Demography of Two Mexican Spotted Owl Populations

Abstract: The Mexican Spotted Owl ( Strix occidentalis lucida ) is a threatened subspecies of the southwestern United States and northern Mexico. Although threatened, little is known about its population status. We studied the demography of the Mexican Spotted Owl in one study area in Arizona and one in New Mexico from 1991 to 1997. We estimated annual survival rates (φ), fecundity rates (   b ), and abundance (   N ) to test the hypothesis that population trends were stationary. Although annual fecundity rates (   b ^ = 0.494 for Arizona and 0.380 for New Mexico) and annual juvenile survival rates ( φ^ = 0.179 for Arizona and 0.109 for New Mexico) differed in magnitude between the study areas, they exhibited similar temporal patterns. Annual survival for territorial owls varied randomly in Arizona but declined linearly in New Mexico. Mean annual survival for territorial owls was 0.814 in Arizona and 0.832 for owls ≥3 years old and 0.644 for owls 1–2 years old in New Mexico. Based on survival and fecundity estimates, the annual rates of change (λ^) indicated that both populations were declining at ≥10% a year. These estimates were corroborated by observed declines in abundance. Some regional factor may have been affecting fecundity, whereas a combination of factors may have been affecting survival. Two possible reasons for the population declines are declines in habitat quality and regional trends in climate.     

Variation in Pesticide Tolerance of Tadpoles among and within Species of Ranidae and Patterns of Amphibian Decline

Abstract: There is significant variation among and within amphibian species with respect to reports of population decline; declining species are often found in environments that are physiograpically similar to environments where the same species is thriving. Because variability exists among organisms in their sensitivity to environmental stressors, it is important to determine the degree of this variation when undertaking conservation efforts. We conducted both lethal (time-to-death) and sublethal (activity change) assays to determine the degree of variation in the sensitivity of tadpoles to a pesticide, carbaryl, at three hierarchical levels: among ranid species, among several populations of a single ranid species (   Rana sphenocephala ), and within populations of R. sphenocephala . We observed significant variation in time to death among the nine ranid species and among the 10 R. sphenocephala populations we tested. Four out of eight R. sphenocephala populations exhibited significantly different times to death among families. The magnitude of the activity change in response to exposure to sublethal carbaryl levels was significantly different among species and within R. sphenocephala populations. Chemical contamination, at lethal or sublethal levels, can alter natural regulatory processes such as juvenile recruitment in amphibian populations and should be considered a contributing cause of declines in amphibian populations.     

Effects of Widespread Fish Introductions on Paedomorphic Newts in Europe

Abstract:  As a result of factors such as global warming, habitat destruction, and species introduction, amphibians are declining worldwide. No one, however, has analyzed the status of polymorphic amphibian species at a national or continental scale, although some local reports exist. Our aim was to report on the loss of intraspecific heterochrony as a loss to diversity in determining the consequences of fish stocking on European populations of paedomorphic newts. Paedomorphosis is a polymorphism in which larval traits are retained in the adult stage. We surveyed 39 paedomorphic populations of the alpine (  Triturus alpestris ) and palmate ( T. helveticus ) newts, all but one of which initially occupied fishless ponds and lakes in France, Italy, Slovenia, Bosnia, Montenegro, and Greece. Exotic fishes were found in 44% of the studied aquatic habitats, with a 100% presence in Montenegro. At all sites paedomorphs disappeared and metamorphs declined. Only fish explained these population changes because alternative factors such as drying were not significant. More catastrophically, fish introductions occurred in habitats known to support the largest populations of newts and even some endemic subspecies. If management and legislative measures are not taken to stop fish stocking, protect paedomorphs as conservation units at national and international levels, and restore natural habitats, all the largest paedomorphic populations may disappear in the near future. Their disappearance would represent a loss of one of the rare, fascinating examples of intraspecific heterochrony.     

Inconclusiveness of Chytridiomycosis as the Agent in Widespread Frog Declines

Abstract:  Although there is considerable evidence to support the hypothesis that the chytrid fungus Batrachochytrium dendrobatidis is the primary agent responsible for widespread declines in amphibian populations, particularly rainforest frog populations in Australia and Central America, I argue the case has not yet been made conclusively. Few specimens were collected at the time of population declines, so it may never be possible to conclusively determine their cause. It remains unclear whether the pathogen is novel where declines have occurred. Although it is not necessary that the infection be novel for it to be implicated in declines, if a preexisting pathogen has only recently caused extinctions, cofactors must be important. Whether the pattern of outbreaks represents a "wave" of extinctions is unclear, but if it does, the rate of spread in Australia is implausibly high for a waterborne pathogen, given the most likely estimates of epidemiological parameters. Although B. dendrobatidis is an amphibian pathogen according to Koch's postulates, the postulates are neither necessary nor sufficient criteria to identify a pathogen. The following key pieces of information are necessary to better understand the impact of this fungus on frog communities: better knowledge of the means and rate of transmission under field conditions, prevalence of infection among frog populations, as distinct from morbid individuals, and the effect of the fungus on frogs in the wild. It is crucial to determine whether there are strains of the fungus with differing pathogenicity to particular frog species and whether host-pathogen coevolution has occurred or is occurring. Recently developed diagnostic tools bring into reach the possibility of addressing these questions and thus developing appropriate strategies to manage frog communities that may be affected by this fungus.     

Demographic Evidence of Illegal Harvesting of an Endangered Asian Turtle

Harvesting pressure on Asian freshwater turtles is severe, and dramatic population declines of these turtles are being driven by unsustainable collection for food markets, pet trade, and traditional Chinese medicine. Populations of big‐headed turtle (Platysternon megacephalum) have declined substantially across its distribution, particularly in China, because of overcollection. To understand the effects of chronic harvesting pressure on big‐headed turtle populations, we examined the effects of illegal harvesting on the demography of populations in Hong Kong, where some populations still exist. We used mark‐recapture methods to compare demographic characteristics between sites with harvesting histories and one site in a fully protected area. Sites with a history of illegal turtle harvesting were characterized by the absence of large adults and skewed ratios of juveniles to adults, which may have negative implications for the long‐term viability of populations. These sites also had lower densities of adults and smaller adult body sizes than the protected site. Given that populations throughout most of the species’ range are heavily harvested and individuals are increasingly difficult to find in mainland China, the illegal collection of turtles from populations in Hong Kong may increase over time. Long‐term monitoring of populations is essential to track effects of illegal collection, and increased patrolling is needed to help control illegal harvesting of populations, particularly in national parks. Because few, if any, other completely protected populations remain in the region, our data on an unharvested population of big‐headed turtles serve as an important reference for assessing the negative consequences of harvesting on populations of stream turtles. Evidencia Demográfica de la Captura Ilegal de una Tortuga Asiática en Peligro     

Emerging infectious disease as a proximate cause of amphibian mass mortality

A newly discovered infectious disease of amphibians, chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis, is implicated in population declines and possible extinctions throughout the world. The purpose of our study was to examine the effects of B. dendrobatidis on the mountain yellow-legged frog (Rana muscosa) in the Sierra Nevada of California (USA). We (1) quantified the prevalence and incidence of B. dendrobatidis through repeat surveys of several hundred R. muscosa populations in the southern Sierra Nevada; (2) described the population-level effects of B. dendrobatidis on R. muscosa population abundance; and (3) compared the mortality rates of infected and uninfected R. muscosa individuals from pre- through post-metamorphosis using both laboratory and field experiments. Mouthpart inspections conducted in 144 and 132 R. muscosa populations in 2003 and 2004, respectively, indicated that 19% of R. muscosa populations in both years showed indications of chytridiomycosis. Sixteen percent of populations that were uninfected in 2003 became infected by 2004. Rana muscosa population sizes were reduced by an average of 88% following B. dendrobatidis outbreaks at six sites, but at seven B. dendrobatidis-negative sites, R. muscosa population sizes increased by an average of 45% over the same time period. In the laboratory, all infected R. muscosa developed fatal chytridiomycosis after metamorphosis, while all uninfected individuals remained healthy. In the field experiment in which R. muscosa tadpoles were caged at infected and uninfected sites, 96% of the individuals that metamorphosed at infected sites died vs. 5% at the uninfected sites. These studies indicate that chytridiomycosis causes high mortality in post-metamorphic R. muscosa, that this emerging disease is the proximate cause of numerous observed R. muscosa population declines, and that the disease threatens this species with extirpation at numerous sites in California's Sierra Nevada.     

Reservoir‐host amplification of disease impact in an endangered amphibian

Emerging wildlife pathogens are an increasing threat to biodiversity. One of the most serious wildlife diseases is chytridiomycosis, caused by the fungal pathogen, Batrachochytrium dendrobatidis (Bd), which has been documented in over 500 amphibian species. Amphibians vary greatly in their susceptibility to Bd; some species tolerate infection, whereas others experience rapid mortality. Reservoir hosts—species that carry infection while maintaining high abundance but are rarely killed by disease—can increase extinction risk in highly susceptible, sympatric species. However, whether reservoir hosts amplify Bd in declining amphibian species has not been examined. We investigated the role of reservoir hosts in the decline of the threatened northern corroboree frog (Pseudophryne pengilleyi) in an amphibian community in southeastern Australia. In the laboratory, we characterized the response of a potential reservoir host, the (nondeclining) common eastern froglet (Crinia signifera), to Bd infection. In the field, we conducted frog abundance surveys and Bd sampling for both P. pengilleyi and C. signifera. We built multinomial logistic regression models to test whether Crinia signifera and environmental factors were associated with P. pengilleyi decline. C. signifera was a reservoir host for Bd. In the laboratory, many individuals maintained intense infections (>1000 zoospore equivalents) over 12 weeks without mortality, and 79% of individuals sampled in the wild also carried infections. The presence of C. signifera at a site was strongly associated with increased Bd prevalence in sympatric P. pengilleyi. Consistent with disease amplification by a reservoir host, P. pengilleyi declined at sites with high C. signifera abundance. Our results suggest that when reservoir hosts are present, population declines of susceptible species may continue long after the initial emergence of Bd, highlighting an urgent need to assess extinction risk in remnant populations of other declined amphibian species.     

Factors Mediating Co‐Occurrence of an Economically Valuable Introduced Fish and Its Native Frog Prey

Habitat characteristics mediate predator–prey coexistence in many ecological systems but are seldom considered in species introductions. When economically important introduced predators are stocked despite known negative impacts on native species, understanding the role of refuges, landscape configurations, and community interactions can inform habitat management plans. We measured these factors in basins with introduced trout (Salmonidae) and the Cascades frog (Rana cascadae) to determine, which are responsible for observed patterns of co‐occurrence of this economically important predator and its native prey. Large, vegetated shallows were strongly correlated to co‐occurrence, and R. cascadae larvae occur in shallower water when fish are present, presumably to escape predation. The number of nearby breeding sites of R. cascadae was also correlated to co‐occurrence, but only when the western toad (Anaxyrus boreas) was present. Because A. boreas larvae are unpalatable to fish and resemble R. cascadae, they may provide protection from trout via Batesian mimicry. Although rescue‐effect dispersal from nearby populations may maintain co‐occurrence, within‐lake factors proved more important for predicting co‐occurrence. Learning which factors allow co‐occurrence between economically important introduced species and their native prey enables managers to make better‐informed stocking decisions. Factores que Median la Co‐Ocurrencia de un Pez Introducido con Valor Económico y su Presa, una Rana Nativa     

Possible Environmental Factors Underlying Amphibian Decline in Eastern Puerto Rico: Analysis of U.S. Government Data Archives

Abstract: The past three decades have seen major declines in populations of several species of amphibians at high elevations in eastern Puerto Rico, a region unique in the humid tropics because of the degree of environmental monitoring that has taken place through the efforts of U.S. government agencies. I examined changes in environmental conditions by examining time-series data sets that extend back at least into the 1980s, a period when frog populations were declining. The data include forest cover; annual mean, minimum, and maximum daily temperature; annual rainfall; rain and stream chemistry; and atmospheric-dust transport. I examined satellite imagery and air-chemistry samples from a single National Aeronautics and Space Administration aircraft flight across the Caribbean showing patches of pollutants, described as thin sheets or lenses, in the lower troposphere. The main source of these pollutants appeared to be fires from land clearing and deforestation, primarily in Africa. Some pollutant concentrations were high and, in the case of ozone, approached health limits set for urban air. Urban pollution impinging on Puerto Rico, dust generation from Africa ( potential soil pathogens), and tropical forest burning ( gaseous pollutants) have all increased during the last three decades, overlapping the timing of amphibian declines in eastern Puerto Rico. None of the data sets pointed directly to changes so extreme that they might be considered a direct lethal cause of amphibian declines in Puerto Rico. More experimental research is required to link any of these environmental factors to this problem.     

Estimating the Effect of Protected Lands on the Development and Conservation of Their Surroundings

Abstract:  The fate of private lands is widely seen as key to the fate of biodiversity in much of the world. Organizations that work to protect biodiversity on private lands often hope that conservation actions on one piece of land will leverage the actions of surrounding landowners. Few researchers have, however, examined whether protected lands do in fact encourage land conservation nearby or how protected lands affect development in the surrounding landscape. Using spatiotemporal data sets on land cover and land protection for three sites (western North Carolina, central Massachusetts, and central Arizona), we examined whether the existence of a protected area correlates with an increased rate of nearby land conservation or a decreased rate of nearby land development. At all sites, newly protected conservation areas tended to cluster close to preexisting protected areas. This may imply that the geography of contemporary conservation actions is influenced by past decisions on land protection, often made for reasons far removed from concerns about biodiversity. On the other hand, we found no evidence that proximity to protected areas correlates with a reduced rate of nearby land development. Indeed, on two of our three sites the development rate was significantly greater in regions with more protected land. This suggests that each conservation action should be justified and valued largely for what is protected on the targeted land, without much hope of broader conservation leverage effects.     

两栖动物种群衰退研究进展

过去50a中，全球范围内许多两栖动物种群显著衰退，一些种类已经灭绝．研究发现，这类现象的出现与人类活动日益加强密切相关，生态环境的破坏，污染的排放、气候的改变、野生动植物疾病的流行等因素是导致这一现象的主要原因.本文总结介绍了近年来国际上本领域的最新研究成果与动态，并对我国的研究现状和未来作了讨论．图1参40     

Patterns of body mass senescence and selective disappearance differ among three species of free-living ungulates

Declines in survival and reproduction with age are prevalent in wild vertebrates, but we know little about longitudinal changes in behavioral, morphological, or physiological variables that may explain these demographic declines. We compared age-related variation in body mass of adult females in three free-living ungulate populations that have been the focus of long-term, individual-based research: bighorn sheep (Ovis canadensis) at Ram Mountain, Canada; roe deer (Capreolus capreolus) at Trois Fontaines, France; and Soay sheep (Ovis aries) on St. Kilda, Scotland. We use two recently proposed approaches to separate contributions to age-dependent variation at the population level from within-individual changes and between-individual selective disappearance. Selective disappearance of light individuals in all three populations was most evident at the youngest and oldest ages. In later adulthood, bighorn sheep and roe deer showed a continuous decline in body mass that accelerated with age while Soay sheep showed a precipitous decrease in mass in the two years preceding death. Our results highlight the importance of mass loss in explaining within-individual demographic declines in later adulthood in natural populations. They also reveal that the pattern of senescence, and potentially also the processes underlying demographic declines in late life, can differ markedly across related species with similar life histories.     

Cooling of bat hibernacula to mitigate white-nose syndrome

White-nose syndrome (WNS) is a fungal disease that has caused precipitous declines in several North American bat species, creating an urgent need for conservation. We examined how microclimates and other characteristics of hibernacula have affected bat populations following WNS-associated declines and evaluated whether cooling of warm, little-used hibernacula could benefit bats. During the period following mass mortality (2013–2020), we conducted 191 winter surveys of 25 unmanipulated hibernacula and 6 manipulated hibernacula across Pennsylvania (USA). We joined these data with additional datasets on historical (pre-WNS) bat counts and on the spatial distribution of underground sites. We used generalized linear mixed models and model selection to identify factors affecting bat populations. Winter counts of Myotis lucifugus were higher and increased over time in colder hibernacula (those with midwinter temperatures of 3–6 °C) compared with warmer (7–11 °C) hibernacula. Counts of Eptesicus fuscus, Myotis leibii, and Myotis septentrionalis were likewise higher in colder hibernacula (temperature effects = –0.73 [SE 0.15], –0.51 [0.18], and –0.97 [0.28], respectively). Populations of M. lucifugus and M. septentrionalis increased most over time in hibernacula surrounded by more nearby sites, whereas Eptesicus fuscus counts remained high where they had been high before WNS onset (pre-WNS high count effect = 0.59 [0.22]). Winter counts of M. leibii were higher in hibernacula with high vapor pressure deficits (VPDs) (particularly over 0.1 kPa) compared with sites with lower VPDs (VPD effect = 15.3 [4.6]). Counts of M. lucifugus and E. fuscus also appeared higher where VPD was higher. In contrast, Perimyotis subflavus counts increased over time in relatively warm hibernacula and were unaffected by VPD. Where we manipulated hibernacula, we achieved cooling of on average 2.1 °C. At manipulated hibernacula, counts of M. lucifugus and P. subflavus increased over time (years since manipulation effect = 0.70 [0.28] and 0.51 [0.15], respectively). Further, there were more E. fuscus where cooling was greatest (temperature difference effect = –0.46 [SE 0.11]), and there was some evidence there were more P. subflavus in hibernacula sections that remained warm after manipulation. These data show bats are responding effectively to WNS through habitat selection. In M. lucifugus, M. septentrionalis, and possibly P. subflavus, this response is ongoing, with bats increasingly aggregating at suitable hibernacula, whereas E. fuscus remain in previously favored sites. Our results suggest that cooling warm sites receiving little use by bats is a viable strategy for combating WNS.     

Effects of Amphibian Chytrid Fungus on Individual Survival Probability in Wild Boreal Toads

Abstract: Chytridiomycosis is linked to the worldwide decline of amphibians, yet little is known about the demographic effects of the disease. We collected capture–recapture data on three populations of boreal toads (Bufo boreas [Bufo = Anaxyrus]) in the Rocky Mountains (U.S.A.). Two of the populations were infected with chytridiomycosis and one was not. We examined the effect of the presence of amphibian chytrid fungus (Batrachochytrium dendrobatidis [Bd]; the agent of chytridiomycosis) on survival probability and population growth rate. Toads that were infected with Bd had lower average annual survival probability than uninfected individuals at sites where Bd was detected, which suggests chytridiomycosis may reduce survival by 31–42% in wild boreal toads. Toads that were negative for Bd at infected sites had survival probabilities comparable to toads at the uninfected site. Evidence that environmental covariates (particularly cold temperatures during the breeding season) influenced toad survival was weak. The number of individuals in diseased populations declined by 5–7%/year over the 6 years of the study, whereas the uninfected population had comparatively stable population growth. Our data suggest that the presence of Bd in these toad populations is not causing rapid population declines. Rather, chytridiomycosis appears to be functioning as a low‐level, chronic disease whereby some infected individuals survive but the overall population effects are still negative. Our results show that some amphibian populations may be coexisting with Bd and highlight the importance of quantitative assessments of survival in diseased animal populations.     

Flood regime and leaf fall determine soil inorganic nitrogen dynamics in semiarid riparian forests.

Flow regulation has reduced the exchange of water, energy, and materials between rivers and floodplains, caused declines in native plant populations, and advanced the spread of nonnative plants. Naturalized flow regimes are regarded as a means to restore degraded riparian areas. We examined the effects of flood regime (short [SIFI] vs. long [LIFI] inter-flood interval) on plant community and soil inorganic nitrogen (N) dynamics in riparian forests dominated by native Populus deltoides var. wislizenii Eckenwalder (Rio Grande cottonwood) and nonnative Tamarix chinensis Lour. (salt cedar) along the regulated middle Rio Grande of New Mexico. The frequency of inundation (every 2-3 years) at SIFI sites better reflected inundation patterns prior to the closure of an upstream dam relative to the frequency of inundation at LIFI sites (> or =10 years). Riparian inundation at SIFI sites varied from 7 to 45 days during the study period (April 2001-July 2004). SIFI vs. LIFI sites had higher soil moisture but greater groundwater table elevation fluctuation in response to flooding and drought. Rates of net N mineralization were consistently higher at LIFI vs. SIFI sites, and soil inorganic N concentrations were greatest at sites with elevated leaf-litter production. Sites with stable depth to ground water (approximately 1.5 m) supported the greatest leaf-litter production. Reduced leaf production at P. deltoides SIFI sites was attributed to drought-induced recession of ground water and prolonged inundation. We recommend that natural resource managers and restoration practitioners (1) utilize naturalized flows that help maintain riparian groundwater elevations between 1 and 3 m in reaches with mature P. deltoides or where P. deltoides revegetation is desired, (2) identify areas that naturally undergo long periods of inundation and consider restoring these areas to seasonal wetlands, and (3) use native xeric-adapted riparian plants to revegetate LIFI and SIFI sites where groundwater elevations commonly drop below 3 m.     

The Pending Extinction of the Uncompahgre Fritillary Butterfly

Previous mark-recapture studies and population size estimates indicated that in the 1980s populations of the endangered Uncompahgre fritillary ( Boloria acrocnema ) declined precipitously, apparently leading to extirpation at its type locality. This locality and a nearby second site, both high in the San Juan Mountains of southwestern Colorado, constitute the total known range of B. acrocnema . A survey of 20 presumptive allozyme loci from one B. acrocnema population and seven populations of the closely related B. improba , sampled along the Rocky Mountain cordillera from the Yukon Territory, Canada, to the Wind River Range of Wyoming, revealed that B. acrocnema populations are less variable genetically than are B. improba populations in the northern part of that species' North American range. Genetic variability was also assayed at 18 presumptive allozyme loci in populations of Boloria titania, which were sampled from five of the same locations for comparison with the B. improba-B. acrocnema clade. Low estimates of heterozygosity indicate that genetic and demographic factors preceded by historic human disturbance are causing the decline of B. acrocnema . In addition, persistent drought conditions in southwestern Colorado throughout the 1980s probably contributed to the population declines. Conservation recommendations include strict enforcement of the ban against collecting pursuant to the Endangered Species Act, careful monitoring of known B. acrocnema sites, and possible translocations. Extreme measures beyond these will be costly and may be inappropriate for this relict species, which has an unknown capacity to persist in the face of local weather fluctuations and predicted regional warming trends.     

Epidemic Disease and the Catastrophic Decline of Australian Rain Forest Frogs

In the montane rain forests of eastern Australia at least 14 species of endemic, stream-dwelling frogs have disappeared or declined sharply (by more than 90%) during the past 15 years. We review available information on these declines and present eight lines of evidence that collectively suggest that a rapidly spreading, epidemic disease is the most likely responsible agent. The extreme virulence of the putative frog patbogen suggests it is likely exotic to Australian rain forests. We propose that exotic pathogens may be responsible for some recent declines of amphibian populations on other continents and that the intercontinental spread of such pathogens is greatly facilitated by human activities such as the thriving international trade in aquarium fish. Our hypothesis may help explain why some amphibian populations in seemingly pristine environments have mysteriously declined.     

Changes in sea urchin populations after the destruction of kelp beds

In St. Margaret's Bay, Nova Scotia, Canada, there are large areas in which sea urchins (Strongylocentrotus droebachiensis) have eliminated beds of kelp (Laminaria spp.). Sites were identified where destruction of kelp beds had taken place 1, 2, 3, 3.5 and 4 years ago. With increase of time since kelp disappearance, the sea urchins showed decreased growth rate, reduced gonad size, but an increase in numbers resulting from high recruitment rates in the first two years after kelp bed destruction. These sea urchin populations, by their browsing, effectively prevent the regeneration of kelp. There is, as yet, no evidence of the sea urchin populations being starved out to allow the kelp to return.     

Remarkable Amphibian Biomass and Abundance in an Isolated Wetland: Implications for Wetland Conservation

Abstract:  Despite the continuing loss of wetland habitats and associated declines in amphibian populations, attempts to translate wetland losses into measurable losses to ecosystems have been lacking. We estimated the potential productivity from the amphibian community that would be compromised by the loss of a single isolated wetland that has been protected from most industrial, agricultural, and urban impacts for the past 54 years. We used a continuous drift fence at Ellenton Bay, a 10-ha freshwater wetland on the Savannah River Site, near Aiken, South Carolina (U.S.A.), to sample all amphibians for 1 year following a prolonged drought. Despite intensive agricultural use of the land surrounding Ellenton Bay prior to 1951, we documented 24 species and remarkably high numbers and biomass of juvenile amphibians (>360,000 individuals; >1,400 kg) produced during one breeding season. Anurans (17 species) were more abundant than salamanders (7 species), comprising 96.4% of individual captures. Most (95.9%) of the amphibian biomass came from 232095 individuals of a single species of anuran (southern leopard frog [Rana sphenocephala ]). Our results revealed the resilience of an amphibian community to natural stressors and historical habitat alteration and the potential magnitude of biomass and energy transfer from isolated wetlands to surrounding terrestrial habitat. We attributed the postdrought success of amphibians to a combination of adult longevity (often >5 years), a reduction in predator abundance, and an abundance of larval food resources. Likewise, the increase of forest cover around Ellenton Bay from <20% in 1951 to >60% in 2001 probably contributed to the long-term persistence of amphibians at this site. Our findings provide an optimistic counterpoint to the issue of the global decline of biological diversity by demonstrating that conservation efforts can mitigate historical habitat degradation.     

Multiple stressors and amphibian declines: dual impacts of pesticides and fish on yellow-legged frogs.

More than 40% of Earth's 5700+ amphibian species have undergone recent declines. Despite the likely involvement of multiple factors in driving these declines, most studies continue to focus on single stressors. In California (USA), separate studies have implicated either introduced fish or pesticides as causal agents. To date, however, no study has simultaneously evaluated the respective roles of these two potential stressors nor attempted to assess their relative importance, information critical for the development of effective conservation efforts and environmental policies. We examined the role and relative effect of fish and pesticides on the mountain yellow-legged frog (Rana muscosa) using unusually detailed data sets for a large portion of R. muscosa's historic range in California's Sierra Nevada. Habitat characteristics and presence/absence of R. muscosa and fish were quantified at each of 6831 sites during field surveys. Pesticide use upwind of each site was calculated from pesticide application records and predominant wind directions. Using generalized additive models, we found that, after accounting for habitat effects, the probability of R. muscosa presence was significantly reduced by both fish and pesticides, with the landscape-scale effect of pesticides much stronger than that of fish. The degree to which a site was sheltered from the predominant wind (and associated pesticides) was also a significant predictor of R. muscosa presence. Taken together, these results represent the strongest evidence to date that windborne pesticides are contributing to amphibian declines in pristine locations. Our results suggest that amphibian declines may have complex multi-factorial causes, and caution that single-factor studies that demonstrate the importance of one factor should not be used as evidence against the importance of other factors.