Timing of oviposition influences the effects of a non-native grass on amphibian development

Land-use change can alter the energy dynamics in aquatic systems by changing the subsidies that form the nutrient base within them. However, experimental evaluations of subsidy change often fail to consider how effects, such as differences in individual growth and survival, may differ under varying ecological contexts experienced in the field. We used a mesocosm approach to investigate how litter (Native Prairie or Non-Native Tall-Fescue Grass) surrounding wetlands and timing of oviposition affected larval amphibian development. We found that survival differed between litter types in the Early-Oviposition treatment, with nearly 100% mortality in Fescue treatments. Conversely, survival  was similar across litter types in the Late Oviposition treatment (~ 43%), and larvae in Late-Fescue treatments metamorphosed more quickly and were larger post-metamorphosis than larvae in Prairie treatments. Follow-up experiments confirmed that low dissolved oxygen (DO) was responsible for high mortality in Early-Fescue treatments; high quantities of Fescue resulted in a microbial bloom that reduced DO to < 2 mg/L for several days, resulting in low hatching success. This effect was eliminated in treatments with supplemental aeration. Finally, we confirmed that experimentally observed DO patterns also occurred in the field. Context (i.e., timing of inundation relative to amphibian breeding) is critical to understanding the effects of subsidies on amphibian populations; early and explosively breeding species may experience catastrophic mortality due to DO depletion; whereas, species that breed later may experience enhanced fitness of recruits. Considering the effects of non-native species across different ecological contexts is necessary for elucidating the extent of their impacts.