The impact of sheep grazing on burrows for pygmy bluetongue lizards and on burrow digging spiders
Bull, Christopher Michael
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Grazing by domestic stock has altered and degraded natural grassland ecosystems worldwide, directly and indirectly impacting the endemic plant and animal species occupying those grasslands. The pygmy bluetongue lizard (Tiliqua adelaidensis) is an Endangered species, restricted now to fragments of native grassland habitat in South Australia, which are predominantly grazed by sheep. These lizards exclusively occupy burrows dug by spiders, and use them as refuges, basking sites and ambush points. They do not dig their own burrows and rely on co-existing spiders for this essential resource. We asked how sheep grazing influences construction and persistence of spider burrows, by comparing burrow dynamics in adjacent grazed and ungrazed grassland habitat. In ungrazed plots spider burrows increased over one spring and summer period, particularly after a summer rain event that softened the soil. In grazed plots more existing burrows were destroyed, presumably by sheep trampling, and fewer new burrows were constructed, leading to a net loss in burrow numbers over the same period. However, in this short study, grazing did not affect the number of pygmy bluetongue lizards or the number of lycosid spiders. Burrows that were lost tended to be shallower and to have smaller diameter entrances than those that were retained, suggesting that the best burrows for lizard refuges were more likely to persist despite sheep activity. However, heavy grazing may have negative impacts on both lizards and spiders, resulting from a reduction in available burrows and in spider digging behaviour.