Author: Gary Jackson

Toxic cane toads pose threat to people, pets

The cane toad is one of more than 37,000 alien species introduced – deliberately or inadvertently – by humans into environments around the world, according to a landmark United Nations report released this week. Cane toads cannot survive for more than three days during periods of dry conditions that normally prevail in arid Australia without access to water. However, dams created by pastoralists that serve as reservoirs for bore-water, have dramatically increased the availability of water in naturally parched semi-arid landscapes. Dams, therefore, provide a network of refuge habitats or “invasion hubs” in which toads congregate during dry seasons and visit on an almost daily basis. The poisonous toads kill both pets and native species when animals bite, lick, or eat them, and they outcompete native species for resources like food and breeding habitat. The map on the right shows the predicted range of toads in semi-arid and arid Australia based on their physiological limits (grey).

For example, an invasive woody shrub can alter thermal and light levels on the ground beneath it, as well as reducing nutrient availability and salinity in the soil (Cox 2004; Benkman et al. 2008; Gonzalez et al. 2008). In marine benthic and terrestrial plant communities, invaders may take up open space, thus restricting settlement opportunities. Any such shift in resource availability might impose selection on habitat selection and use by native species.

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Foraging within areas that contain high densities of toads is a consistent trait of taxa impacted by these invasive anurans [57]. Especially near the end of the dry-season, when waterbodies shrink, both yellow-spotted monitors and cane toads are concentrated in the same places within an increasingly parched landscape, particularly at the invasion front where toad density is highest. Such encounters may be dangerous for goannas, even for individuals that have learnt taste aversion to toads [44]. In keeping with that interpretation, Llewelyn et al. [21] reported a single case of a yellow-spotted monitor from a long-colonised population eating a cane toad; and we witnessed a similar case (a yellow-spotted monitor consuming a road-killed toad) in the same region during the present study.

More generally, multiple anthropogenically-induced changes to natural ecosystems may have synergistic effects, intensifying the impacts beyond that expected from either threat in isolation. To assess correlates (and thus, potential determinants) of the relative abundance of varanids, we ran GLMMs with all combinations of factors within the “MuMIn” package [52] on the overall datasets to rank alternative models. Our models included (i) time since toad invasion (categorical variable with four levels; uninvaded, recently invaded, mid-term invaded, and long-term invaded) to test the effect of invasion history on the number of lace monitors or yellow-spotted monitors encountered per day. Plausibly, our counts of varanid lizards also might be affected by biotic and abiotic factors that could influence the abundance of animals, or modify our ability to detect the animals even if they are present. As an introduced species spreads outside its native range, it initiates a complex array of evolutionary processes that can produce clear effects over a timeframe of years or decades.

Study

Dung beetles were introduced to Australia to improve nutrient cycling in rangeland ecosystems and disrupt the life cycles of livestock parasites. However, predation by cane toads has reduced the important ecosystem services that dung beetles provide for the environment and pastoral industry. At first sight, it would seem that local abiotic conditions pose a challenge to the invader (for which they are novel) but not the local taxa (which have evolved in those circumstances).

Those temporal shifts make it difficult to attribute specific faunal shifts to cane toad invasion. For example, some declines in predator populations coincident with toad invasion in tropical Australia were caused by stochastic weather events, not toads (Brown et al. 2011). The importance of invader-driven catastrophe for conservation issues has distracted attention from the possibility that invasion benefits a subset of native taxa (King et al. 2006; Hagman and Shine 2007). For example, the invader may provide an additional food source for predators and additional hosts for parasites. The net effect of an invasive species on any given native taxon will be the sum total of negative and positive effects.