Invasive exotic species are considered to be one of the most important drivers of biodiversity loss across the world. Encroachment from alien species can change habitats thus contributing to global amphibian declines. Leachates from invasive plants can add metabolites (tannins) or toxins (phenolics) from decomposing leaf litter to aquatic systems and thereby potentially impact the development and fitness of larval amphibians. Amur honeysuckle (Lonicera maackii) is an invasive shrub that dominates many of the edge habitats of the Midwest and Eastern United States and is the predominant woody vegetation along many streams and ponds. Wood frogs (Rana sylvatica) range throughout this area and are one of the first to breed in the spring. Thus, R. sylvatica larvae (tadpoles) are potentially more susceptible to L. maackii leachates than most other species because leachates from leaves dropped in the fall are at higher concentrations in water than later in the spring. We looked at invasive plant chemical impacts on tadpole development in R. sylvatica by examining tadpole digestive efficiency and metamorph fitness (performance & energetic) of larvae raised under varying concentrations in leaf “teas” from three sources: 1) Amur honeysuckle (L. maackii), 2) native mixed hardwood leaf litter, and 3) pure water control.
Our results suggest that digestive efficiencies, time to metamorphosis, and fitness performance of metamorphs are all reduced in frogs raised in honeysuckle leaf teas in comparison to native leaf teas or a water control. Under the highest concentration of L. maackii tea (1 g dry leaves/L), mean caloric assimilation efficiency was less than 70%, while efficiencies of tadpoles raised under similar mixed hardwood tea concentrations and the water control were near 80%. Uptake of tannins by individual frog tadpoles may disrupt their digestion, thereby reducing absorption of nutrients, and leading to slower growth rates and decreased fitness of metamorphs. Fitness in metamorphs was determined on the basis of jumping performance (predator escape mechanism) and body fat content (energetic reserves). We found that frogs raised in higher concentrations of L. maackii teas had significantly lower fitness than frogs raised in mixed hardwood teas or the water control. This indicates that the degradation of leaves from invasive plants, by changing water chemistry in ponds and streams, may reduce survival rates of developing amphibian larvae, thereby contributing in yet another way to the global decline of amphibians.