After the crash: How do predators adjust following the invasion of a novel toxic prey type?

The ability of a native predator to adjust to a dangerously toxic invasive species is key to avoiding an ongoing suppression of the predator's population and the trophic cascade of effects that can result. Many species of anurophagous predators have suffered population declines due to the cane toad's (Rhinella marina: Bufonidae) invasion of Australia; these predators can be fatally poisoned from attempting to consume the toxic toad. We studied one such toad‐vulnerable predator, the yellow‐spotted monitor (Varanus panoptes: Varanidae), testing whether changes to the predator's feeding behaviour could explain how the species persists following toad invasion. Wild, free‐roaming lizards from (1) toad‐naïve and (2) toad‐exposed populations were offered non‐toxic native frogs and slightly toxic cane toads (with parotoid glands removed) in standardized feeding trials. Toad‐naïve lizards readily consumed both frogs and toads, with some lizards displaying overt signs of illness after consuming toads. In contrast, lizards from toad‐exposed populations consumed frogs but avoided toads. Repeated encounters with toads did not modify feeding responses by lizards from the toad‐naïve populations, suggesting that aversion learning is limited (but may nonetheless occur). Our results suggest that this vulnerable predator can adjust to toad invasion by developing an aversion to feeding on the toxic invader, but it remains unclear as to whether the lizard's toad‐aversion arises via adaptation or learning.     

The impact of invasive cane toads on native wildlife in southern Australia

Commonly, invaders have different impacts in different places. The spread of cane toads (Rhinella marina: Bufonidae) has been devastating for native fauna in tropical Australia, but the toads' impact remains unstudied in temperate‐zone Australia. We surveyed habitat characteristics and fauna in campgrounds along the central eastern coast of Australia, in eight sites that have been colonized by cane toads and another eight that have not. The presence of cane toads was associated with lower faunal abundance and species richness, and a difference in species composition. Populations of three species of large lizards (land mullets Bellatorias major, eastern water dragons Intellagama lesueurii, and lace monitors Varanus varius) and a snake (red‐bellied blacksnake Pseudechis porphyriacus) were lower (by 84 to 100%) in areas with toads. The scarcity of scavenging lace monitors in toad‐invaded areas translated into a 52% decrease in rates of carrion removal (based on camera traps at bait stations) and an increase (by 61%) in numbers of brush turkeys (Alectura lathami). The invasion of cane toads through temperate‐zone Australia appears to have reduced populations of at least four anurophagous predators, facilitated other taxa, and decreased rates of scavenging. Our data identify a paradox: The impacts of cane toads are at least as devastating in southern Australia as in the tropics, yet we know far more about toad invasion in the sparsely populated wilderness areas of tropical Australia than in the densely populated southeastern seaboard.     

Behavioural responses of reptile predators to invasive cane toads in tropical Australia

The ecological impact of an invasive species can depend on the behavioural responses of native fauna to the invader. For example, the greatest risk posed by invasive cane toads (Rhinella marina Bufonidae) in tropical Australia is lethal poisoning of predators that attempt to eat a toad; and thus, a predator's response to a toad determines its vulnerability. We conducted standardized laboratory trials on recently captured (toad‐naïve) predatory snakes and lizards, in advance of the toad invasion front as it progressed through tropical Australia. Responses to a live edible‐sized toad differed strongly among squamate species. We recorded attacks (and hence, predator mortality) in scincid, agamid and varanid lizards, and in elapid, colubrid and pythonid snakes. Larger‐bodied predators were at greater risk, and some groups (elapid snakes and varanid lizards) were especially vulnerable. However, feeding responses differed among species within families and within genera. Some taxa (notably, many scincid and agamid lizards) do not attack toads; and many colubrid snakes either do not consume toads, or are physiologically resistant to the toad's toxins. Intraspecific variation in responses means that even in taxa that apparently are unaffected by toad invasion at the population level, some individual predators nonetheless may be fatally poisoned by invasive cane toads.     

Behavioural responses of carnivorous marsupials (Planigale maculata) to toxic invasive cane toads (Bufo marinus)

The arrival of a toxic invasive species may impose selection on local predators to avoid consuming it. Feeding responses may be modified via evolutionary changes to behaviour, or via phenotypic plasticity (e.g. learning, taste aversion). The recent arrival of cane toads (Bufo marinus) in the Northern Territory of Australia induced rapid aversion learning in a predatory marsupial (the common planigale, Planigale maculata). Here, we examine the responses of planigales to cane toads in north‐eastern Queensland, where they have been sympatric for over 60 years, to investigate whether planigale responses to cane toads have been modified by long‐term exposure. Responses to toads were broadly similar to those documented for toad‐naïve predators. Most Queensland planigales seized (21 of 22) and partially consumed (11 of 22) the first toad they were offered, but were likely to ignore toads in subsequent trials. However, unlike their toad‐naïve conspecifics from the Northern Territory, the Queensland planigales all survived ingestion of toad tissue without overt ill effects and continued to attack toads in a substantial proportion of subsequent trials. Our data suggest that (i) learning by these small predators is sufficiently rapid and effective that selection on behaviour has been weak; and (ii) physiological tolerance to toad toxins may be higher in planigales after 60 years (approximately 60 generations) of exposure to this toxic prey.     

School for Skinks: Can Conditioned Taste Aversion Enable Bluetongue Lizards (Tiliqua scincoides) to Avoid Toxic Cane Toads (Rhinella marina) as Prey?

The invasion of cane toads (Rhinella marina) through Australia imperils native predators that are killed if they consume these toxic anurans. The magnitude of impact depends upon the predators’ capacity for aversion learning: toad impact is lower if predators can learn not to attack toads. In laboratory trials, we assessed whether bluetongue lizards (Tiliqua scincoides) – a species under severe threat from toads – are capable of learned taste aversion and whether we can facilitate that learning by exposing lizards to toad tissue combined with a nausea‐inducing chemical (lithium chloride). Captive bluetongues rapidly learned to avoid the ‘unpalatable’ food. Taste aversion also developed (albeit less strongly) in response to meals of minced cane toad alone. Our data suggest that taste aversion learning may help bluetongue lizards survive the onslaught of cane toads, but that many encounters will be fatal because the toxin content of toads is so high relative to lizard tolerance of those toxins. Thus, baiting with nausea‐inducing (but non‐lethal) toad products might provide a feasible management option to reduce the impact of cane toad invasion on these native predators.     

A trophic cascade initiated by an invasive vertebrate alters the structure of native reptile communities

Invasive vertebrates are frequently reported to have catastrophic effects on the populations of species which they directly impact. It follows then, that if invaders exert strong suppressive effects on some species then other species will indirectly benefit due to ecological release from interactions with directly impacted species. However, evidence that invasive vertebrates trigger such trophic cascades and alter community structure in terrestrial ecosystems remains rare. Here, we ask how the cane toad, a vertebrate invader that is toxic to many of Australia's vertebrate predators, influences lizard assemblages in a semi‐arid rangeland. In our study area, the density of cane toads is influenced by the availability of water accessible to toads. We compared an index of the abundance of sand goannas, a large predatory lizard that is susceptible to poisoning by cane toads and the abundances of four lizard families preyed upon by goannas (skinks, pygopods, agamid lizards and geckos) in areas where cane toads were common or rare. Consistent with the idea that suppression of sand goannas by cane toads initiates a trophic cascade, goanna activity was lower and small lizards were more abundant where toads were common. The hypothesis that suppression of sand goannas by cane toads triggers a trophic cascade was further supported by our findings that small terrestrial lizards that are frequently preyed upon by goannas were more affected by toad abundance than arboreal geckos, which are rarely consumed by goannas. Furthermore, the abundance of at least one genus of terrestrial skinks benefitted from allogenic ecosystem engineering by goannas where toads were rare. Overall, our study provides evidence that the invasion of ecosystems by non‐native species can have important effects on the structure and integrity of native communities extending beyond their often most obvious and frequently documented direct ecological effects.     

A native dasyurid predator (common planigale,Planigale maculata) rapidly learns to avoid a toxic invader

Abstract Interactions between invasive species and native fauna afford a unique opportunity to examine interspecific encounters as they first occur, without the complications introduced by coevolution. In northern Australia, the continuing invasion of the highly toxic cane toad Bufo marinus poses a threat to many frog‐eating predators. Can predators learn to distinguish the novel toxic prey item from native prey (and thus, avoid being poisoned), or are longer‐term genetically based changes to attack behaviour needed before predators can coexist with toads? To predict the short‐term impact of cane toads on native predators, we need to know the proportion of individuals that will attack toads, the proportion surviving the encounter, and whether surviving predators learn to avoid toads. We quantified these traits in a dasyurid (common planigale, Planigale maculata) that inhabits tropical floodplains across northern Australia. Although 90% of naïve planigales attacked cane toads, 83% of these animals survived because they either rejected the toad unharmed, or killed and consumed the prey snout‐first (thereby avoiding the toxin‐laden parotoid glands). Most planigales showed one‐trial learning and subsequently refused to attack cane toads for long time periods (up to 28 days). Toad‐exposed planigales also avoided native frogs for up to 9 days, thereby providing an immediate benefit to native anurans. However, the predators gradually learnt to use chemical cues to discriminate between frogs and toads. Collectively, our results suggest that generalist predators can learn to distinguish and avoid novel toxic prey very rapidly – and hence, that small dasyurid predators can rapidly adapt to the cane toad invasion. Indeed, it may be feasible to teach especially vulnerable predators to avoid cane toads before the toads invade, by deploying low‐toxicity baits that stimulate taste‐aversion learning.     

How do goannas find sea turtle nests?

Squamate reptiles rely heavily on visual and chemical cues to detect their prey, so we expected yellow‐spotted goannas (Varanus panoptes) which are predators of sea turtle nests on mainland beaches in northern Australia would use these cues to find sea turtle nests. Ghost crabs (Ocypode ceratophthalmus and Ocypode cordimanus) are also common on Australian sea turtle nesting beaches and frequently burrow into sea turtle nests. However, the potential for ghost crab burrowing activity at sea turtle nests to signal the location of a nest to goannas has not been investigated. Here, we used camera traps and presence of tracks at nests to record goanna activity around selected nests during the incubation period and 10 days after hatchling turtles emerged from their nests. We also recorded the number of ghost crab burrows around nests to evaluate ghost crab activity. Our results indicated that nest discovery by goannas was independent of nest age, but that the nest visitation rate of goannas and crabs increased significantly after a nest had been opened by a goanna or after hatchlings had emerged from the nest. There was no apparent connection between ghost crab burrows into a nest and the likelihood of that nest being predated by goannas.     

Adaptation or preadaptation: why are keelback snakes (<Emphasis Type="Italic">Tropidonophis mairii</Emphasis>) less vulnerable to invasive cane toads (<Emphasis Type="Italic">Bufo marinus</Emphasis>) than are other Australian snakes?

Biological invasions can expose native predators to novel prey which may be less nutritious or detrimental to predators. The introduction and subsequent spread of cane toads (Bufo marinus) through Australia has killed many anuran-eating snakes unable to survive the toad’s toxins. However, one native species, the keelback snake (Tropidonophis mairii), is relatively resistant to toad toxins and remains common in toad-infested areas. Is the keelback’s ability to coexist with toads a function of its ancestral Asian origins, or a consequence of rapid adaptation since cane toads arrived in Australia? And does the snake’s feeding preference for frogs rather than toads reflect an innate or learned behaviour? We compared keelback populations long sympatric with toads with a population that has encountered toads only recently. Unlike toad-vulnerable snake species, sympatry with toads has not affected keelback toxin tolerances or feeding responses: T. mairii from toad-sympatric and toad-naïve populations show a similar sensitivity to toad toxin, and a similar innate preference for frogs rather than toads. Feeding responses of neonatal keelbacks demonstrate that learning plays little or no role in the snake’s aversion to toads. Thus, behavioural aversion to B. marinus as prey, and physiological tolerance to toad toxins are pre-existing innate characteristics of Australian keelbacks rather than adaptations to the cane toad’s invasion of Australia. Such traits were most likely inherited from ancestral keelbacks that adapted to the presence of bufonids in Asia. Our results suggest that the impact of invasive species on native taxa may be strongly influenced by the biogeographic histories of the species involved.     

Does intraspecific niche partitioning in a native predator influence its response to an invasion by a toxic prey species?

Abstract The introduced and highly toxic cane toad (Bufo marinus) is rapidly spreading across northern Australia where it may affect populations of large terrestrial vertebrate predators. The ecological impact of cane toads will depend upon the diets, foraging modes and habitat use of native predators, and their feeding responses to cane toads. However, intraspecific niche partitioning may influence the degree of vulnerability of predators to toxic prey, as well as the time course of the impact of alien invaders on native species. We studied the diet of the northern death adder Acanthophis praelongus and their feeding responses to cane toads. In the laboratory, death adders from all size classes and sexes readily consumed frogs and cane toads. Diets of free ranging A. praelongus from the Adelaide River floodplain were more heterogeneous. Juvenile snakes ate mainly frogs (39% of prey items) and small scincid lizards (43%). Both sexes displayed an ontogenetic dietary shift from lizards to mammals, but adult males fed on frogs (49%) and mammals (39%) whereas adult females (which grew larger than males) fed mainly on mammals (91%) and occasionally, frogs (9%). Feeding rates and body condition of adult snakes varied temporally and tracked fluctuations in prey availability. These results suggest that cane toads may negatively affect populations of northern death adders in the Darwin region. However, we predict that different size and sex classes of A. praelongus will experience differential mortality rates over different timescales. The initial invasion of large toads may affect adult males, but juveniles may be unaffected until juvenile toads appear the following year, and major affects on adult female death adders may be delayed until annual rainfall fluctuations reduce the availability of alternative (rodent) prey.     

Behavioural responses of native predators to an invasive toxic prey species

One important impact of invasive species may be to modify the behaviour of native taxa. For example, the invasion of highly toxic cane toads (Bufo marinus) kills many anurophagous native predators, but other predators learn to recognize and avoid the toxic invader. We exposed native fish (northern trout gudgeons, Mogurnda mogurnda) and Dahl's aquatic frogs (Litoria dahlii) to cane toad tadpoles, then monitored the predator's responses during subsequent trials. Both the frogs and fish initially attacked toad tadpoles, but rapidly learned not to do so. Fish and adult frogs retained their aversion for at least a week, whereas recently metamorphosed frogs did not. Clearly, the spread of cane toads through tropical Australia can modify feeding responses of native aquatic predators. For predators capable of rapid avoidance learning, the primary impact of cane toads may be on foraging behaviour rather than mortality.     

Toad’s tongue for breakfast: exploitation of a novel prey type,the invasive cane toad,by scavenging raptors in tropical Australia

Although interest in the ecological impacts of invasive species has largely focused on negative effects, some native taxa may benefit from invader arrival. In tropical Australia, invasive cane toads (Bufo marinus) have fatally poisoned many native predators (e.g., marsupials, crocodiles, lizards) that attempt to ingest the toxic anurans, but birds appear to be more resistant to toad toxins. We quantified offtake of dead (road-killed) cane toads by raptors (black kites (Milvus migrans) and whistling kites (Haliastur sphenurus)) at a site near Darwin, in the Australian wet-dry tropics. Raptors readily took dead toads, especially small ones, although native frogs were preferred to toads if available. More carcasses were removed in the dry season than the wet season, perhaps reflecting seasonal availability of alternative prey. Raptors appeared to recognize and avoid bufotoxins, and typically removed and consumed only the toads’ tongues (thereby minimizing toxin uptake). The invasion of cane toads thus constitutes a novel prey type for scavenging raptors, rather than (as is the case for many other native predators) a threat to population viability.     

Invasive cane toads might initiate cascades of direct and indirect effects in a terrestrial ecosystem

Understanding the impacts that invasive vertebrates have on terrestrial ecosystems extends primarily to invaders’ impacts on species with which they interact directly through mechanisms such as predation, competition and habitat modification. In addition to direct effects, invaders can also initiate ecological cascades via indirect population level effects on species with which they do not directly interact. However, evidence that invasive vertebrates initiate ecological cascades in terrestrial ecosystems remains scarce. Here, we ask whether the invasion of the cane toad, a vertebrate invader that is toxic to many of Australia’s vertebrate predators, has induced ecological cascades in a semi-arid rangeland. We compared activity of a large predatory lizard, the sand-goanna, and abundances of smaller lizards preyed upon by goannas in areas of high toad activity near toads’ dry season refuges and areas of low toad activity distant from toads’ dry season refuges. Consistent with the hypothesis that toad invasion has led to declines of native predators susceptible to poisoning, goanna activity was lower in areas of high toad activity. Consistent with the hypothesis that toad-induced goanna decline lead to increases in abundance the prey of goannas, smaller lizards were more abundant in areas of high toad activity. Structural equation modelling showed a positive correlation between goanna activity and distance from dry season refuge habitats used by toads. The abundances of small lizards was correlated negatively with goanna activity and distance from dry season refuges of toads. Our findings provide support for the notion that invasions by terrestrial vertebrates can trigger ecological cascades.     

Cane toads a threat to West Indian wildlife: mortality of Jamaican boas attributable to toad ingestion

The notorious “cane toad” (Bufo marinus) is considered to be one of the 100 worst invasive species in the world. A native of South and Central America, Mexico, and the Rio Grande Valley of the United States, this large toad was intentionally introduced to islands in the Caribbean, and subsequently throughout the southern Pacific, as a biological control agent to combat sugar cane pests. Unfortunately, the primary result of those introductions has been deleterious impacts on native biotas, primarily through competition and predation. More recently, the cane toad has devastated populations of amphibian-eating predators in Australia, through the ingestion of this highly toxic anuran. Elsewhere, however, the impact of the toad on native predators has not been documented. Here we report the first evidence that the cane toad is impacting native predators in other geographic regions. Specifically, we document death due to cane toad poisoning in the endemic and threatened Jamaican boa (Epicrates subflavus). To our knowledge, this is the first report of cane toads causing mortality in naturally occurring predators outside of Australia. Like all members of the genus, B. marinus secretes a powerful bufogenin toxin, which is often fatal if ingested by naïve species that have not co-evolved with Bufo species. Our results should therefore serve as a warning that other endemic predator species in the West Indies and elsewhere may be at risk. Thus, efforts to control the population growth and spread of cane toads may be of even greater conservation concern than previously recognized.     

Foraging responses of predators to novel toxic prey: effects of predator learning and relative prey abundance

Learning to avoid toxic prey items may aid native predators to survive the invasion of highly toxic species, such as cane toads Bufo marinus in tropical Australia. If the predators’ initial aversion is generalized, native prey that resemble the toxic invader may receive a benefit through accidental mimicry. What ecological factors influence the acquisition of learned avoidance (and hence, the impact of invasion on both predators and native prey)? We conducted laboratory experiments to evaluate how the relative abundance of toad tadpoles compared to palatable native tadpoles (Litoria caerulea and L. rubella) affected the ability of native aquatic predators to discriminate between these two prey types. Both fish (northern trout gudgeon, Mogurnda mogurnda) and frogs (Dahl's aquatic frog, Litoria dahlii) learned to discriminate between toads and frogs within an eight‐day period. Higher abundance of toad tadpoles relative to frog tadpoles enhanced rates of predator learning, and thus reduced predation on toads and increased predation on native tadpoles. In the field, spatial and temporal variation in the relative abundance of cane toads compared to native frogs may influence the rates at which these novel toxic items are deleted from predator diets, and the duration of predator protection afforded to natives that resemble the invader.     

Something different for dinner? Responses of a native Australian predator (the keelback snake) to an invasive prey species (the cane toad)

Predictions from foraging theory suggest that the probability a native predator will incorporate a novel type of prey (such as an invasive species) into its diet depends upon the potential benefits (e.g., nutrient input) vs. costs (e.g., handling time) of ingesting it. Cane toads (Bufo marinus) were introduced to Australia in 1935 and are highly toxic to many frog-eating snakes, thus there was strong selection to delete toads from the diet of these species. What has happened, however, to the feeding responses of an Australian snake species that is able to consume toads without dying? Our field surveys in northeastern Queensland show that, despite their high tolerance to toad toxins (compared to other native snakes), keelbacks (Tropidonophis mairii) feed primarily on native frogs rather than cane toads. This pattern occurs because the snakes show active prey preferences; even under standardized conditions in the laboratory, snakes are more likely to consume frogs than toads. When they are force-fed, snakes frequently regurgitate toads but not frogs. Thus, despite the high availability of the abundant toads, these invasive anurans are largely avoided as prey. This probably occurs because consumption of toads, although not lethal to keelbacks, causes significant sublethal effects and confers little nutritional benefit. Hence, keelback populations are not threatened by toad invasion, but neither do the snakes benefit substantially from the availability of a new type of potential prey.     

Out of the frying pan: Reintroduction of toad‐smart northern quolls to southern Kakadu National Park

Invasive species are a leading cause of native biodiversity loss. In Australia, the toxic, invasive cane toad Rhinella marina has caused massive and widespread declines of northern quolls Dasyurus hallucatus. Quolls are fatally poisoned if they mistakenly prey on adult toads. To prevent the extinction of this native dasyurid from the Top End, an insurance population was set up in 2003 on two toad‐free islands in Arnhem Land. In 2015, quolls were collected from one of these islands (Astell) for reintroduction. We used conditioned taste aversion to render 22 of these toad‐naïve quolls toad averse. Seven quolls received no taste aversion training. The source island was also predator‐free, so all quolls received very basic predator‐aversion training. In an attempt to re‐establish the mainland population, we reintroduced these 29 northern quolls into Kakadu National Park in northern Australia where cane toads have been established for 13 years. The difference in survival between toad‐averse and toad‐naive quolls was immediately apparent. Toad‐naive quolls were almost all killed by toads within 3 days. Toad‐averse quolls, on the other hand, not only survived longer but also were recorded mating. Our predator training, however, was far less effective. Dingo predation accounted for a significant proportion of toad‐smart quoll mortality. In Kakadu, dingoes have been responsible for high levels of quoll predation in the past and reintroduced animals are often vulnerable to predation‐mediated population extinction. Dingoes may also be more effective predators in fire degraded landscapes. Together, these factors could explain the extreme predation mortality that we witnessed. In addition, predator aversion may have been lost from the predator‐free island populations. These possibilities are not mutually exclusive but need to be investigated because they have clear bearing on the long‐term recovery of the endangered northern quoll.     

Spatial and temporal variation in the morphology (and thus, predicted impact) of an invasive species in Australia

The impact of an invasive species is unlikely to be uniform in space or time, due to variation in key traits of the invader (e.g. morphology, physiology, behaviour) as well as in resilience of the local ecosystem. The weak phylogeographic structure typical of an invasive population suggests that much of the variation in an invading taxon is likely to be generated by the environment and recent colonisation history. Here we describe effects of the environment and colonisation history on key morphological traits of an invader (the cane toad Bufo marinus ). These "key traits" (body size and relative toxicity) mediate the impact of toads on Australian native predators, which often die as a consequence of ingesting a fatal dose of toad toxin. Measurements of museum specimens collected over >60 yr from a wide area show that seasonal variation in toad body size (due to seasonal recruitment) effectively swamps much of the spatial variance in this trait. However, relative toxicity of toads showed strong spatial variation and little seasonal variation. Thus, the risk to a native predator ingesting a toad will vary on both spatial and temporal scales. For native predators capable of eating a wide range of toad sizes (e.g. quolls, varanid lizards), seasonal variation in overall toad size will be the most significant predictor of risk. In contrast, gape-limited predators restricted to a specific range of toad sizes (such as snakes) will be most strongly affected by the relative toxicity of toads. Gape-limited predators will thus experience strong spatial variation in risk from toad consumption.     

Impacts of eggs and tadpoles of the invasive cane toad (Bufo marinus) on aquatic predators in tropical Australia

Invasive species affect native ecosystems in a variety of ways, and the magnitude of impact may depend upon many factors. In an invading species such as the cane toad (Bufo marinus), the multiphasic life history creates a potential for impact to differ between life history stages. Previous research on the impact of cane toads in Australia has focused on fatal poisoning of predators that ingest terrestrial stages of the toad, but aquatic stages (eggs, larvae) are toxic also. We exposed nine native Australian fish species and one native Australian turtle species to the eggs and larvae of toads. Strong species differences were evident, both in palatability (propensity to attack the egg or larva), and in subsequent responses (e.g. taste and reject the item, vs. ingest it). Toad eggs were less likely than toad tadpoles to be attacked, but also less likely to be rejected before ingestion (probably because the non‐toxic jelly coat masks the presence of toxins in the ovum). As a result, predators were far more likely to be killed by ingesting toad eggs than toad tadpoles. Fortuitously, the spatial and temporal availability of toad egg masses restricts encounter rates with predators, so that overall ecological impact may be low despite the high vulnerability of a fish or turtle that encounters such an egg mass. Understanding such ontogenetic shifts in the nature of interactions and magnitude of impact is crucial if we are to understand the overall ecological impact of invasive species.     

Effects of an invasive species on refuge‐site selection by native fauna: The impact of cane toads on native frogs in the Australian tropics

Invasive species can induce shifts in habitat use by native taxa: either by modifying habitat availability, or by repelling or attracting native species to the vicinity of the invader. The ongoing invasion of cane toads (Rhinella marina) through tropical Australia might affect native frogs by affecting refuge‐site availability, because both frogs and toads frequently shelter by day in burrows. Our laboratory and field studies in the wet‐dry tropics show that native frogs of at least three species (Litoria tornieri, Litoria nasuta and Litoria dahlii) preferentially aggregate with conspecifics, and with (some) other species of native frogs. However, the frogs rarely aggregated with cane toads either in outdoor arenas or in standardized experimental burrows that we monitored in the field. The native frogs that we tested either avoided burrows containing cane toads (or cane toad scent) or else ignored the stimulus (i.e. treated such a burrow in the same way as they did an empty burrow). Native frogs selected a highly non‐random suite of burrows as diurnal retreat sites, whereas cane toads were less selective. Hence, even in the absence of toads, frogs do not use many of the burrows that are suitable for toads. The invasion of cane toads through tropical Australia is unlikely to have had a major impact on retreat‐site availability for native frogs.