Regarding Timothy B. Wheeler's recent article on declining amphibian populations, the mysterious disappearance of many frogs may be due to an invasive plant species known as buckthorn ("Alarming U.S. decline seen in environment's sentinels," May 23).

Buckthorn was imported from Europe in the 19th century and was initially employed for garden hedges. But since then it has spread throughout the woodlands of America.

Buckthorn is notorious for shading other native plants, inhibiting their growth and allowing increased visibility for amphibian predators. Research by Seth Magle of the Lincoln Park Zoo confirms denser numbers of predators wherever buckthorn resides.

Moreover, buckthorn engages in a form of chemical warfare. Its leaves are full of the toxin emodin. When buckthorn leaves fall, the chemical finds its way into frog ponds, and research by Richard King of Northern Illinois University suggests that the presence of emodin in ponds can decimate embryonic frog populations.

Tony Favero, Half Moon Bay, Calif.