Saturday, April 27, 2013

Northern Broad-headed snake ecology

Hoplocephalus bungaroides. Photo from Wikipedia

Conservation of highly specialized animals require detailed information on habitat use, dispersal and movement patterns. This kind of data often is often difficult to gather, especially for endangered species because the animals are rare, and because research methods cannot further endanger the species. As a result,  knowledge of many endangered taxa is based on studies performed at only a single site where the species is abundant and easily observed. These kind of sites are atypical of conditions that pertain over most of the species’ range.

One such species is the broad-headed snake (Hoplocephalus bungaroides), an elapid that has drastically declined since European settlement of Australia. Broad-headed snakes rely on  habitat with specific features: they shelter beneath thin, sun-exposed exfoliated rocks on sandstone rock outcrops with western or north-western aspects. These retreat sites allow snakes to thermoregulate during winter and spring. Hoplocephalus bungaroides also exhibit life history traits that render them vulnerable to disturbance: they depend on high rates of adult survival;  breed only every 3 to 4 years; have low fecundity, 3 to 4 offspring per litter; they may take six years to mature; low rates of dispersal; and a small geographic range. All of these traits contribute to the endangered status of broad-headed snake. The habitat of H. bungaroides has become fragmented, and subject to vegetation overgrowth and removal of shelter-sites (exfoliated rock) for landscaping and gardening.

Genetic data show that the intensively-studied southern population belongs to a genetically distinct clade, with another isolated, evolutionarily significant unit identified in the north of the species range. Those two clades diverged approximately 800 000 years ago. Vegetation, temperatures and potential prey species differ between the northern and southern parts of the species’ range.

In a recently published paper, Croak et al. (2013) captured and radio-tracked 9 adult broad-headed snakes at sites in the northern part of the species’ distribution, to evaluate the generality of results from prior studies most of which had been conducted at a southern study site. The authors identify critical habitat components for this northern population. They found snakes spent most of winter beneath sun-warmed rocks then shifted to tree hollows in summer. Thermal regimes within retreat-sites support the hypothesis that this shift is thermally driven. Intervals between successive displacements were longer than in the southern snakes but dispersal distances per move and home ranges were similar. The northern snakes showed non-random preferences both in terms of macrohabitat by avoiding of some vegetation types and selecting microhabitats, that is th  hollow-bearing trees. Despite many consistencies, the ecology of this species differs enough between southern and northern extremes of its range that managers need to incorporate information on local features to most effectively conserve this threatened reptile.

Citation
Croak BM, Crowther MS, Webb JK, Shine R (2013) Movements and Habitat Use of an Endangered Snake, Hoplocephalus bungaroides (Elapidae): Implications for Conservation. PLoS ONE 8(4): e61711. doi:10.1371/journal.pone.0061711.

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