The long-term effects of toads on the Australian environment are difficult to determine, however some effects include "the depletion of native species that die eating cane toads; the poisoning of pets and humans; depletion of native fauna preyed on by cane toads; and reduced prey populations for native insectivores, such as skinks."
Precipitous declines in populations of the Northern Quoll (Dasyurus hallucatus) have been observed after toads have invaded an area. There are a number of reports of declines in goanna and snake populations after the arrival of toads. For example, local populations of Varanus panoptes dropped by up to 90% when their habitat was invaded by cane toads. The preliminary risk assessment of cane toads in Kakadu National Park stated that the predation of the cane toad by native wildlife is the greatest risk to biodiversity. Other factors, such as competition with native wildlife for resources, and the predation of the cane toad on native wildlife, were considered much lower risk factors but requiring further study. In the Northern Territory, goanna predation on cane toads has been linked to a rise in the amount of undamaged salt water crocodile eggs. Cane toads were present within a few days of the crocodiles hatching in April 2007.
Numerous native species have been reported as successfully preying on toads. Some birds, such as the Black Kite (Milvus migrans), have learned to attack the toad's belly, avoiding the poison-producing glands on the back of the head. Anecdotal reports in the Northern Territory suggest that a native frog, Dahl's Aquatic Frog (Litoria dahlii), is able to eat the tadpoles and live young of the toad without being affected by the poison that often kills other predators. This may account for slower than expected infestations of toads in certain areas of the Northern Territory, although later research carried out jointly by several Australian Universities casts doubt on these reports. Some snakes have been reported to have adapted smaller jaws so that they are unable to swallow large cane toads which have large quantities of poison.
Another study, however, notes that the cane toad is adapting to a wider environmental range and may in the future be spreading into habitats currently not available.
In 2009 it was found that the native meat ant is immune to the toad's poison and can successfully prey upon young cane toads. Whereas native frogs and toads have natural reflexes to avoid the meat ants, the cane toads do not tend to try to escape the ants, rather standing still when attacked waiting for the toxin to kill the attacker. More: http://en.wikipedia.org/wiki/Cane_toads_in_Australia