The impact of cane-toads on the trophic cascades of freshwater ecosystems in the Northern Territory

A project undertaken at the Institute of Wildlife Research University of Sydney and supervised by Mike Letnic

The cane toad is one of the world’s most successful invasive species. Since they were introduced to Queensland in 1935 they have invaded much of northern Australia and their range is still expanding. Cane toads contain high concententrations of bufotoxin which is poisonous to most Australian vertebrates, including crocodiles. Freshwater crocodiles (Crocodylus johnstoni) prey upon and can be poisoned by cane toads. In the Victoria River District of the Northern Territory there have been drastic reductions in crocodile populations following the arrival of cane toads (Fig. 1). This reduction in crocodile numbers was coincident with observations of dead crocodiles. Dissection of these crocodiles indicated that cane toads were present in the stomachs of many of the dead animals. However, in other river systems of the Northern Territory such as the Daly and Finniss Rivers, cane toads appear to have litle impact on freshwater crocodile populations (Fig. 1). Why the impacts of cane toads have been so different is unknown but it is possible that cane toads may suffer from the effects of the hot-dry climate of the Victoria River and as a result attempt to rehydrate in the river where they are likley to be preyed upon and subsequently kill crocodiles. Alternatively, the Victoria River may provide less food for crocodiles than more northern rivers where rainfall is higher. Consequently, crocodiles in the Victoria River may rely more upon terrestrial prey such as cane toads than crocodiles in the Daly and Finniss river systems.

Because crocodile populations on the Victoria River have declined following the arrival of cane toads, it is likely that there will be major flow-on effects within freshwater and riparian ecosystems. Studies in both aquatic and terrestrial environments have shown that the disruption or total cessation of interactions between top-order predators and their prey species can have dramatic effects on the organization and function of ecosystems.  These effects include the irruption of prey species and an increase in the abundance of smaller predators owing to the absence of competition or predation by larger carnivores and changes in the composition of plant communities owing to altered-plant-herbivore relationships. In the future, we will continue to monitor the longer-term responses of crocodile populations to cane toads in the rivers of Northern Australia

Collaborators

Dr Tim Dempster (Sintef Norway), Professor Rick Shine (University of Sydney), Dr Jonathon Webb (University of Sydney), Dr Robyn Delaney (Parks and Wildlife Service of the Northern Territory).

Captions to Figures

Figure 1. Non-hatchling crocodile density (A) before (1997) and after (2005, 2007) the arrival of cane toads at four sites on the Daly River, and (B) before (2005) and after (2007) the arrival of cane toads at four sites on the Victoria River. On the Daly River, cane toads (Bufo marinus) arrived at Claravale in 2002, Oolloo upstream and downstream in 2003 and Beeboom in 2004. On the Victoria River, cane toads arrived at Victoria River Gorge and Wickham in 2005, at Pigeonhole in 2006 and Longreach in 2006.

Figure 2. An approximately 120 cm TL freshwater crocodile (Crocodylus johnstoni) from the Daly River with a cane toad in its jaws.

Figure 3. Dr Mike Letnic dissecting a dead freshwater crocodile. The dissection revealed a cane toad in the stomach of the crocodile.

Figure 4. Dr Tim Dempster with a cane toad removed from the stomach of a dead crocodile on the Victoria River.

Publications

Somaweera R, Shine R, Webb J, Dempster T & Letnic M (2012). "Why does vulnerability to toxic invasive cane toads vary among populations of Australian freshwater crocodiles". Animal Conservation doi:10.1111/j.1469-1795.2012.00578.x

 
Figure 1. See captions at left.

 

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