Amur honeysuckle (Lonicera maackii) is an invasive edge shrub species that is currently invading and dominating stream and pond banks throughout the eastern parts of the US and Canada. Our previous studies suggest that litter of this shrub decomposes much faster in water than native riparian litter, and that leaf teas from L. maackii measured tannin concentrations nearly twice (1.8x) the level of those found in native leaf teas. Wood frogs (Lithobates sylvaticus) are found in many areas invaded by L. maackii, and their aquatic tadpoles may be negatively impacted by this shrub’s allochthonous litter. Our experiments found decreased digestive assimilation and survival of tadpoles and reduced fitness in metamorphs when raised in L. maackii vs. native plant leaf teas. We hypothesized similar deleterious impacts on digestive efficiencies in tadpoles of a common congener (Lithobates clamitans) raised on diets that included L. maackii leaf matter vs. diets with native plant leaf matter. The results of each replicate diet test were compared to sets of control tadpole replicates that were fed only an algae diet. Feeding experiments were carried out over four weeks, and nutritional analyses were performed on dried food and fecal output material.
Assimilation efficiencies were calculated from laboratory-based tests of caloric content, from bomb calorimetry. Consumption rates varied among diets; tadpoles consumed 57% and 64% of the energetic value consumed in the control diet from L. maackii and native leaf diets, respectively. Assimilation efficiencies of calories consumed also varied based on the diet type. Control diets of algae alone were assimilated at 76.9% of their caloric content, whereas tadpoles consumed 56.2% and 68.4% of the calories in the native plant and L. maackii diets, respectively. The significantly lower energy processing by tadpoles on both leaf diets suggests that leaf matter is digested less easily by tadpoles than non-fibrous algae. The lack of significant differences between the energy processing of L. maackii and native plant leaves suggests that it is not the allochthonous litter type but the chemicals released into the water from honeysuckle that have the greatest impact on larval anuran development.