Insuring against extinction and increasing local adaptation
28th-31st March 2010
Convened by: Ary Hoffman, Carla Sgro
Most conservation efforts focus on threatened species where genetic diversity has been lost, usually as a consequence of small population size resulting from habitat loss and fragmentation. Such populations face high risk of extinction, and the option of assisted migration to alleviate these threats has been the focus of increasing debate. One type of assisted migration that is being increasingly discussed in the literature is genetic (rather than species) translocation. This process focuses on moving individuals or gametes from one population to another (genetic translocation or assisted migration) within a species’ current/historical range as a way of enabling gene flow and conserving/enhancing adaptive potential.
There are two circumstances under which genetic translocation might be considered. The first concerns populations of threatened species that have suffered severe reductions in genetic diversity and where dispersal processes have been disrupted by habitat fragmentation. In this case, the risk of outbreeding depression (and introducing conditional deleterious alleles) should be carefully weighed against the risk that on-going loss of genetic diversity poses to the long term persistence of populations. The second set of circumstances involves cases where there is strong local adaptation (ecotype differentiation). Moving individuals from warm-adapted populations to colder locations may increase the probability of adaptation, and thus persistence of cold-adapted populations under a warming environment. This issue of genetic translocation to maximize population adaptability is pertinent to on-going landscape restoration.
In this workshop we explored in depth the potential of genetic translocation as a conservation tool in a future of increasing uncertainty and environmental change. The workshop participants explored situations where the benefits of genetic translocation might be realised, as well as situations where there might be risks associated with genetic translocation. The overall aim of genetic translocations is to establish viable populations that are capable of persisting in the face of environmental change. To achieve this, several problems have to be overcome, including the likelihood of outbreeding depression and heterosis under environmental change. The former was regarded as having been overemphasized in the conservation literature as managers focused too much on preserving genetic “purity”, whereas the benefits of the latter were often considered as being underappreciated. Outbreeding depression was considered to be problematic only when there had been substantial adaptation to different environmental conditions and/or a long history of isolation between populations/taxa. However it was also noted that the fitness benefits and costs of population crosses were rarely measured, particularly across multiple environments.
For threatened and non-threatened species, workshop participants suggested that levels of genetic diversity within populations needed to be considered, and also the likely benefits of introducing genes into target populations through decreasing levels of inbreeding depression but also aiding in future adaptation. These issues needed to be considered within a risk assessment framework on a case-by-case basis. It was likely that more funds would be available for assessing potential outbreeding issues in threatened organisms. However, threatened organisms were typically in crisis so there was a greater risk of inaction. For these organisms, there were also likely to be fewer potential source populations with fewer genetic options available when planning genetic translocations.
For non-threatened species, there was more time available to assess populations and genetic traits, and locally adapted genes were more likely to occur across multiple populations. Potential source populations could be more easily selected for desired levels of genetic diversity and genetic differentiation. When predicting the likely effects of genetic translocations, it should be possible to introduce small numbers of individuals and monitor them over generations to test for fitness changes over time. The issue of local adaptation was discussed at some length, along with potential problems in its measurement, requiring multiple environments and a consideration of abiotic as well as biotic factors and interactions between them. In the absence of genetic data, it may be possible to use environmental matching and data from related species as much as possible when evaluating adaptation. It was felt that new populations might often be successfully created by mixing genotypes from several populations – which could then evolve further by selection and provide a level of pre-adaptation, although this issue needed to be assessed through modelling. The occurrence of local adaptation needed to be explored further to see if generalizations across different groups of organisms might be possible.
Participants discussed components needed for a risk assessment around genetic introductions. These might include levels of suspected inbreeding depression and heterosis, population size, and rates of gene flow. An initial decision tree was developed. The risk of inbreeding depression could be estimated indirectly such as from chromosomal differences between populations. It was felt that translocation benefits needed to emphasize population size, because of theory and data indicating that populations need to be maintained at a size around a thousand or more to ensure evolutionary potential. Overall the workshop participants felt that there were enormous benefits likely to be gained from genetic translocations not only for threatened organisms but also for dominant species whose persistence might be threatened by climate change.
Participants: Ary Hoffman, Carla Sgro, Stephen Williams, Mark Eldridge, Andrey Young, Richard Frankham, Dave Coates, Liz James, Paul Sunnucks, Margaret Byrne, Nicki Mitchell, Ian Mansergh, Colin Yates, Andrew Lowe, Adam Miller, Elise Furlan.
