Monday, 27 February 2012

Predators vs poisons: the ups and downs of biological control

Ever since Darwin, islands and island groups have been known as prominent natural laboratories of evolution. Their isolation leads to radiation of species from a single common ancestor, the finches and giant tortoises of the Galapagos Islands providing a classic example. But a small population restricted in range also means that many island species are extremely susceptible to external factors, rapid extinction being the ultimate result - as can be seen from the dodo onwards. Living as I do on an island (New Zealand counts within the terms of this discussion, as I will explain) has led me to explore what a foreign invasion can do to a local population.

Either through direct hunting or the actions of imported Polynesian dogs and rats, almost half the native vertebrate fauna was wiped out within a few centuries of humans arriving in New Zealand; so much for the myth of pre-technological tribes living in ecological harmony! But the deliberate introduction of a new species to pray on another is now a much-practised and scientifically-supported technique. One of the late Stephen Jay Gould's most moving essays concerned the plight of the Partula genus of snails on the Society Islands of Polynesia. The story starts with the introduction of edible Achatina snails to the islands as food, only for some to escape and become an agricultural pest. In 1977 the Euglandina cannibal wolfsnail was brought in as a method of biological control, the idea being that they would eat the crop munchers. Unfortunately, the latest wave of immigrant gastropods ignored the Achatina and went after the local species instead. The results were devastating: in little more than a decade, many species of Partula had become extinct in their native habitat.

(As an interesting aside, the hero of Gould's Partula vs. Euglandina story is gastropod biologist Henry Crampton, whose half century of research into the genus is presumably no longer relevant in light of the decimation of many species. Yet Crampton, born in 1875, worked in typical Victorian quantitative fashion and during a single field trip managed to collect 116,000 specimens from just a single island, Moorea. I have no idea how many individual snails existed at the time, but to me this enormous number removed from breeding population in the name of scientific research was unlikely to do anything for the genus. I wonder whether comparable numbers of organisms are still being collected by researchers today: somehow I doubt it!)

The Society Islands is not the only place where the deliberate introduction of Euglandina has led to the unintended devastation of indigenous snail species: Hawaii and its native Achatinella and Bermuda's Poecilozonites have suffered a similar fate to Partula. Gould used the example of the Partula as a passionate plea (invoking 'genocide' and 'wholesale slaughter') to prevent further inept biological control programmes, but do these examples justify banning the method in totality?

The impetus for this post came from a recent visit to my local wetlands reserve, when my daughters played junior field biologists and netted small fish in order to examine them in a portable environment container (alright, a jam jar) - before of course returning them to the stream alive. The main fish species they caught was Gambusia, which originates from the Gulf of Mexico but was introduced to New Zealand in the 1930s as a predator of mosquito larvae. However, akin to Euglandina it has had a severe impact on many other fish species and is now rightly considered a pest. In fact, it's even illegal to keep them in a home aquarium, presumably just in case you accidentally aid their dispersion. Australia has also tried introducing Gambusia to control the mosquito population, but there is little data to show it works there either. The latter nation also provides a good illustration of environmental degradation via second- and third-hand problems originating from deliberate introduction. For example, the cane toad was imported to control several previously introduced beetle species but instead rapidly decimated native fauna, including amphibians and reptiles further up the food chain, via toad-vectored diseases.

Gambusia: the aggressive mosquito fish
Gambusia affinis: a big problem in a small fish

This isn't to say that there haven't been major successes with the technique. An early example concerns a small insect called the cottony cushion scale, which began to have a major impact on citrus farming in late Nineteenth Century California. It was brought under control by the introduction of several Australian fly and beetle species and without any obvious collateral damage, as the military might phrase it. But considering the extinction history of New Zealand since humans arrived, I've been amazed to discover just how many organisms have been deliberately introduced as part of biological control schemes, many in the past quarter century. For instance, twenty-one insect and mite species have been brought over to stem the unrestrained growth of weeds such as ragwort and gorse, although the rates of success have been extremely mixed (Old man's beard proving a complete failure, for example). As for controlling unwelcome fauna in New Zealand, a recent promising research programme involves the modification of parasites that could inhibit possum fertility. This is something of a necessity considering possums (first imported from Australia in the 1830s and now numbering around sixty million) are prominent bovine tuberculosis vectors.

Stephen Jay Gould was a well-known promoter of the importance of contingency within evolution, and how a re-run of any specific branch of life would only lead to a different outcome. So the question has to be asked, how do biologists test the effect of outsider species on an ecosystem (i.e. within laboratory conditions) when only time will show whether the outcome is as intended? No amount of research will show whether an unknown factor might, at an unspecified time during or after the eradication programme, have a negative impact. It could have been argued in the past that the relative cheapness of biological control compared to alternatives such as poison or chemicals made it the preferable option. However, I imagine the initial costs, involving lengthy testing cycles, mean that it is no longer a cut price alternative.

Considering the recent developments in genetic modification (GM), I wonder whether researchers have been looking into ways of minimising unforeseen dangers? For example, what about the possibility of tailoring the lifespan of the control organism? In other words, once the original invasive species has been eliminated, the predator would also rapidly die out (perhaps by something as simple as being unable to switch to an alternative food source, of which there are already many examples in nature). Or does that sound too much like the replicant-designing Dr Eldon Tyrell in Blade Runner?

One promising recent use of GM organisms as a biological control method has been part of the fight to eradicate disease-carrying (female) mosquitos. Any female offspring of the genetically altered male mosquitos are incapable of flight and thus are unable to infect humans or indeed reproduce. However, following extremely positive cage-based testing in Mexico, researchers appear to have got carried away with their achievements and before you could say 'peer review' they conducted assessments directly in the wild in Malaysia, where I assume there is little GM regulation or public consultation. Therefore test results from one location were extrapolated to another with a very different biota, without regard for knock-on effects such as what unwelcome species might come out of the woodwork to fill the gap in the ecosystem. When stakes are so high, the sheer audacity of the scientists involved appears breathtaking. Like Dr Tyrell, we play god at our peril; let us hope we don't come to an equally sticky end at the hands of our creation...