A field study of spatial ecology and movements of a threatened snake species, Hoplocephalus bungaroides

Abstract

Information on movement patterns, home range sizes and site fidelity of endangered fauna may provide a basis for conservation planning (size and location of reserves; vulnerability to habitat fragmentation; feasibility of natural recolonisation of ‘restored’ habitats, etc.). To obtain such information, we surgically implanted miniature radio transmitters in 25 individual broad-headed snakes Hoplocephalus bungaroides, a small (mean snout-vent length = 57 cm, mean mass = 51 g) viviparous elapid snake reliant upon sandstone outcrops in southeastern Australia. We also carried out a mark-recapture study of this threatened species. Our telemetered snakes spent long periods of time sequestered inside retreat-sites (rocks, crevices, tree hollows) and thus were active on only 21% of days. Gravid females had small home ranges (mean size = 0·05 ha, convex polygon method) and remained near cliffs during summer, whereas most males and non-gravid females moved long distances (up to 780 m) away from cliff tops during summer and had larger home ranges (mean size = 3·3 ha). Movements by the snakes were more frequent and extensive when they were in the woodland (mean interval between successive moves = 2·9 days; mean displacement per move = 159 m) than when they were in the rock outcrops (means = 6·3 days, 37 m). Home ranges were larger in summer than in spring, and were larger in 1994–95 than in other years. Home ranges of males showed little spatial or temporal overlap in spring, but females were found within the areas used by males. During summer there was little temporal or spatial overlap of home ranges of adults, which suggests that snakes may actively avoid conspecifics of either sex while foraging in the forest. Many adult snakes showed strong site fidelity, frequently returning to the same rocks where they were initially captured. Dispersal of juvenile snakes from their birth sites was relatively limited (maximum recorded distance = 375 m after 6 months). Our results have significant implications for conservation of this threatened taxon. The fixed home ranges and high site fidelity of adults coupled with the low dispersal rates of juvenile broad-headed snakes mitigate against successful natural recolonisation of rehabilitated habitats. Thus, habitat restoration may need to be combined with release of captive-bred progeny, or field-caught neonates. The exclusive home ranges of adult snakes suggest that social interactions (avoidance of conspecifics) may limit population densities, and hence that (i) removal of some neonates may not reduce future densities of adults, and (ii) habitat destruction is likely to be a more serious long-term threat than removal or killing of individual snakes. The large home ranges, and frequent and extensive movements, of snakes in woodland indicate that conservation reserves must incorporate large forest areas adjacent to sandstone outcrops.