Indirect facilitation of a native mesopredator by an invasive species: are cane toads re-shaping tropical riparian communities?

Top predators can suppress mesopredators both by killing them and by motivating changes in their behavior, and there are numerous examples of mesopredator release caused by declines in top predator populations. Demonstrated cases of invasive species triggering such releases among vertebrate trophic linkages (indirect facilitation), however, are rare. The invasive cane toad, Bufo marinus, has caused severe population-level declines in some Australian predators via lethal toxic ingestion. During a long-term study of the direct impacts of cane toads on predatory monitor lizards in tropical Australia, we documented significant, marked increases in annual counts of a mesopredator, the common tree snake (Dendrelaphis punctulatus). Mean snake counts during surveys of 70-km river transects at two sites increased from <1 individual per survey during 2001–2006, to 8–18 per survey in 2007. These increases occurred approximately 3 years following the arrival of cane toads, and 1–3 years after 71–96 % population declines in three species of predatory monitor lizards (Varanus panoptes, V. mertensi, and V. mitchelli). These data suggest a mesopredator release: the dramatic reduction of predatory monitor lizards caused increases in the tree snake by decreasing predation risk. The increases in tree snake counts were not attributable to either abiotic factors, or a trophic subsidy. The severe declines of predatory monitor lizards, coupled with recent evidence of cascading effects on their prey, suggest that cane toads are re-shaping riparian communities in tropical Australia through both direct negative effects and indirect facilitation.     

Midbrain auditory sensitivity in toads of the genus Bufo (amphibia — bufonidae) with different vocal repertoires

Summary South American male toads Bufo chilensis emit a release call in contact with other individuals and a soft amplectic call, B. spinulosus males emit a release call while isolated in breeding areas, and B. arenarum produces a release call plus an intense mating call. Release calls of the 3 species measure 72–86 dB SPL RMS at 20 cm in front of the animal and the mating call of B. arenarum is 84–87 dB SPL at 4 m.Audiograms obtained with multiunit recordings in the torus semicircularis (TS) show a low frequency region (LFR), centered at 352, 356 and 491 Hz, and a high frequency region (HFR), centered at 1199, 1161 and 1423 Hz, in B. chilensis, B. spinulosus and B. arenarum, respectively. Center frequencies (CFs) in the HFR are in gross correspondence with average dominant frequencies (DFs) of the vocalizations of these species. Best thresholds (BTs) in the HFR are similar between B. chilensis and B. arenarum while in B. spinulosus average BTs are 10.8 and 13.5 dB higher, respectively. The similar auditory thresholds between B. chilensis and B. arenarum denote a conservative nature of auditory sensitivity among these anura.Abbreviations AP amphibian papilla - BP basilar papilla - BT best threshold - CF center frequency - DF dominant frequency - HFR high frequency region - LFR low frequency region - TS torus semi-circularis - SPL sound pressure level     

Effects of invasive cane toads on Australian mosquitoes: Does the dark cloud have a silver lining?

Research on the ecological impacts of invasive organisms typically looks only for negative impacts, ignoring the possibility that the wider community might see benefits in some of these effects. To truly understand the impact of invasive species, we need to look as broadly as possible, and incorporate studies on a diversity of variables. The spread of the South American cane toad (Bufo marinus) through tropical Australia is widely viewed as an ecological catastrophe, but anecdotal reports suggest that the invasion of toads may reduce the numbers of mosquitoes (and thus, potentially, the risk they pose to human health). We conducted experiments to determine whether the presence of toad tadpoles affects survival rates, adult body sizes and/or rates of oviposition of four species of disease-carrying mosquitoes. In the laboratory, the presence of toad tadpoles significantly reduced the sizes of adult mosquitoes at emergence, and also reduced survival rates of the larvae of one mosquito species. In field trials, mosquitoes were less likely to oviposit in waterbodies containing toad tadpoles. Accordingly, these data suggest (but do not prove) that toad invasion may reduce mosquito abundance. More generally, any overall evaluation of the impact of an invasive species needs to consider possible benefits (e.g. to human health) as well as negative effects (e.g. to native species). Both types of information are essential to inform community decisions about the management of feral taxa such as the cane toad in Australia.     

Sexual selection in cane toads Rhinella marina： A male＇s body size affects his success and his tactics

Abstract Male body size can play an important role in the mating systems of anuran amphibians. We conducted labora- tory-based trials with cane toads Rhinella （Bufo） marina from an invasive population in the wet-dry tropics of northern Australia, to clarify the effects of a male＇s body size on his reproductive success and behavior （mate choice）. Males were stimulated with a synthetic hormone to induce reproductive readiness. Larger body size enhanced a male toad＇s ability to displace a smaller rival from amplexus, apparently because of physical strength： more force was required to dislodge a larger than a smaller amplectant male. A male＇s body size also affected his mate-choice criteria. Males of all body sizes were as likely to attempt amplexus with another male as with a female of the same size, and preferred larger rather than smaller sexual targets. However, this size prefe- rence was stronger in larger males and hence, amplexus was size-assortative. This pattern broke down when males were given access to already-amplectant male-female pairs： males of all body sizes readily attempted amplexus with the pair, with no size discrimination. An amplectant pair provides a larger visual stimulus, and prolonged amplexus provides a strong cue for sex iden- tification （one of the individuals involved is almost certainly a female）. Thus, a male cane toad＇s body size affects both his ability to defeat rivals in physical struggles over females, and the criteria he uses when selecting potential mates, but the impacts of that selectivity depend upon the context in which mating occurs     

Postglacial recolonization of North America by spadefoot toads: integrating niche and corridor modeling to study species’ range dynamics over geologic time

Understanding the factors that shape species’ distributions is a key topic in biogeography. As climates change, species can either cope with these changes through evolution, plasticity or by shifting their ranges to track the optimal climatic conditions. Ecological niche modeling (ENM) is a widespread technique in biogeography that estimates the niche of the organism by using occurrences and environmental data to estimate species’ potential distributions. ENMs are often criticized for failing to take species’ dispersal abilities into consideration. Here, we attempt to fill this gap by combining ENMs with dispersal and corridor modeling to study the range dynamics of North American spadefoot toads (Scaphiopodidae) over the Holocene. We first estimated the current and past distributions of spadefoot toads and then estimated their past distributions from the Last Glacial Maximum (LGM) to the present day. Then, we estimated how each taxon recolonized North American by using dispersal and corridor modeling. By combining these two modeling approaches we were able to 1) estimate the LGM refugia used by the North American spadefoot toads, 2) further refine these projections by estimating which of the putative LGM refugia contributed to the recolonization of North America via dispersal, and 3) estimate the relative influence of each LGM refugium to the current species’ distributions. The models were tested using previously published phylogeographic data, revealing a high degree of congruence between our models and the genetic data. These results suggest that combining ENMs and dispersal modeling over time is a promising approach to investigate both historical and future species’ range dynamics.     

Do toads have a jump on how far they hop? Pre-landing activity timing and intensity in forelimb muscles of hopping Bufo marinus

During jumping or falling in humans and various other mammals, limb muscles are activated before landing, and the intensity and timing of this pre-landing activity are scaled to the expected impact. In this study, we test whether similarly tuned anticipatory muscle activity is present in hopping cane toads. Toads use their forelimbs for landing, and we analysed pre-landing electromyographic (EMG) timing and intensity in relation to hop distance for the m. coracoradialis and m. anconeus, which act antagonistically at the elbow, and are presumably important in stabilizing the forelimb during landing. In most cases, a significant, positive relationship between hop distance and pre-landing EMG intensity was found. Moreover, pre-landing activation timing of m. anconeus was tightly linked to when the forelimbs touched down at landing. Thus, like mammals, toads appear to gauge the timing and magnitude of their impending impact and activate elbow muscles accordingly. To our knowledge these data represent the first demonstration of tuned pre-landing muscle recruitment in anurans and raise questions about how important the visual, vestibular and/or proprioceptive systems are in mediating this response.     

No ecological opportunity signal on a continental scale? Diversification and life‐history evolution of African true toads (Anura: Bufonidae)

The niche‐filling process predicted by the “ecological opportunity” (EO) model is an often‐invoked mechanism for generating exceptional diversity in island colonizers. Whether the same process governs lineage accumulation and trait disparity during continental colonization events is less clear. Here, we test this prediction by investigating the rate dynamics and trait evolution of one of Africa's most widespread amphibian colonizers, the true toads (Bufonidae). By reconstructing the most complete molecular phylogeny of African Bufonidae to date, we find that the diversification of lineages in Africa best conforms to a constant rate model throughout time and across subclades, with little support for EO. Evolutionary rates of life‐history traits have similarly been constant over time. However, an analysis of generalists and specialists showed a shift toward higher speciation rates associated with habitat specialization. The overall lack of EO signal can be interpreted in a number of ways and we propose several explanations. Firstly, methodological issues might preclude the detection of EO. Secondly, colonizers might not experience true EO conditions and due to the size, ecological heterogeneity and age of landmasses, the diversification processes might be more complex. Thirdly, lower speciation rates of habitat generalists may have affected overall proliferation of lineages.     

Edge preference of retinal and tectal neurons in common toads (Bufo bufo) in response to worm-like moving stripes: the question of behaviorally relevant ‘position indicators’

Summary Previous experiments have shown that during prey-catching behavior (orienting, snapping) in response to a worm-like moving stripe common toads.Bufo bufo (L.) exhibit a contrast-and direction-dependent edge preference. To a black (b) stripe moving against a white (w) background (b/w), they respond (R^*) preferably toward the leading (l) rather the trailing (t) edge (R _l ^* > R _t ^* ), thus displaying head preference. If the contrastdirection is reversed (w/b), the stripe's trailing edge is preferred (R _l ^* < R _t ^* ), hence showing tail preference. In the present study, neuronal activities of retinal classes R2 and R3 and tectal classes T5(2) and T7 have been extracellularly recorded in response to leading and trailing edges of a 3 ° × 30 ° stripe simulating a worm and traversing the centers of their excitatory receptive fields (ERF) horizontally at a constant angular velocity in variable movement direction (temporo-nasal or naso-temporal).The behavioral contrast-direction dependent edge preferences are best resembled by the responses (R) of prey-selective class T5(2) neurons (R_l R_t=101 for b/w, 0.31 for w/b) and T7 neurons (R_lR_t=61 for b/w, 0.41 for w/b); the T7 responses may be dendritic spikes. This property can be traced back to off-responses dominated retinal class R3 neurons (R_lR_t=61 for b/w, 0.51 for w/b), but not to class R2 (R_lR_t =1.21 for b/w and 0.91 for w/b). The respective edge preference phenomena are independent of the direction of movement.When stimuli were moved against a stationary black-white structured background, the head preference to the black stripe and the tail preference to the white stripe were maintained in class R3, T5(2), and T7 neurons. If the stripe traversed the ERF together with the structured background in the same direction at the same velocity, the responses of tectal class T5(2) and T7 neurons were strongly inhibited, particularly in the former. Responses of retinal R2 neurons in comparable situations could be reduced by about 50%, while class R3 neurons responded to both the stimulus and the moving background structure.The results support the concept that the prey feature analyzing system in toads applies principles of (i) parallel and (ii) hierarchial information processing. These are (i) divergence of retinal R3 neuronal output contributes to stimulus edge positioning and (in combination with R2 output) area evaluation intectal neurons and to stimulus area evaluation and (in combination with R4 output) sensitivity for moving background structures inpre tectal neurons; (ii) convergence of tectal excitatory and pretectal inhibitory inputs specify the property of prey-selective tectal T5(2) neurons which are known to project to bulbar/spinal motor systems.Abbreviations ERF excitatory receptive field - IRF inhibitory receptive field - N nasal - T temporal - R _w response to a worm-like stripe moving in the direction of its longer axis - R _A response to an antiworm-like stripe whose longer axis is oriented perpendicular to the direction of movement - R _l response to the leading edge of a worm-like moving stripe - R _t response to the trailing edge of a worm-like moving stripe - b/w black stimulus against a white background - w/b white stimulus against a black background - sm structured moving background - ss structured stationary background - u minimal structure width of a structured background consisting of rectangular black and white patches in random distribution - HRP horseradish peroxidase     

Does thermoregulatory behaviour of green toads (Bufo viridis) constrain geographical range in the west? A comparison with the performance of syntopic natterjacks (Bufo calamita)

(1)

Behavioural regulation of body temperature (t_b) was monitored in 23 free-ranging Bufo calamita (Bc) and 17 syntopic Bufo viridis (Bv) at Urmitz (Rhineland-Palatinate, Germany) using temperature-sensitive transmitters implanted to the abdominal cavity.

(2)

In field t_b varied between +0.5 and 37.4 °C in Bc and between +0.6 and 33.7 °C in Bv. Maximum t_b of a Bc measured during an experimental trial was 38.8 °C.

(3)

Natterjack toads avoided environmental temperature extremes by burrowing actively into moist sandy soil (2-90 cm deep), whereas green toads hid exclusively in mammal burrows or pre-existing subterranean cavities. Shelter choice did not vary between summer and winter.

(4)

Average t_b of Bc exceeded significantly that of Bv during summer (26.7 °C vs. 24.7 °C), while the reverse was true during winter (4.2 °C vs. 7.2 °C). Following hibernation the body condition of Bv was significantly lower than that of Bc.

(5)

We conclude that green toads fail to colonise regions west of the Rhine valley because of a combination of winter temperatures impeding foraging trips for prolonged periods, the choice of warm hibernacula increasing metabolic costs and/or predation risk and reduced fecundity.

     

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.