Hoegh-Guldberg, O., Hughes, L., McIntyre, S., Lindenmayer, D.B., Parmesan, C., Possingham, H.P., Thomas, C.D. (2008). ECOLOGY: Assisted Colonization and Rapid Climate Change. Science, 321(5887), 345-346. DOI: 10.1126/science.1157897
In last Friday's Science, Hoegh-Guldberg et al., discuss various conservation measures to save those species that may otherwise go extinct as a result of global warming if they can’t migrate on their own or evolve quickly enough to adapt to altered climate patterns.
Of course, the best solution would be to save a species in its own habitat. But sadly, that's not always possible. The paper proposes 2 last ditch resorts if everything else fails or is impractical. The first of these is the storage of frozen gametes with the hope that the species may be re-created, so to speak, in the future if and when the environmental conditions improve. There is no guarantee, however, that the future world will be a more hospitable place for any species that may be on the verge of extinction today. Even assuming that it will be, one would still need to save enough genetic variety to assure the establishment of viable populations.
The second proposal is the main theme of their discussion: assisted colonization or migration of threatened species.
My main interest is with the conservation of threatened terrestrial invertebrates in general and gastropods in specific. In Tentacle No. 16 (p. 17), I discussed the possible vernalization (cold-induced triggering of reproduction) requirements of terrestrial snails and raised the possibility that if global warming trends continue certain snail species may face extinction unless something was done to save them. Starting with the idea of assisted colonization, I can now envisage 3 potential scenarios.
This is an original figure, not from Hoegh-Guldberg et al. Although my ideas are not specific to any particular part of the world, for the sake of argument, I am using a map of North America.
1. Species A has a continuous range extending from southern U.S. to northern U.S. The southern populations (As) are likely to be genetically differentiated and better adapted to warmer climates than are the northern populations (An). Hoegh-Guldberg et al., propose that: "Moving individuals from 'warm-adapted' populations to historically colder locations may increase the probability of subsequent adaptation as the climate changes."
2. Species B exists in 2 disjunct populations, one in the south (Bs) and the other in the north (Bn). The populations in between may have disappeared millions of years ago as a result of changes in habitat or climate arising from natural events or just recently as a result habitat destruction by humans. This situation may be seen as a special case of scenario #1. Hoegh-Guldberg et al., propose that: “Dispersal processes that have been disrupted by loss of habitat connectivity could be restored by colonization.”
3. Species C has always been endemic to the south. Hoegh-Guldberg et al., propose that: “...translocation of species to locations outside their historic range where conditions will be suitable in the medium- to long-term may be the only strategy to prevent [their] extinction.”
This is the most troublesome suggestion. Not only would it be difficult to find suitable habitats for a species hundreds of kilometers away from its native range, but the introduction of a species to a community where it has never existed before could spell trouble for the already-existing native members.
Yes, of course, it has been done many times before inadvertently and in some cases, without apparent detrimental results. For example, several introduced European slugs coexist, seemingly peacefully, with the native snails and slugs in eastern North American forests. Ironically, however, it has usually been the intentional introductions of alien species that ended in disaster. A case in point is the near-destruction of the Partula snails of the Society Islands by the intentionally introduced carnivorous snail Euglandina rosea (more info).
If E. rosea, an endemic of southern U.S., one day faced extinction due to rapid climatic change, it would deserve to be saved as much as any other species. But would we really want to translocate E. rosea to northeast forests where the native species would be unprepared to face such a voracious predator and where E. rosea's only competitor would be the much smaller Haplotrema concavum?