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.

Do drivers intentionally target wildlife on roads?

Despite frequent reliance on surveys to document public attitudes towards conservation issues (such as invasive-species control), only rarely do researchers assess the validity of statements made by the public in response to such surveys. Therefore, how well responses match actual behaviour remains an open question. We conducted a survey asking drivers if they had seen and/or run over (intentionally or not) snakes, native frogs or invasive cane toads (Rhinella marina) on roads in the Northern Territory of Australia. To compare actual driver behaviour to the survey responses, we also carried out field experiments where we quantified the rates at which model snakes, frogs and toads (and controls) were run over on a rural highway. Our results show a discrepancy between survey responses and driver behaviour: for example, 25% of the people we surveyed indicated that they intentionally run over cane toads, yet field experiments showed that model toads were run over no more frequently than expected by chance, or than any other type of model.

Noisy neighbours at the frog pond: effects of invasive cane toads on the calling behaviour of native Australian frogs

Invasive species can disrupt the communication systems that native biota use for reproductive interactions. In tropical Australia, invasive cane toads (Rhinella marina) breed in many of the same waterbodies that are used by native frogs, and males of both the invader and the native taxa rely on vocal signals to attract mates. We conducted playback experiments to test the hypothesis that calls of toads may influence the calling behaviour of frogs (Limnodynastes convexiusculus and Litoria rothii). Male L. convexiusculus adjusted their calling rate and the variance in inter-call interval in response to a variety of sounds, including the calls of cane toads as well as those of other native frog species, and other anthropogenic noise, whereas L. rothii did not. Within the stimulus periods of playbacks, male L. convexiusculus called more intensely during long silent gaps than during calling blocks. Thus, males of one frog species reduced their calling rate, possibly to minimise energy expenditure during periods of acoustic interference generated by cane toads. In spite of such modifications, the number of overlapping calls (within stimulus periods) did not differ significantly from that expected by chance. In natural conditions, the calls of cane toads are continuous rather than episodic, leaving fewer gaps of silence that male frogs could exploit. Future work could usefully quantify the magnitude of temporal (e.g. diel and seasonal) and spatial overlap between calling by toads and by frogs and the impact of call-structure shifts on the ability of male frogs to attract receptive females.

Predation on a blotched bluetongue lizard (Tiliqua nigrolutea) by a highlands copperhead (Austrelaps ramsayi) in the Blue Mountains, Australia

This note documents a predation event on a juvenile Blotched Bluetongue Lizard (Tiliqua nigrolutea) by a Highlands Copperhead (Austrelaps ramsayi) in the Blue Mountains, Australia.  The diet of elapid snakes in the genus Austrelaps consists mainly of frogs and small skinks. Adults of the larger Tiliqua species may be too large for Austrelaps species to consume, and juveniles of these larger Tiliqua are possibly approaching the larger end of consumable prey items for Austrelaps.

Do green and golden bell frogs (Litoria aurea) occupy habitats with fungicidal properties?

The Green and Golden Bell frog Litoria aureo is in major decline in Australia, where its distribution is now confined mainly to the east coast of New South Wales (NSW). Infection by the newly emerged amphibian fungal pathogen Batrachochytrium dendrobatidis has been identified as one of the main threats affecting L. aurea. Surprisingly, some of the sites in NSW sustaining the largest populations of this species are industrial and urban habitats that are often disturbed and polluted, which could protect L. aurea from chytrid infection if pollution had fungicidal capacity.The aim of this study was to characterise the trace metal concentration of several L. aurea breeding sites in the Sydney and IIlawarra regions of NSW and to evaluate the fungicidal efficacy of the main "'ace metals identified. Selected L. aurea sites were sampled throughout the breeding season (September to February) to establish the concentration of trace metals in both surface sediment and waters. Physico-chemical parameters including pH and salinity were also measured. Of the trace metals identified, copper and zinc were consistently elevated across sites. Over 50% of sites exceeded the National Sediment Quality Guideline for both copper and zinc concentration, and over 90% of sites exceeded the National Water Quality Guideline for these metals. Consequently, we evaluated their effect on the growth and survival of a laboratory culture of B. dendrobatidis,These tests were performed in media containing dissolved metal concentrations of 0.02 - 0.65 mgL^-1 Cu and 0.24 - 5.0 mgL^-1 Zn. Growth rates were inferred by total fungal density in liquid culture (based on spectral absorbance measurements), final dry weight, and the density of zoospores in fungal cultures grown for 28 days. Both copper and zinc were found to reduce the growth and proliferation of B. dendrobatidis, but in a non-linear manner. This suggests that L. aurea may be gaining some protection from B. dendrobatidis infection at several of the sites examined.

Sperm competition and offspring viability at hybridization in Australian tree frogs, Litoria peronii and L. tyleri

Hybridization between closely related species often leads to reduced viability or fertility of offspring. Complete failure of hybrid offspring (post-zygotic hybrid incompatibilities) may have an important role in maintaining the integrity of reproductive barriers between closely related species. We show elsewhere that in Peron's tree frog, Litoria peronii, males more closely related to a female sire more offspring in sperm competition with a less related rival male. Observations of rare 'phenotypic intermediate' males between L. peronii and the closely related L. tyleri made us suggest that these relatedness effects on siring success may be because of selection arising from risks of costly hybridization between the two species. Here, we test this hypothesis in an extensive sperm competition experiment, which shows that there is no effect of species identity on probability of fertilization in sperm competition trials controlling for sperm concentration and sperm viability. Instead, there was a close agreement between a male's siring success in isolation with a female and his siring success with the same female in competition with a rival male regardless of species identity. Offspring viability and survival, however, were strongly influenced by species identity. Over a 14-day period, hybrid offspring suffered increasing mortality and developed more malformations and an obvious inability to swim and right themselves, leading to compromised probability of survival. Thus, hybridization in these sympatric tree frogs does not compromise fertilization but has a strong impact on offspring viability and opportunity for reinforcement selection on mate choice for conspecific partners.

Detecting the impact of invasive species on native fauna: Cane toads (Bufo marinus), frillneck lizards (Chlamydosaurus kingii) and the importance of spatial replication

An understanding of which native species are severely impacted by an anthropogenic change (such as the arrival of an invasive species) and which are not is critical to prioritizing conservation efforts. However, it is difficult to detect such impacts if the native taxa exhibit strong stochastic variations in abundance; a ‘natural’ population decline might be wrongly interpreted as an impact of the invader. Frillneck lizards (Chlamydosaurus kingii) are large iconic Australian agamids, and have been reported to decline following the invasion of toxic cane toads. We monitored three populations of the species in the savanna woodland of tropical Australia over a 7-year period bracketing toad arrival. One population crashed, one remained stable and one increased. Hence, studies on any single population might have inferred that cane toads have negative, negligible or positive effects on frillneck lizards. With the benefit of spatial replication, and in combination with observations of prey choice by captive lizards, our data suggest that invasive cane toads have had little or no effect on frillneck abundance.

Individual variation in thermal performance curves: swimming burst speed and jumping endurance in wild-caught tropical clawed frogs

The importance of studying individual variation in locomotor performance has long been recognized as it may determine the ability of an organism to escape from predators, catch prey or disperse. In ectotherms, locomotor performance is highly influenced by ambient temperature (Ta), yet several studies have showed that individual differences are usually retained across a Ta gradient. Less is known, however, about individual differences in thermal sensitivity of performance, despite the fact that it could represent adaptive sources of phenotypic variation and/or additional substrate for selection to act upon. We quantified swimming and jumping performance in 18 wild-caught tropical clawed frogs (Xenopus tropicalis) across a Ta gradient. Maximum swimming velocity and acceleration were not repeatable and individuals did not differ in how their swimming performance varied across Ta. By contrast, time and distance jumped until exhaustion were repeatable across the Ta gradient, indicating that individuals that perform best at a given Ta also perform best at another Ta. Moreover, thermal sensitivity of jumping endurance significantly differed among individuals, with individuals of high performance at low Ta displaying the highest sensitivity to Ta. Individual differences in terrestrial performance increased with decreasing Ta, which is opposite to results obtained in lizards at the inter-specific and among-individual levels. To verify the generality of these patterns, we need more studies on individual variation in thermal reaction norms for locomotor performance in lizards and frogs.

Invasive cane toad triggers chronic physiological stress and decreased reproductive success in an island endemic

Understanding the mechanisms that afford invasive species their ecological success as important agents of global change is key to addressing their biodiversity impacts. Species invasions that occur on small islands are especially detrimental and suggest that invaders intensify their ecological impacts by exploiting novel ecological functions. However, it remains unknown whether such strong impacts are also a consequence of an invader's indirect effect (e.g. causing physiological stress or reproductive failure) on island species. Therefore, it is valuable to quantify the physiological mechanisms through which invasive species can exert indirect effects on the performance, and ultimately the fitness of island endemics. In this study, we investigated whether the invasive cane toad (Rhinella marina) caused indirect competitive impacts on the endemic Fijian ground frog (Platymantis vitiana) on the small (60 ha) Viwa Island, Fiji. We used large (4 × 10 000 m2), natural and replicated enclosures to monitor ground frog stress hormone levels, reproductive hormone cycle, body condition, breeding and survival in the presence/absence of the cane toad. We conducted monthly sampling to analyse annual patterns in testosterone for males, estradiol and progesterone for females, corticosterone for both sexes and body condition of ground frogs in replicated enclosures or natural habitats with high/low cane toad densities. We also measured survival and reproductive success of ground frogs in enclosures. Results showed that in both enclosures and natural habitats with high cane toad densities, ground frogs had a significant reduction in body condition, increased urinary corticosterone metabolites and suppressed sex steroid metabolites. Most importantly, annual field surveys showed significant reduction in ground frog reproductive success (fewer eggs were laid in enclosures with toads present); however, survival was not severely reduced. Our study clearly demonstrated that on small islands, invasive species may exploit broader ecological roles with strong indirect effects that amplify their impacts beyond those seen on continents. Overall, the effects of cane toad competition had the capacity to strongly reduce ground frog reproductive success. We strongly advocate management actions that either minimize invasion or limit the strength of invasive-native species interactions (e.g. through habitat conservation) to prevent further extinctions on islands.

Cost and feasibility of a barrier to halt the spread of invasive cane toads in arid Australia: incorporating expert knowledge into model-based decision-making

1. Active engagement with practitioners is a crucial component of model-based decision-making in conservation management; it can assist with data acquisition, improve models and help narrow the 'knowing-doing' gap.
2. We worked with practitioners of one of the worst invasive species in Australia, the cane toad Rhinella marina, to revise a model that estimates the effectiveness of landscape barriers to contain spread. The original model predicted that the invasion could be contained by managing artificial watering points on pastoral properties, but was initially met with scepticism by practitioners, in part due to a lack of engagement during model development.
3. We held a workshop with practitioners and experts in cane toad biology. Using structured decision-making, we elicited concerns about the original model, revised its structure, updated relevant input data, added an economic component and found the most cost-effective location for a barrier across a range of fixed budgets and management scenarios. We then conducted scenario analyses to test the sensitivity of management decisions to model revisions.
4. We found that toad spread could be contained for all of the scenarios tested. Our modelling suggests a barrier could cost $4·5 M (2015 AUD) over 50 years for the most likely landscape scenario. The incorporation of practitioner knowledge into the model was crucial. As well as improving engagement, when we incorporated practitioner concerns (particularly regarding the effects of irrigation and dwellings on toad spread), we found a different location for the optimal barrier compared to a previously published study (Tingley et al. 2013).
5. Synthesis and applications. Through engagement with practitioners, we turned an academic modelling exercise into a decision-support tool that integrated local information, and considered more realistic scenarios and constraints. Active engagement with practitioners led to productive revisions of a model that estimates the effectiveness of a landscape barrier to contain spread of the invasive cane toad R. marina. Benefits also include greater confidence in model predictions, improving our assessment of the cost and feasibility of containing the spread of toads.

Influence of temperature on evaporative water loss and cutaneous resistance to water vapour diffusion in the orange-thighed frog (Litoria xanthomera)

We evaluated the effect of ambient temperatures between 25 and 43°C on the rate of evaporative water loss (EWL) in eight adult Litoria xanthomera (average body mass = 7.3 ± 0.6 g). Frogs were placed in a cylindrical chamber that permitted them to fully conceal their ventral surfaces using a water-conserving posture. Their EWL was 7.1 ± 0.7 mg g^–1 h^–1 at 25°C and reached 28.0 ± 2.5 mg g^–1 h^–1 at 43°C. Agar replicas of the frogs were used to evaluate boundary-layer resistances associated with the EWL measurements and, thus, to permit evaluation of cutaneous resistance to vapour diffusion (r_c) in live frogs. The r_c of L. xanthomera was stable over the temperature range of 25–35°C, averaging about 28 s cm^–1, and then declined stepwise with ambient temperatures above 37°C. The highest r_c recorded for each individual over the range of temperatures studied averaged 32.0 ± 1.2 s cm^–1. The thermolabile nature of r_c demonstrates a well developed thermoregulatory control of EWL in this species, a trait very similar in pattern and extent to that previously measured in the closely related Litoria chloris.

Movement patterns of southern bell frogs (Litoria raniformis) in response to flooding

Within the semiarid regions of New South Wales, Australia, the endangered southern bell frog (Litoria raniformis) occupies a landscape that is characterised by unpredictable rainfall and periodic flooding. Limited knowledge of the movement and habitat-occupancy patterns of this species in response to flood events has hampered conservation efforts. We used radio-tracking to assess changes in movement patterns and habitat occupancy of L. raniformis (n = 40) over three different periods (November, January and April/May) that coincided with the flooding, full capacity and subsequent drying of waterbodies within an irrigation landscape. We assessed (1) the use of permanent and ephemeral habitats in response to flooding and drying and (2) distances moved, turning angles and dispersion of frogs during wetland flooding, full capacity and drying. Individuals remained in permanent waterbodies in November but had abandoned these areas in favour of flooded ephemeral waterbodies by January. As the ephemeral waterbodies dried, radio-tracked individuals moved back into permanent waterbodies. The movement patterns of radio-tracked individuals were significantly different in the three radio-tracking periods, but did not differ significantly between sexes. Individuals moved significantly greater distances over 24 h, in straighter lines and movements were more dispersed while they occupied ephemeral waterbodies during January than when they occupied permanent waterbodies during November and April/May. Local weather conditions did not influence movement patterns when all three tracking periods were modelled together using a single linear stepwise regression. The dynamic distribution of habitat patches over space and time, combined with changing patterns of resource utilisation and movement of L. raniformis, highlights the importance of incorporating both permanent and ephemeral habitat patches into conservation plans. Reductions in flood frequency and extent of ephemeral wetlands due to modified flooding regimes have the capacity to limit dispersal of this species, even when permanent waterbodies remain unchanged.

Investigation into the design and development of the Toad Interactive Multimedia Program (TIMMP) and its use by first year biology undergraduates learning dissection procedures and in situ functional anatomy

Investigates how first year biology undergraduates learn dissection when they have access to such interactive multimedia technology. Related design and developmental factors were evaluated in relation to how they helped students learn the content in the multimedia program. Through the use of interactive technology students can learn at their own pace without the need for actual animal dissection.

The disappearing mammal fauna of northern Australia : context, cause, and response

This article provides a context to, attempts an explanation for, and proposes a response to the recent demonstration of rapid and severe decline of the native mammal fauna of Kakadu National Park. This decline is consistent with, but might be more accentuated than, declines reported elsewhere in northern Australia; however, such a comparison is constrained by the sparse information base across this region. Disconcertingly, the decline has similarities with the earlier phase of mammal extinctions that occurred elsewhere in Australia. We considered four proximate factors (individually or interactively) that might be driving the observed decline: habitat change, predation (by feral cats), poisoning (by invading cane toads), and novel disease. No single factor readily explains the current decline. The current rapid decline of mammals in Kakadu National Park and northern Australia suggests that the fate of biodiversity globally might be even bleaker than evident in recent reviews, and that the establishment of conservation reserves alone is insufficient to maintain biodiversity. This latter conclusion is not new; but the results reported here further stress the need to manage reserves far more intensively, purposefully, and effectively, and to audit regularly their biodiversity conservation performance.

The power of myth : the (non) impact of invasive cane toads (Bufo marinus) on domestic chickens (Gallus gallus)

Myths about invasive species are widespread in the general community, even when contrary to scientific evidence. Public revulsion against invasive cane toads (Bufo marinus) in Australia has encouraged the belief that toads pose a significant risk to domestic poultry, by poisoning fowls that eat toads or that drink water contaminated by toads. Although discredited by scientists in 1938, within 2 years of the toads’ introduction to Australia, the myth continues to flourish. We conducted experimental trials to evaluate the vulnerability of chickens to toad-contaminated water, and to toad ingestion. No ill effects were seen, with one chicken consuming 45 small toads without falling ill. Thus, available evidence suggests that cane toads do not imperil domestic poultry.