Factors affecting the vulnerability of cane toads (Bufo marinus) to predation by ants

Abstract

Experimental evidence on the determinants of prey vulnerability is scarce, especially for vertebrates in the field. Invasive species offer robust opportunities to explore prey vulnerability, because the intensity of predation on or by such animals has not been eroded by coevolution. Around waterbodies in tropical Australia, native meat ants (Iridomyrmex reburrus) consume many metamorph cane toads (Bufo marinus, an invasive anuran). We document the determinants of toad vulnerability, especially the roles of toad body size and ant density. Larger metamorphs were attacked sooner (because they attracted more ants), but escaped more often. Overall, smaller toads were more likely to be killed. Ant densities influenced toad responses, as well as attack rate and success. Data on the immediate outcomes of attacks underestimate mortality: more than 73% of apparent 'escapees' died within 24 h. Because mortality during this period was independent of toad size, predation was less size selective than suggested by immediate outcomes.

Larger Body Size at Metamorphosis Enhances Survival, Growth and Performance of Young Cane Toads (Rhinella marina)

“…Smaller body size also renders a young cane toad more vulnerable to desiccation [51], [52], predation by carnivorous ants [55], cannibalism [54], and mortality after lungworm infection [53]. Thus, for this species, we can confidently conclude that (a) toad body size at metamorphosis can be reduced by multiple biotic and abiotic characteristics of the larval environment [43], [44], [45], [46], (b) that any reduction in body size at metamorphosis is likely to impose subsequent fitness costs, and (c) the strength of any link between metamorph mass and subsequent survival and growth rates will vary through time and space, at least partly because of variation in the availability of environmental resources.…”

“…[19], [55], [73], [74], [75], [76], [77]. Thus, larval history (particularly effects on body size caused by high levels of stress by competition and predation) can reduce the chances of survival for post-metamorphic and juvenile anurans [12], [23], [78], [79].…”

“…Both in the laboratory and the field, experiments have shown that body size at metamorphosis in cane toads can be reduced by exposure to alarm cues from injured conspecifics [45], [50] and by the presence of tadpoles of native frog species [43]. In other work, smaller body size in immediately post-metamorphic toads has been shown to increase the individual’s vulnerability to desiccation [51], [52], to mortality caused by lungworm infection [53], and to predation by cannibalistic conspecifics [54] and predatory ants [55]. In order to evaluate the plausibility of reducing toad recruitment by reducing body sizes at metamorphosis, we conducted experimental trials to measure the effects of body size (mass) at metamorphosis on growth rates, survival, and performance of metamorph and juvenile cane toads.…”

The Interacting Effects of Ungulate Hoofprints and Predatory Native Ants on Metamorph Cane Toads in Tropical Australia

“…It is used by cane toads as a breeding site [27] mostly when the pool diameter averages 25 m and the edge is unvegetated. This area is also a major watering-point for domestic stock, which leave deep footprints in the mud along the waterbody margins.…”

“…These species are dominant members of Australian ant communities because of their abundance, biomass, high activity rates and aggressiveness [48], [49]. They overlap widely with cane toads in Australia in terms of distributional range, diel activity cycles and preferred habitats, with the species Iridomyrmex reburrus being a major predator on post-metamorphic toads at our study site [27], [31], [32]. Unlike many vertebrate predators, these ants are not affected by the toad's toxins [50].…”