The extra-limital spread of an invasive species via 'stowaway' dispersal: toad to nowhere?

The mechanisms by which invasive species spread through new areas can influence the spatial scale of their impact. Although previous research has focused on 'natural' dispersal rates following initial introductions, human-aided inadvertent dispersal by 'stowaways' on commercial and domestic transport is thought to be a major contributor to long-distance dispersal. Few data exist to support this assumption. Cane toads Bufo marinus were introduced to north-eastern Australia in 1935, and have since dispersed rapidly through the tropics. Based on information accumulated by community groups in Sydney, 400 km south of the cane toads' current Australian distribution, we document high rates of translocation (at least 50 toads arriving in Sydney per year). Most toads were translocated on commercial truck transport carrying landscaping and building materials from the current range of the cane toads in New South Wales and Queensland, and resulted in highly clumped locations of toad arrival reflecting primary truck transport destinations. Most introductions involved single toads (68 of 102 translocation events), but some introductions involved two to 19 animals. Adults of both sexes were represented equally but juveniles were rarely detected. High rates of translocation of adult toads of both sexes suggest that the eventual distribution of cane toads in Australia may be limited by the animals' bioclimatic tolerances rather than by an inability to reach suitable habitats, even in areas far distant from the toads' current range.     

Stimulation of ammonium ion excretion in the toad urinary bladder by an extract of human urine

It is well known that ammonium ion excretion is increased during metabolic acidosis in mammals. The purpose of this study was to determine whether we could isolate from human urine during metabolic acidosis a factor that would stimulate NH₄⁺ and/or H⁺ excretion in toad urinary bladder. Extracts of urine from six human subjects collected during NH₄Cl-induced acidosis were prepared. These extracts were tested for their effect on NH₄⁺ excretion in hemibladders mounted between plastic chambers. The extracts significantly increased NH₄⁺ excretion in the toad urinary bladder. We found no effect on H⁺ excretion by these extracts. This ammoniuretic activity was not present in the urine when the same individuals were in metabolic alkalosis. We conclude that during metabolic acidosis a humoral factor is present which stimulates the excretion of NH₄⁺. The factor could act as a permease in the bladder cell or as a stimulator of an NH₄⁺ transport system.     

Do invasive cane toads (Chaunus marinus) compete with Australian frogs (Cyclorana australis)?

Abstract Despite widespread concern about the ecological impacts of invasive species, mechanisms of impact remain poorly understood. Cane toads (Chaunus [Bufo] marinus) were introduced to Queensland in 1935, and have now spread across much of tropical Australia. One plausible impact of toad invasion concerns competition between toads and native frogs, but there has been no previous experimental evaluation of this possibility. We examined interactions between toads and a morphologically similar species of native frog (Cyclorana australis) by manipulating toad and frog densities within large outdoor enclosures beside a floodplain in the wet‐dry tropics of the Northern Territory. Toads differed from frogs significantly in dietary composition and feeding rates, even in comparisons controlling for body‐size differences between these two taxa. Perhaps reflecting the abundant insect biomass, manipulating anuran densities or the presence of the putatively competing species did not influence food intake or dietary composition. However, the presence of toads suppressed activity levels of native frogs. The degree to which the invasion of cane toads influences attributes such as the activity levels, food intake and dietary composition of native frogs warrants further study, but our study suggests that competitive effects are likely to be minor compared with other pathways (such as direct poisoning during ingestion attempts) by which toads can affect frog populations.     

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.