ReviewAssisted colonization: Integrating conservation strategies in the face of climate change
Introduction
The impacts of climate change on species and ecological systems have already become evident (Parmesan, 2006, Williams et al., 2007). Plant and animal range shifts are consistent with climate change predictions, and phenological events are changing in response to seasonal warming patterns (Parmesan and Yohe, 2003, Root et al., 2003). Similar range shifts have occurred in response to historical changes in climate over geological time scales (Petit et al., 2008), and previous climate change rates during glacial periods have occasionally matched or exceeded the current fluctuations; however, the current and predicted rates of human-induced climate change greatly exceed the vast majority of fluctuations observed in the past 420,000 years (Hoegh-Guldberg et al., 2007, Petit et al., 1999), and they are occurring during an inter-glacial period expected to be relatively stable (NRC, 2002, NRC, 2006). These changes pose a challenge to species persistence, occurring on temporal scales that require species to rapidly shift their ranges and/or adapt (McLachlan et al., 2005, Root and Schneider, 2006) as existing environmental conditions are replaced by novel climate regimes (Williams et al., 2007). Additional stressors, such as destruction of habitat, invasive species, diseases, and alteration of disturbance regimes, also pose major threats to biodiversity and may be intensified by a changing climate (Frelich and Reich, 2009, Parmesan and Yohe, 2003).
Conservation strategies that incorporate uncertainty associated with climate change impacts will be necessary to minimize biodiversity loss (Coenen et al., 2008, Lawler et al., 2010, Root and Schneider, 2006). Key components of these “climate change-integrated conservation strategies” (Hannah et al., 2002) include predictive models of species responses to climate change (Hannah et al., 2002, Pearson and Dawson, 2003), an adaptive rather than static approach to management (Lawler et al., 2010), and conservation planning for alternative future climate scenarios (Brooke, 2008). There is also increasing recognition that a dynamic view of community composition may need to be adopted, since differential responses of species to climate change will likely result in novel or “no-analog communities” (Williams and Jackson, 2007).
Assisted colonization – also referred to as assisted migration, assisted translocation, and managed relocation (hereafter, assisted colonization) – is a conservation strategy that has been proposed to mitigate the effects of climate change on biodiversity (Hoegh-Guldberg et al., 2008a, McLachlan et al., 2007). Assisted colonization refers to the physical relocation of a species to a location outside its existing or historical range that is predicted to be favorable for persistence under future climate projections. While much uncertainty exists about assisted colonization, the strategy is thought to be most applicable for species characterized by small populations, restricted dispersal ability and adaptive potential (Ozinga et al., 2009, Petit et al., 2008, Primack and Mao, 1992), and inhabiting low-connectivity landscapes (Trakhtenbrot et al., 2005).
Due to its highly manipulative nature, assisted colonization has been the subject of recent discussion and criticism within the conservation community (Fazey and Fischer, 2009, Ricciardi and Simberloff, 2009, Sax et al., 2009, Schwartz et al., 2009, Seddon et al., 2009, Vitt et al., 2009). Central to this discussion is the risk that a species will become invasive, causing adverse ecological or economic impacts when introduced to a new range. While much research has attempted to identify factors pre-disposing a species to be invasive, traits of invasiveness can remain difficult to predict (Kolar and Lodge, 2001, Ricciardi and Simberloff, 2009). Social and political resistance associated with this ecological uncertainty, as well as economic constraints related to implementation, are expected to complicate decision-making about when and to what extent assisted colonization should be implemented (Hoegh-Guldberg et al., 2008a, Hunter, 2007, McLachlan et al., 2007). Richardson et al. (2009) demonstrate how biologists can begin to incorporate these complex social and economic factors into assisted colonization risk assessment and decision-making. Nonetheless, there remains much controversy and uncertainty about the widespread applicability of assisted colonization as a tool for conserving biodiversity in response to climate change.
Given the uncertainty and criticism surrounding assisted colonization, we sought to identify management practices that have the potential to contribute to an integrated conservation strategy of climate change mitigation and include assisted colonization as one option. In addition to physical movement of species, other traditional conservation actions, such as management for increased habitat connectivity, have been proposed as alternatives for indirectly facilitating natural range shifts (Hunter, 2007, Vitt et al., 2009). We expand upon this idea to examine in detail the conservation strategies that have the potential to mitigate climate change impacts. In the remainder of this paper, we (1) provide a detailed summary of current discussion and criticism of assisted colonization, focusing on factors that influence whether this method is broadly applicable to a wide range of species, (2) examine whether the existing conservation practices of management for habitat connectivity and conservation genetics have the potential to assist the colonization and adaptation of species in response to climate change, and (3) outline an integrated strategy of conservation that has potential to address a larger range of climate change impacts than is possible by focusing solely on assisted colonization.
The primary focus of this paper is on ecological concerns. Although decision-makers should take account of the social, political, and economic contexts of assisted colonization and climate change (Richardson et al., 2009), an in-depth examination of these factors is beyond the scope of this paper.
Section snippets
Discussion surrounding assisted colonization
The recent increased attention to assisted colonization within the conservation community results from the perception that moving populations of a species from one region and introducing them outside the species’ native or historical range is highly manipulative (Hoegh-Guldberg et al., 2008a, Hoegh-Guldberg et al., 2008b, McLachlan et al., 2007), and therefore undesirable compared to less intrusive approaches. Nonetheless, the desired outcomes of assisted colonization efforts are aligned with
Improving habitat connectivity as part of an integrated conservation strategy
Habitat loss and fragmentation perforates the landscape, leaving small and discontinuous patches of habitat, thus limiting species range shifts in response to climate change. Fragmentation results in four landscape-scale effects that function as mechanisms for these limitations: (1) reduction in the amount of habitat, (2) dis-aggregation of habitat into an increasing number of disparate fragments or patches, (3) decrease in the size of fragments, and (4) increased isolation of habitat remnants (
Conservation genetics as part of an integrated conservation strategy
In addition to geographic range shifts, genetic plasticity and in situ adaptation also play important roles in persistence of species facing climate change (Bradshaw and Holzapfel, 2006, Davis and Shaw, 2001, Davis et al., 2005, Parmesan, 2006). Modeling studies highlight the importance of population genetic variation and the rate of adaptation for determining times to extinction and range shifts in response to climate change (Burger and Lynch, 1995, Pease et al., 1989), while numerous field
Discussion
We propose an integrated strategy of conservation in the face of climate change that includes management for habitat connectivity, understanding and using conservation genetics to aid adaptation, and when necessary, assisted colonization of species that still cannot shift their ranges and appear likely to go extinct. Implementing this integrated approach will be more effective than focusing solely on assisted colonization for conserving biodiversity in the face of current and future climate
Conclusions
Given the unprecedented rate of climate change and the degree to which plant and animal range shift or adaptation is required for species to persist, conservation biologists and ecologists seeking to conserve biodiversity face an immense challenge. A key component of this challenge is the need for decision-making that minimizes risk under future climate scenarios. The success of assisted colonization is unclear, due to the difficulty of predicting responses of individuals in colonizing and
Acknowledgements
We thank T.R. Fiutak, L.E. Frelich, N.R. Jordan, and J.A. Perry, for providing guidance, insights, and discussions that aided in the development of this paper. We also thank R.G. Shaw, K.S.G. Sundar, B. Breen, M. Dixon, J. Stucker, M. Wilson, and 2 anonymous reviewers for valuable suggestions and edits that improved the manuscript. S.R.L. was supported during work on the manuscript by a National Science Foundation IGERT Grant: Risk Analysis for Introduced Species and Genotypes (NSF DGE-0653827).
References (117)
Plant invasions, interspecific hybridization, and the evolution of new plant taxa
Trends Ecol. Evol.
(1992)- et al.
Limits to evolution at range margins: when and why does adaptation fail?
Trends Ecol. Evol.
(2007) - et al.
Landscape structure as an indicator of biodiversity: matrix effects on species richness
Agr. Ecosyst. Environ.
(2003) How much habitat is enough?
Biol. Conserv.
(2001)- et al.
Assisted colonization is a techno-fix
Trends Ecol. Evol.
(2009) - et al.
Matrix habitat and species richness in tropical forest remnants
Biol. Conserv.
(1999) - et al.
Body size, niche breadth, and ecologically scaled responses to habitat fragmentation: mammalian predators in an agricultural landscape
Biol. Cons.
(2003) - et al.
Biodiversity management in the face of climate change: a review of 22 years of recommendations
Biol. Conserv.
(2009) - et al.
Communicating clearly about conservation corridors
Landscape Urban Plan.
(2001) - et al.
The influence of matrix type on flower visitors of Centaurea jacea L
Agr. Ecosyst. Environ.
(2003)