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.     

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.     

Plasma osmolality, urine composition and tissue water content of the toad Bufo viridis Laur. in nature and under controlled laboratory conditions

The compositions of plasma and urine were studied in toads (Bufo viridis) which were collected from three locations in Israel, and compared with toads which were kept under constant laboratory conditions for nearly 2 years. Plasma osmolality was rather constant (over 310 mOsm kg-1 H2O) during the whole year in the active toads. Urea was the most variable osmolyte in the plasma, and accounted for the higher osmolality in southern population. Urine osmolality fluctuated in a circannual fashion both in freshly captured and in the toads under constant laboratory conditions. Water content of the tissues was constant throughout the year, independent of the plasma osmolality. It is concluded that high plasma urea concentration and the excretory system (kidneys and the urinary bladder) are important in sustaining constant plasma osmolality in active toads. Both mechanisms change annually and form the basis for the high terrestriality of this species.     

Rapid expansion of the cane toad (Bufo marinus) invasion front in tropical Australia

Abstract Cane toads (Bufo marinus) are large toxic anurans that have spread through much of tropical Australia since their introduction in 1935. Our surveys of the location of the toad invasion front in 2001 to 2005, and radiotracking of toads at the front near Darwin in 2005, reveal much faster westwards expansion than was recorded in earlier stages of toad invasion through Queensland. Since reaching the wet‐dry tropics of the Northern Territory, the toads have progressed an average of approximately 55 km year⁻¹ (mean rate of advance 264 m night⁻¹ along a frequently monitored 55‐km road transect during the wet season of 2004–2005). Radiotracking suggests that this displacement is due to rapid locomotion by free‐ranging toads rather than human‐assisted dispersal; individual toads frequently moved >200 m in a single night. One radiotracked toad moved >21 800 m in a 30‐day period; the fastest rate of movement yet recorded for any anuran. Daily displacements of radiotracked toads varied with time and local weather conditions, and were highest early in the wet season on warm, wet and windy nights. The accelerated rate of expansion of the front may reflect either, or both: (i) evolved changes in toads or (ii) that toads have now entered an environment more favourable to spread. This accelerated rate of expansion means that toads will reach the Western Australian border and their maximal range in northern Australia sooner than previously predicted.     

Effect of oestradiol dipropionate and testosterone propionate on protein, RNA and DNA contents and RNase and DNase activity in the liver of the toad (Bufo melanosticus)

A single injection of oestradiol dipropionate increased the HSI and protein, RNA and DNA contents and decreased the RNase and DNase activities of the liver of male and female toads. The minimum effective dose of oestrogen required to induce most of these changes was found to be 1 microgram/g (single injection), but the liver RNA content increased at the dose of 0.5 microgram/g. Oestrogen in a dose of 0.1 microgram/g did not cause any of these changes in male and female toads. Testosterone propionate (0.1, 0.5, 1 or 2 micrograms/g, single injection) was mostly ineffective in these respects, while in male toads higher doses of testosterone (1 and 2 micrograms/g) enhanced the liver RNA content only. The oestrogenic responses occurred earlier in female toads than in males. The liver protein and DNA contents increased from the 3rd day in female and on the 5th day in male toads. The liver RNA reached the higher level from the 2nd day in female and from the 3rd day in male. The RNase and DNase activities were reduced from the 2nd and 3rd day, respectively, in female and on the 5th day in male toads.     

Anuran mating calling circuits: inhibition by prostaglandin

Male American toads (Bufo americanus) were induced to mating call in response to electronically simulated, conspecific mating calls. The injection of prostaglandin (PG) F2 alpha caused suppression of mating call answering. Neural correlates of mating calling were triggered by electrical stimulation of the anterior preoptic nucleus in the isolated brainstem of male, Northern leopard frogs (Rana p. pipiens). The addition of PGF2 alpha to the bath completely abolished the correlates of mating calling without changing the correlates of pulmonary respiration. The suppression of mating calling shown here, along with the suppression of release signalling described by Diakow and Nemiroff (1981), supports the hypothesis of a close interrelation between the neural circuits of these two calls. The suppression of the neural correlates of mating calling in an isolated preparation shows a central site of action of the PG. The retention of normal correlates of pulmonary respiration, even after suppression of mating calling correlates, suggests that the generation of mating calling patterns involves the extension and pulsing of the expiratory phase of breathing.     

Altitudinal distribution and blood values in the toad, Bufo spinulosus Wiegmann

1. Red blood cell (RBC) count, RBC size, hematocrit, cell and blood hemoglobin concentrations and plasma total solid concentration were measured in 16 lowland (from near sea level up to 2700 m) and 18 highland (3200 up to close to 4500 m) adult toads (Bufo spinulosus). 2. Lowland toads showed higher hematocrit values than highland toads, but their blood hemoglobin concentration and plasma solid concentration were not significantly different. 3. Highland toads had smaller RBC size, higher corpuscular hemoglobin concentration, a trend toward larger RBC count and a considerably smaller body size. These features may contribute to their successful life at high altitude.     

Renal response of euryhaline toad (Bufo viridis) to acute immersion in tap water, NaCl, or urea solutions

Green toads (Bufo viridis) were acclimated to either tap water, 230 mOsmol NaCl kg-1 H2O (saline), 500 mOsmol NaCl kg-1 H2O (high saline), or 500 mmol L-1 urea. Renal functions for each acclimation group were studied on conscious animals that had one ureter chronically catheterized. Reciprocal immersion of tap-water- and saline-acclimated toads in the opposite solution did not stress the animals osmotically, and plasma osmolality increased or decreased by no more than 15%. However, urine osmolality and ionic composition changed immediately and profoundly on exposure to the other solution. Exposure of tap-water-acclimated toads to saline decreased urine flow by 30%, whereas the reciprocal immersion led to an increase of 30%. Immersion of tap-water-acclimated toads in high saline led to immediate cessation of urine flow, whereas immersion of 500 NaCl- or urea-acclimated toads in tap water led to a large increase in urine flow, with an overshoot that lasted 10 h (as a result of either salt or urea diuresis). Urine flow then stabilized at a level 5-6 times higher than the value attained at high-salt environment. On immersion of 500 urea-acclimated toads in 500 NaCl, urine flow doubled, accompanied by a change in ion composition, without change in the osmolality. In all experimental conditions, plasma potassium concentration was maintained within a narrow range. The results show that the toad's kidneys contributed efficiently both to osmo- and ionoregulation in a wide range of ambient solutions.     

The ecological impact of commercial beehives on invasive cane toads (<Emphasis Type="Italic">Rhinella marina</Emphasis>) in eastern Australia

Understanding the factors that affect an invasive species’ viability and distribution has vital implications for biocontrol. In Australia, invasive cane toads (Rhinella marina) are anecdotally reported to utilise commercial beehives as a prey resource, but that interaction has never been studied in detail. We investigated the impact of apiaries on cane toads in northern New South Wales via mark-recapture surveys, dissections, and camera-trap observations. Cane toads were the most frequent visitors to apiaries, followed by bandicoots and corvid birds. Cane toads at apiaries were more abundant and in better body condition (i.e., larger mass relative to snout-urostyle length) than were toads at nearby control sites. Toads at beehives contained more prey items per stomach (mostly bees, which were never recorded in the stomachs of toads from other sites), and adult female toads at beehives had larger livers and ovaries relative to body size. We conclude that commercial apiaries attract cane toads, influence their diets, and increase their feeding rates and reproductive capacity. Like other habitat modifications wrought by agricultural activities, honey bee colonies provide resources that facilitate the spread of cane toads through an otherwise harsh landscape matrix. Minor modifications to beehives could exclude toads, thereby eliminating their positive impact on the invader.     

Field trials of chemical suppression of embryonic cane toads (Rhinella marina) by older conspecifics

1. Laboratory experiments have shown that the viability of embryos of the invasive cane toad (Rhinella marina) can be reduced by exposure to chemical cues from older conspecific larvae. These effects (very strong in laboratory trials) may offer an exciting new approach to controlling this problematic invasive species in Australia. However, the degree to which the method works in natural environments has yet to be assessed.
2. Our experiments in the laboratory and in seminatural outdoor waterbodies show that chemical cues from tadpoles do indeed suppress the growth, development, and survival of conspecific larvae that are exposed as embryos and do so in a dose‐dependent manner; higher tadpole densities cause greater suppression of embryos.
3. In seminatural outdoor waterbodies, suppressor‐exposed tadpoles were less than half as likely to survive to metamorphosis as were controls, and were much smaller when they did so and hence, less likely to survive the metamorph stage. Additionally, female cane toads were less likely to oviposit in a waterbody containing free‐ranging (but not cage‐enclosed) tadpoles, suggesting that the presence of tadpoles (rather than the chemical cues they produce) may discourage oviposition.
4. Broadly, our results suggest that the suppression effect documented in laboratory studies does indeed occur in the field also, and hence that we may be able to translate that approach to develop new and more effective ways to reduce rates of recruitment of peri‐urban populations of cane toads in their invasive range.

     

Mechanisms of competition between tadpoles of Australian frogs (Litoria spp.) and invasive cane toads (Rhinella marina)

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The effect of salt acclimation of the water uptake and osmotic permeability of the skin of the toad (Bufo viridis, L.)

1. Water uptake in vivo, and water fluxes across the isolated skin were studied in salt (NaCl) acclimated toads. 2. Water uptake of acclimated toads maintained in the solution of acclimation, decreased with the environmental salinity. 3. The osmotic water permeability (Pos) of the skin increased upon salt (NaCl) acclimation, both in vivo and in vitro. 4. Pos of the skin of toads acclimated to non-permeant solutes such as sucrose (230 mmol/l) or mannitol (400 nmol/l), was greatly reduced. 5. Oxytocin (syntocinon) increased the Pos both in tap water and salt acclimated toads. In high salt (greater than 200 mmol/l NaCl) acclimated toads however, the increased Pos and water flux at larger osmotic gradients, could not be stimulated further by the hormone. 6. The adaptive nature of the selective changes in the permeability properties of the skin under salt acclimation conditions is discussed.     

Observations on myiasis by the calliphorid,Bufolucilia silvarum,in the eastern American toad (Bufo americanus americanus) from southeastern Wisconsin

Larvae of certain species of blowflies (Calliphoridae) cause myiasis in amphibians which may result in significant mortality, yet there are few reports from North America. In this study, we observed primary myiasis in a population of juvenile eastern American toads (Bufo americanus americanus) collected during May-July 1998 from southeastern Wisconsin (USA). Nine (6%) of 140 toads were infected by the green blow fly (Bufolucilia silvarum) with a mean intensity of 10.5 +/- 7.2 (range = 1-24). Weekly parasite prevalence and mean intensity remained low, ranging from 0-20% and 2 +/- 1.4 to 14 +/- 6, respectively. We found: 1) flies lay eggs on healthy toads, 2) eggs hatch with first instar maggots penetrating under the skin, 3) maggots develop to mature third instars within 5-7 days, 4) maggots leave the host and form pupa within 8-11 days of hatching, and 5) maggots pupate within 7-9 days at room temperature. All infected toads died within 1-2 wk as a result of the infection. The low prevalence observed in this study and other reports of this species from mammalian and bird carcasses indicated that B. silvarum is probably a facultative parasite of toads and other amphibians in the United States. This is the first report of B. silvarum causing myiasis in Wisconsin amphibians and the first report in eastern American toads in the United States.     

External and internal control of patterns of feeding, growth and gonadal function in a temperate zone anuran, the toad Bufo bufo

Young toads were brought into the laboratory before hibernation in the autumn and exposed to alternating periods of low temperature and high temperature and feeding, thus simulating climatic cycles. In newly metamorphosed, non-hibernating toads, food intake was initially low and growth was stagnant, but subsequently food intake and lean growth increased strongly. Presumably, growth was initiated independently of feeding. In one-year-old, non-hibernating toads, food intake and growth declined with time, but high rates of feeding and growth were restored in the toads exposed to simulated hibernation. In both metamorphosed toads and one-year-old, hibernating toads, a period of predominant fat deposition succeeded predominant lean growth before food intake again declined to low levels. Initiation of sexual maturation in the female toads coincided with suspended growth, whereas sexual maturation in males was not correlated with changes in growth rate. In the female, but not the male toads, sexual maturation was greatly advanced under the experimental conditions, resulting in adult female sizes far below those typical of the wild population.     

Relationship between nutritional state and testes function, together with observations on patterns of feeding, in the toad, Bufo bufo bufo

The effects of 10 weeks of starvation, restricted feeding, or ad lib. feeding on nutritional state and testes functions were investigated in male toads that had artificially hibernated in a refrigerator before being placed at room temperature. Food intake increased for about four weeks in both the fed groups. After this initial period the toads on restricted food ate their ration for the remaining part of the experiment, and they developed fat bodies of sizes characteristic of male toads during late summer in nature. The toads fed ad lib. increased food intake to about twice that of those on restricted food. After two weeks of high feeding rate food intake rapidly declined and remained low to the end of the experiment, when the fat bodies were larger than normally seen in nature. At the end of the experiment spermatogenetic activity had declined to a low level in the starving toads, whereas it was high in the fed toads. Starvation for 10 weeks did not affect the late stages of spermatogenesis. Interstitial cells and thumbpads remained reduced in the starving toads, but were restored in the fed toads. The importance of the annual cycle in nutritional state for gonadal cycles in male and female toads is discussed.     

Morphometrical and acoustical comparison between diploid and tetraploid green toads

In the present paper we compare, on the basis of morphometrical characters and acoustical properties of the advertisement calls, a sample of 158 male green toads (Bufo viridis complex) collected in 12 breeding populations of south Kazakstan and north Kyrgyzstan. The samples of three populations resulted in only diploid toads (2n = 22), those of eight populations in only tetraploid toads (2k=44) whereas in one locality diploid, tetraploid and many triploid toads were collected. Diploid toads show significantly larger body size and proportionally larger head and shorter limbs than both tetraploids and triploids, whereas no evident morphometrical differences were observed between triploids and tetraploids. Diploid advertisement calls have spectral and temporal properties that significantly differ from those of both triploid and tetraploid advertisement calls. In particular, diploids produce significantly longer calls with higher pulse-rates and lower frequencies than those of tetraploids. We address the question of the factors that could be responsible for these differences and we discuss four hypotheses: (1) the direct effect of polyploid mutation, (2) genetic drift, (3) reproductive character displacement and (4) environmental selection.     

Angiotensin II elicits water seeking behavior and the water absorption response in the toad Bufo bufo

Fully hydrated toads, Bufo bufo were acclimated to a simulated terrestrial habitat, with access to shelters and water. To get from the shelters to the water, the toads had to walk across the pan of an Ohaus balance and the body weights were recorded on a computer. Toads were placed inside shelters immediately following injection of human angiotensin II (A II), Thr(8)-saralasin, or Ringer's in the dorsal lymph sac, and their behavior was recorded continuously by video surveillance. The injection doses were 1-100 microg/100 g body weight A II and 100 microg/100 g body weight saralasin dissolved in 0.1 ml Ringer's; control animals received the same volume of Ringer's. The latency from injection to the initiation of water absorption behavior (WR) was significantly shorter in both A-II- and saralasin-injected toads, compared to controls. A-II- and saralasin-injected toads also spent significantly more time in the water than controls. The bladder depots when WR was terminated were significantly larger in A-II- or saralasin-injected toads than in controls. The stimulatory action of Thr(8)-saralasin, an antagonist of A II in mammals, on WR behavior in B. bufo suggests differences in receptor structure and/or receptor distribution between amphibians and mammals.     

Toads in the backyard: why do invasive cane toads (Rhinella marina) prefer buildings to bushland?

Like many invasive species, cane toads (Rhinella marina) in Australia concentrate in the disturbed habitats created by human activity, rather than in pristine areas. We surveyed cane toads in the wet–dry tropics of the Northern Territory to assess the abundances, body sizes, sexes, behaviour, hydration state and feeding rates of toads around buildings compared to those in areas remote from buildings, and conducted experimental trials to assess the effects of building-related variables (lights and increased toad densities) on the foraging success of toads. Toads around buildings were smaller than bushland conspecifics, and adult sex-ratios were female-biased. Toads were more sedentary around buildings than in the bush, but their feeding rates (based on direct observations and faeces production post-capture) were similar. That similarity, despite twofold-higher densities of competing toads around building, reflected the strong enhancement of feeding rates due to artificial lights attracting insects (in our experimental trials, a threefold increase regardless of the number of competing toads). Toads collected from around buildings were apparently in better hydric condition. Thus, access to water also may attract toads to buildings. The relative scarcity of adult male toads around buildings likely reflects waterbody-centred reproductive activities, whereas the concentration of females and juveniles around buildings is driven largely by access to the insects attracted by artificial light. We conclude that buildings enhance the persistence of cane toad populations and may facilitate their spread.     

Osmoregulation of the cane toad, Bufo marinus, in salt water

Adult cane toads, B. marinus, survived in salinities up to 40% sea-water (SW). Pre-exposure to 30, then 40% SW, increased the survival time of toads in 50% SW. Plasma from toads acclimated to salt water is hyperosmotic to the environment--a result of increased plasma sodium, chloride and urea concentrations. When toads were placed in tap-water and 20% SW, all significant changes to plasma sodium, chloride, urea and osmotic pressure occurred within the first 2 days of exposure. When toads were placed in 30 and 40% SW environments, the increases in plasma sodium and chloride concentrations occurred within the first 2 days of exposure while urea and total osmotic pressure continued to rise until some time between 2 and 7 days exposure.     

Leptospires in the marine toad (Bufo marinus) on Barbados

Leptospires were isolated from the kidneys of four of 211 toads (Bufo marinus) caught on Barbados. Two of the isolates were identified as Leptospira interrogans serovar bim in the Autumnalis serogroup (the most common cause of leptospiral illness on Barbados), and two as possibly new serovars in the Australis serogroup. Sera from 198 of the toads were examined by the leptospire microscopic agglutination test. Forty-two (21%) were positive at titers of greater than or equal to 1:100, and 54 (27%) at greater than or equal to 1:50. The predominating serogroups were Australis (50%), Autumnalis (23%) and Panama (13%). The agglutination tests on the culture-positive toads showed that serologic studies alone may be of limited value in these animals. Bufo marinus can harbor pathogenic leptospires, and it may be a significant source of the Autumnalis serogroup infections in the Caribbean.