A Framework for Debate of Assisted Migration in an Era of Climate Change

The Torreya Guardians are trying to save the Florida torreya (Torreya taxifolia Arn.) from extinction (Barlow Hannah et al. 2005; Peterson et al. 2006), forcing species to colonize new terrain to survive. The focus of the Torreya Guardians is an “assisted migration” program that would introduce seedlings to forests across the Southern Appalachians and Cumberland Plateau (http://www.TorreyaGuardians.org). Their intent is to avert extinction by deliberately expanding the range of this endangered plant over 500 km northward. Because planting endangered plants in new environments is relatively simple as long as seeds are legally acquired and planted with landowner permission, the Torreya Guardians believe their efforts are justified. Introducing this species to regions where it has not existed for 65 million years is “easy, legal, and cheap” (Barlow McLaughlin et al. 2002; Root et al. 2003; Thomas et al. 2004). Researchers typically conclude that mitigating climate change and providing reserve networks that foster connectivity and movement should be a priority (e.g., Hannah et al. 2002). Ecologists must recognize, however, that even optimistic estimates of natural movement may be insufficient for species to keep pace with climate change. Assisted migration is a contentious issue that places different conservation objectives at odds with one another. This element of debate, together with the growing risk of biodiversity loss under climate change, means that now is the time for the conservation community to consider assisted migration. Our intent here is to highlight the problem caused by a lack of a scientifically based policy on assisted migration, suggest a spectrum of policy options, and outline a framework for moving toward a consensus on this emerging conservation dilemma. Land management agencies, in particular, must confront the issue of assisted migration in terms of their own management and their regulation of others' management efforts. Natural resource agencies are tasked with two mandates: preserving biodiversity and managing species of concern. Preserving biological diversity may include species diversity, habitat integrity, a historical construct of community structure, ecosystem function, or, more likely, a combination of these objectives (Grumbine 1994). As stewards of publicly owned land, agencies develop plans to accomplish these goals, and typically they lack a policy about new species that might be introduced for conservation purposes. Government agencies also play a role in regulation of the intentional movement of species. In the United States federal and state natural resource agencies are the arbiters of the management and movement of species (Czech the obvious violation made by envelope models in their assumption of uniform climatic tolerances across a species' range; the problem of how to include biotic interactions in determining the influence of climate on species; and sizeable uncertainty in climatic predictions themselves (Guisan Kullman 1998), but they must also accept the likelihood that restricting population spread to natural mechanisms may result in the extinction of species that might otherwise have survived. This position represents an attempt to balance the benefits and risks associated with assisted migration (Fig. 1). The hallmark of this position is the expectation that assisted migration is necessary to preserve biodiversity despite recognized risks. Risks can be minimized through careful restrictions on actions, planning, monitoring, and adaptive management. This position may range broadly between positions 1 and 2 (Fig. 1). Uncertainty as to the impacts of introducing species to novel environments and uncertainty in model predictions of the need to move organisms will demand constraints on assisted migration. Thus, proposals for assisted migration may require evidence of imminent threat, a quantitative model of predicted outcome of assisted migration, and an assisted migration management plan. Such a migration plan would be informed by scientific information on the topics described below, perhaps vetted by a board of experts charged with implementing the precautionary principle while allowing the establishment of pilot projects and easing policy restrictions when advances in research suggest it worthwhile. There are obvious costs to constraining assisted migration projects. For example, assisted migration proposals would require substantial data and thus could only be implemented for a few species of highest concern. In addition, supportive evidence may be disputed, which could result in costly delays in assisted migration actions. It is important for academics, advocates, and managers to discuss the role that assisted migration should play in the conservation of species. For example, a liberal policy of assisted migration, as described in position 1, may be irreversible, so we advocate a broad and open discussion before such actions are taken. Legitimate philosophical and scientific differences between positions may be difficult to reconcile. However, our shared concern about the biological consequences of climate change provides important common ground. The only policy options we categorically reject are the two that are currently being implemented. Maverick, unsupervised translocation efforts run the risk of undermining current conservation work and do not reflect a consensus among interested parties. We more strongly reject the far more ubiquitous “business as usual” scenario that is the current de facto policy. Data and models suggest that extinctions are likely to be numerous and imminent given the range shifts and contractions currently underway. Even a policy rejecting assisted migration will have to offer alternative approaches to prevent species extinction. Basic ecological research will play a role in resolving the issues raised by assisted migration. Here, we briefly outline five areas in which new information would help collapse the vertical axis in Fig. 1, resulting in better-informed policy: estimation and monitoring of species distributions, biogeographic modeling, community interactions, long-distance dispersal (LDD), and genetic diversity. We lack basic information about the current distribution of most species even in some developed nations (NAS 1993). Previous efforts to inventory the distribution and abundance of species in the United States, such as the National Biological Survey (NAS) failed under political pressure. There are no current efforts to implement programs monitoring species response to global change at a national scale, although the proposed U.S. National Ecological Observation Network (NEON) might provide a framework to address this lack of basic distributional data (http://www.neoninc.org). In Europe successful long-term monitoring programs show that it is possible to monitor range shifts across a wide spectrum of species (Thomas et al 2004; Parmesan Berry et al. 2002; Matthews et al. 2004). Others are working to improve modeling methods and assess the accuracy of biogeographic range modeling (Gelfand et al. 2005; Guisan Elith et al. 2006; Wright et al. 2006). Narrowly distributed and infrequent species may be particularly difficult to model accurately (Stockwell Trakhtenbrot et al. 2005). Improved LDD estimates are essential for two reasons. First, they are needed to predict which species do and which do not require an assisted migration intervention. Second, information on potential or historical LDD may be desirable even if a species is unlikely to disperse into areas made available by climate change. For example, consider a species capable of dispersal but facing an impermeable matrix on its poleward range edge. An assisted migration program for this species might attempt to simulate (as closely as possible) its “natural” potential for dispersal if such dispersal could occur. Novel technology would help in the estimation of and that could be on many organisms or seeds would help the dispersal at by the number of long-distance for key species. could or such it is at emerging problems in global change evidence suggests that genetic variation is adaptive 2004). This an on programs of assisted migration to source populations For populations in many species may be for they the that successful population expansion the et al. 2004). Such populations might the most suitable to introduce of a species' current range Furthermore, individuals from the of the range would likely be the most common under natural spread. the one might to from populations the in these populations may be most threatened by climate change Hellmann et These are time and but they will be for an assisted-migration Under a scientific into these topics will our to identify appropriate for assisted migration and to implement in a way that ecological scientific will not these policy First, the of species biology the of assisted-migration plans for of the many species likely to be threatened by managers will be to range shifts based on broad of A has only limited success in likely invasive species 1999; & 2001). We expect that future will themselves in a position to invasive species for in and with in in processes such as LDD and species interactions means that more ecological information will not improve our to predictions (Clark et al. 2003). typically to provide information that the risks and benefits of assisted migration will require to uncertainty about (Clark et al. 2001). If this uncertainty is policies will have to adaptive of scientific the magnitude of climate-driven extinctions in policy and will the even more We advocate management strategies with the to to emerging from basic and but we cannot for To an extent this will have to be with we not the we The current that data for often provide for about assisted migration. research on assisted migration will be needed before can to informed policies of assisted migration: there a that should the of assisted of species should be as for should populations be introduced to minimize ecological such as these should be and by a broad of and policy A consensus that the risks and of alternative approaches to assisted migration and suggests strategies would be a significant toward a policy on this The alternative of to what is an of our and We from the U.S. of for Research to