Reversing Reptile Decline

Cane toads have had a devastating impact on many native species and our reptile populations have suffered some of the worst impacts.  There is research showing massive declines in numbers of Yellow Spotted Monitors (around 95%), Merten’s Water Monitors, Mitchell’s Water monitors, King Brown Snakes, Death Adders, Blue-tongue Lizards and in some areas Frill-necked lizards.  Observation evidence suggests other reptile species are declining because they eat toads at some stage in their life cycles.

Large monitors (Goannas) like Varanus panoptes have been really hard hit by the arrival of cane toads and have disappeared from most areas.  An animal that was once a very common sight across the top end is now very rarely seen.  Research from the Daly River area shows an initial decline of over 90% and this decline looks to be continuing to local extinction in many areas.

In Darwin we have been able to keep some of these animals because people have done such a good job of reducing the numbers of cane toads.  Under the leadership of FrogWatch (www.frogwatch.org.au) the people of Darwin have managed to keep toad numbers quite low, with toad densities of less than 10% of some equivalent north Queensland population centres. 

More importantly Darwin has managed to almost eliminate toad breeding in and around the Darwin area and this has had particularly important outcomes for the reptiles in Darwin. Studies show that a “normal” cane toad population has around 50% pre adult toads in the population, in Darwin this seems to be about 2.5%.

Our research is suggesting that it is small cane toads that are responsible for much of the damage to reptile numbers.  Many reptiles such as our smaller monitors like the Spotted Tree Monitor, Varanus scalaris are too small to consider a large cane toad as a meal, but small cane toads are just the right size.  This also applies to the juveniles of the larger species like V. panoptes and many other reptiles, including Frill-necked Lizards.

This is clearly shown in a comparison between different areas of Darwin. In areas like the Botanic Gardens and the Gardens Park Golf Links toads have managed to breed and the area has been subjected to floods of small toads.  The reptile population in these areas has been devastated and there are no V. panoptes in that area today, where once they were a key feature of our urban wildlife.

Just a few kilometres up the road at East Point there is a barrier fence put in place by FrogWatch and the Larrakia Rangers and this has made managing toads easier.  As a result of the toad control work done in the area toads have never bred on East point and there have never been small cane toads there.  As a result the population of reptiles still appears to be healthy.

Yellow Spotted monitors have bred on East Point over the past few wet seasons and they are regularly sighted by visitors to the area.

Our project Reversing Reptile Decline is designed to protect the biodiversity, particularly the reptiles, of the coastal fringe of Darwin from Vesteys’ Beach through to Lee Point and to find out more about this population of monitors and other reptiles (Blue-tongue lizards and Frill-necked lizards) and get a more accurate picture of the populations remaining in Darwin.  We are hoping we can grow those populations and get reptiles to spread back into the gardens and golf course areas.