Joseph M. Kiesecker

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.

Synergism between trematode infection and pesticide exposure: a link to amphibian limb deformities in nature?

The apparently rapid increase in the prevalence of amphibian limb deformities has led to substantial interest from ecologists and public health professionals. Hypotheses proposed to explain the deformities fall into two broad categories: chemical contaminants and trematode infection. Although there are convincing experimental demonstrations that certain factors can lead to some deformities, the causes for recent increases in amphibian malformation remain controversial. Moreover, no experimental studies on amphibian deformities have been conducted in the field, and no studies have attempted to examine the synergistic effects of trematode infection and exposure to chemical contaminants. Here, I present the results of field and laboratory experiments that link increased trematode infection, and increased limb deformities, to pesticide exposure. Field experiments conclusively demonstrated that exposure to trematode infection was required for the development of limb deformities in wood frogs, Rana sylvatica. However, deformities were more common at sites adjacent to agricultural runoff. Laboratory experiments corroborated the association between pesticide exposure and increased infection with pesticide-mediated immunocompetency as the apparent mechanism. Given the conservative contaminant exposure levels used [Environmental Protection Agency (EPA) drinking water standards] and the widespread use of many pesticides, these negative impacts may help to explain pathogen-mediated amphibian declines in many regions.

Policy Development for Biodiversity Offsets: A Review of Offset Frameworks

Biodiversity offsets seek to compensate for residual environmental impacts of planned developments after appropriate steps have been taken to avoid, minimize or restore impacts on site. Offsets are emerging as an increasingly employed mechanism for achieving net environmental benefits, with offset policies being advanced in a wide range of countries (i.e., United States, Australia, Brazil, Colombia, and South Africa). To support policy development for biodiversity offsets, we review a set of major offset policy frameworks—US wetlands mitigation, US conservation banking, EU Natura 2000, Australian offset policies in New South Wales, Victoria, and Western Australia, and Brazilian industrial and forest offsets. We compare how the frameworks define offset policy goals, approach the mitigation process, and address six key issues for implementing offsets: (1) equivalence of project impacts with offset gains; (2) location of the offset relative to the impact site; (3) “additionality” (a new contribution to conservation) and acceptable types of offsets; (4) timing of project impacts versus offset benefits; (5) offset duration and compliance; and (6) “currency” and mitigation replacement ratios. We find substantial policy commonalities that may serve as a sound basis for future development of biodiversity offsets policy. We also identify issues requiring further policy guidance, including how best to: (1) ensure conformance with the mitigation hierarchy; (2) identify the most environmentally preferable offsets within a landscape context; and (3) determine appropriate mitigation replacement ratios.

FOREST CANOPY AND THE PERFORMANCE OF LARVAL AMPHIBIANS

Freshwater ponds inhabited by larval amphibians can become encroached by forest vegetation. Surveys reveal that gradients formed by canopy cover can act as a selective sieve for the distribution of amphibian larvae among ponds. In eastern North America, many species, including spring peepers (Pseudacris crucifer), are usually absent from ponds where forest canopies have closed over the pond basin. Relatively fewer species, including wood frogs (Rana sylvatica), are commonly present in both closed and open canopy ponds. Long-term observations reveal that canopy development is associated with extinction of spring peeper populations and persistence of wood frog populations. We evaluated performance of spring peeper and wood frog larvae in a set of experiments designed to assess three predictions: (1) that conditions in closed canopy ponds (e.g., lower temperature) are associated with depressed growth rates, (2) that the impact of canopy cover will be more severe on an open canopy specialist vs. a canopy generalist species, and (3) that abiotic differences between closed and open canopy ponds may provide a sufficient explanation for performance patterns. In support of the first prediction, a field transplant experiment showed that both species grew slower in closed vs. open canopy ponds. We found some support for the second prediction during two separate experiments. During the field experiment, the impact of canopy on spring peeper growth was similar to the effect on wood frogs. However, in a common garden experiment (in which physical conditions tended to coverge), spring peepers raised on substrates and water from closed canopy ponds grew substantially slower than conspecifics in the presence of water and substrate from open canopy ponds. By contrast, wood frogs grew faster in the closed canopy treatment. Finally, while recognizing that abiotic variation appears to play an important role in performance across the canopy gradient, we rejected the third hypothesis. We found that a biotic manipulation (food addition) partially mitigated depressed growth rates in closed canopy ponds. This result suggests that biotic factors (e.g., variation in abundance and composition of periphyton) also contribute to performance differences across the canopy gradient.

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.

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.

Development by design: blending landscape‐level planning with the mitigation hierarchy

Compensatory mitigation, or biodiversity offsets, provide a mechanism for maintaining or enhancing environmental values in situations where development is being planned, despite detrimental environmental impacts. Offsets are generally intended as an option for addressing any remaining environmental impacts of a development plan, after efforts have been made to avoid, minimize, or restore on-site impacts. Although offset programs require that developers adhere to the mitigation hierarchy to avoid, minimize, and restore biodiversity on-site before considering an offset for residual impacts, no quantitative guidelines exist for this decision-making process. What criteria are needed to require that impacts be minimized or avoided altogether? Here, we examine how conservation planning can provide a way to address this issue. By blending landscape-level conservation planning with application of the mitigation hierarchy, we can ensure that the use of biodiversity offsets is consistent with sustainable developmen...

Epidermal ‘alarm substance’ cells of fishes maintained by non-alarm functions: possible defence against pathogens, parasites and UVB radiation

Many fishes possess specialized epidermal cells that are ruptured by the teeth of predators, thus reliably indicating the presence of an actively foraging predator. Understanding the evolution of these cells has intrigued evolutionary ecologists because the release of these alarm chemicals is not voluntary. Here, we show that predation pressure does not influence alarm cell production in fishes. Alarm cell production is stimulated by exposure to skin-penetrating pathogens (water moulds: Saprolegnia ferax and Saprolegnia parasitica), skin-penetrating parasites (larval trematodes: Teleorchis sp. and Uvulifer sp.) and correlated with exposure to UV radiation. Suppression of the immune system with environmentally relevant levels of Cd inhibits alarm cell production of fishes challenged with Saprolegnia. These data are the first evidence that alarm substance cells have an immune function against ubiquitous environmental challenges to epidermal integrity. Our results indicate that these specialized cells arose and are maintained by natural selection owing to selfish benefits unrelated to predator–prey interactions. Cell contents released when these cells are damaged in predator attacks have secondarily acquired an ecological role as alarm cues because selection favours receivers to detect and respond adaptively to public information about predation.

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

A Framework for Implementing Biodiversity Offsets: Selecting Sites and Determining Scale

Biodiversity offsets provide a mechanism for maintaining or enhancing environmental values in situations where development is sought despite detrimental environmental impacts. They seek to ensure that unavoidable negative environmental impacts of development are balanced by environmental gains, with the overall aim of achieving a net neutral or positive outcome. Once the decision has been made to offset, multiple issues arise regarding how to do so in practice. A key concern is site selection. In light of the general aim to locate offsets close to the affected sites to ensure that benefits accrue in the same area, what is the appropriate spatial scale for identifying potential offset sites (e.g., local, ecoregional)? We use the Marxan site-selection algorithm to address conceptual and methodological challenges associated with identifying a set of potential offset sites and determining an appropriate spatial scale for them. To demonstrate this process, we examined the design of offsets for impacts from development on the Jonah natural gas field in Wyoming.