Impacts of sheep grazing on burrow use by spiders and pygmy bluetongue lizards (Tiliqua adelaidensis)

Author: Jess Clayton

Clayton, Jess, 2018 Impacts of sheep grazing on burrow use by spiders and pygmy bluetongue lizards (Tiliqua adelaidensis), Flinders University, College of Science and Engineering

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Habitat fragmentation and alteration as a result of anthropogenic land-use has drastically impacted ecosystems worldwide. Native grassland habitats are perhaps one of the most destroyed due to their suitability to ploughing and grazing livestock. While many species have the potential to be negatively impacted by changes to their ecosystem, the impact on certain species is likely to have wider implications for ecosystem function.

Ecosystem engineers are species that provide resources to other species through their actions. Burrowing organisms provide a wide range of ecosystem services to the environment, one of which is refuge space for other organisms. For some species burrows provide refuge from predation, while for others they provide respite from an otherwise uninhabitable climate. Impacts of habitat destruction on engineering species may therefore influence a wide range of other species within an ecosystem.

The pygmy bluetongue lizard is an Endangered species, restricted to fragmented native grasslands in the Mid North region of South Australia. It obligatorily occupies burrows dug by wolf and trapdoor spiders which act as ecosystem engineers in these grasslands. Native grasslands in this region are predominantly grazed by sheep, thus, sheep grazing has the potential to drastically impact on both lizard and spider populations.

This study aimed to determine how lizards persist with their burrow-engineers, despite the potential for spiders and lizards to cause fatality to one another. It also aimed to investigate how sheep grazing affects spider burrows, lizards and spiders. Twelve 30 x 30m plots were monitored monthly during two Austral spring-summer lizard activity seasons. Within each plot I monitored all spider burrows and their occupants to determine the dynamics of burrow use. Sheep grazing was introduced into half of the study plots in each season to investigate the effects of grazing on lizard habitat.

The results presented in this thesis show that lizards displayed spatial and temporal niche partitioning, selecting empty burrows, and showing a preference for trapdoor spider burrows. This selection of vacated burrows and of a particular subset of burrows has likely played a major role in their ongoing persistence utilising burrows engineered by these spider groups.

Sheep grazing had differential effects on pygmy bluetongue lizards and the spider groups. Grazing resulted in a decline in wolf spiders and their burrows, but did not reduce the abundance of trapdoor spiders, their burrows, or of the lizards. Grazing also resulted in a decline of reproductive output for both spider groups. Preferential selection of trapdoor spider burrows by lizards is likely to play a role in the retention of these lizards in native grasslands.

This thesis provides new information for conservation of the pygmy bluetongue lizard, identifying interactions between co-existing species and impacts of grazing. As well as direct benefits to future translocations of these lizards, I have provided insights that can benefit other grazed land, native grasslands, and burrow-occupying communities. The knowledge gained through this study has broad implications for the management of grassland habitats and provides insights into the interactions of cryptic grassland species and their response to disturbance.

Keywords: Pygmy bluetongue, conservation, ecology, grazing, spider, grassland

Subject: Biodiversity and Conservation thesis

Thesis type: Doctor of Philosophy
Completed: 2018
School: College of Science and Engineering
Supervisor: A/Prof Michael Gardner