Body size at metamorphosis is a key trait in species (such as many anurans) with biphasic life-histories. Experimental studies have shown that metamorph size is highly plastic, depending upon larval density and environmental conditions (e.g. temperature, food supply, water quality, chemical cues from conspecifics, predators and competitors). To test the hypothesis that this developmental plasticity is adaptive, or to determine if inducing plasticity can be used to control an invasive species, we need to know whether or not a metamorphosing anuran’s body size influences its subsequent viability. For logistical reasons, there are few data on this topic under field conditions. We studied cane toads (Rhinella marina) within their invasive Australian range. Metamorph body size is highly plastic in this species, and our laboratory studies showed that larger metamorphs had better locomotor performance (both on land and in the water), and were more adept at catching and consuming prey. In mark-recapture trials in outdoor enclosures, larger body size enhanced metamorph survival and growth rate under some seasonal conditions. Larger metamorphs maintained their size advantage over smaller siblings for at least a month. Our data support the critical but rarely-tested assumption that all else being equal, larger body size at metamorphosis is likely to enhance an individual’s long term viability. Thus, manipulations to reduce body size at metamorphosis in cane toads may help to reduce the ecological impact of this invasive species.

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Summary1. Native to the Americas, cane toads Bufo marinus are an invasive species causing substantial ecological impacts in Australia. We need ways to control invasive species such as cane toads without collateral damage to native fauna. 2. We explored the feasibility of suppressing survival and growth of cane toad tadpoles via competition with the tadpoles of native frogs. Compared to the invasive toads, many native frogs breed earlier in the season and their tadpoles grow larger and have longer larval periods. Hence, adding spawn or tadpoles of native frogs to toad-breeding sites might increase tadpole competition, and thereby reduce toad recruitment. 3. Our laboratory trials using tadpoles of eight native frog species gave significant results: the presence of six of these species (Cyclorana australis, C. longipes, Litoria caerulea, L. dahlii, L. rothii and L. splendida) reduced toad tadpole survival and ⁄ or size at metamorphosis. Litoria caerulea also increased the duration of the larval period of cane toad tadpoles. Tadpoles of the other two frog species (Litoria rubella and Litoria tornieri) did not affect survival or growth of larval cane toads any more than did an equivalent number of additional toad tadpoles. Native frog species with larger tadpoles exerted greater negative effects on toad tadpoles than did native species with smaller tadpoles. 4. Synthesis and applications. Encouraging the general public to construct and restore waterbodies in peri-urban areas to build up populations of native frogs -especially the much-loved green tree frog Litoria caerulea -could help to reduce recruitment rates of invasive cane toads in Australia.

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