Andrew R. Blaustein

Complex causes of amphibian population declines

Amphibian populations have suffered widespread declines and extinctions in recent decades. Although climatic changes, increased exposure to ultraviolet-B (UV-B) radiation and increased prevalence of disease have all been implicated at particular localities, the importance of global environmental change remains unclear. Here we report that pathogen outbreaks in amphibian populations in the western USA are linked to climate-induced changes in UV-B exposure. Using long-term observational data and a field experiment, we examine patterns among interannual variability in precipitation, UV-B exposure and infection by a pathogenic oomycete, Saprolegnia ferax. Our findings indicate that climate-induced reductions in water depth at oviposition sites have caused high mortality of embryos by increasing their exposure to UV-B radiation and, consequently, their vulnerability to infection. Precipitation, and thus water depth/UV-B exposure, is strongly linked to El Niño/Southern Oscillation cycles, underscoring the role of large-scale climatic patterns involving the tropical Pacific. Elevated sea-surface temperatures in this region since the mid-1970s, which have affected the climate over much of the world, could be the precursor for pathogen-mediated amphibian declines in many regions.

Projected climate-induced faunal change in the Western Hemisphere

Climate change is predicted to be one of the greatest drivers of ecological change in the coming century. Increases in temperature over the last century have clearly been linked to shifts in species distributions. Given the magnitude of projected future climatic changes, we can expect even larger range shifts in the coming century. These changes will, in turn, alter ecological communities and the functioning of ecosystems. Despite the seriousness of predicted climate change, the uncertainty in climate-change projections makes it difficult for conservation managers and planners to proactively respond to climate stresses. To address one aspect of this uncertainty, we identified predictions of faunal change for which a high level of consensus was exhibited by different climate models. Specifically, we assessed the potential effects of 30 coupled atmosphere-ocean general circulation model (AOGCM) future-climate simulations on the geographic ranges of 2954 species of birds, mammals, and amphibians in the Western Hemisphere. Eighty percent of the climate projections based on a relatively low greenhouse-gas emissions scenario result in the local loss of at least 10% of the vertebrate fauna over much of North and South America. The largest changes in fauna are predicted for the tundra, Central America, and the Andes Mountains where, assuming no dispersal constraints, specific areas are likely to experience over 90% turnover, so that faunal distributions in the future will bear little resemblance to those of today.

Population differences in responses of red-legged frogs (Rana aurora) to introduced bullfrogs

We studied eight populations of the red-legged frog, Rana aurora, to examine responses of allotopic and syntopic tadpoles to the bullfrog, Rana catesbeiana, an introduced predator of R. aurora. We also assessed predation rates by R. catesbeiana on syntopic and allotopic populations of R. aurora. Syntopic R. aurora tadpoles significantly reduced their activity and increased their refuge use when presented with the chemical cues of both tadpoles and adult R. catesbeiana. In contrast, allotopic tadpoles did not significantly alter their behavior in the presence of either R. catesbeiana adults or larvae. Predation by R. catesbeiana was lower in syntopic than in allotopic populations of R. aurora tadpoles. Our results show differential responses of syntopic and allotopic R. aurora tadpoles to larval and adult R. catesbeiana. Syntopic tadpoles avoid predation by R. catesbeiana more efficiently than do tadpoles from allotopic populations. Apparently, individuals that are unfamiliar with novel, introduced organisms may not possess adaptations that would prevent a negative encounter.

Pathogenic fungus contributes to amphibian losses in the pacific northwest

Abstract Worldwide declines in amphibian populations have been the subject of numerous recent reports, and numerous hypotheses have been constructed to address the causes. There is no evidence for a single cause for the declines. We identify and describe the spread of a pathogenic fungus that appears to be largely responsible for egg mortality in one population of western toad Bufo boreas. This is the first study documenting contemporary mortality in an amphibian population with identification of the attributed pathogenic species. The fungus we identify is circumglobally distributed and we suggest that this fungal infection could also be a major contributor to other amphibian populations declines.

PARASITE (RIBEIROIA ONDATRAE) INFECTION LINKED TO AMPHIBIAN MALFORMATIONS IN THE WESTERN UNITED STATES

Parasites and pathogens can influence the survivorship, behavior, and very structure of their host species. For example, experimental studies have shown that trematode parasites can cause high frequencies of severe limb malformations in amphibians. In a broad-scale field survey covering parts of California, Oregon, Washington, Idaho, and Montana, we examined relationships between the frequency and types of morphological abnormalities in amphibians and the abundance of trematode parasite infection, pH, con- centrations of 61 pesticides, and levels of orthophosphate and total nitrate. We recorded severe malformations at frequencies ranging from 1% to 90% in nine amphibian species from 53 aquatic systems. Infection of larvae by the trematode Ribeiroia ondatrae was associated with, and functionally related to, higher frequencies of amphibian limb malfor- mations than found in uninfected populations (#5%). Parasites were concentrated around the basal tissue of hind limbs in infected anurans, and malformations associated with infection included skin webbings, supernumerary limbs and digits, and missing or mal- formed hind limbs. In the absence of Ribeiroia, amphibian populations exhibited low (0- 5%) frequencies of abnormalities involving missing digits or distal portions of a hind limb. Species were affected differentially by the parasite, and Ambystoma macrodactylum , Hyla regilla, Rana aurora, R. luteiventris, and Taricha torosa typically exhibited the highest frequencies of abnormalities. None of the water-quality variables measured was associated with malformed amphibians, but aquatic snail hosts (Planorbella spp.) were significant predictors of the presence and abundance of Ribeiroia infection. Morphological comparisons of adult specimens of Ribeiroia collected from different sites and raised in experimental definitive hosts suggested that all samples represented the same species— R. ondatrae. These field results, coupled with experimental research on the effects of Ribeiroia on amphibians, demonstrate that Ribeiroia infection is an important and widespread cause of amphibian limb malformations in the western United States. The relevance of trematode infection to declines of amphibian populations and the influence of habitat modification on the pathology and life cycle of Ribeiroia are emphasized as areas requiring further research.

The complexity of deformed amphibians

Many amphibian populations have disappeared or are in decline throughout the world. In addition, more than 60 different species of amphibians with severe abnormalities have been found in the US and several other countries. These complex, perhaps interrelated phenomena are associated with important current challenges in conservation biology. Although intense research, beginning in the early 1990s, has led to a better understanding of why amphibian populations are declining, there is still a basic lack of knowledge about the causes and implications of amphibian deformities.

The complexity of amphibian population declines: understanding the role of cofactors in driving amphibian losses

Population losses and extinctions of species are occurring at unprecedented rates, as exemplified by declines and extinctions of amphibians worldwide. However, studies of amphibian population declines generally do not address the complexity of the phenomenon or its implications for ecological communities, focusing instead on single factors affecting particular amphibian species. We argue that the causes for amphibian population declines are complex; may differ among species, populations, and life stages within a population; and are context dependent with multiple stressors interacting to drive declines. Because amphibians are key components of communities, we emphasize the importance of investigating amphibian declines at the community level. Selection pressures over evolutionary time have molded amphibian life history characteristics, such that they may remain static even in the face of strong, recent human‐induced selection pressures.

POTENTIAL MECHANISMS UNDERLYING THE DISPLACEMENT OF NATIVE RED-LEGGED FROGS BY INTRODUCED BULLFROGS

The bullfrog (Rana catesbeiana) is a widespread invasive species that may displace several species of native ranid frogs throughout its introduced range. Although this pattern is well known, the underlying mechanism of displacement remains unclear. Previous work has suggested that interactions with bullfrogs may contribute to the popu- lation decline of native red-legged frogs ( Rana aurora) in Oregon, USA. Interactions be- tween these species appear to be strongly context dependent and potentially influenced by habitat modification. To gain a more detailed understanding of this displacement, we studied the effects of food-resource distribution, a factor that can be influenced by human habitat alteration, on competitive interactions between larval red-legged frogs and larval bullfrogs. The presence of bullfrog larvae had strong negative effects on the performance of red- legged frog larvae. However, this effect was dependent on whether food resources were clumped or scattered. Survivorship to metamorphosis and mass at metamorphosis were reduced when red-legged frog tadpoles were exposed to bullfrogs in clumped-resource ponds. In contrast, the presence of bullfrogs had a negligible effect on larval performance of red-legged frogs in scattered-resource ponds. Behavioral observations indicate that a passive interference mechanism is likely to be responsible for the outcome of interactions between bullfrogs and red-legged frogs. Our results suggest that clumped resources can intensify interspecific competition, and this may influence the success of exotics when human-induced habitat alteration affects resource distribution. Understanding the context-dependent nature of interactions will be necessary if we are to predict invasion success and control the impact of exotics on natives.

Amphibian defenses against ultraviolet-B radiation.

SUMMARY As part of an overall decline in biodiversity, amphibian populations throughout the world are disappearing. There are a number of potential causes for these declines, including those related to environmental changes such as increasing ultraviolet‐B (UV‐B) radiation due to stratospheric ozone depletion. UV‐B radiation can kill amphibian embryos or can cause sublethal effects that can harm amphibians in later life stages. However, amphibians have defenses against UV‐B damage that can limit damage or repair it after exposure to UV‐B radiation. These include behavioral, physiological, and molecular defenses. These defenses differ interspecifically, with some species more able to cope with exposure to UV‐B than others. Unfortunately, the defense mechanisms of many species may not be effective against increasing persistent levels of UV‐B radiation that have only been present for the past several decades due to human‐induced environmental damage. Moreover, we predict that persistent UV‐B–induced mortality and sublethal damage in species without adequate defenses could lead to changes in community structure. In this article we review the effects of UV‐B radiation on amphibians and the defenses they use to avoid solar radiation and make some predictions regarding community structure in light of interspecific differences in UV‐B tolerance.

Ambient Ultraviolet Radiation Causes Mortality in Salamander Eggs

Previous research has shown that amphibian species have differential sensitivity to ultraviolet-B (UV-B) radiation. In some anuran species, ambient levels of UV-B cause mortality in embryonic stages and hatching success is significantly reduced. Projected increases in UV-B may affect an increasing number of species. The adverse effects of UV-B may eventually be manifested at the population level and may ultimately contribute to population declines. Using field experiments, we investigated the effects of ambient UV-B on salamander (Ambystoma gracile) embryos developing at natural oviposition sites. We show that the hatching success of eggs of A. gracile shielded from UV-B is significantly higher than those not shielded from UV-B. See full-text article at JSTOR