Ecological implications of reptile mesopredator release in arid South Australia – paper review

Mesopredator release has been well documented in mammals, for example in North America the reduction in coyote numbers led to an increase in smaller carnivore activity, which ultimately led to the significant declines in many bird species within the area (Crooks & Soule´, 1999). Despite the important ecological role that large varanids and snakes play, there are few examples of mesopredator release within reptiles. Read & Scoleri (2015) have helped to begin to fill this gap, in their study on mesopredator release in the sand goanna (Varanus gouldii) and the potential impacts of such release.

The area studied in Read & Scoleri’s (2015) research was the Arid Recovery Reserve near Roxby Downs in northern South Australia. Three treatments were used including a “reintroduced mammals” treatment, in which feral animals had been removed and four native mammals were re-introduced. A second treatment was “exotics” which was an area where foxes, cats, and rabbits had not been removed. The third treatment was the “control”, in which feral animals had been removed, however, no native mammals were re-introduced.

Sand goanna tracks and foraging digs were used to identify the amount of activity in each treatment due to the inefficiency of trapping for adult goannas in pit traps. The researchers found significantly more activity in both the reintroduced mammals treatment and control, compared to the exotics treatment. It was also found that the activity of tracks and foraging digs did not significantly differ between the control and reintroduced mammals treatment. This is of importance due to the competition for prey and resources between V. gouldii and the reintroduced mammals including the Western Barred Bandicoot (Parameles bougainville) and the Greater Bilby (Macrotis lagotis). It appears then, that the competition between V. gouldii and native mammals has less of an influence on the ecosystem than does the exclusion of feral predators.

With that being said, it was found that several species of smaller lizard declined in the Arid Recovery Reserve when foxes and other introduced animals were removed. These species include the geckos, Diplodactylus conspicillatus and Diplodactylus tessellatus. As both these geckos are prey items of V. gouldii, this finding led the authors to believe that the decrease within the area was the result of sand goanna numbers increasing upon the removal of feral predators. The consequences of this could potentially be a trophic cascade including increases in abundance of small invertebrates (preyed on by small lizards), and decreases in larger invertebrates such as centipedes (prey items of the sand goanna). These findings will hopefully guide future management in fox and cat control and provide an understanding of the potential wider implications to the ecosystem.

Thanks to Garry Ogston for providing this review. To download a copy of the full article please click here

Reference

Read, J. L. & Scoleri, V. (2015). Ecological implications of reptile mesopredator release in arid South Australia. Journal of Herpetology, 49(1), 64-69. doi:10.1670/13-208