Maladaptive traits in invasive species: in Australia, cane toads are more vulnerable to predatory ants than are native frogs

Abstract

1. If a species is translocated outside its native range, some of its traits (evolved to match conditions in the ancestral range) likely will be maladaptive. Identifying ways in which the invader are poorly suited to its new range might provide novel opportunities for biocontrol. 2. The spread of cane toads (Bufo marinus, native to central and South America) through tropical Australia has created major ecological problems. Although many native predators cannot deal with the toxins of the invasive toads, 'meat ants' (Iridomyrmex reburrus) kill and consume many metamorph toads. Might this be a mismatch between the invader and its newly invaded range, whereby the morphology, locomotor ability and/or behaviour of cane toads renders them vulnerable to a predator that poses little danger to native anurans? 3. To explore this possibility, we measured habitat use and activity patterns in meat ants, metamorph cane toads and metamorphs of seven native frog species (Litoria bicolor, L. caerulea, L. dahlii, L. nasuta, L. rothii, Limnodynastes convexiusculus, Opisthodon ornatus) in standardized enclosures in the laboratory. 4. Unlike the frogs, (1) toads selected open microhabitats and were active diurnally, thus increasing encounter rates with meat ants; (2) toads failed to detect and evade approaching ants; (3) toads exhibited poor locomotor ability (short slow hops, reflecting their small size and short limbs); and (4) toads frequently relied on an ineffective defence mechanism (crypsis) when attacked. 5. In combination, these traits rendered cane toad metamorphs far more susceptible to predation by meat ants than were any of the native frogs tested. That vulnerability presumably reflects lack of coevolution between cane toads and Australian ants. 6. The inability of invasive toads to escape predatory native ants might be exploited to reduce cane toad numbers, by manipulating ant densities and/or locations during periods of toad metamorph emergence.