Department of Zoology and Tropical Ecology, James Cook University, Townsville City QLD 4811, AUSTRALIA.
The impact of the eggs, hatchlings and tadpoles of the introduced cane toad, Bufo marinus , on native aquatic fauna in northern Queensland, Australia was investigated using a series of replicated laboratory and artificial pond experiments. Specifically, the project investigated: (1) the toxic effects of Bufo on native aquatic species, (2) predation by B. marinus tadpoles on native aquatic species, (3) competition between B. marinus tadpoles and native aquatic species, and (4) higher order effects produced by Bufo on other trophic interactions within native aquatic animal assemblages.
The toxic effects of Bufo on native aquatic fauna were always associated with the consumption of Bufo; there was no evidence that toxins are released from Bufo into solution. Native aquatic species exhibited considerable inter- and intraspecific variation in their susceptibility to B. marinus toxins. Bufo were highly toxic to certain species but were non-toxic to others. Interspecific variation in toxic effects was not related to the number of Bufo ingested, and there was no clear pattern of distribution of vulnerability among species within higher taxa. Intraspecific variation in responses to toxins may result from (1) differences in the number of Bufo ingested by individuals, (2) individual variation in resistance to B. marinus toxins, or (3) individual variation in toxicity of Bufo.
Two "susceptible" native aquatic taxa (fish and anuran larvae) were chosen for detailed studies. Native fish (barramundi: Lates calcarifer; sooty grunter: Hephaestus fuliginosus) usually learned with minimal trauma to avoid B. marinus tadpoles. Populations of these species are therefore unlikely to experience significant declines in water bodies where they co-occur with Bufo. Anuran larvae, however, exhibited considerable interspecific variation in their ability to detect and avoid B. marinus toxins. Artificial pond experiments demonstrated that populations of species which have limited ability to detect and avoid B. marinus toxins (Litoria bicolor, L. nigrofrenata, Limnodynastes ornatus) experienced significant increases in mortality when exposed to Bufo. However, the toxic effects of Bufo on L. ornatus tadpoles indirectly facilitated the survival of eggs and hatchlings of later breeding native anurans by reducing the intensity of predation on these early life history stages by L. ornatus tadpoles.
Bufo tadpoles were not significant predators of native anuran eggs, hatchlings or tadpoles, but did compete with native tadpoles (L. ornatus). The outcome of competition between B. marinus tadpoles and L. ornatus tadpoles was determined by their order of introduction into ponds. Generally, each species performed better when added to ponds before the other species, and performed worse when added to ponds after the other species, as compared to when both species were added to ponds simultaneously. However, the toxic effects of B. marinus eggs and hatchlings on L. ornatus tadpoles reversed these competitive priority effects and allowed late breeding Bufo to perform as well as, or better than, conspecifics which were added to ponds prior to L. ornatus.
The results demonstrate that B. marinus eggs, hatchlings and tadpoles may have a significant impact on the composition and dynamics of native aquatic communities, and in particular, on native larval anuran communities.