Pesticide alters oviposition site selection in gray treefrogs

Poison in the nursery: Mercury contamination in the tadpole-rearing sites of an Amazonian frog

Artisanal and small-scale gold mining (ASGM) has become a major threat for Neotropical forests. This technique for obtaining gold is a substantial driver of small-scale deforestation and the largest contributor of Hg emissions to both the atmosphere and freshwater systems globally. Previous studies have demonstrated the impacts of Hg accumulation on various aquatic ecosystems and organisms. However, its consequences in other, more discrete systems such as phytotelmata (water-holding plant structures), and the organisms therein, have so far gone unnoticed. Here, we show high concentrations of Hg (mean ± SD: 1.43 ± 2.19 ppm) in phytotelmata and other small pools, the aquatic microenvironments used by the Neotropical poison frog Dendrobates tinctorius as tadpole-rearing sites. In 17 % of the cases, we detected Hg concentrations above the severe effect level (SEL = 2 ppm) for freshwater sediments. Hg concentrations varied depending on pool characteristics and tended to increase in proximity to known ASGM sites. We did not find an effect of Hg concentration on the number of D. tinctorius tadpoles in a given pool. Tadpoles were found in pools with concentrations of up to 8.68 ppm, suggesting that D. tinctorius fathers do not avoid pools with high Hg levels for tadpole deposition. While further research is needed to determine the potential effects of Hg on tadpole development, we found an intriguing tendency for tadpoles in later developmental stages to have lower body condition when occurring in pools with higher Hg concentrations. Our findings provide evidence of relevant Hg concentrations in the terrestrial water systems used by phytotelm-breeding anurans, and highlight the need of further field and experimental studies investigating the implications of Hg contamination for tadpole development and behaviour and the overall conservation of Amazonian biodiversity.

Avoidance behavior of juvenile common toads (Bufo bufo) in response to surface contamination by different pesticides

Most agricultural soils are expected to be contaminated with agricultural chemicals. As the exposure to pesticides can have adverse effects on non-target organisms, avoiding contaminated areas would be advantageous on an individual level, but could lead to a chemical landscape fragmentation with disadvantages on the metapopulation level. We investigated the avoidance behavior of juvenile common toads (Bufo bufo) in response to seven pesticide formulations commonly used in German vineyards. We used test arenas filled with silica sand and oversprayed half of each with different pesticide formulations. We placed a toad in the middle of an arena, filmed its behavior over 24 hours, calculated the proportion of time a toad spent on the contaminated side and compared it to a random side choice. We found evidence for the avoidance of the folpet formulation Folpan® 500 SC, the metrafenone formulation Vivando® and the glyphosate formulation Taifun® forte at maximum recommended field rates for vine and a trend for avoidance of Wettable Sulphur Stulln (sulphur). No avoidance was observed when testing Folpan® 80 WDG (folpet), Funguran® progress (copper hydroxide), SpinTorTM (spinosad), or 10% of the maximum field rate of any formulation tested. In the choice-tests in which we observed an avoidance, toads also showed higher activity on the contaminated side of the arena. As video analysis with tracking software is not always feasible, we further tested the effect of reducing the sampling interval for manual data analyses. We showed that one data point every 15 or 60 minutes results in a risk of overlooking a weak avoidance behavior, but still allows to verify the absence/presence of an avoidance for six out of seven formulations. Our findings are important for an upcoming pesticide risk assessment for amphibians and could be a template for future standardized tests.

Frog occupancy of polluted wetlands in urban landscapes

Urban sprawl and the rising popularity of water-sensitive urban design of urban landscapes has led to a global surge in the number of wetlands constructed to collect and treat stormwater runoff in cities. However, contaminants, such as heavy metals and pesticides, in stormwater adversely affect the survival, growth, and reproduction of animals inhabiting these wetlands. A key question is whether wildlife can identify and avoid highly polluted wetlands. We investigated whether pond-breeding frogs are attempting to breed in wetlands that affect the fitness of their offspring across 67 urban wetlands in Melbourne, Australia. Frog species richness and the concentration of contaminants (heavy metals and pesticides) were not significantly related, even in the most polluted wetlands. The proportion of fringing vegetation at a wetland had the greatest positive influence on the number of frog species present and the probability of occurrence of individual species, indicating that frogs inhabited wetlands with abundant vegetation, regardless of their pollution status. These wetlands contained contaminant levels similar to urban wetlands around the world at levels that reduce larval amphibian survival. These results are, thus, likely generalizable to other areas, suggesting that urban managers could inadvertently be creating ecological traps in countless cities. Wetlands are important tools for the management of urban stormwater runoff, but their construction should not facilitate declines in wetland-dependent urban wildlife.

Stormwater wetlands can function as ecological traps for urban frogs

Around cities, natural wetlands are rapidly being destroyed and replaced with wetlands constructed to treat stormwater. Although the intended purpose of these wetlands is to manage urban stormwater, they are inhabited by wildlife that might be exposed to contaminants. These effects will be exacerbated if animals are unable to differentiate between stormwater treatment wetlands of varying quality and some function as "ecological traps" (i.e., habitats that animals prefer despite fitness being lower than in other habitats). To examine if urban stormwater wetlands can be ecological traps for frogs, we tested if survival, metamorphosis-related measures, and predator avoidance behaviors of frogs differed within mesocosms that simulated stormwater wetlands with different contaminant levels, and paired this with a natural oviposition experiment to assess breeding-site preferences. We provide the first empirical evidence that these wetlands can function as ecological traps for frogs. Tadpoles had lower survival and were less responsive to predator olfactory cues when raised in more polluted stormwater wetlands, but also reached metamorphosis earlier and at a larger size. A greater size at metamorphosis was likely a result of increased per capita food availability due to higher mortality combined with eutrophication, although other compensatory effects such as selective-mortality removing smaller individuals from low-quality mesocosms may also explain these results. Breeding adults laid comparable numbers of eggs across wetlands with high and low contaminant levels, indicating no avoidance of the former. Since stormwater treatment wetlands are often the only available aquatic habitat in urban landscapes we need to better understand how they perform as habitats to guide management decisions that mitigate their potential ecological costs. This may include improving wetland quality so that fitness is no longer compromised, preventing colonization by animals, altering the cues animals use when selecting habitats, pretreating contaminated water prior to release, providing off-line wetlands nearby, or simply not constructing stormwater treatment wetlands in sensitive areas. Our study confirms the potential for urban stormwater treatment wetlands to function as ecological traps and highlights the need for greater awareness of their prevalence and impact at landscape scales.

Predation risk and patch size jointly determine perceived patch quality in ovipositing treefrogs, Hyla chrysoscelis

Two of the most important factors determining community structure and diversity within and among habitat patches are patch size and patch quality. Despite the importance of patch size in existing paradigms in island biogeography, metapopulation biology, landscape ecology, and metacommunity ecology, and growing conservation concerns with habitat fragmentation, there has been little investigation into how patch size interacts with patch quality. We crossed three levels of patch size (1.13 m² , 2.54 m² and 5.73 m² ) with two levels of patch quality (fish presence/absence, green sunfish [Lepomis cyanellus] and golden shiners [Notemigonus crysoleucus]) in six replicate experimental landscapes (3 × 2 × 6 = 36 patches). Both fish predators have been previously shown to elicit avoidance in ovipositing treefrogs. We examined how patch size and patch quality, as well as the interaction between size and quality, affected female oviposition preference and male calling site choice in a natural population of treefrogs (Hyla chrysoscelis). Females almost exclusively oviposited in the largest fishless patches, indicating that females use both risk, in the form of fish predators, and size itself, as components of patch quality. Females routinely use much smaller natural and experimental patches, suggesting that the responses to patch size are highly context dependent. Responses to fish were unaffected by patch size. Male responses largely mimicked those of females, but did not drive female oviposition. We suggest that patch size itself functions as another aspect of patch quality for H. chrysoscelis, and serves as another niche dimension across which species may behaviorally sort in natural systems. Because of strong, shared avoidance of fish (as well as other predators), among many colonizing taxa, patch size may be a critical factor in species sorting and processes of community assembly in freshwater habitats, allowing species to behaviorally segregate along gradients of patch size in fishless ponds. Conversely, lack of variation in patch size may concentrate colonization activity, leading to intensification of species interactions and/or increased use of lesser quality patches.

Forest canopy, water level, and biopesticide interact to determine oviposition habitat selection in Aedes albopictus

Understanding how interacting abiotic and biotic factors influence colonization rates into different habitat types is critical for both conserving and controlling species. For example, the rapid global spread of Asian tiger mosquitoes, Aedes albopictus, has reduced native species abundances and produced disease outbreaks. Fortunately, bacterial endospores of two Bacillus species (biospesticide) are highly lethal to Ae. albopictus larvae and have been commercially developed to reduce populations. Oviposition habitat selection is the first defense Ae. albopictus females possess against any control substance added to breeding sites, and considerable variation exists in their response to biopesticides. In a field experiment, I crossed the presence/absence of biopesticides, with two canopy (open, closed) and water (high, low) levels at 64 breeding sites, to examine if these interacted to influence oviposition site choice. Avoidance of biopesticide was most pronounced in closed canopy sites and those with low water levels, as all main effects and two-way interactions influenced oviposition. Oviposition habitat selection represents a possible mechanism of resistance to biopesticides and other methods used to kill mosquito larvae. Future experiments examining how larval density and mortality modify these results should allow for more effective control of this highly invasive species.

Effects of a commonly used glyphosate-based herbicide formulation on early developmental stages of two anuran species

Environmental contamination, especially due to the increasing use of pesticides, is suggested to be one out of six main reasons for the global amphibian decline. Adverse effects of glyphosate-based herbicides on amphibians have been already discussed in several studies with different conclusions, especially regarding sublethal effects at environmentally relevant concentrations. Therefore, we studied the acute toxic effects (mortality, growth, and morphological changes) of the commonly used glyphosate-based herbicide formulation Roundup® UltraMax on early aquatic developmental stages of two anuran species with different larval types (obligate vs. facultative filtrating suspension feeders), the African clawed frog (Xenopus laevis) and the Mediterranean painted frog (Discoglossus pictus). While X. laevis is an established anuran model organism in amphibian toxicological studies, we aim to establish D. pictus as another model for species with facultative filtrating larvae. A special focus of the present study lies on malformations in X. laevis embryos, which were investigated using histological preparations. In general, embryos and larvae of X. laevis reacted more sensitive concerning lethal effects compared to early developmental stages of D. pictus. It was suggested, that especially the different morphology of their filter apparatus and the higher volume of water pumped through the buccopharynx of X. laevis larvae lead to higher exposure to the formulation. The test substance induced similar lethal effects in D. pictus larvae as it does in the teleost standard test organism used in pesticide approval, the rainbow trout (Oncorhynchus mykiss), whereas embryos of both species are apparently more tolerant and, conversely, X. laevis larvae about two times more sensitive. In both species, early larvae always reacted significantly more sensitive than embryos. Exposure to the test substance increased malformation rates in embryos of both species in a concentration-dependent manner, but not at environmentally relevant concentrations. However, the assumed field safety, based on calculated surface water concentrations of the active ingredient (glyphosate), should be validated with realistic field data and buffer strips have to be urgently regarded to any aquatic amphibian habitat.

Predator diversity reduces habitat colonization by mosquitoes and midges

Changes in predator diversity via extinction and invasion are increasingly widespread and can have important ecological and socio-economic consequences. Anticipating and managing these consequences requires understanding how predators shape ecological communities. Previous predator biodiversity research has focused on post-colonization processes. However, predators can also shape communities by altering patterns of prey habitat selection during colonization. The sensitivity of this non-consumptive top down mechanism to changes in predator diversity is largely unexamined. To address this gap, we examined patterns of dipteran oviposition habitat selection in experimental aquatic habitats in response to varied predator species richness while holding predator abundance constant. Caged predators were used in order to disentangle behavioural oviposition responses to predator cues from potential post-oviposition consumption of eggs and larvae. We hypothesized that because increases in predator richness often result in greater prey mortality than would be predicted from independent effects of predators, prey should avoid predator-rich habitats during colonization. Consistent with this hypothesis, predator-rich habitats received 48% fewer dipteran eggs than predicted, including 60% fewer mosquito eggs and 38% fewer midge eggs. Our findings highlight the potentially important links between predator biodiversity, prey habitat selection and the ecosystem service of pest regulation.

Agricultural chemicals: life changer for mosquito vectors in agricultural landscapes?

BACKGROUND

Although many mosquito species develop within agricultural landscapes where they are potentially exposed to agricultural chemicals (fertilizers and pesticides), the effects of these chemicals on mosquito biology remain poorly understood. This study investigated the effects of sublethal concentrations of four agricultural chemicals on the life history traits of Anopheles arabiensis and Culex quinquefasciatus mosquitoes.

METHODS

Field and laboratory experiments were conducted to examine how sublethal concentrations of four agricultural chemicals: an insecticide (cypermethrin), a herbicide (glyphosate), and two nitrogenous fertilizers (ammonium sulfate and diammonium phosphate) alter oviposition site selection, emergence rates, development time, adult body size, and longevity of An. arabiensis and Cx. quinquefasciatus.

RESULTS

Both mosquito species had preference to oviposit in fertilizer treatments relative to pesticide treatments. Emergence rates for An. arabiensis were significantly higher in the control and ammonium sulfate treatments compared to cypermethrin treatment, while emergence rates for Cx. quinquefasciatus were significantly higher in the diammonium phosphate treatment compared to glyphosate and cypermethrin treatments. For both mosquito species, individuals from the ammonium sulfate and diammonium phosphate treatments took significantly longer time to develop compared to those from cypermethrin and glyphosate treatments. Although not always significant, males and females of both mosquito species tended to be smaller in the ammonium sulfate and diammonium phosphate treatments compared to cypermethrin and glyphosate treatments. There was no significant effect of the agrochemical treatments on the longevity of either mosquito species.

CONCLUSIONS

These results demonstrate that the widespread use of agricultural chemicals to enhance crop production can have unexpected effects on the spatial distribution and abundance of mosquito vectors of malaria and lymphatic filariasis.

Bioinsecticide and leaf litter combination increases oviposition and reduces adult recruitment to create an effective ovitrap for Culex mosquitoes

Mosquito egg traps, aquatic habitats baited with oviposition attractant and insecticide, are important tools for surveillance and control efforts in integrated vector management programs. The bioinsecticide Bacillus thuringiensis subsp. israelensis (Bti) is increasingly used as an environmentally friendly alternative to chemical insecticides and the combination of Bti with a simple oviposition attractant like leaf litter to create an effective egg trap seems appealing. However, previous research suggests that Bti may itself alter oviposition, and that leaf litter may dramatically reduce Bti toxicity. Here we present results from field experiment designed to link the effects of litter and Bti on mosquito oviposition habitat selection and post-colonization survival to production of adult mosquitoes. Tripling litter increased Culex spp. oviposition nearly nine-fold, while Bti had no effect on oviposition. Neither factor altered egg survival, thus larval abundance reflected the effects of litter on oviposition. Both Bti and litter reduced larval survival by ∼60%. We found no evidence that increased litter reduced Bti toxicity. Adult production was dependent upon both litter and Bti. In the absence of Bti, effects of litter on oviposition translated into three-fold more adults. However, in the presence of Bti, initial increases in oviposition were erased by the combined negative effects of Bti and litter on post-colonization survival. Thus, our study provides field evidence that combined litter and Bti application creates an effective ovitrap. This combined treatment had the highest oviposition and the lowest survival, and thus removed the greatest number of mosquitoes from the landscape.

Effects of an environmentally relevant temporal application scheme of low herbicide concentrations on larvae of two anuran species

Cultivation of herbicide-tolerant crops involves repeated applications of the complementary herbicide throughout the growing season, while in conventional corn production, herbicide application is restricted to the beginning of cultivation. Repeated application of herbicides increases both the likelihood an organism will be exposed to the herbicide and the concentration it may be exposed to. We examined effects of short and pulsed exposure of the cycloxydim-based herbicide formulation Focus® Ultra at doses close to the calculated LC5 (0.01 and 0.5 mg a.i. L(-1)) and LC10 values (0.05 and 1.0 mg a.i. L(-1)) on early premetamorphic and prometamorphice larvae of two anuran model organisms, Xenopus laevis and Discoglossus scovazzi. In addition, larvae were repeatedly exposed, i.e. at all considered developmental stages. The herbicide did not induce effects on body size at and time to metamorphosis or increase deformation rates in both species. Exposure to calculated LC5 values did not increase mortality or cause clinical signs in both species. At calculated LC10 values, narcotic effects were seen in all developmental stages. There was no clear evidence of developmental-specific mortality. Metamorphic success was independent of time point and duration of application in X. laevis. Only repeated exposure significantly increased mortality at metamorphosis in D. scovazzi. Narcosis may result in increased mortality under field conditions due to rise of predation risk. Different sensitivity of the test species to the compound was attributed to their physiological properties. Different filtering rates were understood as an accompanying factor influencing exposition.

Behavioral responses of the Strawberry Poison Frog (Oophaga pumilio) to herbicide olfactory cues: possible implications for habitat selection and movement in altered landscapes

Recent research has focused on the importance of behavior in mediating the effects of landscape change on amphibian populations and communities. Factors such as chemical contaminants may affect habitat selection and movement of amphibians in human-altered habitats and contribute to landscape-level patterns of distribution and abundance. The objective of this study was to determine if the Strawberry Poison Frog (Oophaga pumilio (Schmidt, 1857)) can use olfactory cues to detect and avoid the glyphosate-based herbicide Roundup™. Fifty frogs were captured in the field in Costa Rica and tested in experimental arenas where they were given a choice between a control and an herbicide treatment. Analysis of time spent in treatment areas revealed a significant interaction between sex and treatment. Analyses of choice at the start and end of the trials indicated that sex and cardinal direction were important factors influencing orientation behavior. These results suggest that males and females differed in their behavioral responses, and that male O. pumilio may use olfactory cues to detect and avoid areas treated with glyphosate-based herbicide. However, the sampled population was male-biased, which resulted in a lower sample size and lower power to detect an effect for females. Further work is needed to better understand amphibian behavioral responses to herbicides, as well as the role of sex and individual variation in modifying these responses.

Preference and avoidance responses by tadpoles: the fungicide pyrimethanil as a habitat disturber

Tadpoles of two amphibian species, the neotropical anuran Leptodactylus latrans and the North American bullfrog Lithobates catesbeianus, were used in experiments to assess their preferred spatial distribution along habitat gradients and, thus, to what extent contamination by the fungicide pyrimethanil could trigger active spatial avoidance. The tadpoles were tested in a non-confined multi-compartment static system with a pyrimethanil contamination gradient through which organisms could move freely. Two samples, with and without (reference) pyrimethanil contamination, taken from outdoor mesocosms, were assayed. Tadpoles showed to be able to detect and move to the most favorable environment by preferring compartments containing reference mesocosm water. Pyrimethanil concentrations from 0.2 to 1.4 mg L(-1) were below lethal levels, but acted as habitat disturber since spatial avoidance was triggered. Avoiders of L. latrans reached almost 50 % at 1.4 mg L(-1). The present data reinforces the hypothesis regarding the risk of plant protection products to act, not only as toxicants, but also as habitat disturber, potentially leading to avoidance-driven population decline of amphibians.

Copper-driven avoidance and mortality in temperate and tropical tadpoles

Amphibians have experienced an accentuated population decline in the whole world due to many factors, one of them being anthropogenic contamination. The present study aimed to assess the potential effect of copper, as a worldwide and reference contaminant, on the immediate decline of exposed population due to avoidance and mortality responses in tadpoles of three species of amphibians across climatic zones: a South American species, Leptodactylus latrans, a North American species, Lithobates catesbeianus, and a European species, Pelophylax perezi. A non-forced exposure system with a copper gradient along seven compartments through which organisms could freely move was used to assess the ability of tadpoles to detect and avoid copper contamination. All species were able to avoid copper at a concentration as low as 100 μg L(-1). At the lowest (sublethal) concentrations (up to 200 μg L(-1)) avoidance played an exclusive role for the population decline, whereas at the highest concentrations (> 450 μg L(-1)) mortality was the response determining population decline. The median concentrations causing exposed population immediate decline were 93, 106 and 180 μg L(-1) for Le. latrans, Li. catesbeianus and P. perezi, respectively. Contaminants might, therefore, act as environmental disruptors both by generating low-quality habitats and by triggering avoidance of tadpoles, which could be an important response contributing to dispersion patterns, susceptibility to future stressors and decline of amphibian populations (together with mortality).

Slipping through the cracks: rubber plantation is unsuitable breeding habitat for frogs in Xishuangbanna, China

Conversion of tropical forests into agriculture may present a serious risk to amphibian diversity if amphibians are not able to use agricultural areas as habitat. Recently, in Xishuangbanna Prefecture, Yunnan Province – a hotspot of frog diversity within China – two-thirds of the native tropical rainforests have been converted into rubber plantation agriculture. We conducted surveys and experiments to quantify habitat use for breeding and non-breeding life history activities of the native frog species in rainforest, rubber plantation and other human impacted sites. Rubber plantation sites had the lowest species richness in our non-breeding habitat surveys and no species used rubber plantation sites as breeding habitat. The absence of breeding was likely not due to intrinsic properties of the rubber plantation pools, as our experiments indicated that rubber plantation pools were suitable for tadpole growth and development. Rather, the absence of breeding in the rubber plantation was likely due to a misalignment of breeding and non-breeding habitat preferences. Analyses of our breeding surveys showed that percent canopy cover over pools was the strongest environmental variable influencing breeding site selection, with species exhibiting preferences for pools under both high and low canopy cover. Although rubber plantation pools had high canopy cover, the only species that bred in high canopy cover sites used the rainforest for both non-breeding and breeding activities, completing their entire life cycle in the rainforest. Conversely, the species that did use the rubber plantation for non-breeding habitat preferred to breed in low canopy sites, also avoiding breeding in the rubber plantation. Rubber plantations are likely an intermediate habitat type that ‘slips through the cracks’ of species habitat preferences and is thus avoided for breeding. In summary, unlike the rainforests they replaced, rubber plantations alone may not be able to support frog populations.

Effects of water contamination on site selection by amphibians: experiences from an arena approach with European frogs and newts

Pesticide residues in breeding ponds can cause avoidance by at least some amphibian species. So far, outdoor experiments have been performed only with artificial pools in areas where the focus species usually occur and new colonization has been observed. Results of this kind of study are potentially influenced by natural disturbances and therefore are of limited comparability. We used an easily manufactured and standardizable arena approach, in which animals in reproductive condition for some hours had a choice among pools with different concentrations of a contaminant. Because there has been much debate on the potential environmental impacts of glyphosate-based herbicides, we investigated the impact of glyphosate isopropylamine salt (GLY-IS), Roundup LB PLUS (RU-LB-PLUS), and glyphosate's main metabolite aminomethylphosphonic acid (AMPA) on individual residence time in water. The following European amphibian species were tested: Common frog (Rana temporaria), Palmate newt (Lissotriton helveticus), and Alpine newt (Ichthyosaura alpestris). The residence time in water was not significantly affected by concentrations below or slightly above the European Environmental Quality Standards for AMPA or the German "worst-case" expected environmental concentrations for GLY-IS and RU-LB-PLUS. Occasionally, microclimatic cofactors (nightly minimum ground temperature, water temperature) apparently influenced the residence time. The major drawback of such quick behavior studies is that results can only be transferred to perception and avoidance of contaminated water but not easily to site selection by amphibians. For example, testing oviposition site selection requires more natural water bodies and more time. Hence, to develop a standard procedure in risk assessment, an intermediate design between an arena approach, as presented here, and previously performed field studies should be tested.

Patch quality and context, but not patch number, drive multi-scale colonization dynamics in experimental aquatic landscapes

Colonization and extinction are primary drivers of local population dynamics, community structure, and spatial patterns of biological diversity. Existing paradigms of island biogeography, metapopulation biology, and metacommunity ecology, as well as habitat management and conservation biology based on those paradigms, emphasize patch size, number, and isolation as primary characteristics influencing colonization and extinction. Habitat selection theory suggests that patch quality could rival size, number, and isolation in determining rates of colonization and resulting community structure. We used naturally colonized experimental landscapes to address four issues: (a) how do colonizing aquatic beetles respond to variation in patch number, (b) how do they respond to variation in patch quality, (c) does patch context affect colonization dynamics, and (d) at what spatial scales do beetles respond to habitat variation? Increasing patch number had no effect on per patch colonization rates, while patch quality and context were critical in determining colonization rates and resulting patterns of abundance and species richness at multiple spatial scales. We graphically illustrate how variation in immigration rates driven by perceived predation risk (habitat quality) can further modify dynamics of the equilibrium theory of island biogeography beyond predator-driven effects on extinction rates. Our data support the importance of patch quality and context as primary determinants of colonization rate, occupancy, abundance, and resulting patterns of species richness, and reinforce the idea that management of metapopulations for species preservation, and metacommunities for local and regional diversity, should incorporate habitat quality into the predictive equation.

Impacts of the herbicide butachlor on the larvae of a paddy field breeding frog (Fejervarya limnocharis) in subtropical Taiwan

Butachlor is the most commonly used herbicide on paddy fields in Taiwan and throughout Southeast Asia. Since paddy fields provide habitat for pond breeding amphibians, we examined growth, development, time to metamorphosis, and survival of alpine cricket frog tadpoles (Fejervarya limnocharis) exposed to environmentally realistic concentrations of butachlor. We documented negative impacts of butachlor on survival, development, and time to metamorphosis, but not on tadpole growth. The 96 h LC(50) for tadpoles was 0.87 mg/l, much lower than the 4.8 mg/l recommended dosage for application to paddy fields. Even given the rapid breakdown of butachlor, tadpoles would be exposed to concentrations in excess of their 96 h LC(50) for an estimated 126 h. We also documented DNA damage (genotoxicity) in tadpoles exposed to butachlor at concentrations an order of magnitude less than the 4.8 mg/l recommended application rate. We did not find that butachlor depressed cholinesterase activity of tadpoles, unlike most organophosphorus insecticides. We conclude that butachlor is likely to have widespread negative impacts on amphibians occupying paddy fields with traditional herbicide application.

Indirect Effects of Pesticides on Mosquito Larvae Via Alterations of Community Structure

We describe how pesticides used for mosquito control alter communities in mosquito breeding sites, and how these alterations affect larval populations of mosquitoes. Lethal and sublethal toxic effects modify biological interactions through density- and trait-mediated changes. Density-mediated effects due to pesticide treatment can lead to indirect positive effects on the target species. For example, recolonization of pests can be amplified due to disturbances of antagonistic species. Trait-mediated effects can result in lethal effects of originally sublethal exposure when the pesticide is combined with additional stress. Such lethal effects can result from changes of behavior or sensitivity. Also the immune capacity and resistance of individuals to parasitic infection could be decreased. Furthermore, pesticide treatment can act independently of toxic effects. For example, habitat and oviposition site selection can be influenced. These examples highlight the diversity of processes to be considered when determining the overall consequences of pesticide treatment. We show that a better understanding of these processes is needed to predict effects of pesticides on population dynamics. Such knowledge would have direct benefits in designing mosquito control strategies.

Predator-Induced Shifts in Mosquito Oviposition Site Selection: A Meta-Analysis and Implications for Vector Control

The global resurgence and emergence of new mosquito-borne diseases and increasing resistance of mosquitoes to chemical pesticides have prompted renewed interest in biocontrol methods that use aquatic predators of mosquito larvae. For disease vectors with complex life cycles, like mosquitoes, in which adults are terrestrial and choose aquatic habitats in which to deposit their offspring, shifts in oviposition site selection may have important consequences for vector population dynamics and epidemiology. While there have been numerous studies of mosquito oviposition site selection, methodology and results vary, making it difficult to evaluate the general importance of predator-induced shifts in oviposition site selection for biocontrol scenarios. Here we use meta-analysis to provide a quantitative framework for examining variation in mosquito oviposition responses to predators. Overall, we find a broad pattern of predator avoidance among mosquito and predator taxa. The primary factor explaining variation in oviposition response appears to be taxonomic and/or life-history related—avoidance is weakest or non-existent inAedesspecies that oviposit eggs above water in container habitats. Responses also varied among predators. Generally, oviposition avoidance was strongest in response to fish and insects, weak or nonexistent in response to notostracans, urodeles, or dipterans, and there is limited evidence that some mosquitoes are attracted to cyclopoid crustaceans. Our results highlight that predator avoidance during oviposition is common, but not ubiquitous, in mosquitoes and needs to be considered when evaluating the likely efficacy of aquatic predators for biocontrol.

A hump-shaped relationship between isolation and abundance of Notonecta irrorata colonists in aquatic mesocosms

We examined the relationship between the isolation of experimental aquatic mesocosms and the abundance of an aquatic insect colonist, Notonecta irrorata, over two years. We used a curve-fitting approach to assess whether linear or quadratic models better describe the relationship between isolation and abundance. For two measures of mesocosm isolation, distance to nearest source and distance to the largest source population, there was a significant quadratic relationship between isolation and abundance. Abundance of colonizing N. irrorata was not found to be significantly related to a third measure of isolation, mesocosm connectivity. These results indicate that the relationship between habitat isolation and colonist abundance may not be a monotonic decline across all spatial scales, a finding that contradicts the usual assumption incorporated in measures of habitat connectivity. Our results suggest that under some circumstances individuals that have undertaken dispersal may bypass patches they encounter early in this process and preferentially settle in patches encountered later. This behavioral preference in conjunction with decreased numbers of potential colonists at sites far from the source environment could lead to the "hump-shaped" colonist abundance by habitat isolation relationship we observed in this study. We suggest that simple assumptions about the relationship between habitat isolation and the probability a site is colonized need to be reexamined and alternative possible forms of this relationship tested.

Pesticide alters habitat selection and aquatic community composition

Anthropogenic chemical contamination is an important issue for conservation of aquatic ecosystems. While recent research highlights that community context can mediate the consequences of contaminant exposure, little is known about how contaminants themselves might determine this context by altering habitat selection and thus initial community composition. Here we show that the insecticide carbaryl and its commercial counterpart Sevin can affect aquatic community composition by differentially altering oviposition and colonization of experimental pools by amphibians and insects. On average, contaminated pools received 20-fold more adult beetle and heteropteran colonists and 12-fold more Culex mosquito and chironomid midge egg masses. On the other hand, ovipositing Anopheles mosquitoes and cricket frogs showed no preference and we have shown previously that gray treefrogs strongly avoid contaminated pools. Overall, initial richness doubled in contaminated pools compared with controls. By affecting colonizing taxa differently and increasing richness, the contaminant may alter the ecological context under which subsequent effects of exposure will unfold. Given that community context is important for evaluating toxicity effects, understanding the net effects of contaminants in natural systems requires an understanding of their effects on community assembly via shifts in habitat selection.