Effects of Chinese tallow leaf litter on water chemistry and surfacing behaviour of anuran larvae

The establishment of exotic invasive species, including plants, has been linked to the decline of some amphibian populations. Of particular concern with invasive plants, from an amphibian conservation perspective, is that they are disproportionately wetland or riparian species. Recent evidence suggests that Chinese tallow (Triadica sebifera), an exotic deciduous tree species, is expanding its range and becoming more abundant where it occurs in the United States. This is particularly relevant to amphibian conservation considering that Chinese tallow tends to invade wetlands, and recent studies have demonstrated that the leaf litter causes mortality of anuran eggs and larvae by reducing the dissolved oxygen and pH of water. The lethal effect of Chinese tallow leaf litter is short lived and concentrated soon after leaf fall, typically December through to February in the south-eastern United States. In this study, we were interested in determining the sub-lethal effects of Chinese tallow leaf litter on the surfacing frequency and air-gulping behaviour of overwintering anuran larvae. Lithobates catesbeianus and L. clamitans clamitans are two frog species that commonly overwinter as aquatic larvae and extensively overlap in range with invasive Chinese tallow, which may expose their tadpoles to the deleterious effects of the leaf litter. We conducted experiments where we exposed tadpoles to four different concentrations of tallow leaf litter and recorded water chemistry and tadpole surfacing frequency. We found that as Chinese tallow concentration increased, oxygen levels decreased. Both anuran species responded similarly to our treatments and dissolved oxygen levels, where tadpoles swam to the water’s surface to air gulp at a significantly higher rate in the treatments with greater tallow concentration. Such changes in behaviour induced by Chinese tallow could have negative consequences on tadpole foraging efficiency and predator avoidance, ultimately reducing fitness. As biological invasions will continue to be an important part of global change, more attention should be given to sub-lethal impacts, as they pertain to fitness.