The comparative population ecology of two semi-aquatic varanids

Abstract

During the dry season in tropical northern Australia, when water and food are scarce but temperatures remain warm enough for activity, many terrestrial varanids lower their levels of activity (some to the point of inactivity). However, previous studies have shown that the semi-aquatic varanid Varanus mertensi remains active throughout the year, maintaining a lower body temperature than terrestrial varanids. Is the example found in V. mertensi indicative of other semi-aquatic monitors? Here I describe, using a suite of novel and innovative sampling and analysis techniques, the population ecology of V. mertensi and another semi-aquatic varanid species Varanus indicus. Specifically, I ask the questions of whether (i) V. indicus responds similarly to seasonality, and (ii) the home range and movement patterns of semi-aquatic varanids also differ from fully terrestrial species. I also consider the likely impact of the invasive and toxic cane toad (Bufo marinus) on these species and other reptiles and to what degree it is possible to quantify the demography of free living varanids (particularly when they are hard to sex in the field). I demonstrate that V. mertensi and V. indicus share broad similarities in life history traits with terrestrial varanids: males travel further in the breeding season; larger animals are more likely to move further; and home ranges overlap between conspecifics. However their semi-aquatic lifestyle, while buffering them (both physiologically and ecologically) from seasonal water shortages, has implications for their use of space (in the case of V. indicus, leading to relatively small home ranges) and their population dynamics (high densities). In addition, I show that Bufo marinus poses a definite threat to these varanids along with many other reptile species. Lastly, I demonstrate that radiotracking is more robust than mark-recapture for population monitoring of varanids.

Date of Award	Jan 2007
Original language	English
Supervisor	Keith Christian (Supervisor), Barry Brook (Supervisor) & Anthony Griffiths (Supervisor)