Distribution of the Mallorcan midwife toad (Alytes muletensis) in relation to landscape topography and introduced predators

The endemic midwife toad of Mallorca (Alytes muletensis) is restricted to a small number of breeding populations in the mountainous northwest of the island. The decline of the species has been attributed to the impacts of introduced species such as the viperine snake (Natrix maura) and green frog (Rana perezi), and toads may be surviving only in areas that are suboptimal for these predators. The influence of landscape features (elevation, aspect and maximum slope) on the distribution of toads and associated predators was therefore investigated using GIS. The presence of toads was positively associated with steep slopes. At sites where they occurred with toads, the distribution of predators was negatively associated with elevation. Reproductive success within toad populations was strongly associated with the number of pools at each site, while reproductive success within individual pools was positively associated with elevation. These findings may be used to optimise the design and location of future reintroduction sites.     

Mass mortality of native anuran tadpoles in tropical Australia due to the invasive cane toad (Bufo marinus)

Specific pathways of the ecological impact of invasive species remain poorly known. Although the spread of toxic cane toads (Bufo marinus) through tropical Australia is widely believed to have caused extensive mortality of native reptiles and mammals, effects of toad ingestion on native anurans have been virtually ignored. Our studies on the Adelaide River floodplain show that the most numerous vertebrate victims of toad invasion are native tadpoles that die when they attempt to consume toad eggs. We documented 11 episodes of mass mortality, totalling >1300 tadpoles of 10 species, in five waterbodies within a single wet-season shortly after the toads invaded. A causal link between toad breeding and tadpole mortality is supported by observations that: (1) in at least 9 of the 11 waterbodies involved, toads bred immediately prior to mortality events; (2) water quality was indistinguishable from that of control ponds, and tadpoles placed in that water remained healthy; (3) dead tadpoles showed no sign of disease; and (4) laboratory trials showed rapid, 100% mortality in native tadpoles exposed to freshly-laid toad eggs. Despite these high mortality rates, toad invasion does not appear to threaten the viability of anuran populations because frogs often breed in ponds not used by toads, and because density-dependent growth and survival within tadpole communities mean that additional mortality may not reduce the total effective recruitment of metamorph frogs from a waterbody.     

The recent decline of a New Zealand endemic: how and why did populations of Archey's frog Leiopelma archeyi crash over 1996-2001?

Dramatic changes have been documented in New Zealand's vertebrate faunas since human settlement, involving major declines and extinctions, but over recent years few species have declined in numbers so rapidly as the terrestrial Archey's frog Leiopelma archeyi (Anura: Leiopelmatidae). Long-term monitoring over more than 20 years revealed a major population reduction of the species over 1996-2001 and L. archeyi is now classified as Nationally Critical under the New Zealand threat classification system. The decline progressed northwards in the Coromandel ranges, and mostly larger (female) frogs survived. On a 100 m² study plot at Tapu Ridge, annual population estimates averaged 433 frogs (SE ±32) over 1984-1994, declining by 88% to average 53 frogs (SE ±8) over 1996-2002. A mean annual survival rate of 82% for most years declined to 33% over 1994-1997. There is mounting evidence to suggest that disease is the major agent of decline, supported by (1) the rapidity and severity of decline, (2) the progressive (south to north) nature of decline, and (3) finding frogs with chytriodiomycosis from Batrachochytrium dendrobatidis at the time of decline. Surprisingly, sympatric populations of the semi-aquatic Leiopelma hochstetteri have not declined dramatically, nor has a western population of L. archeyi at Whareorino, despite chytridiomycosis occurring in some frogs there. Sustaining and restoring populations of L. archeyi in New Zealand raises major challenges for conservation management.     

Environmental change as a cause of natterjack toad (Bufo calamita) declines in Britain

An investigation was made into factors which have influenced the decline of natterjack toads Bufo calamita in Britain over the last 40 years. Natterjacks are limited in Britain to two types of habitats, coastal dunes and inland heaths. By comparing the relative declines of the species on these two habitats with other changes which have occurred on them, it became clear: (1) That Bufo calamita has declined much more extensively on heaths than on dunes: (2) that this selective decline cannot be attributed to climatic changes, public pressure or direct development; (3) that large-scale changes in the flora of heathland areas have occurred concurrently with natterjack declines. These changes, which in general have not been reflected on dunes, have occurred subsequent to altered patterns of land use on the heathlands. Cessation of grazing and increased forestry activity have permitted widespread encroachment by tall vegetation, which creates shade, both changes being directly undesirable for natterjacks, and more importantly, enabling the common toad Bufo bufo to enter as a successful competitor. This latter factor is thought to have been one of the most important causes of natterjack declines on heathland.     

Habitats of the British amphibians (2): suburban parks and gardens

An investigation was made into the use of park and garden ponds as breeding sites by amphibians in Sussex, England. Of the five common British species, three were widespread in the study area; in order of abundance (of records) these were: Rana temporaria > Triturus vulgaris > Bufo bufo. Although more or less randomly distributed with respect to each other, records of amphibians generally became scarce near the most intensively developed areas of town centres. Neither pond volume, age nor local geology seemed to exert significant effects on species distribution. Aquatic vegetation was also unimportant, but fish did prove to be a relevant factor: toads tended to be absent from ponds not containing fish, whereas newts demonstrated a slight preference for such pools. Neither pH nor ionic score correlated with the distribution of any of the three species individually, but large old ponds of high ionic scores were associated with a greater abundance of toads compared with frogs in situations where the two species occurred together. Surveys indicated that in the study area about 15% of gardens contained ponds and of these at least 50% were used as breeding sites by amphibians.     

Distribution and environmental limitations of an amphibian pathogen in the Rocky Mountains, USA

Amphibian populations continue to be imperiled by the chytrid fungus (Batrachochytrium dendrobatidis). Understanding where B. dendrobatidis (Bd) occurs and how it may be limited by environmental factors is critical to our ability to effectively conserve the amphibians affected by Bd. We sampled 1247 amphibians (boreal toads and surrogates) at 261 boreal toad (Bufo boreas) breeding sites (97 clusters) along an 11° latitudinal gradient in the Rocky Mountains to determine the distribution of B. dendrobatidis and examine environmental factors, such as temperature and elevation, that might affect its distribution. The fungus was detected at 64% of all clusters and occurred across a range of elevations (1030-3550 m) and latitudes (37.6-48.6°) but we detected it in only 42% of clusters in the south (site elevations higher), compared to 84% of clusters in the north (site elevations lower). Maximum ambient temperature (daily high) explained much of the variation in Bd occurrence in boreal toad populations and thus perhaps limits the occurrence of the pathogen in the Rocky Mountains to areas where climatic conditions facilitate optimal growth of the fungus. This information has implications in global climate change scenarios where warming temperatures may facilitate the spread of disease into previously un- or little-affected areas (i.e., higher elevations). This study provides the first regional-level, field-based effort to examine the relationship of environmental and geographic factors to the distribution of B. dendrobatidis in North America and will assist managers to focus on at-risk populations as determined by the local temperature regimes, latitude and elevation.     

Impact of lakeshore development on green frog abundance

Many amphibian species exhibit metapopulation spatial dynamics and temporally are faced with local population extinction and re-colonization. These natural population fluctuations can exhibit stochastic effects when human-caused alteration and fragmentation of habitats occur during sensitive life-cycle events. In this study, we explored the effects of shoreline development on adult green frogs Rana clamitans melanota on lakes (n=24) of northern Wisconsin. We estimated green frog abundance using both auditory and direct observation surveys. The immediate shoreline habitat was mapped and placed into a Geographical Information System (GIS) for analysis. Adult green frog populations were significantly lower on lakes with varying degrees of shoreline house and cottage development than lakes with little or no development. A negative linear relationship existed between shoreline development densities and the number of adult green frogs. However, house and cottage densities alone did not directly explain this reduction. Analysis of variance (ANOVA) identified that the amount of suitable habitat, not development density, significantly affected green frog abundance. Therefore, greater development densities significantly decrease breeding habitat quality, resulting in lower adult frog abundance. These and other findings suggest that lakeshore development regulations are not protecting sensitive amphibian species.     

Beaver (Castor canadensis) as a surrogate species for conserving anuran amphibians on boreal streams in Alberta, Canada

We explored the use of beaver (Castor canadensis) as a surrogate species for amphibian conservation on small (1st-4th-order) streams in the Boreal Foothills of west-central Alberta. Anuran call surveys indicated that beaver create breeding habitat for the boreal chorus frog (Pseudacris maculata), wood frog (Rana sylvatica) and western toad (Bufo boreas). No calling males of any species were recorded on unobstructed streams. Wood frog, the most abundant species, exhibited high rates of juvenile recruitment on beaver ponds. Pitfall traps captured more wood frogs on beaver ponds versus unobstructed streams, and most individuals (84%) were young-of-year. Abundance of young-of-year was strongly correlated with percent landscape occupied by beaver ponds indicating that anurans captured along streams originated in beaver ponds. Based on a novel combination of a digital elevation model and aerial photographs examined with GIS, statistical models showed that the probability of beaver pond occurrence on streams was positively associated with stream order and dependent on the interacting effects of distance to nearest forestry cutblock and availability of beaver foods (Populus spp.). We propose that the distribution and abundance of beaver ponds could be determined over large areas quickly and inexpensively by remote sensing and used to identify and monitor amphibian habitat, and possibly, populations. This work establishes the pre-eminence of beaver-created wetlands as amphibian habitat in the Boreal Foothills and that the incorporation of dam-building patterns into forest management strategies could aid amphibian conservation.     

Terrestrial activity and conservation of adult California red-legged frogs Rana aurora draytonii in coastal forests and grasslands

The federally threatened California red-legged frog Rana aurora draytonii occupies both aquatic and terrestrial habitats in its adult life stage. The terrestrial activities of this species are not well known and require documentation to assist in the development of appropriate levels of protection under the US Endangered Species Act. We studied the terrestrial activities of radio-tagged red-legged frogs (n=8-26) inhabiting a coastal watershed in Santa Cruz County, California, during 1997-1998. In particular, we investigated (1) the use of terrestrial habitats by non-migrating adults in relation to season, breeding chronology, and precipitation, and (2) adult migration behavior, including seasonal timing, duration, distances traveled, and the use of corridors. Non-migrating red-legged frogs occupied terrestrial habitats briefly (median=4-6 days) following infrequent summer rains, but resided nearly continuously on land (median=20-30 days) from the onset of the winter wet-season until breeding activities commenced 1-2 months later. All of the non-migrating frogs remained within 130 m of their aquatic site of residence (median <25 m). Intervals spent on land were again brief during mid/late winter (median=1-4 days), despite frequent and copious rainfall. Adult migration to and from breeding sites occurred from late October through mid-May (wet season). We monitored 25 migration events between aquatic sites that were 200-2800 m apart. Short distance movements (<300 m) were completed in 1-3 days, longer movements required up to 2 months. Most migrating frogs moved overland in approximately straight lines to target sites without apparent regard to vegetation type or topography. Riparian corridors were neither essential nor preferred as migration routes. Frogs traveling overland occurred in upland habitats as far as 500 m from water. Approximately 11-22% of the adult population was estimated to migrate to and from breeding sites annually, whereas the bulk of the adult population was resident at these sites. Adequate protection of red-legged frog populations inhabiting relatively undeveloped landscapes is liable to be achieved by retaining an array of suitable habitat elements within at least 100 m of occupied aquatic sites, and by imposing seasonal limitations on detrimental human activities occurring within this zone. Specific protections for migrating frogs are probably unwarranted in forest and rangeland environments because dispersal habitat is ubiquitous and migrating frogs are widely distributed across the landscape in space and time.     

Monitoring brown frogs Rana arvalis and Rana temporaria in 120 south Swedish ponds 1989-2005. Mixed trends in different habitats

The two brown frogs Rana arvalis and Rana temporaria were monitored at 57 localities (that contained a total of 120 ponds) for up to 17 years each during the years 1989-2005. The analysis summarized below only accounts for trends within sites actually usable for frogs during the analysis period, large scale habitat losses are thus not accounted for. R. arvalis populations tended to increase over the study period. R. temporaria populations displayed no significant change. However, both species displayed significant fluctuations from year to year. These were not correlated between the two species. Localities with permanent ponds tended to display more positive population trends than localities with temporary ponds and ponds in pastures tended to display more positive trends than those in forests. For ponds in cropped fields, where only R. temporaria were found, the trend were also generally negative. Thus, long-term trends suggest that neither species is in decline. Nevertheless, the population trends observed in more exploited habitats are less positive than those in relatively unexploited habitats. We conclude frog populations in agricultural habitats should be more carefully studied to identify the factors behind the decline. The negative trend in temporary ponds are cause for a closer analysis of the effects of weather factors on frog population dynamics.     

The ecology of extinction: population fluctuation and decline in amphibians

Even among widespread species with high reproductive potentials and significant dispersal abilities, the probability of extinctions should be correlated both with population size variance and with the extent of population isolation. To address how variation in demographic characteristics and habitat requirements may reflect on the comparative risk of species decline, I examined 617 time series of population census data derived from 89 amphibian species using the normalized estimate of the realized rate of increase, ΔN, and its variance. Amphibians are demonstrably in general decline and exhibit a great range of dispersal abilities, demographic characteristics, and population sizes. I compared species according to life-history characteristics and habitat use. Among the populations examined, census declines outnumbered increases yet the average magnitudes for both declines and increases were not demonstrably different, substantiating findings of amphibian decline. This gives no support for the idea that amphibian population sizes are dictated by regimes featuring relatively rare years of high recruitment offset by intervening years of gradual decline such that declines may outnumber increases without negative effect. For any given population size, those populations living in large streams or in ponds had significantly higher variance than did populations of completely terrestrial or other stream-dwelling amphibians. This could not be related to life-history complexity as all the stream-breeding species examined have larvae and all of the wholly terrestrial species have direct development without a larval stage. Variance in ΔN was highest amongst the smallest populations in each comparison group. Estimated local extinction rates averaged 3.1% among pond-breeding frogs, 2.2% for pond-breeding salamanders, and negligible for both stream-breeding and terrestrial direct-developing species. Recoveries slightly exceeded extinctions among European pond-breeding frogs but not among North American pond-breeding frogs. Less common species had greater negative disparities between extinctions and recoveries. Species with highly fluctuating populations and high frequencies of local extinctions living in changeable environments, such pond- and torrent-breeding amphibians, may be especially susceptible to curtailment of dispersal and restriction of habitat.     

Infection of an invasive frog Eleutherodactylus coqui by the chytrid fungus Batrachochytrium dendrobatidis in Hawaii

The chytrid fungus Batrachochytrium dendrobatidis has contributed to declines and extinctions of amphibians worldwide. B. dendrobatidis is known to infect the frog Eleutherodactylus coqui in its native Puerto Rico. E. coqui was accidentally introduced into Hawaii in the late 1980s, where there are now hundreds of populations. B. dendrobatidis was being considered as a biological control agent for E. coqui because there are no native amphibians in Hawaii. Using a DNA-based assay, we tested 382 E. coqui from Hawaii for B. dendrobatidis and found that 2.4% are already infected. We found infected frogs in four of 10 study sites and on both the islands of Hawaii and Maui. This is the first report of B. dendrobatidis in wild populations in Hawaii. As the range of E. coqui expands, it may become a vector for the transmittance of B. dendrobatidis to geographic areas where B. dendrobatidis does not yet exist.     

Climatic effects on calling and spawning of the natterjack toad Bufo calamita: Discriminant analyses and applications for conservation monitoring

Unlike the other anurans native to Britain, the rare natterjack toad Bufo calamita has a protracted breeding season. Surviving populations are regularly monitored for assessment of population trends and conservation requirements. We have examined the factors influencing calling and spawning behaviour at two British sites and used discriminant analysis to identify climatic conditions associated with reproductive activity. These conditions include rainfall over a short period preceding spawning, though not necessarily on the day itself, and preferably following a spell of dry weather; maximum temperatures on the day before spawning of at least 10°C, and a series of preceding minimum temperatures of at least 5°C (on average) over 2–4 nights. We anticipate that this kind of climatic information, which is accessible without actually visiting toad sites, should maximise the efficiency of monitoring operations by allowing judicious use of limited resources.     

Changes in status of the frog (Rana temporaria) and the toad (Bufo bufo) on part of the East Anglian Fenland in Britain

An attempt has been made to determine fluctuations in status of the frog (Rana temporaria) and the toad (Bufo bufo) on parts of the Fens (10 km squares TL38, 39, 48 and 49). Information from Fenland workers indicated that both species were common from at least as far back as 1900 up until the 1930s. Severe declines occured in the 1940s, 1950s and 1960s, and by the 1970s both species were very rare. Improved drainage and the marked change from pasture to arable, with associated loss of suitable wet and damp habitat, are believed to have been responsible for the early decreases. Again, loss of habitat was likely to be the main factor causing the later declines, but pesticides probably also contributed. At present, about half of the dykes containing water at a depth which frogs in other areas normally select for spawning dry out before tadpole metamorphosis would be completed.     

Frogs under friendly fire: How accurately can the general public recognize invasive species?

Members of the general public interact with wildlife in many ways, and an inability to distinguish between species can have significant implications for conservation. For example, attempts by environmentally-concerned private citizens to control invasive species may cause collateral damage unless people can reliably distinguish native fauna from the invader. We tested the Australian public’s ability to distinguish invasive cane toads (Bufo marinus) from native frogs at egg, tadpole, subadult and adult life-history stages. Errors were common, especially for eggs and tadpoles (27-31% error rates) and less so for subadult and adult toads and frogs (5-43% error rates). Accuracy of identification was higher in people living in areas where toads occur (compared to other parts of Australia or overseas), and similar in men and women (but with a decrease in older men). Ability to identify anurans was increased by toad-identification awareness programs and membership of “toad-busting” community groups, but direct killing of cane toads by the general public may inflict substantial “friendly fire” on native frogs. In the absence of any clear evidence for ecological benefits from toad-killing, we suggest that such collecting activities should be conducted only in areas where toads are known to occur, and under the supervision of trained personnel (to identify any anuran before it is killed), rather than as an ad hoc activity pursued independently by local residents. More generally, conservation activities that involve public participation should carefully evaluate the potential rates and consequences of species misidentification by well-intentioned but untrained people.     

Declines in forage availability for bumblebees at a national scale

We assessed national scale changes in the forage plants of bumblebees in Britain, as a means of providing quantitative evidence for the likely principal cause of declines in bumblebee species. We quantified the relative value of native and long-established plant species as forage (nectar and pollen) resources for bumblebees by collating visitation data from 14 field sites across Britain. Twentieth Century changes in range and frequency of these forage plants were assessed using data from the New Atlas of the British and Irish Flora (1930-1969 to 1987-1999) and the Countryside Surveys of Britain (1978-1998). Forage plants declined in both large-scale range and local-scale frequency between the two survey periods. These changes were of greater magnitude than changes in other native plant species, reflecting serious reductions in quality of foraging habitats for bees as well as a general decline in insect-pollinated plants. Seventy-six percent of forage plants declined in frequency within 1-km squares, including those (e.g. Trifolium pratense) of particular value for threatened bumblebee species. We consider how our findings relate to other recorded changes in the British flora, how they may help to explain declines in bumblebees and how they could contribute to a conservation strategy.     

A survey of natterjack toad Bufo calamita distribution and breeding success in the north merseyside sand-dune system,England

In spring 1978, adult natterjack toads Bufo calamita assembled at 101 out of 132 sites containing water in the north Merseyside sand-dunes. A total of 2001 spawn strings was counted in 95 sites, 95% of the spawn being deposited before 11 May. Metamorphosis was confirmed at 28 sites and probably occurred at a further 12 sites distributed throughout the dunes. Most spawning took place in shallow water in slacks or excavated scrapes. Deep ponds, ditches and bomb craters were little used. Unsuccessful breeding was mainly due to shallow water bodies drying up before metamorphosis could occur. Of sites where metamorphosis was confirmed, 82% were scrapes which had been excavated for conservation purposes.Water-table trends in the dune system and various methods of censusing the adult toad population are discussed.     

Anthropogenic and natural disturbance lead to differing patterns of gene flow in the Rocky Mountain tailed frog, Ascaphus montanus

Understanding the influence of habitat alteration on population structure and persistence is critical for effective conservation strategies. Timber harvest and wildfire are two of the most prevalent disturbances across temperate forests, yet the long-term effects of these two forces on population connectivity have rarely been studied. We studied populations of the Rocky Mountain tailed frog (Ascaphus montanus) across landscapes that have experienced either timber harvest or broad scale fires. Rocky Mountain tailed frogs generally require forested habitat, and are a species of concern in managed forests. We used landscape genetic techniques to test the explanatory power of alternative paths of connectivity across both burned and harvested forests and identified topographical and climatic variables that significantly influenced gene flow. We found that timber harvest and fire led to different patterns of genetic connectivity. Widespread terrestrial gene flow was maintained across previously burned areas and connectivity was primarily limited by solar radiation. In contrast, gene flow among populations in managed forests followed riparian corridors, presumably to avoid the loss of cover due to timber harvest. Gene flow along riparian corridors in Rocky Mountain tailed frogs differs from a population of the closely related coastal tailed frog (Ascaphus truei), for which gene flow appears to occur primarily over land. These results suggest management of Rocky Mountain tailed frogs should focus on maintaining riparian buffer zones, which may be less effective for the coastal species. Thus, consideration of landscape differences is essential even for conservation of closely related, morphologically similar species.     

Terrestrial vertebrate communities at black-tailed prairie dog (Cynomys ludovicianus) towns

We capitalized on a regional-scale, anthropogenic experiment—the reduction of black-tailed prairie dog (Cynomys ludovicianus) towns across the Great Plains of North America—to test the hypothesis that decline of this species has led to declines in diversity of native grassland vertebrates of this region. We compared species richness and species composition of non-volant mammals, reptiles and amphibians at 36 prairie dog towns and 36 paired sites in the Panhandle Region of Oklahoma during the summers and falls of 1997, 1998 and 1999. We detected 30 species of mammals, 18 species of reptiles and seven species of amphibians. Comparisons between communities at prairie dog towns and paired sites in the adjacent landscape indicated that while richness per se was not necessarily higher in towns, they did harbor significantly more rare and imperiled species. Species that were positively associated with prairie dog towns during one or both seasons (summer and fall) included badgers (Taxidea taxus), eastern cottontails (Sylvilagus floridanus), coyotes (Canis latrans), grasshopper mice (Onychomys leucogaster), swift fox (Vulpes velox), pronghorn antelopes (Antilocapra americana), striped skunks (Mephitis mephitis), white-tailed deer (Odocoileus virginianus), cattle, thirteen-lined ground squirrels (Spermophilus tridecemlineatus), black-tailed jackrabbits (Lepus califonicus), barred tiger salamanders (Ambystoma tigrinum), plains spadefoot toads (Scaphiopus bombifrons), Great Plains toad (Bufo cognatus), Woodhouse's toad (Bufo woodhousii), prairie rattlesnakes (Crotalis viridis), western plains garter snakes (Thamnophis radix), Texas horned lizards (Phrynosoma cornutum), and ornate box turtles (Terrapene ornata).     

Amphibian population declines in montane southern Mexico: resurveys of historical localities

Declines of amphibian populations have been well documented in the US, Canada, and Central America, but little is known regarding the status of amphibian populations in Mexico. In 2000, we surveyed 43 transects from 3 upland regions in Guerrero and Oaxaca, Mexico. We found 161 adult amphibians belonging to 39 species, representing only 19-48% of the anuran fauna known from these regions. We found one dead (Eleutherodactylus saltator) and one dying frog (Ptychohyla erythromma) from two different streams near Chilpancingo, Guerrero. Both frogs were infected with Batrachochytridium dendrobatidis, a pathogenic fungus involved in other declines of amphibian populations. We collected 368 tadpoles; 60 (19%) tadpoles from 9 different streams among the three regions were missing mouthparts, which is indicative of infection by chytrid fungus. We report additional data from the state of Chiapas, Mexico, that document declines, disease, and apparent extirpations from that region as well. Overall, we report 31 populations (representing 24 species) that appear to have been extirpated, including populations of as many as 11 endemic species that have been missing for 16-40 years and may be extinct.