The (non) impact of invasive cane toads on freshwater crocodiles at Lake Argyle in tropical Australia

The most substantial (and to date, unexplained) heterogeneity in the impact of toxic cane toads (Rhinella marina) on the native fauna of tropical Australia involves freshwater crocodiles (Crocodylus johnstoni); some populations have experienced catastrophic mortality whereas others have been unaffected. A trend for higher impact in more arid areas suggests that Western Australian (Kimberley region) crocodile populations may be at high risk. We monitored crocodile densities and body sizes, and the spread of cane toads, at a large water body (Lake Argyle) in the eastern Kimberley. Toads arrived on the lakeshore in early 2009, and spread to cover >300km of lakeshore, and colonize all of the larger islands within a 24-month period. Physical removal of >10000 toads by a community group depressed toad abundances, but only briefly. Spool-tracking showed that toads moved extensively along the lakeshore (up to 90m per night), often into floating vegetation in the lake. Crocodiles thus encountered toads (3-15% of crocodiles were <2m from a toad when sighted), and we recorded 36 cases of toads being seized by crocodiles. Nonetheless, crocodile mortality was rare, and crocodile numbers did not decrease through time, nor differ between toad-infested versus toad-free areas of the lake. Although ingestion of a single adult toad may be fatal to a freshwater crocodile, and the Lake Argyle crocodiles encounter and consume these toxic anurans, the population-level impact of toad invasion has been trivial. Hence, the Lake Argyle crocodile population may not warrant immediate active management to reduce the impact of cane toads.