Sheep grazing indirectly impacts endangered Australian pygmy bluetongue lizards (Tiliqua adelaidensis) by negative effects on the spiders that dig their burrows

Background/Question/Methods

The pygmy bluetongue lizard, Tiliqua adelaidensis, is an endangered skink, endemic to native grasslands of South Australia, which are now predominantly grazed by sheep. It obligatorily occupies spider burrows, using them as refuges, ambush points, and basking sites. Previous research reported that reduced grass cover from grazing benefits lizards, but sheep may also trample and destroy the burrows that are an essential lizard resource. Management of the few remaining populations requires an understanding of the overall impact of the grazing regime. We assessed how sheep grazing affected spiders and their burrows, as well as the abundance of pygmy bluetongue lizards.  Each month, over one austral spring and summer period (September 2012 - March 2013), we monitored 12 30m X 30m plots within a 70 ha site, near Burra, South Australia.

Within each plot, we counted number of burrows, number of burrow occupants (lizards and spiders) and number of new or lost burrows. Six study plots were exposed to sheep grazing (4 sheep/ hectare) in the second half of the study, while the other six plots remained ungrazed. 

Results/Conclusions

Spider burrow dynamics were influenced both by seasonal changes and by sheep grazing activity. In ungrazed plots, the mean number of spider burrows per plot remained stable until January, then increased over the next two months, coinciding with a significant increase in newly constructed spider burrows. In contrast, the mean number of lycosid spiders in burrows declined from spring to summer, while the mean number of pygmy bluetongue lizards per plot did not change.

 In the grazed plots, after grazing commenced there was a decrease in the mean number of spider burrows per plot and a smaller increase in the mean number of unoccupied burrows per plot than in the ungrazed controls. There was also a decline in the number of newly constructed burrows, and an increase in the number of recorded burrows that were lost between surveys in grazed plots.

We conclude that sheep grazing may have a negative impact on the number of spider burrows, suitable for pygmy bluetongue lizard occupancy. Appropriate management will require a grazing regime that balances between maintaining grass cover reduction while minimizing trampling effects that destroy burrows.