SFA researcher finds Chinese tallow leaf litter reduces amphibian breeding in mesocosm trials

Caleb Mellas, a doctoral student at Stephen F. Austin State University, presented multi-year mesocosm experiments testing how leaf-litter species and concentration affect oviposition and tadpole survival for local amphibians. Mellas said Chinese tallow (Triadica sebifera) leaf litter decomposes rapidly, produces darker, tannin-rich water and can sharply lower dissolved oxygen and pH compared with native leaf types such as loblolly pine and shumard oak.

In a 2024 cafeteria-style mesocosm experiment, Mellas placed 35 tanks with different leaf-litter species (tallow, oak, pine, and a no-litter control) at two concentrations and allowed local tree frogs to select oviposition sites. He reported the following findings from the March–May breeding period: - Dissolved oxygen and pH were lowest in tallow treatments; pine treatments retained higher dissolved oxygen. - Egg masses, tadpoles and breeding pairs were observed least frequently in tanks with tallow leaf litter, particularly at higher concentrations. - End-of-season dip-net surveys found no tadpoles in high-tallow treatments, while pine and control treatments had substantially higher tadpole abundance.

Mellas told the group he replicated and expanded the experiment in a second, full factorial study that also varied wetland size (large 800‑liter tanks vs. small 80‑liter tanks) and predator presence (Mosquitofish). Preliminary results reinforce that tallow decomposition drives rapid oxygen consumption and acidification; small ephemeral wetlands showed faster turnover of water chemistry and could be more vulnerable to short-term anoxic events. He said tadpoles were absent from many tallow treatments in 2024 but a limited number appeared in subsequent trials, suggesting tallow exposure can cause high larval mortality and/or avoidance by adults.

Why it matters: Chinese tallow is a widespread invasive tree in the region; its leaf litter alters water chemistry in ways that can make wetlands less suitable for amphibian reproduction. Mellas said those changes can scale up to reduced amphibian recruitment, shifts in wetland community composition and potential indirect effects on mosquito dynamics and human-wildlife interactions.

Methods and controls: Mellas described leaf-litter concentrations (grams per liter), weekly water-quality monitoring (DO, pH, temperature, conductivity, turbidity and TDS) and nightly counts of adult males, females, breeding pairs and presence/absence of eggs and tadpoles. Mesocosms were open to allow tree-frog access and arranged in randomized blocks; treatments were replicated five times.

Next steps: Mellas said he is expanding analyses to include water-clarity spectrometer data and climate-context experiments on decomposition under elevated temperatures; he is seeking to transition portions of the research to undergraduate or master's projects to maintain the mesocosm array and continue monitoring.