies will affect other aspects of forest ecosystems and sustainability. At a minimum, reporting on progress toward sustainability should include tracking invasive species' social, economic, and ecological impacts. The review also should recommend the most effective methods for quantifying invasive-species metrics. This will be an important application for many of the advances in database development, spatial technologies, modeling, and other analytical methods identified elsewhere in this article.
Develop improved cost estimates to inform policy and management decisions
Critics charge that government policies and implementing agencies often fail to adequately consider the risks posed by invasive species when making decisions about international trade or evaluating pest control options (Campbell 2001). Yet remarkably little quantitative information exists about the past and projected costs of species invasions for US forests. When available, such information has affected policymaking—for example, in justifying the Animal and Plant Health Inspection Service's impending rules for treatment of solid wood packing materials (69 Fed. Reg. 55719–55733 ).
More and better quantitative estimates of invasive-species costs would substantially strengthen decisionmaking by federal legislators and agencies, making it possible to weigh the outcomes of alternative policy and management actions (e.g., using cost–benefit or cost-effectiveness analyses, as encouraged by the US Office of Management and Budget) in terms that resonate with policymakers and the public. In keeping with sustainable forestry's goals, estimates of past and projected costs and benefits of policies should go beyond conventional economic analyses of market values, such as timber losses and pest suppression expenses. They should also quantify a wide range of nonmarket environmental and social values relevant to sustainability that may be degraded, such as watershed protection, biodiversity, and aesthetic and other amenity values (Naylor 2000, Leung et al. 2002).
Quantifying these nonmarket values will require surmounting certain technical problems. Objective quantifications of invasive species' environmental impacts generally are lacking (NRC 2002), making it difficult to translate these impacts into economic terms. Even with good information on environmental impacts, economic techniques for assigning dollar values to nonmarket goods and services are often costly and controversial. Values can be more easily estimated for well-documented services, such as human health or aesthetic protection, than for complex global services, such as carbon sequestration or climate regulation. Also, standard techniques discount benefits that accrue far into the future—a particular problem for invasive species, since expenditures on prevention or control often precede by decades the benefits derived from avoided harm. Alternative economic approaches should be explored for assessing how invasive species alter the capacity of forest ecosystems to deliver valued services (Daily 1997). Analyses might, for example, compare levels of ecosystem functioning with and without a specific invader present, or evaluate whether removing a species is a cost-effective way to reduce fire frequency or severity (Wainger and King 2001).
International trade decisions will be a key application for improved cost or risk estimates (Leung et al. 2002). Recent trade agreements have left federal agencies struggling to reconcile trade policies that potentially enhance invasion opportunities with their obligations to protect US agriculture and natural resources from invasive pests (Campbell 2001). Further, World Trade Organization (WTO) agreements only allow member nations to impose trade restrictions if they meet high standards of evidence of harm. Balanced economic analyses of imported species and commodities that weigh the full range of risks and benefits and make comparisons to available domestic substitutes could significantly improve the agencies' regulatory decisions, their ability to propose trade restrictions, and the acceptance of agency decisions by affected stakeholders.
Better cost and benefit estimates also could improve pest suppression programs. The speed and sequence of decisions and management actions during the progression of a biological invasion greatly influence effectiveness (Hobbs and Humphries 1995, Naylor 2000); yet invasive species can confound timely decisions (figure 4). Rapid-response decision rules could help resolve this dilemma by providing standard procedures for agencies to follow in times of emergency. At present, agencies develop methods to assess each new situation. Decision rules also would provide a clear rationale for appropriators to fund rapid responses.
Rapid-response decision rules would use objective criteria to assess the risks of a new species invasion, the potential costs and benefits of responding at different points in the invasion process, and the optimal timing for switching from an eradication campaign to suppression. Measuring the benefits of any given response will rest on demonstrating the invasive species' impacts on ecosystem services and other valued qualities. Risk assessments typically integrate situation-specific information, including species' ecological characteristics and their invasiveness and impacts elsewhere; ecosystem qualities that affect invasion vulnerability; and vectors that facilitate invasions. While methods for analyzing and making predictions based on such information continue to improve, risk assessments will always involve some level of uncertainty caused by insufficient information (Anderson et al. 2004). Scenario analysis is one approach that could enable decisionmaking even when uncertainty is great. For example, examining the cost-effectiveness of early eradication under different risk scenarios could help establish the threshold risk level that would justify rapid action.
Build distributed information resources to serve diverse decisionmakers
Diverse people and organizations make decisions every day that determine the impacts of invasive species on US forests (table 2). Unfortunately, the United States lacks the information infrastructure that would enable these decisionmakers to make consistently informed choices (NRC 2002). Despite proliferation of related online databases and information resources, the present US information system on forest invaders is incomplete and fragmented, and access to and analyses of these data are weak, mirroring the global status of invasive-species databases (Ricciardi et al. 2000). Most databases are not yet widely used, especially among the private sector and local governments—entities that own more than half of US forested lands. Detailed information is not readily available on the distribution and impacts of some of the most devastating forest invaders. Notably absent are anticipatory systems for preventive action that would routinely identify emerging pathways, probable new invaders, or ecosystems vulnerable to future invasions. Moreover, standards and protocols for data and information are recent developments, and modern data management practices have not yet been widely adopted by ecologists or forest managers (Green et al. 2005).
The nation now needs a more comprehensive and dynamic information system on invasive species that presently do or could affect US sustainable forestry (figure 5). Realizing the full power of invasive-species databases will require new tools to mine and link these resources. Internet search engines and algorithms, Web crawlers, and other data-mining and classification techniques could aid in conducting targeted data queries across databases, in linking invasive species information with other data to deliver customized decision support products, and in automating risk alerts (figure 5). Ideally, the system should support distributed decisionmaking and collaborative public processes and deliver database query results in formats easily integrated into forest planning and management. It also should support rapid mobilization by government agencies and forest managers in the face of imminent threats, for example, by providing diagnostic tools and up-to-date biological information and management options. Possible economic or legal impediments to developing spatial data that describe forest conditions or identify the locations of invasive species on private landholdings should be assessed now to help set feasible expectations for the system.
Modification of some key databases would facilitate their use in risk identification (figure 5). The usefulness of the US Department of Agriculture (USDA) Forest Service's national Forest Inventory and Analysis, or FIA, database for identifying areas at risk of invasion would be improved by the inclusion of more data on alternative hosts (e.g., understory plants) and better links to sites with data layers on climate, soils, and high-risk locations (e.g., where host plants are sold in nurseries or bought by sawmills). The USDA's Port Interception Network (PIN) could be used routinely to identify high-risk pathways, if initial occurrences of known forest pests were systematically compared with variables that affect pathways, such as trade volume, commodity type, or shipping technologies and packing material (Haack 2001). The PIN's value for this purpose is currently limited, because it primarily includes pests of “quarantine significance” rather than all intercepted species, and because access is restricted (NRC 2002). Coordinated reporting could significantly speed public and private responses to emerging threats. One possibility is a “national clearing-house” that allows state agencies, tree-care companies, forest managers, landowners, and others to voluntarily report verified sightings (Kelly and Tuxen 2003).
Developing critical new tools and concepts
Significant needs will always exist for more information about the biology of invasive species and their behaviors in new places. Such information is essential for honing predictions and for designing effective measures to counter individual high-impact invaders. Against that general backdrop, several specific technical and conceptual advances will be essential for addressing invasive species in sustainable forestry.
Advance technologies for detection, identification, and monitoring
Accurate species identification is a key challenge in detection and monitoring. The number of trained taxonomists has dwindled. Physical characters used to identify species can be subtle and difficult to distinguish for nonspecialists, and taxonomic information is incomplete for many groups. Some closely related taxa, particularly different strains or hybrids of pathogens, lack obvious morphological differences. Identification can be further confounded if a pathogen causes various symptoms on multiple hosts, mimics symptoms identical to those caused by other pathogens, or undergoes rapid genetic changes that alter its epidemiology (Brasier 2001). For other taxa, species identifications often must be based on intercepted larvae, seeds, and spores that lack the distinguishing features of later life-history stages (Haack 2001).
Innovations from molecular biology, biotechnology, and digital imaging now hold the promise of providing new and easier approaches for detecting and identifying invasive species. Polymerase chain reaction technologies already are being used to detect plant pathogens in nursery stock (Davidson et al. 2003). As these technologies advance, real-time capabilities using microarray chips could be developed to assay for known pathogens of particular hosts at ports or during preshipment certification. The Central Science Laboratory at Sand Hutton, United Kingdom, is piloting this approach with a microchip that can screen samples for the presence of 250 potato pathogens (Ian Barker, Immunological and Molecular Methods Team, Central Science Laboratory, personal communication, 3 August 2004). DNA microprobes have similar potential for identifying insect larval stages and eggs found within cargo and packing materials (Kethidi et al. 2003). Although developing such identification techniques seems a major undertaking today, this investment will yield significant long-term benefits through improved inspection efficiency and reduced damage. Combining genetic information with improved digital imaging could provide powerful tools for future diagnostic identifications. Supporting the necessary online image and DNA libraries will require comparable advances in data development, archiving, and delivery.
Expert systems that automate risk identification at ports of entry could improve import screening and help fill the pressing need for rapid, flexible, and scientifically valid procedures that meet WTO criteria (Campbell 2001). Development and validation of systems that objectively evaluate intentional introductions might build on current techniques for predicting invasiveness that consider ecological traits, climate similarities between source environments and destinations, or host distributions (Sutherst et al. 1995, Reichard and Hamilton 1997) and that incorporate stochastic aspects of introductions (Mack 2000). Systems integrating data on imports (e.g., product records, receiving ports, containerized freight destinations), US habitat distribution (e.g., climate, soils, forest types), and pest distributions in originating countries could automatically target inspections toward commodities or other vectors known to harbor high-risk species or to come from countries that previously were sources of contaminated cargo. Such expert systems might also inform government inspection goals by providing information on contamination rates and probability of detection, and thereby reveal the extent to which the current inspection rate at US entry ports (2% in 2002) misses contaminants on imported wood products and packaging materials (NRC 2002). Applying expert systems will require setting acceptable levels of risk through transparent processes that consider the full risks of new forest invaders (including risks to biodiversity and ecosystem function) as well as the benefits of commerce (NRC 2002).
Tracking the advance or retreat of invasive species across large areas, often in remote locations, is a particular difficulty for sustainable forestry. Information must be geographically comprehensive, yet sensitive and frequent enough to encounter relatively small populations of invaders when they are easier to eradicate. Additional methods for developing and analyzing remote sensing outputs would help. Researchers have begun to explore applications of remote imagery (e.g., satellite, hyperspectral, multispectral, and aerial photography) for identifying the distribution or impacts of invasive pathogens, insects, and plants and for characterizing their spatial dynamics (Bonneau et al. 1999, Kelly and Meentemeyer 2002, Underwood et al. 2003). Patterns of defoliation, crown dieback, and tree mortality aid in monitoring certain species. Such analyses can pinpoint high-risk locations for more detailed ground surveys. Digital “sketch mapping” is being explored as a means of providing aerial survey results within days of flight. Accelerated research on remote sensing should be coupled with efforts to make the outputs easy to integrate into the activities of forest managers and state forestry organizations.
Evolve ecosystem and landscape management approaches
Despite their capacity to alter ecosystem processes, thus far invasive species have been poorly integrated into the conceptual framework for forest ecosystem management. This shortcoming is unfortunate, because spatial patterns of physical and biological processes across forests can retard or promote species invasions and could be managed to reduce invasion risks (With 2002).
For example, roads, railways, vehicles, and foot traffic provide conduits for various pests to spread, while contiguous populations of host plants and alternative hosts can enhance the spread of invasive pathogens and insects (Parendes and Jones 2000, Trombulak and Frissell 2000, Jules et al. 2002). Adjacent land uses, the size of habitat fragments, and the edge-to-interior ratio of forests all affect invasions (With 2002). Disturbances can open up habitat for invaders or, alternatively, disrupt species' dispersal corridors (With 2002). Fire suppression can intensify pathogen outbreaks by altering stand dynamics, whereas severe wildfires after years of fire suppression can facilitate the spread of invasive plants (Harrod and Reichard 2001, Krakowski et al. 2003). Silvicultural practices affect vulnerability to and recovery from pathogens and insects, as well as the relative abundance of nonnative understory plants (Gottschalk 1993, Houston 1997, Thysell and Carey 2001). Plantation forestry and land-use history may also affect understory communities (Harrington and Ewel 1997), and in some parts of the world plantation trees have become invasive (Richardson 1999). Some invasive species alter fire frequency, hydrology, and other ecosystem processes—changes that may favor further invasions by the same or other species (Vitousek et al. 1996). Interactions among these processes can influence invasive species' spread and success (With 2002).
This empirical and theoretical understanding of how landscape structure and ecosystem condition affect invasions now needs to be synthesized and transformed into management prescriptions that can be tested and improved through adaptive management. These prescriptions should aim to eliminate invasion pathways and opportunities and to minimize impacts where invasive species are established. The synthesis should consider common forest management practices, such as patterns of timber harvest and thinning, fire suppression and burn frequency, revegetation after disturbance, road building, and hunting and recreational uses, as well as natural disturbance regimes. It also should integrate how fragmentation and matrix land uses affect invasion vulnerability. Potential mitigation approaches should be identified, such as whether closing logging roads after harvest reduces invasion risks, or whether interplanting or buffers can slow the spread of host-specific invaders. An important issue will be whether fundamentally different approaches are required for managing forests with few invaders in comparison to those that are highly altered by repeated past invasions; for example, heavily invaded forests might tolerate a greater level of disturbance without experiencing a fundamental shift in species composition.
Spatially explicit models are increasingly important in forest management, conservation, and restoration (Dale 2003). They offer another approach for understanding how landscape structure affects invasions (Hart and Gardner 1997) and for designing effective monitoring and suppression programs against individual high-impact invaders. Models have ranged from static predictive mapping of species based on habitat suitability (Peterson and Vieglais 2001) to various dynamic population- or individual-based models (Sharov et al. 1998, Higgens et al. 2000). Some have projected ecological and resource losses, evaluated the cost-effectiveness of different management strategies, or screened potential invaders on the basis of how interactions with native species might affect their spread. Modeling has proved particularly useful for justifying and targeting resources for large, expensive management efforts (Sharov et al. 1998, NRC 2002). Time-series scenarios of spread under different management strategies now guide multistate gypsy moth (Lymantria dispar) suppression (Sharov and Liebhold 1998) and could be applied to other large-scale programs.
Spatial analyses could further explore how suppressing invaders at certain locations can reduce invasions elsewhere by altering source–sink dynamics. Improved understanding of spatial population dynamics might lead to general practices for managing invasive species for extinction—that is, encouraging local or regional extinctions by altering habitat fragmentation or connectivity (With 2002), by suppressing populations to maximize extinction probability, or by other means (Anderson et al. 2004). Further integrating economic tools with spatial population models (Sharov and Liebhold 1998, Sharov et al. 1998, Higgens et al. 2000, Leung et al. 2002) could help to optimize control strategies by distributing resources where and when a species' invasiveness and harmfulness are greatest, the treatment costs are lowest, or the chances of success are highest (Wainger and King 2001).
Effective management of invasive species across large forested areas ultimately will require participation by a mosaic of owners and managers having differing goals, cultures, and contexts for their actions (table 2, figure 6). Improved understanding of such variation could aid in developing processes, incentives, or policies that spur multiple landowners and managers to develop shared management objectives; to voluntarily contribute to large-scale prevention, monitoring, or management efforts; or to accept and adopt necessary control technologies.
Better integration of invasive-species issues into tools for forest ecosystem management will be essential. Early forest-planning models optimized harvest schedules for sustainable yield and were not designed to address spatial patterns of ecosystem processes such as wildfire or species invasions. Next-generation forest planning models now being developed incorporate landscape processes (Sessions et al. 1999), offering the potential to better integrate invasive species concerns into routine forest management planning. Meanwhile, even relatively simple approaches, such as GIS-based predictive mapping of site susceptibility to harmful alien species (figure 7), could help raise awareness among forest users and managers about potential risks in currently noninfested areas.
Address interactions with climate and other global change processes
Several global processes, including climate and land-use change, economic globalization, and alteration of nutrient cycles, are contributing to escalating rates of species invasions and impacts (Vitousek et al. 1996, Mooney and Hobbs 2000). Climate and land-use change alter the physical environment and disturbance regimes in ways that can favor nonnative species and alter forest ecosystem vulnerability to invasions (Dale et al. 2001). Economic globalization is not only changing the pathways and rates of species transfers between nations, but also the economic forces affecting local land-use decisions, and thereby indirectly influencing disturbance regimes and invasion opportunities.
The combined influences of global change processes will accelerate over this century, creating major shifts in the distribution of species, ecological communities, and ecosystems. Climate change alone may cause many ecological communities to disassemble, as species ranges shift at different rates in response to a changing climate and interacting variables such as hydrology, fire frequency, atmospheric carbon dioxide, and land use (Mooney and Hobbs 2000, Simberloff 2000, Dale et al. 2001, Hansen et al. 2001). Feedback loops could emerge that amplify these effects, as invasive species that further alter ecosystem functions become widely established (Vitousek et al. 1996). The ranges of many forest pests may shift, insect outbreak behaviors may intensify, and pathogen impacts may increase (Williams and Liebhold 2002, Logan et al. 2003). The social, economic, and ecological value of these systems could change radically and often may be degraded.
Managers, policymakers, and others who implement sustainable forestry need a far better conceptual approach for understanding interacting global change processes and making robust choices in the face of incomplete knowledge. New modeling and scenario-based approaches are needed to examine how interacting global change processes may alter forest ecosystem vulnerability and invasion patterns, rates, and impacts (Simberloff 2000). These approaches should identify the ecosystems at greatest risk and help evaluate alternative management and policy options. To aid in building institutional response capabilities, they should describe pending changes over time scales that are relevant to harvest rotation and other business cycles, to the development of government policies, and to public values. Past models emphasized continental-scale shifts in species and ecosystem ranges: now needed are models of population, community, and ecosystem dynamics at scales that match local and regional decision-making.
Important questions will arise about how managers should set appropriate and realistic goals for sustaining the social, economic, and environmental values of changing forest ecosystems—particularly since the feasible baselines for accomplishing these three outcomes could shift substantially. Maintaining present or restoring historical conditions, including species composition, often will not be possible, and sometimes may not even be desirable. One approach would be to specify desired ecosystem services—such as ecological functions, level of biological diversity, and economic outputs—rather than species-specific goals (Daily 1997). The process of specifying these services would need to incorporate the values different stakeholders assign to various services. Sustaining the delivery of desired ecosystem services will require new methods for measuring and monitoring ecosystem state and vulnerability relative to these objectives.
Forests afford a special opportunity for managing ecosystems to prevent, mitigate, or direct adaptation to global change. Many are already manipulated to achieve specific objectives, and methods exist for accomplishing these manipulations at large spatial scales. Forest dynamics are slow, because trees have long generation times, and these systems exhibit significant inertia in changing environments (Iverson et al. 2004). Preempting, minimizing, or reversing the worst impacts of biological invasions exacerbated by global change may often be necessary. Specific invaders or functional groups of invasive species might be targeted on the basis of their probable long-term impacts on ecosystem function and resilience in a future system that has changed and perhaps will change further. Inhibiting, or at least slowing, certain changes in species assemblages sometimes may be desired (e.g., to retain a reservoir of species for restoration or human-assisted range expansions; Hansen et al. 2001). Managers also might direct changes in species composition toward specific ecological outcomes, for example, where rates of species migration are too low to match rates of habitat change or where migratory corridors do not exist. Scientific advances discussed elsewhere in this paper would aid in designing, evaluating, and implementing such management interventions.
We have set forth an interdisciplinary, outcome-based science strategy for reducing the impacts of invasive species on sustainable forestry. This strategy addresses several persistent challenges that have emerged over the past decade as activity related to invasive species has burgeoned. New institutional mechanisms are needed to engage diverse players. Appropriate tools are lacking to tackle invasive species at large spatial scales. Despite explosive growth in online information resources, this information still needs to be translated into actionable knowledge. And invasive species have not yet been well addressed within the context of multiple societal goals or interacting global change processes. Forestry provides a strong platform for addressing these challenges because of its sustainability frameworks, well-developed approaches for ecosystem management, integration of spatial technologies into planning and management, and knowledge base on global change. Consequently, action on the priorities identified here ultimately should benefit not only sustainable forestry but also broader efforts to reduce the harmful impacts of invasive species.
This article is the result of a workshop sponsored by the National Commission on Science for Sustainable Forestry (NCSSF) and held at the National Center for Ecological Analysis and Synthesis (NCEAS) in April 2003. We are grateful to NCSSF for funding this project and to NCEAS for providing on-site support. We also appreciate the efforts of three anonymous reviewers whose comments greatly improved the manuscript.
A sample of possible future, recent, and past invaders of significance to sustainable forestry in the United States, listed in order of first US detection.
Social scales of influence and major activities for entities affecting invasive species in US sustainable forestry.