While his classmates were getting ready for bed, Pieter Johnson spent many midnights standing waist-deep in Santa Clara County ponds. Wearing chest-high waders and a head lamp, he slid slowly into the murky waters to avoid startling his prey: Pacific treefrogs.
The late nights paid off in April when -- less than a year after his graduation -- Science published Johnson's discovery that a tiny parasite causes limb deformities in Pacific treefrogs. The finding may shed new light on the worldwide decline in frog populations -- a trend blamed on environmental pollutants.
Attention to deformed frogs grew in 1995 when a group of American schoolchildren made a highly publicized discovery of amphibians with extra and missing limbs. Suspicion immediately fell on environmental agents like pesticides. But Johnson, a biological sciences major looked deeper. Starting in December 1996 and continuing beyond his graduation in June 1998, he zeroed in on four Santa Clara County ponds that were home to Pacific treefrogs. All four were habitats for snails that are hosts of the parasitic flatworm Ribeiroia.
Other scientists had proposed parasites as an explanation for frog deformities as early as 1990. But until now, most researchers dismissed that theory because parasites also were present in normal frogs. Johnson, 23, says the parasite shows up in both, but only damages frogs infected at a specific time in their lifecycle.
To test their hypothesis, Johnson and colleagues took frog eggs from parasite-free waters in Mendocino County. They let the eggs hatch into tadpoles and placed them in containers with concentrations of the parasite similar to those found in the Santa Clara County ponds. The result: deformities showed up in 85 percent of the frogs. Alan E. Launer, '81, MS '82, research associate at Stanford's Center for Conservation Biology, and two investigators from other universities also contributed to the paper.
Johnson, who is continuing his work at Claremont McKenna College, cautions that the paper doesn't mean environmental factors don't play a role in the frog deformities. It simply raises the question of why the parasite is so prevalent now. "I don't think humans are completely blameless," says Johnson. "Our research does not indicate the all-clear."
