02 December 2008

A failed case of assisted colonization

Assisted colonization (or assisted migration) has been proposed as a potential solution for the conservation of the species threatened by global warming. The basic idea is simple: individuals of a threatened species will be collected and moved to a suitable habitat further north where they are less likely to be affected by climate change.

I have been developing the ideas expressed in my previous post into a more coherent manuscript. By coincidence, the reader Andrew Broome just recently sent a link to a relevant paper. Interestingly, the subject paper1, by Stringer & Parrish (2008), discusses a failed attempt to establish a colony of the land snail Placostylus hongii, a native of New Zealand, on an island. In this case, the snail species was not threatened by climate change, but by habitat loss and nonindigenous predators (rats, etc.). Nevertheless, the results, because they exemplify the uncertainties of colonization, assisted or natural, are quite relevant for assisted colonization research.

Eleven individuals (4 adults and 7 juveniles) of P. hongii were introduced to a limestone island, called, well, Limestone Island, near the northern tip of North Island of New Zealand in August 2002. They were monitored at uneven internals and initially appeared to have been doing well and growing. But by March 2003, all were dead. The deaths are unlikely to have been from old age, because the species’ adult lifespan is estimated to be at least 30 years. (That’s a long life for a snail.)

The authors cannot pinpoint a cause for the snails' failure to survive and establish a long-term colony on the island. Although they partly blame the drought and high temperatures experienced in the summer of 2003/2004.

This example illustrates the difficulty of successfully predicting the outcomes of assisted colonization events even in cases when the biology of the subject species had been well studied. We know very little about the biology and ecology of most other terrestrial gastropod species to be confident that transplanted species will survive without creating unintended and detrimental traumas in the receiver communities–even if assisted colonization operations were logistically and economically feasible.

1Ian A. N. Stringer and G. Richard Parrish. 2008. Transfer of captive-bred Placostylus hongii snails to Limestone Island. DOC Research & Development Series 302. 18 p. pdf

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