Radio-Tagged Tarantulas Help Take on Deforestation
John Pickrell in England
for National Geographic News
|June 3, 2003|
Eight-legged, wriggly, and potentially menacingtarantulas don't figure in many people's top ten favorite furry animals, though Memphis Zoo, Tennessee biologist and curator Steve Reichling might disagree. In fact, his longtime passion for the largest of spiders has led to their enlistment in a unique experiment.
Reichling has embarked on a novel, decades-long project: to map and quantify Belizean forest degradation with the assistance of 50 or more tarantulas surgically implanted with radio transponders.
Like much of Central and South America, Belize's rainforests have been altered by successive human cultures beginning with the Maya. For most of human history this has involved sustainable methods of land use such as small-scale slash-and-burn farming. However, modern large-scale agriculture is destroying rainforest at an unprecedented rate.
Reichling and his veterinarian colleague Chris Tabaka are collecting information about the success of tarantula populations in rainforest sites, and using it as a measure of the degree of damage to these habitats. The project, which began in March 2000, is expected to continue for at least 20 years, to follow what happens as the current tarantula residents die out and are replaced.
Tarantulas are particularly suitable bio-indicators to map environmental change, because they can live for 20 years and remain in one burrow for life, making them easy to relocate year after year, says Reichling.
So-called bio-indicator species are those that scientists monitor to help understand how environmental change is affecting their habitat as a whole. Bio-indicators have included mammals, plants, and other species. For example, lichens, some of which need pristine air to flourish, have been used to indicate levels of atmospheric pollution in cities.
Though there are many suitable long-lived plant and vertebrate species to monitor tropical ecosystems, most invertebrates have relatively short lifespans, said Stuart Longhorn, who works on tarantula conservation genetics at The Natural History Museum in London. "Tarantula spiders are one of a few exceptions, and can provide a totally different viewpoint [from] studies of vertebrates and plants alone," said Longhorn.
In order to quantify the impact of agriculture on natural habitat over the next 20 years or more, Reichling is following the fortunes of tarantula communities at three different sites in and around Belize's Lamanai Archeological Reserve. Each site consists of mature old-growth forest, long-barren open pastures, traditionally managed forest, and sites newly cleared for farming.
The researchers have already radio-tagged 50 individuals of two common Belizean species of tarantula: the cinnamon tarantula, Crassicrus lamanai, which usually inhabits sun-baked clearings, and the redrump tarantula, Brachypelma vagans, which much prefers dense forest. Changing proportions of the two species at forested sites should offer a quantitative measure of when deforestation has progressed to such a degree that the plot is no longer suitable for its original assemblage of species.
"We are especially interested in documenting an expected shift on the recently cleared areas [from a redrump-dominated site] to a cinnamon-tarantula-biased site," said Reichling. As old redrumps die out, if the habitat is no longer suitable, they won't be replaced, he said.
To monitor species numbers and survivorship year after year, the pair had to come up with a reliable way of marking tarantulas. That's a significant problem in a species that molts on an annual basis, shedding any markers. Some kind of internal tag was necessary, but such a procedure had never been attempted. The solution was to surgically implant rice-grain-size radio transponders that passively transmit a serial number when scanned with a portable device.
"I'd been bumbling around with various schemes of leg painting and this just seemed like such an obvious use for [radio] transponders," said Reichling. "I got the idea from my zoo job, tagging the reptiles and thinking that the spiders I work with are larger than some of the animals we were marking."
Tabaka has perfected a technique to implant the tags into tarantula abdomens. The procedure can have complications, however. Spider blood doesn't clot for a start, so botched surgery can be fatal. "Imagine trying to implant a marble in a balloon filled with water, without deflating it!" said Reichling
In addition, Belizean tarantulas are often covered with stinging spines, which can be breathed in, or damage human skin and eyes, so protective clothing must be worn. Despite the complications, the surgeries typically go smoothly and most tarantulas emerge unscathed.
With the aid of a portable scanner the scientists can scan the burrows each year to see if tarantulas are still in residence. If they find burrows that don't turn up a signal at any of the sites, Reichling says they'll catch and tag new residents.
Studying the whole range of spiders in a habitat has previously been used to measure pristine or altered environments, said Paul Hillyard, arachnologist at The Natural History Museum in London, "but surgically tagging invertebrates is certainly novel." However, the surgical procedure is very invasive, and potentially dangerous to the tarantula, he cautioned.
Though the study has yielded few results as yet, some of the habitat has already been totally destroyed in two of the three study sites, "to make way for a soccer field and a Maya ruin visitor center," said Reichling. "I almost lost heart but decided to go ahead anyway, and document this tremendous disturbance."
The project is detailed in a recent edition of the journal Biologist.
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