Larval interactions with an invasive species (the Cane ToadRhinella marina) affect life-history traits in an Australian anuran (the Marbled FrogLimnodynastes convexiusculus)

Abstract

The viability of a metamorph anuran can be influenced by its body size and the time it has taken to complete larval development. In a laboratory experiment, we show that the presence of tadpoles of the invasive Cane Toad Rhinella marina causes tadpoles of a native frog (Marbled Frog Limnodynastes convexiusculus) to metamorphose later and at smaller sizes. These effects may render frog metamorphs more vulnerable to desiccation and predation, but render them less vulnerable to Cane Toads. Marbled Frogs prey upon other anurans, including the highly toxic Cane Toad metamorphs. Small, late-emerging metamorph frogs are unlikely to encounter metamorph toads small enough to ingest; and hence, are less likely to be fatally poisoned by consuming the toxic invader. Developing in the presence of larval Cane Toads thus increases the native taxon's ability to survive the presence of toads postmetamorphosis. Predicting the ecological impacts of an invasive species on native taxa with biphasic life histories (such as most anurans) thus requires information on interactions in both aquatic and terrestrial environments. The expression of phenotypic plasticity in one phase may influence fitness in a subsequent phase, in complex and non-intuitive ways. (Tyler et al. 1983). Cane Toads and Marbled Frogs (Limnodynastes convexiusculus) often breed in the same ponds at the same time (Crossland et al. 2008). Collection, husbandry and experimental designWe collected 10 clutches of Marbled Frog eggs from a temporary pond (131°18'46.74" E, 12°34'15.33" S) on 27 February 2007, the morning after they were deposited. The clutches were taken to our research facility less than 5 km from the collection site, and all clutches hatched within three days. Frog eggs and tadpoles were housed in an opensided shed, initially in plastic containers (38 cm long x 26 cm wide x 20 cm high). Tadpoles were reared on a diet of frozen lettuce provided ad libitum. Water was partially changed twice weekly. On 23 May 2007, 10 tadpoles at stage 25 of development (Gosner 1960) were randomly selected from each clutch, and a tadpole from each clutch was added to each of 10 bins (60 cm long x 40 cm wide x 40 cm high, filled to 5cm below the rim; i.e. 10 tadpoles total per container). To half of these bins (randomly selected), we added 10 Cane Toad tadpoles between Gosner stages 26-30, collected from a pond 10 km away (131°18'57.07" E, 12°38'24.72" S). We used mid-stage toad tadpoles to minimise toxic effects, in case any toad tadpoles died and were consumed by frog tadpoles (Hayes et al. 2009). All containers contained frozen lettuce ad libitum as food, and water was partially changed weekly. Over the course of the experiment, water temperature ranged from 26 to 30°C. We did not include additional treatments of different densities of frog tadpoles because our aim was to simulate effects of toad invasion; at least initially, these anurans add to total anuran biomass rather than replacing native taxa (Greenlees et al. 2006(Greenlees et al. , 2007. Measurements an...