Ecology and management of Eastern bearded dragon : Pogona barbata

Abstract

There is a global effort to research the impacts of habitat fragmentation and degradation on wildlife. There is, however, still a need for greater understanding of ecological processes at work in fragmented and degraded habitat in peri-urban and agricultural areas. The study was undertaken in order to enable insights to be gained to provide a basis for management of fragmented habitat, using a single species as a focus. Eastern Bearded Dragon Pogona barbata is a reptile icon in Australia, but which is regarded anecdotally as being in decline, and for which little is known of the ecology. The aim of the study was to research the biology and spatial ecology of a species that had a dependent relationship on structural habitat of the nature of that found in peri-urban areas. Pogona barbata was selected as being an appropriate species in being terrestrial l /arboreal, having a territorial social structure, being an ecological generalist species, and being of a suitable size for study in relation to the size of fragmented habitat in western Sydney. Wild populations in relatively undisturbed habitat in the peri-urban area of western Sydney, Australia, were observed from 2000 to 2002, with additional data gained from museum collections and road-killed animals. Morphometrics were analysed by taking the ratio of a character to snout-vent length, and plotting that value against snout-vent length in order to more clearly see allometric change. Detailed tracking of animals using spool threads was used to collect data on microhabitat use, spatial activity and home range. The distribution of P. barbata across its range coincided with open forest and woodland habitat, which in some areas is subject to intensive urban and agricultural impacts. Pogona barbata is found to be insectivorous as a juvenile and omnivorous as an adult. As hatchlings grow, the relative length of the tail increases (positive allometry) until sexual maturity, and this coincides with an insectivorous diet. The onset of sexual maturity occurs just after the diet becomes omnivorous, and the relative tail length decreases (negative allometry) to a value that can be less than that of hatchlings. The gracile body form of juveniles is different from the robust body form of adults, and this is consistent with the difference in diet and microhabitat use. Factors affecting the persistence of the species were identified. Eggs were eaten by foxes on a regular basis, and hatchlings and juveniles were very rare, indicating an unbalanced demographic structure. The apparent rarity of hatchlings and juveniles may be due to crypsis. Road death was found to remove important breeding individuals from the population. An improved method of estimating testis volumes in lizards using three dimensions was used, which showed that the anatomy of P. barbata testes is bilaterally asymmetrical. It is currently assumed that reptile testes are at least bilaterally regular, but this is not the case in P. barbata. The outcome of this finding is that reproductive and other studies that rely on measurement of a single testis in two dimensions may miss significant data. Testis volumes for P. barbata in western Sydney in the past 20 years are highly abnormal, which coincides with increasing air pollution in the area. Extremes of testis condition were absent testes, very small testes, and extreme differences of testis size in individual animals. Resource partitioning was found, with microhabitat use, to be significantly different for juveniles, males and females. Juveniles and hatchlings used dense habitat with a high incidence of fine and coarse woody debris. The fine matter in the habitat upon which adult females also rely is lost in frequent fires that are used to control bushfire hazard. Pogona barbata is generally regarded as a sit-and-wait predator, on the basis of its being commonly observed on obvious perches. This study demonstrates that such perches are not the most important component of its habitat and that its foraging mode is that of a grazing omnivore. This study appears to be the first that addresses a seasonal shift in microhabitat use for a reptile in general terms. The pattern of daily activity changes with season, and changes in daily habitat preference based on age, sex and season, indicated that P. barbata required a diverse structural habitat. This habitat is affected by frequent hazard reduction fires. A new method of estimating animal home range is presented. The method consists of daily spool trace polygons collected by GPS and analysed in ArcView 3.2, with a spatial analysis output that indicates an estimated total home range and a confidence interval for that estimate. A typical result is an acceptable estimate within 20-30 sequential days of tracking an animal. That outcome is a considerable improvement on the currently accepted minimum of 50 independent days, being at least 100 calendar days. Pogona barbata ‘breaks the rules’ for a number of concepts in the biology of reptiles, and in this study has displayed some new insights in testis morphology, ontology and ecology. It was rare in the areas studied, lending support to the perception that the species is in decline. Reproductive suppression was suspected to affect the population in western Sydney. Pogona barbata has shown a range of characteristics that makes it useful as an umbrella species for the purpose of managing fragmented and degraded habitat in peri-urban and agricultural areas. A range of biological and ecological measures is required to enable effective habitat management of any species, and this study has provided some appropriate techniques for that purpose.