Saturday, 10 May 2014

Darwin's dilemma

What would Darwin do? 
Introduced parasites are a threat to biodiversity when hosts lack effective defences against such parasites. Several parasites have recently colonized the Galapagos Islands, threatening native bird populations, such as the parasitic nest fly Philornis downsi which has been implicated in the decline of endangered species of Darwin’s finches, in particular the mangrove finch (Camarhynchus heliobates).

The native range of this parasite is known to be the Island of Trinidad and also Brazil, but it appears to have been introduced to the Galapagos Islands (probably firstly to Santa Cruz) with imported fruit, as the adults feed on fruit. However, the fly lays its eggs in the nests of Darwin’s finches and other land birds in the Galapagos.

Once the eggs hatch, the fly larvae feed on the blood of nestlings and were first discovered in finch nests on the island of Santa Cruz in 1997, although retrospective examination of insect collections show that the fly was actually present in the Galapagos Islands as early as 1964. Since then the parasite has spread to 12 of the 13 main Galapagos Islands and its larvae have been found in some years, in all of Darwin’s finch nests. The blood sucking larvae can cause chick mortality of up to 100% in some nests in some years. Because of this high impact, it has been given the highest risk ranking amongst introduced insects/parasites.

However scientists have been busy thinking of ways to control the pest, before Darwin’s finches run out of time and evolve no longer, becoming extinct – and all down to us, humans.

Darwin’s finches can be encouraged to ‘self-fumigate’ nests with cotton fibres that have been treated with a permethrin-based insecticide, which the birds collect to line their nests with. Nests with permethrin-treated cotton had significantly fewer parasites than control nests, and nests containing at least one gram of cotton were virtually parasite-free. Nests directly fumigated with permethrin had fewer parasites and fledged more offspring than nests treated with water.

Overall, 50 out of 60 nestlings (83%) fledged from experimental nests, compared to just 29 of 54 nestlings (54%) from control nests. The study shows that Darwin’s finches can control this parasite with the help of permethrin-treated cotton, and that fumigation increases fledging success. There are currently no other effective methods for controlling this parasite.

Self-fumigation may thus be a viable approach for combating this parasite in the nests of Darwin’s finches, in particular the Mangrove finch which is the most critically endangered species of Darwin’s finch, with a population of less than 100 individuals restricted to a home range of less than 1 km square, on Isabela Island. Around 60 cotton dispensers could treat this entire population and self-fumigation may be a particularly efficient approach because mangrove finches often build their nests high in mangrove trees, where they are relatively inaccessible.

Once again we are faced with a dilemma. I wonder what Darwin himself would make of this intervention, even though I'm sure he would have been delighted at the cleverness of the idea.  Would he observe and make notes of the eventual demise of “his” finches that so helped him to prove the process of evolution and make his name, or would he step in, regarding this as a totally “unnatural” human induced problem and therefore not “real” evolution at all.

I would be most interested in your thoughts having read this piece?  

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