Centuries ago, a tree was plucked out of its native ecosystems and introduced to the U.S. Chinese tallow (Triadica sebifera) is a showy tree with waxy seeds and heart-shaped leaves.
Every autumn, its leaves turn crimson or orange before falling to the ground – or the water. “Chinese tallow invades wetlands and riparian areas in the Southeast,” says U.S. Forest Service scientist Daniel Saenz.
In wetlands, the fallen leaves can kill frog and toad eggs, as Saenz and others showed in 2012.
“Chinese tallow leaves change the pH of water,” says Saenz. “The leaves also decompose much faster than leaves from native trees.”
The swift decay dumps nutrients into the water. Bacteria feast on the extra nutrients, and their populations explode, making the water look hazy. “A film of bacteria forms on the surface of the water,” says Saenz. “This can all happen in less than 48 hours.”
More bacteria require more dissolved oxygen, leaving less for frog and toad larvae.
Saenz and SRS wildlife biologist Cory Adams suspected that even small amounts of tallow litter would harm tadpoles.
The scientists studied tadpoles from two native frog species – American bullfrog (Lithobates catesbeianus) and bronze frog (Lithobates clamitans).
The results of their study were published in the Herpetological Journal.
The scientists collected Chinese tallow leaves from the Stephen F. Austin Experimental Forest. They put leaves in small mesh bags and let them steep in five-gallon aquaria.
“We tested three concentrations of Chinese tallow litter, plus a control,” says Saenz. The most concentrated treatment had one gram of leaves – equal to about four dry leaves – for every liter of water.
As Chinese tallow concentration increased, dissolved oxygen in the water decreased. The scientists also measured pH and salinity. Then, they added tadpoles. “We recorded their position in the water column and watched them breathe,” says Saenz.
Tadpoles have several options for acquiring oxygen. Like fish, they have gills. They can even respire through their skin. Tadpoles also have lungs, which usually do not work very well until metamorphosis.
“In treatments with more Chinese tallow and less dissolved oxygen, tadpoles had to use their lungs more frequently,” says Saenz. “They swam to the water’s surface to gulp air much more often.”
In a world of lurking predators, even small changes in behavior could affect tadpole survival. “Every time a tadpole surfaces, it becomes a target for predators,” says Saenz.
It takes time and energy for tadpoles to swim to the surface. Based on tadpole behavior in the lab, high concentrations of tallow litter could force tadpoles to make over 30 extra trips to the water surface every hour.
Over the course of a day, the extra trips could add up to hundreds of meters. “That’s a lot of time spent swimming instead of foraging,” says Saenz.
Bronze frog tadpoles exposed to Chinese tallow also spent more time floating near the surface of the water.
“We have little doubt that hypoxia is the driver,” says Saenz. However, the exact mechanisms are unclear. Their air-filled lungs could buoy the tadpoles. They could also be seeking more oxygen, as oxygen levels are higher near the water’s surface.
The study is likely the first to link invasive plants, hypoxia, and altered amphibian behavior.
“In nature, other buffers or facilitators could play a role,” says Saenz. “Natural wetlands vary in depth, and so would the energy expenditures of making these extra trips.”
Saenz and Adams visited natural wetlands at the Davy Crockett National Forest in eastern Texas. “We wanted more context for the water chemistry values we measured in the lab,” says Saenz.
The scientists sampled water chemistry in 51 wetlands. They sampled before autumn, so no leaves had fallen into the water, and there were no Chinese tallow trees in the area.
Oxygen concentrations in the natural wetlands varied. However, they tended to be high – about twice as high as the lowest concentration treatment.
The lowest treatment had 0.15 grams of dry tallow leaves per liter of water. Larvae exposed to this concentration in the lab gulped air significantly more often than larvae in the control groups.
“Our study suggests that even modest Chinese tallow invasions could significantly alter water chemistry,” says Saenz. “That could, in turn, affect the behavior and survival of native frogs and toads.”
For more information, email Daniel Saenz at firstname.lastname@example.org.