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Peter Hovingh has spent 16 years figuring out where and when amphibians spread through the Great Basin of western Utah and eastern Nevada, discovering that the region's geography helped form its biological history.

Traces of ancient cataclysms and volcanic eruptions show up in the patterns of distribution of these humble frogs, toads and salamanders.All amphibians are highly dependent upon water, needing it to breed and keep their skins moist. They also are restricted by temperature, as they are unable to survive in cold mountain areas.

Hovingh - a biologist who worked for the Veterans Administration in Salt Lake City - is now semi-retired and focusing his efforts on his own field work. Recently he outlined his work during a meeting of the Utah Association of Herpetologists at the University of Utah.

The one amphibian whose distribution doesn't tell much about upheavals in ancient times is the spadefoot toad, because it is more mobile than others. This is "an animal that can live anywhere in the desert - and it does."

The big, hardy toads have spread into every valley of the Great Basin, up to about 6,000 feet elevation, where the pinyon-juniper forests begin. One population was in a well about eight miles from the nearest water.

"Any valley floor, you could find spadefoot toads in a cow watering trough," Hovingh told the group. All other amphibians are far more restricted, as they "are very dependent upon water."

The tiger salamander is another amphibian whose distribution involves unusual factors. In this case, many populations are the result of escaped fishing bait.

Hovingh sorted out which populations of tiger salamanders are introductions by studying old surveys, checking for connections with other drainages and seeing whether populations survive from year to year.

Once he had crossed off the introduced populations, he found those that remained were restricted to the east of the Jordan River-Bear River drainages. Another native population is in California.

"One could propose that 15 million years ago, this entire region was an eastern hardwood or an Asian-type hardwood forest," Hovingh said. The forest may have had gingko trees and dawn redwoods, and tiger salamanders thrived in its streams.

But then the land began rising. About 5 million years ago the present habitat of mountain ranges and intervening basins formed.

The Mediterranean-type climate disappeared from the region. As the mountains rose, the changes "separated the two populations" of salamanders, leaving a great gap between the Utah tiger salamanders and those in Nevada.

The spread of leopard frogs is another story of adaptation to changing conditions. Many populations are probably introduced ones, used to feed birds or bass at such sites as Ruby Lake National Wildlife Refuge or Locomotive Springs state waterfowl area.

Hovingh can tell that these populations aren't native because they keep dying out.

The remaining populations of leopard frogs apparently were affected by the appearance and disappearance of a great lake over the past 600,000 years, the latest manifestation of which was Lake Bonne-ville.

"The different lake levels are very important to distributions," he said. Every time deep water flooded habitat, frogs were wiped out.

Volcanism also affects habitat.

Spotted frogs probably moved into this region about a million years ago, he believes. Then volcanic activity in southeastern Idaho broke up this animal's range between the Upper Snake River and the Bonneville Basin.

The spotted frog is an example of an animal that probably managed to survive floods and spread out when the great lake receded. Heber Valley is the lowest elevation in the region that was not flooded by Lake Bonneville and its predecessors, he said.