Quantifying the ecological role of surface rocks on reptile conservation in agricultural landscapes
Abstract
Surface rocks provide critical habitat for reptiles and are a common feature in agricultural landscapes. Despite this, there is limited quantitative evidence describing the use of surface rock by biodiversity and its conservation significance in landscapes. Recent advances in soil amelioration practices have triggered a resurgence in the large-scale removal of surface rock from commodity production regions. Rock removal practices may have substantial impacts on biodiversity, particularly for range-restricted and rock-dependent reptile species. In this thesis, I aim to explore the ecological role of surface rocks for reptiles in agricultural landscapes through answering four questions: (i) what is the potential scale of surface rock use by reptiles and impact of removal (Chapter 1); (ii) what landscape and rock features contribute to use (Chapter 2); (iii) can surface rock be restored (Chapter 3 and 4); and (iv) how might improved management of surface rock habitats benefit reptile conservation (Chapter 5).
Chapter 1 investigates how many terrestrial squamate species depend on, or are associated with, rock habitats globally. I found that over a quarter of squamate species globally are associated with rock habitats, and over 10% completely depend on rock habitats. Further, I found that both rock-dependent and rock-associated species were more likely to be threatened and had significantly smaller range sizes than species not associated with rocks, suggesting these species could be particularly vulnerable to loss and degradation of rocky habitat. These findings highlight the extent of rock use by reptiles globally and how poorly understood, and consequently poorly protected, these habitat types are.
Chapter 2 revealed the landscape factors and rock attributes that influence the selection of retreat sites by reptiles for overwintering. Specifically, I found that reptiles favoured elevated sites with limited canopy cover. They also preferred smaller rocks characterised by a larger surface area to volume ratio. Further, I found that small patches of surface rock supported high reptile numbers, indicating that these habitats are critical for the persistence of reptiles over winter.
Chapter 3 detailed the outcomes of rock restoration in grazing landscapes. Specifically, it tests for interactive effects of rock addition and grazing treatments on measures of reptile diversity. I found that rock addition increased the abundance and diversity of reptiles, but grazing did not influence these effects, either alone, or in combination with rock addition. These findings provide experimental evidence that surface rock addition is an effective restoration method to improve conservation outcomes for reptiles in agricultural landscapes.
Chapter 4 further explores the colonisation of restored rock by herpetofauna and investigates how restored rock compares to existing surface rock habitats. I found that reptiles and frogs responded quickly and positively to added rock. However, sites with added rock supported fewer reptile species than reference sites and were colonised only by a subset of the reptile community. This chapter provides evidence that rock restoration is an effective management strategy but should complement rather than replace the protection of existing surface rock areas.
Finally, Chapter 5 integrates the management outcomes of all previous chapters and summarises the state of knowledge around surface rock management for reptile conservation.