LEWIS SPRINGS, Ariz. – Phil Rosen is knee-deep in a
disaster this spring day. Just a few years ago, native leopard
frogs filled algae-covered pools in this side drainage of the San
Pedro River, one of the last free-flowing rivers in the Southwest,
70 miles southeast of Tucson.
Now, Rosen keeps
turning up bad news. He wades up the trickle, plunging his
collection net under the watercress. “Crayfish,” the biologist
says, showing the mucky contents of his net. “Two more crayfish,”
he says, wishing he’d find a native frog.
Rosen
is disappointed: Another aquatic system looks to have been usurped
by the West’s menagerie of non-native
invaders.
For Rosen, a doctoral candidate in
herpetology (the study of reptiles and amphibians) at the
University of Arizona, the story is getting old. Over the past 20
years, many of the populations of native amphibians he studies in
Arizona and New Mexico have disappeared. Four of Arizona’s five
remaining species of native aquatic frogs have hit critical status
in recent years. And the biggest factor, contend Rosen and his
colleague Cecil Schwalbe, a University of Arizona ecologist, is the
rapid spread of non-native bullfrogs, crayfish and predatory
fishes.
Other factors also contribute to killing
native frogs and salamanders, including pesticide runoff,
smelter-related acid rain and increased ultraviolet radiation
caused by our depletion of the ozone layer. But in the case of
Arizona’s amphibians, Rosen and Schwalbe say radical, man-made
habitat changes have provided a perfect environment for
native-gobbling aliens.
A
regional mix-up
Left to themselves, the natural
wetlands and rivers might have resisted exotic invasions. But they
were not left alone.
Water developers – local,
state and federal – built hundreds of dams, reducing and flattening
out the radical swings in water flow.
Ranchers
dug thousands of stock-watering tanks near perennial springs,
creating thousands of ponds where none existed before. Flows to
marshes were diverted or impounded, or entrenched in channels.
And the Arizona Game and Fish Department, like
other state game agencies, promoted sport fishing by stocking new
artificial lakes with non-native bass, trout and even, until the
early 1970s, bullfrogs, a native of the eastern United
States.
Throw in a human population boom and “you
have a mess,” says Schwalbe.
“Before all this
habitat modification, there wasn’t much standing water in Arizona,”
he says. “But now you’ve created a bunch of perfect environments
for all the Great Plains and Southeastern predators we’ve brought
in. No wonder we’ve got an explosion of exotic invaders.”
Or, as Rosen says, “Abrupt flooding and frequent
drying define native communities in the Southwest. Take that away
and you take away the natives’ advantages.”
The
changes have already decimated the region’s native fish
populations. Forty kinds of non-native fish have overwhelmed 33
natives, wiping out five of them and leaving 19 others threatened
or endangered. Now exotic fishes, voracious frogs and crayfish are
wiping out amphibians.
The invaders have moved
beyond the state’s lowland reservoirs and rivers to take over
adjacent wetlands and even infiltrate the higher waters of
southeastern Arizona’s “sky island” mountain ranges.
Wherever they go, they chew through local
populations of smaller leopard frogs, Mexican garter snakes and
Sonoran turtles. In the larger streams and impoundments, the fish
scarf up larval leopard frogs. Elsewhere, crayfish eat native frogs
and their tadpoles as well as other organisms, including aquatic
vegetation. And the bullfrogs … well, don’t get Cecil Schwalbe
started.
“Bullfrogs are the primary threat to the
remaining native frogs in Arizona,” says Schwalbe. “I mean, they
say bullfrogs are competitors. Hell, they just eat the leopard
frogs. We’ve opened their stomachs and found virtually everything
they exist near: bats, snakes, turtles, even a red-wing blackbird.
We’ve found them sitting every meter along the pond edge, by the
hundreds. They’re monsters!’
After 15 years of
study, Schwalbe and Rosen say the spread of bullfrogs has now
relegated native leopard frogs mainly to scattered and peripheral
pockets – two or three canyons here, a number of remote cattle
tanks there. So precarious looks the future of one species, the
Chiricahua leopard frog (Rana chiricahuensis), that U.S. Fish and
Wildlife Service officials say they will list it as threatened or
endangered later this year.
A
trickle of hope
And yet Rosen and Schwalbe harbor
a hope: Though the spread of invasive species on dry land may be
irreversible, in the aquatic system victories remain
possible.
The two first experimented with
eliminating bullfrogs from a test wetland at the San Bernardino
National Wildlife Refuge in the extreme southeast corner of
Arizona. They trapped, speared and shot hundreds, but they couldn’t
match the evasiveness – or fecundity – of the bullfrogs, which
rebounded quickly. They also moved leopard frogs into fenced, newly
created, bullfrog-free ponds. Such projects on a remote federal
wildlife refuge have allowed the researchers to develop recovery
techniques without inconveniencing or alienating ranchers already
smarting from federal land grazing shut-downs caused by
environmental litigation.
But Rosen and Schwalbe
are also working with ranchers. In one of these projects, the
researchers have teamed with ranchers Matt and Anna Magoffin,
Arizona Game and Fish frog specialist Mike Sredl and the Malpai
Borderlands Group, a private nonprofit confederation of ranchers in
southeast Arizona, to protect a beleaguered population of
Chiricahua leopard frogs on a private ranch adjacent to the San
Bernardino refuge.
For months during the drought
year of 1994-95, the Magoffins and their children hauled a thousand
gallons of water a week to keep the frogs wet and alive in an
earthen cattle pond. Since then the researchers and their
collaborators have received a grant to secure more permanent water
sources and drill additional wells. The upshot: the Magoffins have
two new wells on their place, and the frogs have a new lease on
life in at least one wet spot.
“We realized that
80 to 90 percent of the surviving leopard frogs are in stock tanks,
so the idea is to use the tanks as permanent stop-gaps,” says
Schwalbe. “At least then we’ll maintain some frogs and have them
for translocation to other likely areas.”
But
these projects are “triage,” as Rosen puts it – emergency
interventions to save teetering populations. He and Schwalbe hope
to score a more lasting counterstrike against non-natives by
restoring the cycles of desert wetlands, which brim with
floodwaters during rainy seasons but dry out at other times. This,
the researchers believe, would let natural conditions select for
native fauna.
“Leopard frogs are very much able
to persist with natural drying, while bullfrog adults have a harder
time,” Rosen explains. In addition, he says, flash floods that
native frogs can hop away from may sweep bullfrog tadpoles and
maladapted slow-water fish away.
Rosen and
Schwalbe advocate removing artificial water developments in
wetlands wherever possible. In some places this would involve
“active management.” At the San Bernardino refuge, for instance,
refuge managers try to halt erosion by encouraging arroyos like
Black Draw to fill with sediment. As the eroded channel fills,
natural cienega and stream conditions will return, according to the
researchers.
In other cases, restoration can be
accomplished by doing nothing. Rosen has urged the U.S. Forest
Service simply to forego dredging the silt out of Rucker Lake, a
large artificial pond in the Chiricahua Mountains that filled with
ash and debris in the wake of the 1994 Rattlesnake Fire. That
non-action would not only save tens of thousands of dollars, but
would also permit Rucker Creek to exist without the lake habitat
that previously supported introduced trout.
“They
should just leave it. Threatened fishes are already coming back in
the canyon,” says Rosen. So far, he has gotten his wish, but local
citizens in nearby Douglas continue to lobby the Forest Service to
reopen what was a popular sports fishery in the lake.
“Fishing at that lake was tradition and even
economic development in Douglas,” says Art Macias, the town’s
economic development director. “When they say putting the lake back
might disturb other species in the streams, I think it’s just
another delaying tactic. That wasn’t a problem before. I don’t see
why it is now.”
Getting the public behind
wetlands restoration won’t be easy. Still, the herpetologists hope
to organize a multi-agency wetlands restoration task force. But the
initiative, Rosen adds, will never work without enlisting private
landowners.
“We need them to help us help these
frogs,” Rosen says, “since all those stock ponds and swimming holes
are both the boon and the bane of the situation now.”
n
Mark Muro lives in Tucson,
Arizona. He writes editorials for The Arizona Daily
Star.
You can contact
…
*Phil Rosen, University of Arizona, Tucson,
AZ 85721 (520/621-3187);
rosen@ccit.arizona.edu.
This article appeared in the print edition of the magazine with the headline Exotic predators swallow the Southwest’s native frogs.
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