We think of spiders as fearsome hunters, spinners of webs and treacherous mates, but construction workers? Yes, that too. Some groups of spiders - trapdoor and wolf spiders - dig tunnels that they use to ambush passing insects. But these tunnels can also provide shelter and accommodation for other animals, including one of the rarest of Australia's lizards - the pygmy blue-tongue lizard. It seems that the lizard's survival depends entirely on the spiders.
The pygmy blue-tongue is a native of South Australia. It's so rare that zoologists thought it extinct for over 30 years and it re-emerged in the public eye in the most unlikely way. In 1992, a dead specimen of this supposedly extinct animal was found in the stomach of a brown snake, found dead on the side of a road. That unexpected discovery prompted intensive surveys of the surrounding area, which found several lizards living in spider burrows.
Like the spiders, the lizards use the burrows for ambush but they also act as nurseries, cooling stations and defensive forts. To understand the relationship between lizards and spiders, Michael Bullfrom Adelaide's Flinders University studied the fates of both species in a single hectare of South Australian land. Over two years, his team spent bursts of two weeks, intensively searching the plot of land for signs of burrows. Each one was probed with a fibre optic camera to see who lay inside.
Earlier studies have found that lizards readily accept artificial burrows and adding these to the local area will halt the decline of the lizard populations. That's all very good, but as Bull writes, "a sustainable supply of natural burrows would be a better option in the longer term".
The team found clear evidence that the spiders were the engineers behind the burrows. Most of the newly constructed burrows had been made in the winter months when the lizards tend to be inactive - this season, with its heavy rainfall and damp soil, provides the best conditions for easy excavation. When spring arrived, the new tunnels were always occupied by spiders but later on in the year, the bluetongues started turning up in burrows that had previously had spidery tenants.
Competition for real estate is fierce but not between species. While spiders and lizards never co-habited, Bull found no evidence that lizards oust spiders from their homes, rather than occupying empty burrows. The two animals also have different needs - spiders are all-too-happy to live in shallower tunnels but lizards seem to have a preference for deeper ones that provide them with more shelter from scorching summer temperatures. These are typically in short demand. If the lizard population is too dense, individuals are forced to adopt less-than-ideal accommodation.
Bull also found that the burrows are gradually lost throughout the year, presumably through natural cave-ins or the trampling hooves of sheep. The deeper burrows were least likely to be lost, either because they are inherently more stable, or because their occupants spend a lot of time in maintaining these more valuable abodes.
Bull's team found a more worrying trend - the numbers of both lizards and spiders fell in the survey's single year. That's unlikely to be due to migration, for the lizards' genetics indicate that they don't move from site to site very much. Their decline is probably related to the fall in spider numbers and the reasons for that are a mystery. Bull suggests that it might be a natural part of their 12-18-month life cycle or due to a rise in the populations of parasitic wasps in spring.
Whatever the reason, it's clear that the survival of the bluetongue lizard depends very much on the burrowing activities of their unwitting eight-legged engineers. Conservationists eager to protect this rediscovered species have two challenges on their hands - save the lizards and build up populations of the spiders they rely on.
It's a pattern that's emerging all over the world. Just last month, I wrote about the success of efforts to save the Large Blue butterfly, which depended entirely on identifying their dependence on a local species of ant that they parasitised. The overarching message is that animals don't exist in isolation. Just as we depend on service providers to man our hospitals, deliver our post and collect our garbage, so too do wild species depend on service providers to build their homes, disperse their seeds or even raise their young.
A common conservation technique is translocation - moving individuals of an endangered species to fresh territories in order to establish new populations. But this solution would only work if the right complement of conditions already exists in the new surroundings. In the case of the bluetongue lizards, moving them to an area devoid of burrowing spiders would be utterly futile.
Reference: Fellows, H., Fenner, A., & Bull, C. (2009). Spiders provide important resources for an endangered lizard Journal of Zoology DOI: 10.1111/j.1469-7998.2009.00600.x
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