Larval responses of three midwestern anurans to chronic, low-dose exposures of four herbicides.

Low levels of agricultural herbicides often contaminate surface water and might persist throughout the growing season, potentially acting as stressors on aquatic organisms. Although low-dose, chronic exposures to agrochemicals are likely common for many nontarget organisms, studies addressing these effects using end-use herbicide formulations are rare. We exposed three common species of tadpoles to conservative levels of atrazine, S-metolachlor, and glyphosate end-use herbicide formulations throughout the larval period to test for survival differences or life-history trait alterations. Exposure to the glyphosate product Roundup WeatherMax at 572 ppb glyphosate acid equivalents (a.e.) resulted in 80% mortality of western chorus frog tadpoles, likely as a result of a unique surfactant formulation. Exposure to WeatherMax or Roundup Original Max at 572 ppb a.e. also lengthened the larval period for American toads. Chronic atrazine and S-metolachlor exposures induced no significant negative effects on survival, mass at metamorphosis, or larval period length at the levels tested. These results highlight the importance of explicitly tying chronic tests to the natural environment and considering contributions of surfactant/adjuvant components to end-use formulation toxicities, even between very similar products.

Leaf litter input mediates tadpole performance across forest canopy treatments.

Understanding the mechanisms limiting the distributions of organisms is necessary for predicting changes in community composition along habitat gradients. In many areas of the USA, land originally cleared for agriculture has been undergoing a process of reforestation, creating a gradient of canopy cover. For small temporary wetlands, this gradient can alter abiotic conditions and influence the resource base of wetland food webs by affecting litter inputs. As distributions of amphibians and many other temporary wetland taxa correlate with canopy cover, we experimentally manipulated shade levels and litter types in pond mesocosms to explore mechanisms limiting species performance in wetlands with canopy cover. Most differences between ponds were mediated by litter type rather than direct effects of shading. Although all three amphibian species tested are open-canopy specialists, spring peepers were the only species to show decreased survival in shaded ponds. Pond litter type generally had strong effects on growth and development rates, with tadpoles of two species in grass litter ponds growing to twice the size of, and metamorphosing 7 days earlier than, those in leaf litter ponds. Contrary to our initial hypothesis, shade level and litter type showed very few significant interactions. Our results indicate that the effects of shading cannot be considered in isolation of vegetation changes in pond basins when evaluating the effects of forest succession on temporary pond communities.