Previous Advances in Threat Assessment and Their Application to Forest and Rangeland Management Review of Methods for Developing Regional Probabilistic Risk Assessments, Part 2: Modeling Invasive Plant, Insect, and Pathogen Species P.B. Woodbury and D.A. Weinstein of organisms, can be more robust than purely statistical approaches. Process-based models may better support P.B. Woodbury, senior research associate, Department of extrapolation beyond the range of available or historical Crop and Soil Sciences, Cornell University, Ithaca, NY data because they use predictor variables that represent 14853; D.A. Weinstein, senior research associate, Depart- physical and biological processes. However, even simple ment of Natural Resources, Cornell University, Ithaca, NY correlative approaches may be useful to quantify the 14853. overlap in spatial distribution of stressors and ecological Abstract receptors as a screening-level analysis. Furthermore, if predictors are chosen carefully, they may represent impor- We reviewed probabilistic regional risk assessment meth- tant processes. For example, data on nonindigenous species odologies to identify the methods that are currently in use may be quite useful for predicting the occurrence of much and are capable of estimating threats to ecosystems from rarer invasive species because the correlation is based on fire and fuels, invasive species, and their interactions with the key processes of human-influenced transport, establish- stressors. In a companion chapter, we highlight methods ment, reproduction, and dispersal of propagules. Ecological useful for evaluating risks from fire. In this chapter, we niche-modeling approaches are useful because they can use highlight methods useful for evaluating risks from invasive data from museum collections in other countries to make species. estimates of potential new range areas in the United States. The issue of invasive species is large and complex Other spatial data such as road networks may also be useful because there are thousands of potential invasive species to predict the number of nonindigenous species or presence and constant movement of new and established plants, of a particular species. Such relationships may also support plant material, pests, and pathogens. Adequate data are not extrapolation to future conditions if there will be more always available to support rigorous quantitative model- roads or a higher traffic volume. ing of the different stages of invasion. However, even a As for any regional stressor, the use of multiple models semiquantitative rule-based approach can help to identify and a weight-of-evidence approach would help to increase locations that contain host species susceptible to specific confidence in predictions of ecological risks from invasive pathogens or insect pests, and where propagules are more species. Two approaches to predicting the risk of Asian likely to enter based on the current locations of the invasive longhorned beetle (Anoplophora glabripennis Motschulsky) species, ports of entry, and methods of spread. Predicting throughout U.S. forests make quite different predictions long-distance movement is much more difficult, as such because they focus on different stages in the process of events are rare, often poorly understood, and are often influ- establishment and spread, thus combining such approaches enced by human behavior. Even so, published methods to should result in more robust predictions. Invasive species make probabilistic predictions of pest establishment could management should be addressed at multiple spatial scales, be expanded to provide quantitative estimates of spread including reducing importation of new species at border beyond an initial port of entry. Many invasive species are crossings and ports, national and regional mapping of loca- transported along roads, and so road networks provide some tions of invasive species, methods to reduce long-distance information about the likelihood of introduction into a new transport, and methods to reduce local movement. region. Keywords: Ecological risk assessment, invasive spe- Models based on fundamental biological and physical cies, probabilistic risk assessment, regional risk assessment, processes, such as population demographics and movement risk analysis. 521 GENERAL TECHNICAL REPORT PNW-GTR-802 Introduction irrigation water, municipal water, hydropower, and flood This review provides an overview of issues in probabilistic control (Zavaleta 2000). risk modeling at the regional scale and suggestions for pro- Various aspects of invasive species biology and ecol- ductive directions for future risk assessments and research. ogy, as well as policy and management issues (NRC 2002), Invasive nonindigenous species are a serious and increas- are addressed in many published reviews. We will review ing threat to many ecosystems throughout the United briefly some key issues, but the focus in this piece is on States (NRC 2002, Pimentel 2005). For example, invasive modeling methods suitable for spatially explicit probabilis- species are implicated as threats for more than half of all tic risk assessments for invasive species. This chapter, and a endangered species in the United States (Wilcove and companion chapter addressing fire (Weinstein and Wood- others 1998). Invasive species are also altering fire regimes, bury, this volume), present results of a project sponsored by hydrology, nutrient cycling, and productivity of ecosystems the U.S. Department of Agriculture (USDA) Forest Service, in the Western United States, particularly rangelands and Western Wildland Environmental Threat Assessment riparian areas (Dukes and Mooney 2004). Plant species Center during its development; but these results should not such as yellow star-thistle (Centaurea solstitialis L.), other be construed to represent the views of the center nor its Centaurea species, and cheatgrass (Bromus tectorum L.) personnel. The overall goal of our project was to identify have overtaken large areas of native ecosystems in the promising methods for analyzing ecological risks to forest, Western United States (LeJeune and Seastedt 2001). Leafy rangeland, and wildland ecosystems from multiple stress- spurge (Euphorbia esula L.), knapweeds (Centaurea sp.), ors. The results of such risk analyses are intended to provide tamarisk (also known as salt cedar, Tamarix ramosissima information useful for strategic planning and management Ledeb.), nonnative thistles, purple loosestrife (Lythrum of wildlands including national forests. The specific goal of salicaria L.), and cheatgrass are some of the most severe this chapter is to identify modeling approaches suitable for problems on national forest lands. For example, the number making spatially explicit, probabilistic estimates of ecologi- of counties in Washington, Oregon, Montana, and Wyoming cal risks from invasive plant, insect, and pathogen species where yellow star-thistle has been found has been increas- throughout large regions such as the Western United States. ing exponentially during the last 100 years (D’Antonio Such modeling approaches ideally should be capable of: and others 2004). Furthermore, the number of new exotic 1. Calculating risk of a detrimental environmental species has increased roughly linearly over this time period, effect. reaching a total of nearly 800 by 1997 (D’Antonio and oth- 2. Using spatially heterogeneous environmental ers 2004). Annual costs of selected nonindigenous species data to drive calculation of risk at different in the United States have been estimated at $120 million points throughout a region. Spatial scales of (Pimentel and others 2005). However, this estimate does not interest include landscape, sub-State region, account for all effects of invasive species on rangelands and State, region of the United States, or the entire forests (Dukes and Mooney 2004), and it is clear that such conterminous United States. costs are substantial. Despite the difficulty in quantifying 3. Relying primarily on available regional (in economic damage, there is substantial evidence suggest- United States, state or multi-State) or national ing that invasive species have many deleterious effects in data. ecosystems in the Western United States, and that improved 4. Being useful for many species, not just a single management of invasive species in wildlands is crucial invasive species. (D’Antonio and others 2004). For example, tamarisk alone 5. Modeling effects of interaction among multiple has been estimated to cost $133 to $285 million per year stressors. (in 1998 U.S. dollars) for lost ecosystem services including 6. Modeling effects of changes in environmental conditions in the future. 522 Advances in Threat Assessment and Their Application to Forest and Rangeland Management We review selected modeling approaches relevant to the In addition, they identify research needs for this field. goals listed above, and more detailed analyses of specific Of relevance to this chapter, they suggest that “Spatially aspects of invasive species assessment and management are explicit, multiscale decision-support systems will contribute provided by other chapters within this broader work. to better decisionmaking through enhanced credibility, an explicit and direct relationship with managing for sustain- Stages of Invasion and Risk Assessment ability, and explicit illustration of trade-offs and the cost of Frameworks inaction.” Presented in one of the articles in this series is a This section provides an overview of the stages of