Study

The use of artificial rocks to restore degraded habitats of the endangered broad-headed snake Hoplocephalus bungaroides and velvet gecko Oedura lesueurii, Morton National Park, New South Wales, Australia

  • Published source details Webb J.K. & Shine R. (2000) Paving the way for habitat restoration: can artificial rocks restore degraded habitats of endangered reptiles. Biological Conservation, 92, 93-99

Summary

In south-eastern Australia, the removal of sandstone 'bush-rocks' for landscaping urban gardens has been very severe. This habitat destruction is largely blamed for the decline of a small nocturnal elapid snake, the broad-headed snake Hoplocephalus bungaroides, which uses sandstone rock as diurnal retreats. Recent studies have shown that juvenile broad-headed snakes feed primarily on velvet geckos Oedura lesueurii, which also rely on sandstone rocks and have also declined with rock removal. Indeed, snake abundance is positively related to gecko abundance and therefore snake numbers might be limited by velvet gecko abundance rather than habitat availability per se. Consequently, in this study the effect of habitat re-creation, using cheap artificial concrete paving slabs as rocks, on the population of velvet geckos is investigated.

Study site: Three study sites more than 1 km distant from each other were selected on the western edge of a sandstone plateau (400 m.a.s.l) in the Morton National Park, 160 km south of Sydney. The experiment ran from November 1994 until February 1996 (when vandals threw the paving stones over the cliffs).

Habitat recreation: Square, grey concrete paving slabs (19 cm wide x 5 cm thick)were used to create sandstone bush-rock habitat. A 4 mm or 8 mm thick piece of wood was glued to each corner of a slab, providing a narrow crevice of 4 mm or a wide crevice of 8 mm. Four pavers, two narrow and two wide crevice widths, were placed 20 cm apart in a square formation on flat exposed bare rock outcrops, 5 m from the cliff edge. Groups of four pavers were places at least 5 m apart: Site 1 had eleven groups, Site 2 had eight groups, and Site 3 had thirteen groups. To assess the effect of temperature on rock use, half of the groups of pavers were shaded using a square steel frame (90 cm wide x 50 cm high) covered with two layers of beige cloth; a square steel frame (minus cloth) was placed over the the unshaded control groups of pavers. Pavers were placed between 18 November 1994 and 11 January 1995.

Thermal characteristics of rocks: Temperature was measured under one shaded and one unshaded paver, with wide crevices, by gluing thermocouples onto the underside (centre) of each paver and the substrate immediately below. Also, temperature under eight natural rocks was recorded. Temperatures were recorded every 20 minutes with a data logger (Campbell CR10, Campbell Scientific, Utah, USA).

Monitoring: Six visit were made from 11 April 1995 to 13 November 1995. The number of vertebrates and invertebrates was recorded under each paver. Velvet geckos present were measured (snout-vent length, tail length), sexed, visually assessed for reproductive status (large testes and spurs on males; eggs visible in gravid females) and individually marked using a unique toe clip.

Thermal characteristics of rocks: The temperature of exposed pavers and of the substratum below were often 15 and 9°C higher than those of shaded pavers. Indeed, temperatures recorded for five sunny days in December 1994 revealed that the average and maximum were significantly greater for unshaded than shaded pavers, but that there was no difference in the minimum temperature (see Table 1). Finally, the temperatures of pavers and natural rocks responded in a similar way to shading and rock thickness.

Paver usage by invertebrates: Flat rock spiders (Gnaphosidae), caterpillars (Lepidoptera), and millipedes (Dilpopoda) were found under pavers. The number of pavers with these invertebrates suggested that they showed a strong preference for exposed pavers with small crevices. However, the total number of millipede individuals did not differ between shaded and unshaded pavers.

Paver usage by geckos: A total of 51 individual velvet geckos were located under 37 pavers. A high proportion of geckos were juvenile (<2 month old, < 32 mm snout-vent length). Geckos showed a strong preference for narrow crevices in unshaded pavers (see Table 2), indeed, eleven pavers with narrow crevices (nine unshaded, two shaded) were used by more than one individual (average = 2.9, range = 2-5 geckos). However, larger geckos prefered pavers with wider crevices and, therefore, as a population matures larger creviced pavers might have been more widely used.

Conclusions: The results suggest that concrete paving slabs are suitable sandstone rock replacements in degraded rock outcrops. The authors suggest that long-term habitat recreation should use larger pavers (30-45 cm wide, 5-10 cm thick) and a variety of crevice widths (up to 10 mm) to maximise the number of micro-habitats for broad-headed snakes and velvet geckos.


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