Effects of multiple stressors on northern leopard frogs in agricultural wetlands

Abstract

Natural and anthropogenic stressors, including parasites and pesticides, may induce oxidative stress in animals. Measuring oxidative stress responses in sentinel species that are particularly responsive to environmental perturbations not only provides insight into host physiology but is also a useful readout of ecosystem health. Newly metamorphosed northern leopard frogs (Lithobates pipiens), a sentinel species, were collected from agricultural and non-agricultural wetlands exposed to varying concentrations of the herbicide atrazine. Significant effects of certain parasites' abundance and their interaction with atrazine exposure on frog oxidative stress were identified. Specifically, increased protein levels were detected in frogs infected with echinostome metacercariae. In addition, the nematode Oswaldocruzia sp. was significantly associated with increased thiol concentration and catalase activity. Significant parasite × atrazine interactions were observed for atrazine exposure and the abundance of Oswaldocruzia sp. on thiol, as thiol concentrations increased with parasite abundance at low atrazine localities and decreased in high atrazine wetlands. In addition, a significant interaction between the abundances of Oswaldocruzia sp. and gorgoderid trematodes on thiol concentrations was observed. These findings demonstrate that studies of oxidative stress on animals in natural ecosystems should account for the confounding effects of parasitism, particularly for amphibians in agricultural landscapes.

Keywords: Amphibians; Lithobates pipiens; atrazine; lysozyme; northern leopard frogs; oxidative stress; parasitism.

Graphical abstract

Fig. 1.

Plot of log10 abundance of Echinostoma spp. (log10 ECH + 1) against log10 total protein concentration (mg g⁻¹ F.W.) in spleens of northern leopard frogs (Lithobates pipiens) collected from agricultural and non-agricultural wetlands in Quebec, Canada in 2004. Results show linear mixed effects model predictions showing 95% confidence interval (shaded area) of marginal effects and raw data points.

Fig. 2.

Plot of log10 abundance of Oswaldocruzia sp. (log10 OSW + 1) against thiol concentration (U.I. × 10⁻³ mg⁻¹ protein) in spleens of northern leopard frogs (Lithobates pipiens) collected from (A) low-atrazine and (B) high-atrazine wetlands in Quebec, Canada in 2004 and its interaction with atrazine exposure. Results show linear mixed effects model predictions showing 95% confidence interval (shaded area) of marginal effects and raw data points.

Fig. 3.

Plot of log10 abundance of Oswaldocruzia sp. (log10 OSW + 1) against log10 catalase activity (nmol H₂O₂ min⁻¹ mg protein⁻¹) in northern leopard frogs (Lithobates pipiens) collected from agricultural and non-agricultural wetlands in Quebec, Canada in 2004. Results show linear mixed effects model predictions showing 95% confidence interval (shaded area) of marginal effects and raw data points.