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...

Monday 30 January 2012

Sell-by date: are old science books still worth reading?

As an outsider to the world of science I've recently been struck by an apparent dichotomy that I don't think I've ever heard discussed, namely that if science is believed by non-practitioners to work on the basis of new theories replacing earlier ones, then are out-of-date popular science (as opposed to text) books a disservice, if not positive danger, to the field?

I recently read three science books written for a popular audience in succession, the contrast between them serving as the inspiration for this post. The most recently published was Susan Conner and Linda Kitchen's Science's Most Wanted: the top 10 book of outrageous innovators, deadly disasters, and shocking discoveries (2002). Yes, it sounds pretty tacky, but I hereby protest that I wanted to read it as much to find out about the authors and their intended audience as the subject material itself. Although only a decade old the book is already out of date, in a similar way that a list of top ten grossing films would be. In this case the book lists different aspects of the scientific method and those involved, looking at issues ranging from collaborative couples (e.g. the Curies) to prominent examples of scientific fraud such as the Chinese fake feathered dinosaur fossil Archaeoraptor.

To some extent the book is a very poor example of the popular science genre, since I found quite a few incorrect but easily verifiable facts. Even so, it proved to be an excellent illustration of how transmission of knowledge can suffer in a rapidly-changing, pop-cultural society. Whilst the obsession with novelty and the associated transience of ideas may appear to somewhat fit in with the principle that a more recent scientific theory always replaces an earlier one, this is too restrictive a definition of science. The discipline doesn't hold with novelty for the sake of it, nor does an old theory that is largely superseded by a later one prove worthless. A good example of the latter is the interrelationship between Newton's classical Law of Gravitation (first published in 1687) and Einstein's General Relativity (1916), with the former still used most of the time (calculating space probe trajectories, etc, etc).

The second of the three books discusses several different variants of scientific practice, although far different from New Zealand particle physicist Ernest Rutherford's crude summary that "physics is the only real science. The rest are just stamp collecting." Stephen Jay Gould's first collection of essays, Ever Since Darwin (1977), contains his usual potpourri of scientific theories, observations and historical research. These range from simple corrections of 'facts' – e.g. Darwin was not the original naturalist on HMS Beagle – to why scientific heresy can serve important purposes (consider the much-snubbed Alfred Wegener, who promoted a precursor to plate tectonics long before the evidence was in) through to a warning of how literary flair can promote poor or even pseudo-science to an unwary public (in this instance, Immanuel Velikovsky's now largely forgotten attempts to link Biblical events to interplanetary catastrophes).

Interestingly enough, the latter element surfaced later in Gould's own career, when his 1989 exposition of the Early Cambrian Burgess Shale fossils, Wonderful Life, was attacked by Richard Dawkins with the exclamation that he wished Gould could think as clearly as he could write! In this particular instance, the attack was part of a wider critique of Gould's theories of evolutionary mechanisms rather than material being superseded by new factual evidence. However, if I'm a typical member of the lay readership, the account of the weird and wonderful creatures largely outweighs the professional arguments. Wonderful Life is still a great read as descriptive natural history and I suppose serves as a reminder that however authoritative the writer, don't take accept everything on face value. But then that's a good lesson in all subjects!

But back to Ever Since Darwin. I was surprised by just how much of the factual material had dated in fields as disparate as palaeontology and planetary exploration over the past thirty-five years. As an example, Essay 24 promotes the idea that the geophysical composition of a planetary body is solely reliant on the body's size, a hypothesis since firmly negated by space probe data. In contrast, it is the historical material that still shines as relevant and in the generic sense 'true'. I've mentioned before (link) that Bill Bryson's bestseller A Short History of Nearly Everything promotes the idea that science is a corpus of up-to-date knowledge, not a theoretical framework and methodology of experimental procedures. But by so short-changing science, Bryson's attitude could promote the idea that all old material is essentially worthless. Again, the love of novelty, now so ingrained in Western societies, can cause public confusion in the multi-layered discipline known as science.

Of course, this doesn't mean that something once considered a classic still has great worth, any more than every single building over half a century old is worthy of a preservation order. But just possibly (depending on your level of post-modernism and/or pessimism) any science book that stands the test of time does so because it contains self-evident truths. The final book of the three is a perfect example of this: Charles Darwin's On the Origin of Species, in this case the first edition of 1859. The book shows that Darwin's genius lay in tying together apparently disparate precursors to formulate his theory; in other words, natural selection was already on the thought horizon (as proven by Alfred Russel Wallace's 1858 manuscript). In addition, the distance between publication and today gives us an interesting insight into the scientist as human being, with all the cultural and linguistic baggage we rarely notice in our contemporaries. In some ways Darwin was very much a man of his time, attempting to soften the non-moralistic side to his theory by subtly suggesting that new can equal better, i.e. a form of progressive evolution. For example, he describes extinct South American mega fauna as 'anomalous monsters' yet our overtly familiar modern horse only survived via Eurasian migration, dying out completely in its native Americas. We can readily assume that had the likes of Toxodon survived but not Equus, the horse would seem equally 'anomalous' today.

Next, Darwin had limited fossil evidence to support him, whilst Nineteenth Century physics negated natural selection by not allowing enough time for the theory to have effect. Of course, if the reader knows what has been discovered in the same field since, they can begin to get an idea of the author's thought processes and indeed world view, and just how comparatively little data he had to work with. For example, Darwin states about variations in the sterility of hybrids whilst we understand, for example that most mules are sterile because of chromosomal issues. Yet this didn’t prevent the majority of mid-Victorian biologists from accepting natural selection, an indication that science can be responsive to ideas with only circumstantial evidence; this is a very long way indeed from the notion of an assemblage of clear-cut facts laid out in logical succession.

I think it was the physicist and writer Alan Lightman who said: "Science is an ideal but the application of science is subject to the psychological complexities of the humans who practice it." Old science books may frequently be dated from a professional viewpoint but can still prove useful to the layman for at least the following reasons: understanding the personalities, mind-sets and modes of thought of earlier generations; observing how theories within a discipline have evolved as both external evidence and fashionable ideas change; and the realisation that science as a method of understanding the universe is utterly different from all other aspects of humanity. Of course, this is always supposing that the purple prose doesn’t obscure a multitude of scientific sins...