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Individual study: Adding artificial burrows to enhance a population of the endangered pygmy blue tongued lizard Tiliqua adelaidensis near Burra, South Australia

Published source details

Souter N. J., Bull M. & Hutchinson M. N. (2004) Adding burrows to enhance a population of the endangered pygmy blue tongue lizard, Tiliqua adelaidensis. Biological Conservation, 116, 403-408

Summary

The endangered pygmy blue tongue lizard Tiliqua adelaidensis was presumed extinct before rediscovery in South Australia in 1992. Several small populations have subsequently been located but each is threatened by habitat loss due to agricultural disturbance. Conservation management may be crucial in preventing local extinctions. The lizards are burrow specialists and occupy vertical spider burrows (thought to be constructed by lycosid and mygalomorph spiders) in order to shelter, regulate temperature and conceal themselves from passing prey and above-ground predators; the burrows are thought to be a population-limiting resource. An experiment was conducted to examine the effect of providing artificial burrows upon a population of pygmy blue tongues and to assess its usefulness as a conservation management technique.

The experiment took place where the pygmy blue tongue lizard was rediscovered in 1992, named the 'Main Site', near the town of Burra, South Australia.

A 1 ha area had been set aside for monitoring of the population and this was surveyed in autumn and spring 2000. The survey was undertaken using five parallel transects in order to estimate the proportion of spider burrows suitable for lizards i.e. burrows in excess of 12 cm long. Similarly, the proportion of 'high quality' burrows (in excess of 30 cm long) was also estimated and these proportions were applied to the actual experimental area, where burrows were counted but not measured.

Field trials showed that adult lizards prefer 30 cm burrows. However, the majority of spider burrows were less than 30 cm yet still contained adult lizards, supporting the hypothesis that natural burrows are a limiting resource.

The experimental area was 300 x 140 m and was divided into 24, 20 x 20 m quadrats (plots) in four rows of four to eight plots per row, each separated by 20 m from adjacent plots. In the first survey (August 2001), every burrow in each quadrat was counted and viewed using an Olympus IF8D4X2-10l optic fibre scope to determine whether it was occupied by a lizard, spider or centipede (larger individuals of which are potential lizard predators).

In three days following the August survey, every alternate plot was allocated as a control, while the remaining 12 plots had 36 artificial burrows (hammering alternating steel rods of 13 mm and 17 mm diameter, 30 cm into the ground, 3 m apart) added. Previous trials showed lizards prefer holes with diameters slightly larger than their heads, it was agreed that any burrow addition program should provide refuge for both adults and juveniles, these diameter rods fulfilling this requirement. A total of 216 small and 216 large burrows were added in 6 x 6 grid patterns equivalent to a burrow density of 900/ha, thus increasing the number of suitable burrows by about 10 times and high quality burrows (about 30 cm or so long) by over 30 fold.

In January 2002, before any new offspring were born, all natural and artificial burrows were surveyed and small burrows which had deteriorated were re-constructed. The third and final survey took place in April (2002), the usual time when litters are first recorded and juveniles have dispersed from their natal burrows. In the third survey, juveniles and adults were differentiated on the basis of size.

Lizard densities: From the first to the third survey, the density of lizards/plot increased in the experimental plots (August average 1.25; January average 4.00; April average 7.25) whilst remaining about the same in the control plots (August average 1.42; January average 1.42; April average 1.50). Concurrently, the density of lizards in the adjacent monitoring area also remained consistent, thus reducing the possibility that the lizards had simply re-distributed themselves to burrows of preferred depths.

The high lizard densities recorded in the experimental plots suggest that the increased availability of 'high quality' burrows may reduce lizard mortality. Therefore movement of lizards to high quality burrows may have had a conservation value even if the overall population in the short-term had remained unchanged.

Recruitment success also increased with burrow density, with a significantly higher proportion of juveniles seen in the experimental plots compared with the controls during the April survey. This could have been a direct result of the enhanced numbers of adults, and/or the greater availability of suitable burrows for juveniles dispersing from their natal burrows.

Predation: Increased burrow density led to increased density of predatory centipedes Scolopendra cf. morsitans which also inhabited the burrows, particularly in the spring (August-January). However, increases also occurred in the already existing burrows suggesting that the centipedes were attracted to the experimental plots independent of the extra burrows provided. The centipedes may have been responding to increased prey density, although there was no evidence to suggest that they actually attacked lizards in the burrows and their increase may have had little detrimental effect on the lizard population.

Dispersal and colonisation: Increased burrow density may reduce pygmy blue-tongue dispersal distances as they do not need to travel as far to find a new burrow. This in turn may reduce the colonisation potential of adjacent suitable habitat. For small populations (often the case for this species) with limited chances of successful dispersal and colonisation anyway, the increased population density as a result of burrow provision is considered to outweigh possible disadvantageous dispersal effects.

Competition: Lizards exhibit a 'sit and wait' hunting strategy and increased burrow density may encourage competitive interactions between neighbouring lizards targeting the same prey. The extent of conspecific interactions were beyond the scope of this study, although two juvenile lizards were found in adjacent burrows less than 10 cm apart suggesting at least a short term tolerance for very near neighbours.

Conclusions: Results from the study support the use of supplementary refuges as a conservation strategy to boost population densities of this endangered reptile. Further studies into potential side effects may be necessary but the enhanced population density and recruitment success in the pygmy blue tongue lizard suggests that this technique is overall, beneficial.


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