Natterjack Toad Bufo Calamita Research Articles

Monitoring the conservation status of an endangered amphibian: the natterjack toad Bufo calamita in Britain

AbstractAdequate monitoring of amphibian populations will be increasingly important if global declines are to be understood and, where possible, reversed. The natterjack toad, Bufo calamita, is the rarest British amphibian and has been the subject of substantial conservation efforts. Monitoring strategies have aimed to quantify both the total numbers of populations and the status of individual populations. Between 1970–1999 the number of natterjack populations increased from 43 to 48 because successful translocations (11) outnumbered extinctions (6). Efforts to monitor individual populations increased over the period 1970–1999. Between 1990–1999 there were no detectable trends in B. calamita population size or breeding success in Britain, although power to detect any trends over 10 years was low. Calling activity and short‐term trends in spawn counts were unreliable predictors of long‐term population viability in the absence of extra information such as toadlet production. Long periods (> 10 years) of spawn counts are needed to demonstrate trends that are reflective of real population changes at the national level. Efforts to develop reliable methods for monitoring B. calamita in Britain could provide useful guidelines for work on some other declining species of amphibians.

Impact of ammonium nitrate on growth and survival of six European amphibians.

We conducted static experiments to assess the effects of ammonium nitrate fertilizer on embryos and larvae of six European amphibians: sharp-ribbed salamander (Pleurodeles waltl), Iberian painted frog (Discoglossus galganoi), western spadefoot toad (Pelobates cultripes), common toad (Bufo bufo), natterjack toad (Bufo calamita), and common tree frog (Hyla arborea). Embryos were exposed to different and environmentally relevant concentrations of ammonium nitrate (0 to 200 mg NO(3)(-)/L) for 15 days. Hatching took place during the experiments. H. arborea was extremely sensitive and had high mortality after 8 days of exposure even at the lower fertilizer levels. D. galganoi and B. bufo were also very sensitive and had significant mortality after 15 days of exposure. The rest of the species did not suffer lethal effects but suffered abnormalities or decreased growth at the highest fertilizer concentrations. Chemical fertilizers or manure could have contributed to the observed decrease of B. bufo and D. galganoi in agricultural areas of the Iberian Peninsula during recent years. H. arborea was the most sensitive species studied. The results of our study showed that environmentally relevant levels of ammonium nitrate can induce mortality and might affect population dynamics of this species in agricultural environments.

Heterospecific call recognition and phonotaxis in the orientation behavior of the marbled newt, Triturus marmoratus

The role of acoustic cues as reference cues for orientation by amphibians has been demonstrated in anurans, the only amphibian group that engages in acoustic communication, but not in urodeles. Orientation responses of marbled newts, Triturus marmoratus, were studied to determine whether heterospecific calls elicited positive phonotaxis. The orientation tests consisted in presenting either a familiar acoustic stimulus, the advertisement calls of natterjack toads (Bufo calamita), or a control stimulus, the advertisement calls of European green toads (Bufo viridis) that the newts would not be expected to recognize. Marbled newts and natterjack toads occur in simpatry, but T. marmoratus and B. viridis are allopatric species. Thus, T. marmoratus is distributed over the northern half of the Iberian Peninsula, whereas B. viridis occurs in the Balearic Islands, but not over the Iberian Peninsula. Newts were released in a circular arena while a recorded chorus of natterjack toads or European green toads played outside the arena to determine whether they displayed positive phonotactic orientation. Our results show that marbled newts performed positive phonotaxis when exposed to the breeding calls of natterjack toads, but not to those of European green toads. Newts chose a compass course in the direction of the advertisement calls of B. calamita. Acoustic information might improve orientation accuracy. This study is the first to provide evidence of heterospecific call recognition and positive phonotactic response in urodeles.

CONTRASTING PATTERNS OF QUANTITATIVE AND NEUTRAL GENETIC VARIATION IN LOCALLY ADAPTED POPULATIONS OF THE NATTERJACK TOAD, BUFO CALAMITA

The relative importance of natural selection and genetic drift in determining patterns of phenotypic diversity observed in nature is still unclear. The natterjack toad (Bufo calamita) is one of a few amphibian species capable of breeding in saline ponds, even though water salinity represents a considerable stress for them. Results from two common-garden experiments showed a pattern of geographic variation in embryonic salinity tolerance among populations from either fresh or brackish environments, consistent with the hypothesis of local adaptation. Full-sib analysis showed increased variation in survival among sibships within population for all populations as osmotic stress was increased (broad-sense heritability increased as salinity raised). Nevertheless, toads native to the brackish water environment had the highest overall survival under brackish conditions. Levels of population genetic differentiation for salinity tolerance were higher than those of neutral genetic differentiation, the latter obtained through the analysis of eight microsatellite loci. Microsatellite markers also revealed little population differentiation, lack of an isolation-by-distance pattern, and moderate gene flow connecting the populations. Therefore, environmental stress tolerance appears to have evolved in absence of geographic isolation, and consequently we reject the null hypothesis of neutral differentiation.

Ecological impact assessment in data-poor systems: a case study on metapopulation persistence.

Legislation on the protection of biodiversity (e.g., European Union Habitat and Bird Directives) increasingly requires ecological impact assessment of human activities. However, knowledge and understanding of relevant ecological processes and species responses to different types of impact are often incomplete. In this paper we demonstrate with a case study how impact assessment can be carried out for situations where data are scarce but some expert knowledge is available. The case study involves two amphibian species, the great crested newt (Triturus cristatus) and the natterjack toad (Bufo calamita) in the nature reserve the Meinweg in the Netherlands, for which plans are being developed to reopen an old railway track called the Iron Rhine. We assess the effects of this railway track and its proposed alternatives (scenarios) on the metapopulation extinction time and the occupancy times of the patches for both species using a discrete-time stochastic metapopulation model. We quantify the model parameters using expert knowledge and extrapolated data. Because of our uncertainty about these parameter values, we perform a Monte Carlo uncertainty analysis. This yields an estimate of the probability distribution of the model predictions and insight into the contribution of each distinguished source of uncertainty to this probability distribution. We show that with a simple metapopulation model and an extensive uncertainty analysis it is possible to detect the least harmful scenario. The ranking of the different scenarios is consistent. Thus, uncertainty analysis can enhance the role of ecological impact assessment in decision making by making explicit to what extent incomplete knowledge affects predictions.

POPULATION ON THE VERGE OF A MUTATIONAL MELTDOWN? FITNESS COSTS OF GENETIC LOAD FOR AN AMPHIBIAN IN THE WILD

The fitness costs of high genetic load in wild populations have rarely been assessed under natural conditions. Such costs are expected to be greatest in small, bottlenecked populations, including those occurring near range edges. Britain is at the northwesterly range limit of the natterjack toad Bufo calamita. We compared fitness attributes in two populations of this amphibian with very different recent histories. Key larval fitness attributes in B. calamita, notably growth rate and metamorph production, were substantially higher in the large outbreeding population (Ainsdale) than in the small and isolated one (Saltfleetby). These differences were manifest under seminatural conditions, when larvae were reared in mesh cages within breeding ponds at the site of the small population, and were exacerbated by high stress treatments. The results indicate that genetic load effects can be sufficiently severe enough to predispose extinction over relatively short time frames, as predicted by extinction vortex models.

GEOGRAPHIC VARIATION IN ASYMMETRIC COMPETITION: A CASE STUDY WITH TWO LARVAL ANURAN SPECIES

The outcome of interspecific competition may be modulated by a large number of factors, both biotic and abiotic. In this paper, we examined experimentally the effects of geographic variation, population density, and abiotic stress on the outcome of interspecific competition between two species of larval anurans. Three different populations of natterjack toad (Bufo calamita) were set to compete with common toad (Bufo bufo) in replicated artificial ponds under different combinations of osmotic stress (freshwater or brackish water) and intraspecific density (high and low). Among the B. calamita populations used, only one of them is sympatric to B. bufo and inhabits a freshwater environment. The two other populations, allopatric to B. bufo, occupy freshwater and brackish water environments, respectively. Survival, length of the larval period, and size at metamorphosis were analyzed at both the specific and the populational interaction levels. Intra- and interspecific competition did occur and generally resulted in a decrease in survival rates, an increase in the duration of the larval period, and a decrease in size at metamorphosis. Water salinity also prolonged the larval period and interacted with intra- and interspecific density, intensifying its effects. Interspecific competition was asymmetric, since B. bufo was competitively superior to B. calamita, causing larger effects on B. calamita than those experienced by B. bufo. However, the three populations of B. calamita showed different competitive effects and responses when they were set to compete with B. bufo. The population sympatric to B. bufo had the stronger competitive effect and response, causing a strong impact on survival, growth, and developmental rate of B. bufo. The allopatric population inhabiting the brackish environment also had a marked effect on B. bufo survival, although only at high density. The other allopatric population only had a significant effect on the size at metamorphosis of B. bufo. The impact of B. bufo on B. calamita was greater for the allopatric populations than for the sympatric one. This pattern suggests that the nature and outcome of ecological interactions are a function of environmental conditions and population history.

Modelling the extinction risk of isolated populations of natterjack toad <i>Bufo calamita</i>

Abstract. Many local populations of the natterjack toad Bufo calamita in Germany are endangered. Due to the fragmentation and destruction of natural habitats by man, toads have often been forced to switch to secondary habitats. The permanent existence of these habitats is uncertain. Habitat and area requirements have been investigated in various parts of Germany in recent years. Our study uses field observations from sites in Halle (Saxony-Anhalt) and List (Schleswig-Holstein) as a basis for analyses of the population dynamics under different environmental conditions. Deterministic trends of these populations are calculated with the help of a Leslie matrix consisting of average parameters for mortality and reproduction. For a more thorough analysis we use a stochastic simulation model in order to assess survival probabilities of local toad populations. This model also takes into account environmental fluctuations affecting mortality and reproduction. Using scenarios from different locations, a sensitivity analysis of the parameters indicates which management options are the most promising to preserve a population. Our results indicate that the mortality rates of juveniles and the availability of spawning ground have the greatest influence on a population’s survival. Consequently, habitat management should focus on these aspects. In addition, we investigate the risk of extinction for different reproductive strategies. Natterjack toad populations observed in the field actually follow a strategy with three breeding periods. We find that this strategy supports the survival of the population better than strategies with less periods, which are more likely to result in a complete breeding failure during one season.

Application of Genetic Bottleneck Testing to the Investigation of Amphibian Declines: a Case Study with Natterjack Toads

Abstract: Declines of amphibians are causing widespread concern and in some cases have been explained by factors such as climate change and the spread of microbial pathogens. A problem remains, however, in the unequivocal demonstration of decline in situations where populations may be undergoing natural fluctuations in abundance. We describe the application of a genetic test for bottlenecks (Cornuet & Luikart 1996) that should distinguish between natural oscillations and true population declines. British natterjack toads ( Bufo calamita ) provide examples of populations that have not declined in recent decades, populations that have declined, and populations in which trends are unclear. Microsatellite allele frequency data from these populations were tested for heterozygote excess and shifts in allele frequency distributions, and inferences from these computations about bottlenecks (i.e., persistently smaller population sizes than the recent means) were compared with demographic information. The genetic test correctly identified bottlenecks and should therefore prove useful in demonstrating whether amphibian declines have occurred where long‐term demographic time series are not available.

A further four polymorphic microsatellite loci in the natterjack toad Bufo calamita

A further four polymorphic microsatellite loci in the natterjack toad Bufo calamita

Terrestrial movements of the natterjack toad Bufo calamita (Amphibia, Anura) in a semi-arid, agricultural landscape

AbstractThe movements of 19 natterjack toads (Bufo calamita) were monitored by telemetry in a semi-arid agricultural landscape in NE Spain. Toads reproduced in two water pools in fields of barley and an ancient clayquarrying zone filled by rainfall. Thirteen toads were followed during the breeding season (weeks 5-17 of 1997) and 11 toads were followed after reproduction (weeks 18 to 46). During the breeding season four toads moved distances of up to 500 m between breeding sites. No significant differences in movement patterns were found between the sexes, but a tendency was observed for males to show higher terrestrial site fidelity than females. Distances moved after the breeding season were substantial. The distance between the furthest positions recorded was from 164 to 1201 m, the maximum distance observed between daily recordings ranged from 125 to 353 m and the maximum overall distances ranged from 567 to 4411 m. No significant differences were observed between males and females or between small and large toads. The movements of these toads in a semi-arid agricultural landscape in Spain are compared with those observed in England and Germany.

Phylogeography of the natterjack toad Bufo calamita in Britain: genetic differentiation of native and translocated populations

The natterjack toad Bufo calamita is rare in Britain, which is at the northwestern edge of its biogeographical range. We investigated the level of genetic differentiation amongst almost all (34 out of 38) of the surviving British populations of this species, and among six new populations established by translocations during the 1980s. For eight microsatellite loci, allele sizes and frequencies were analysed using samples from each of these populations. The populations clustered into three robustly differentiated groups, each of which corresponded with a geographical region (east/southeast England, Merseyside and Cumbria). The Cumbrian populations showed a further weak geographical substructuring into northern and southern clades. The populations in south Cumbria were genetically more diverse than those in any of the other regions, as judged by the mean numbers of alleles per locus and the mean heterozygosity estimates. The translocated populations clustered close to their founders and, with one exception, did not differ significantly with respect to mean allele numbers, heterozygosity or polymorphism level. However, significant genetic differentiation (as measured by unbiased RST) was found between all but one of the founder‐translocation pairs. The implications of this phylogeographic study for the future conservation of B. calamita in Britain are discussed.

Archive contributions to a molecular phylogeography of the toad Bufo calamita in Britain.

A complete phylogeographic analysis of any species requires sampling throughout its biogeographical range. In the case of the natterjack toad Bufo calamita in Britain, recent local extinctions have left substantial areas of its historical range without extant populations. We therefore obtained tissue samples of archived Bufo calamita from four museums in the United Kingdom. A range of tissues (tongue, liver, skin, lung, and larval tail) was sampled from a total of 33 individual animals. DNA was extracted and eight polymorphic microsatellite loci were scored. One or more loci were amplified successfully from 27 individuals, and sufficient data were obtained from regions with few or no surviving populations to supplement a phylogeographic analysis based on extant populations.

Effects of predator interactions, prey palatability and habitat structure on survival of natterjack toad Bufo calamita larvae in replicated semi-natural ponds

The significance of predation and aquatic habitat structures to the survivorship of natterjack toad Bufo calamita larvae was investigated by manipulating predator numbers and pond characteristics in a series of replicated semi-natural pools over three consecutive years Two species of fish (common carp Cyprinus carpio and perch Perca fluviatilis) increased the survival of small tadpoles severalfold by selectively consuming predatory invertebrates, but a third species of fish (rudd Scardinius erythrophthalmus) devoured tadpoles and invertebrates indiscriminately Survival of larger tadpoles later in larval development was less affected by the reduction of invertebrate predation pressure from carp and perch, probably because abiotic factors (pond desiccation and anoxia) were stronger agents of tadpole mortality In ponds of low pH (ca 4 5) there was greatly increased spawn mortality and reduced tadpole growth rates but no significant change in tadpole predation compared with circumneutral controls Neutralisation of acid ponds to pH 7 by addition of Ca(OH)2 restored spawn viability and tadpole growth rates to control levels without affecting predation level Addition of organic nutrients stimulated tadpole growth rates significantly m ohgotrophic ponds but not sufficiently to improve survival of small larvae in the face of predation Extensive growths of macrophytes increased predator numbers up to more than twofold but effects on tadpole mortality rates differed between experiments Replacement of natural substrates by concrete basins substantially increased tadpole survival throughout development, probably because both predation by invertebrates and abiotic mortality factors were ameliorated Predation was a strong force early in natterjack tadpole development irrespective of chemical and biological conditions within ponds, but became much less important compared with abiotic factors as an agent of mortality at later tunes

Factors Affecting Population Densities of Adult Natterjack Toads Bufo calamita in Britain

The population densities of adult natterjack toads (Bufo calamita, Laurenti) were found to vary over an order of magnitude between different localities for this species in Britain. Adult toad population density correlated positively with average toadlet production over the previous decade at each site. Average toadlet production in turn correlated with breeding pond density at natterjack sites and the adult toad population size at one study site increased subsequent to an artificial increase of pond density. Toadlet production within particular ponds was positively correlated with spawn deposition rate and there was no evidence of density-dependent regulation of toadlet production in two natterjack breeding ponds studied over several years. The proportion of breeding seasons in which greater than the median number of toadlets was produced per hectare of total natterjack habitat was positively correlated with pond density at natterjack sites. Taken together, these data suggest that adult natterjack population density is primarily regulated by the average rate of metamorphic success at a site, and this in turn is dependent upon stochastic rather than density-dependent processes operating at the level of the breeding ponds.

Conservation of the natterjack toad Bufo calamita in Britain over the period 1970–1990 in relation to site protection and other factors

The number of known sites (populations) documented by conservationists for the natterjack toad Bufo calamita between 1970 and 1990 in Britain rose from 21 to 40, excluding those resulting from translocations. The progressive reduction in rate of discovery of undocumented sites suggests that few further populations will be located. The real overall number of natterjack sites has probably changed little over this period, with successful translocations approximately compensating for the few (three to five) concurrent extinctions. More than 90% of matterjacks in Britain now live in the vicinity of just five river estuaries. Habitat protection, mainly the result of scheduling land as Sites of Special Scientific Interest (SSSI), has been increasingly effective in safeguarding natterjacks, the proportion of safeguarded sites rising from 60% in 1970 to 83% by 1990. However, the legislation for species protection has proved much less effective. Much of the recent damage on unscheduled sites was a result of agricultural improvements by landowners ignorant of the presence of natterjacks on their land. The breeding records from natterjack sites are used to examine the possible impacts of radioactivity and climate change, against which site protection alone is no safeguard. Recommendations are made for conserving natterjack populations in the future.

The Effect of Food Level on Competition in Tadpoles: Interference Mediated by Protothecan Algae?

1. The development of natterjack toad (Bufo calamita Laurenti) tadpoles was monitored under two levels of food and with or without competitors (Rana temporaria L.) using a replicated pond system. The design prevented physical interactions between species but allowed B. calamita access to R. temporaria faeces. 2. Food supplements accelerated growth with or without competitors. The presence of competitors depressed B. calamita growth, even under conditions of high food. 3. B. calamita raised under high food metamorphosed earlier than those raised under low food, but neither food nor competitors affected survival. 4. The growth-inhibiting alga, Prototheca, was detected in the faeces of both R. temporaria and B. calamita. B. calamita raised with R. temporaria for several weeks produced larger numbers of protothecan cells in their faeces than those raised without competitors. 5. Inferior competitors may be diverted from high quality food resources by the attractive properties of Prototheca in faeces. In such a situation superior competitors may exert a negative effect even under high food levels.

Long-term management of a natterjack toad (Bufo calamita) population in southern Britain

Abstract We have investigated the dynamics of an isolated population of natterjack toads (Bufo calamita) over the period 1972-1991. During this time the habitat was manipulated by clearance of scrub woodland and by creation and restoration of ponds. The adult toad population increased at least threefold during the study, with a nadir in the late 1970s and a maximum in the late 1980s. Female population size, as judged by spawn string counts, varied according to metamorphic success rates three years previously. Ponds suitable for natterjack breeding on chemical and topographical grounds nevertheless varied both in their attractiveness to natterjacks and in their success rates in sustaining larvae through to metamorphosis; those desiccating at intermediate times were more successful than highly ephemeral or longer-lasting water bodies. Artificial eutrophication of one naturally acid pond by addition of Ca((OH)2 increased its attractiveness to natterjacks for several years but did not sustain high levels of toadlet production. Female natterjacks survived up to twice as long as the oldest males, a difference possibly attributable to selective predation by the snake Natrix natrix. Invertebrates, especially beetles (Coleoptera) and ants (Hymenoptera) were the primary food of both natterjacks and sympatric common toads (B. bufo) at the study site. Addition of fish (Leuciscus idus) to breeding pools reduced numbers of predatory invertebrates and increased toadlet production in two separate experiments. Optimal terrestrial habitat for natterjacks was sustained only for 2-3 years after scrub clearance, after which seral succession rendered it unsuitable.

Effects of body size and timing of reproduction on reproductive success in female natterjack toads (Bufo calamita)

This study estimated variability in components of reproductive success in female natterjack toads (Bufo calamita, Laurenti, 1768) as a function of their body size and timing of reproduction. Two components of reproductive success were analysed: fecundity and metamorphic success. Fecundity, as a function of body size, tended to increase exponentially. Age had a negligible effect once body size was statistically controlled. Egg size was also related to female body size but showed greater variation. Metamorphic success depended on the timing of reproduction. Later breeders suffered an exponential decline in metamorphic success because females spawned at recently‐filled ponds of ephemeral duration. Since female body size decreased as the breeding season progressed, it may be inferred that larger, early‐breeding females would have a differential reproductive success. The inverse relationship between the timing of reproduction and body size may be based on the energetic physiology of vitellogenesis, and on the potential trade‐offs between the allocation of energy to growth—higher in smaller females—and reproduction.

Effects of Egg Size and Density on Metamorphic Traits in Tadpoles of the Natterjack Toad (Bufo calamita)

Effects of Egg Size and Density on Metamorphic Traits in Tadpoles of the Natterjack Toad (Bufo calamita)