In Australia, large native predators are fatally poisoned when they ingest invasive cane toads (Rhinella marina). As a result, the spread of cane toads has caused catastrophic population declines in these predators. Immediately prior to the arrival of toads at a floodplain in the Kimberley region, we induced conditioned taste aversion in free-ranging varanid lizards (Varanus panoptes), by offering them small cane toads. By the end of the 18-month study, only one of 31 untrained lizards had survived longer than 110 days, compared to more than half (nine of 16) of trained lizards; the maximum known survival of a trained lizard in the presence of toads was 482 days. In situ aversion training (releasing small toads in advance of the main invasion front) offers a logistically simple and feasible way to buffer the impact of invasive toads on apex predators.

Ecosystem engineers directly or indirectly affect the availability of resources through changing the physical state of biotic and/or abiotic materials. Fossorial ecosystem engineers have been hypothesized as affecting fire behaviour through altering litter accumulation and breakdown, however, little evidence of this has been shown to date. Fire is one of the major ecological processes affecting biodiversity globally. Australia has seen the extinction of 29 of 315 terrestrial mammal species in the last 200 years and several of these species were ecosystem engineers whose fossorial actions may increase the rate of leaf litter breakdown. Thus, their extinction may have altered the rate of litter accumulation and therefore fire ignition potential and rate of spread. We tested whether a reduction in leaf litter was associated with sites where mammalian ecosystem engineers had been reintroduced using a pair-wise, cross-fence comparison at sites spanning the Australian continent. At Scotia (New South Wales), Karakamia (Western Australia) and Yookamurra (South Australia) sanctuaries, leaf litter mass (À24%) and percentage cover of leaf litter (À3%) were significantly lower where reintroduced ecosystem engineers occurred compared to where they were absent, and fire behaviour modelling illustrated this has substantial impacts on flame height and rate of spread. This result has major implications for fire behaviour and management globally wherever ecosystem engineers are now absent as the reduced leaf litter volumes where they occur will lead to decreased flame height and rate of fire spread. This illustrates the need to restore the full suite of biodiversity globally.

Our best hope of developing innovative methods to combat invasive species is likely to come from the study of high-profile invaders that have attracted intensive research not only into control, but also basic biology. Here we illustrate that point by reviewing current thinking about novel ways to control one of the world’s most well-studied invasions: that of the cane toad in Australia. Recently developed methods for population suppression include more effective traps based on the toad’s acoustic and pheromonal biology. New tools for containing spread include surveillance technologies (e.g., eDNA sampling and automated call detectors), as well as landscape-level barriers that exploit the toad’s vulnerability to desiccation—a strategy that could be significantly enhanced through the introduction of sedentary, range-core genotypes ahead of the invasion front. New methods to reduce the ecological impacts of toads include conditioned taste aversion in free-ranging predators, gene banking, and targeted gene flow. Lastly, recent advances in gene editing and gene drive technology hold the promise of modifying toad phenotypes in ways that may facilitate control or buffer impact. Synergies between these approaches hold great promise for novel and more effective means to combat the toad invasion and its consequent impacts on biodiversity.

Most ecological research on cane toads (Rhinella marina) has focused on invasive populations in Australia, ignoring other areas where toads have been introduced. We radio-tracked and spool-tracked 40 toads, from four populations on the island of Hawai’i. Toads moved extensively at night (mean 116 m, from spool-tracking) but returned to the same or a nearby retreat-site each day (from radio-tracking, mean distance between successive retreat sites 11 m; 0 m for 70% of records). Males followed straighter paths during nocturnal movements than did females. Because moist sites are scarce on the highly porous lava substrate, Hawai’ian toads depend on anthropogenic disturbance for shelter (e.g. beneath buildings), foraging (e.g. suburban lawns, golf courses) and breeding (artificial ponds). Foraging sites are further concentrated by a scarcity of flying insects (negating artificial lights as prey-attractors). Habitat use of toads shifted with time (at night, toads selected areas with less bare ground, canopy, understory and leaf-litter), and differed between sexes (females foraged in areas of bare ground with dense understory vegetation). Cane toads in Hawai’i thrive in scattered moist patches within a severely arid matrix, despite a scarcity of flying insects, testifying to the species’ ability to exploit anthropogenic disturbance.

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.

Summary1. Invasive species pose ecological threats in many areas, but attempts to control invaders by introducing other exotic species may cause further unanticipated problems. If we can use predators native to the introduced range to assist in control of the invader, the risks of collateral damage are lower. 2. In tropical Australia, high desiccation rates restrict newly-transformed (metamorph) cane toads Bufo marinus to the margins of waterbodies, rendering the metamorphs vulnerable to predatory ants (Iridomyrmex reburrus). By adding bait (catfood) to selected areas, we increased ant densities (and thus, toad mortality) more than fourfold. 3. Over 50% of attacks by ants in the field were immediately fatal to the metamorph toads, and most 'escapee' toads (88%) died of their injuries within 24 h after the attack. 4. When we increased ant densities by artificial baiting, 98% of metamorph toads were encountered, and 84% attacked, within the two-minute observation period. Collateral damage to native fauna appears to be low, but warrants closer examination. 5. Synthesis and applications. Manipulating the foraging locations of native predatory ants can substantially increase their off-take of invasive toads. More generally, vulnerabilities of invasive species to predators native to the introduced range may facilitate control of invader numbers with little collateral damage to the rest of the fauna.

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Summary 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.