Scott M. Weir

Ecological risk of anthropogenic pollutants to reptiles: evaluating assumptions of sensitivity and exposure.

A large data gap for reptile ecotoxicology still persists; therefore, ecological risk assessments of reptiles usually incorporate the use of surrogate species. This necessitates that (1) the surrogate is at least as sensitive as the target taxon and/or (2) exposures to the surrogate are greater than that of the target taxon. We evaluated these assumptions for the use of birds as surrogates for reptiles. Based on a survey of the literature, birds were more sensitive than reptiles in less than 1/4 of the chemicals investigated. Dietary and dermal exposure modeling indicated that exposure to reptiles was relatively high, particularly when the dermal route was considered. We conclude that caution is warranted in the use of avian receptors as surrogates for reptiles in ecological risk assessment and emphasize the need to better understand the magnitude and mechanism of contaminant exposure in reptiles to improve exposure and risk estimation.

Unraveling the relative importance of oral and dermal contaminant exposure in reptiles: insights from studies using the western fence lizard (Sceloporus occidentalis).

Despite widespread recognition of significant data deficiencies, reptiles remain a relatively understudied taxon in ecotoxicology. To conduct ecological risk assessments on reptiles frequently requires using surrogate taxa such as birds, but recent research suggests that reptiles have significantly different exposure profiles and toxicant sensitivity. We exposed western fence lizards, Sceloporus occidentalis, to the same quantities of three model chemicals via oral (gavage) and dermal (ventral skin application) exposure for either 24 or 48 hours. Three phthalate esters (di-methyl phthalate [DMP], di-iso-butyl phthalate [DIBP], and di-n-octyl phthalate [DNOP]) were chosen as model chemicals because they represent a gradient of lipophilicity but are otherwise structurally similar. Overall, the more lipophilic phthalates (DIBP and DNOP) were found to have higher concentrations in tissues than the less lipophilic DMP. Significant differences in tissue concentrations between DIBP and DNOP were tissue-dependent, suggesting that delivery to a site of action following exposure is not only a simple function of lipophilicity. In dermal treatments, DMP usually had fewer detections (except in ventral skin samples), suggesting that lipophilicity (log Kow>2) is a requirement for uptake across the skin. In general, tissue residues were greater in oral treatments than dermal treatments (significant in adipose and liver tissue), but differences were driven strongly by differences in DMP which did not appear to be absorbed well across skin. When differences in tissue residue concentrations between oral and dermal exposure did occur, the difference was not drastic. Taken together these results suggest that dermal exposure should be considered in risk assessments for reptilian receptors. Dermal exposure may be an especially important route for reptiles as their ectothermic physiology translates to lower energetic demands and dietary exposure compared to birds and mammals.

High tolerance to abiotic stressors and invasion success of the slow growing freshwater snail, Melanoides tuberculatus

Considerable research has been conducted to determine traits common to invasive species with the goal of predicting, preventing, or managing invasions. The importance of physiological tolerance to abiotic stressors in the ability of invasive species to establish and displace native species has been hypothesized to be important although there are few actual tests of the hypothesis in the literature. In freshwater molluscs it has been suggested that high fecundity is the most important trait for invasion success and that physiological tolerance to abiotic conditions is unlikely to play a significant role. We examined the tolerance to abiotic stressors using a known invasive snail species (Melanoides tuberculatus) that has a much slower growth rate and fecundity than a native species it has displaced (Biomphalaria glabrata). We tested the hypothesis that M. tuberculatus would have significantly greater tolerance to natural and anthropogenic abiotic stressors (cadmium, malathion, temperature extremes, and desiccation) which may provide a mechanism for displacement of B. glabrata. A time-to-event analysis was used to determine relative tolerance between species. M. tuberculatus was significantly more tolerant to the tested abiotic stressors than B. glabrata with the exception of low temperature (5°C). Stress tolerance may partly explain the ability of M. tuberculatus to displace B. glabrata despite having a much lower growth rate and fecundity. These results also suggest that M. tuberculatus is likely to have a strong advantage in disturbed or polluted habitats. Identifying those traits most important for the invasion success of particular species could be used to better inform removal strategies and may allow for improved predictions of invasion potential.

Acute toxicity of herbicide formulations and chronic toxicity of technical-grade trifluralin to larval green frogs (Lithobates clamitans)

Fewer toxicity studies have been performed on herbicides than on insecticides despite heavier use of herbicides and evidence of herbicide formulation toxicity to amphibians. We conducted acute and chronic toxicity tests with the herbicide trifluralin (2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)aniline) on tadpoles. Herbicide formulations had lower median lethal concentrations than an insecticide formulation and technical-grade trifluralin. Chronic trifluralin exposure resulted in significantly smaller tadpoles at low concentrations (20 µg/L) compared with controls and 200-µg/L treatments.

Insights into reptile dermal contaminant exposure: Reptile skin permeability to pesticides

There is growing interest in improving ecological risk assessment exposure estimation, specifically by incorporating dermal exposure. At the same time, there is a growing interest in amphibians and reptiles as receptors in ecological risk assessment, despite generally receiving less research than more traditional receptors. Previous research has suggested that dermal exposure may be more important than previously considered for reptiles. We measured reptile skin permeability to four pesticides (thiamethoxam, malathion, tebuthiuron, trifluralin) using ventral skin samples. All four pesticides penetrated the skin but generally had low permeability. There was no apparent relationship between physicochemical properties and permeability coefficients. Malathion had a significantly greater permeability rate at all time points compared to the other pesticides. Tebuthiuron had a greater permeability than thiamethoxam. Reptiles and mammals appear to have similar skin permeability suggesting that dermal exposure estimates for mammals may be representative of reptiles.

The effects of pesticide exposure on ultraviolet-B radiation avoidance behavior in tadpoles

Effects of contaminants on behavior may have important consequences on wildlife populations because behaviors such as predation, predator avoidance, reproduction, and social interaction can affect population dynamics. As a common environmental stressor, ultraviolet-B (UVB) radiation causes various deleterious effects and some aquatic organisms actively avoid UVB radiation in water. However, the extent to which environmental contaminants can impair UVB avoidance has not been evaluated, which may cause greater UVB exposure and toxicity. In the present study, we used Xenopus laevis tadpoles to determine if acute exposure to sublethal concentrations of agricultural chemicals can alter tadpole response to UVB radiation. We exposed tadpoles to four pesticides (malathion, endosulfan, α-cypermethrin, and chlorothalonil) for 96 h. At the end of the exposure, tadpoles were transferred to tanks divided into UVB and no-UVB areas. We observed tadpoles for 30 min and recorded time spent in the UVB area. We compared the proportion of time tadpoles spent in the UVB area among different concentrations for each pesticide. There was no significant difference between FETAX control and solvent control tadpoles. When combined, control tadpoles spent less than half of the time in the UVB area indicating that X. laevis tadpoles exhibit UVB avoidance behavior. Tadpoles exposed to 5 μg/L endosulfan spent significantly more time under UVB than control tadpoles. Other pesticides had no effect on tadpole UVB avoidance behavior. Our results suggest that some neurotoxic pesticides can affect UVB avoidance in larval amphibians, which may increase their exposure and subsequently the risk of UVB-induced damage. The present study highlights the importance of examining the interaction between two stressors that co-occur across broad spatial scales and to consider behavioral alteration when evaluating the risk of pesticides to amphibians.

Managing the risk of invasive species: How well do functional traits determine invasion strategy and success?

Our results to date support those of Driver et al. (1991), Mineau et al. (1990) and others who have demonstrated the relative importance of nondietary exposure. It is increasingly clear that all avian regulatory assessments of liquid pesticide formulations are fundamentally flawed. Without a consideration of exposure routes other than the dietary route, government regulators have no credible basis for assessing the safety of any pesticide to birds. The faster we accept this fact, and resources are redirected to the necessary research, the sooner we will be able to provide the tools needed for risk assessment.

Direct and indirect effects of petroleum production activities on the western fence lizard (Sceloporus occidentalis) as a surrogate for the dunes sagebrush lizard (Sceloporus arenicolus)

The dunes sagebrush lizard (Sceloporus arenicolus) is a habitat specialist of conservation concern limited to shin oak sand dune systems of New Mexico and Texas (USA). Because much of the dunes sagebrush lizards habitat occurs in areas of high oil and gas production, there may be direct and indirect effects of these activities. The congeneric Western fence lizard (Sceloporus occidentalis) was used as a surrogate species to determine direct effects of 2 contaminants associated with oil and gas drilling activities in the Permian Basin (NM and TX, USA): herbicide formulations (Krovar and Quest) and hydrogen sulfide gas (H2S). Lizards were exposed to 2 concentrations of H2 S (30 ppm or 90 ppm) and herbicide formulations (1× or 2× label application rate) representing high-end exposure scenarios. Sublethal behavioral endpoints were evaluated, including sprint speed and time to prey detection and capture. Neither H2S nor herbicide formulations caused significant behavioral effects compared to controls. To understand potential indirect effects of oil and gas drilling on the prey base, terrestrial invertebrate biomass and order diversity were quantified at impacted sites to compare with nonimpacted sites. A significant decrease in biomass was found at impacted sites, but no significant effects on diversity. The results suggest little risk from direct toxic effects, but the potential for indirect effects should be further explored.

Organochlorine pesticides in squamate reptiles from southern Arizona, USA.

Despite recognition of the lack of reptile ecotoxicology data, the taxon remains poorly studied. Contaminant body burdens are useful in demonstrating exposures to contaminants do occur and may provide insight regarding risks. The purpose of this study was to determine organochlorine pesticide burdens in various tissues of terrestrial reptiles opportunistically collected in Arizona. Heptachlor, DDE, and endrin were the most common analytes detected in fat samples. Liver samples contained methoxychlor and heptachlor at greater frequency than other organochlorines. Investigations into chronic low-level exposures are rare for reptiles and research is needed to determine critical body residues associated with adverse impacts.