Invasive species are a global concern due to their influence on ecosystem health and biodiversity. Wetlands are especially at risk as they host disproportionately large numbers of invaders. Exacerbating this issue, established invaders can facilitate other potential invaders by inhibiting native species. For example, phytochemical input (leaf litter) from invasive wetland plants can reduce water quality creating conditions that favor new invaders. Additionally, wetlands are vulnerable to a diversity of chemicals (e.g. road salt) that also reduce water quality. While both invasive plant litter and contaminants can independently reduce water quality, these stressors commonly coexist in nature and likely interact. Therefore, we investigated whether shifts in wetland plant communities from native to invasive interact with NaCl to facilitate a non-native amphibian (African-clawed frog). We asked: (1) Can invasive plant litter facilitate a non-native amphibian? (2) Does NaCl interact with invasive plant litter to facilitate a non-native amphibian? We exposed early stage native leopard frog and invasive African-clawed frog embryos to factorial combinations of seven leaf litter treatments (3-native; 3-invasive; water-control) and two NaCl treatments (0g/L-NaCl; 1g/L-NaCl). We measured water quality parameters, amphibian development (time-to-hatching), and assessed tadpole susceptibility to a lethal concentration of NaCl (time-to-death assay).
Leaf litter from invasive plants (common reed, autumn olive, and purple loosestrife) collectively caused a 20% reduction in dissolved oxygen and a 48% increase in turbidity compared to leaf litter from native plants (cattails, blueberry, steeplebush). We found a significant effect of leaf litter treatment (p<0.001) and NaCl (p=0.003) on native leopard frog development but not invasive African-clawed frog development (p = 0.16 and 0.25, respectively). Leopard frog embryos reared in invasive leaf litter or sublethal NaCl hatched faster compared to those in native leaf litter or NaCl-free water. To understand how leaf litter treatment affected tadpole susceptibility to lethal concentrations of NaCl, we conducted a time-to-death assay. In the absence of leaf litter (filtered-water control), African-clawed tadpoles were 54% less susceptible to NaCl compared to leopard frog tadpoles. When reared in leaf litter from invasive plants compared to native plants, leopard frog tadpoles were more susceptible to NaCl (p=0.002) but African-clawed tadpoles were less susceptible to NaCl (p<0.001). Collectively, these results suggest that native amphibians were more sensitive to invasive leaf litter and NaCl compared to invasive amphibians, and provide evidence that some wetland invasive plants may interact with NaCl to facilitate invasion by other non-native species.