February 8, 2011

Disappearing Frogs and Shifting Baselines

I've studied mountain yellow-legged frogs for the last 15 years, all in the High Sierra, the last stronghold of these beasts. As a consequence of my experiences, when I think of mountain yellow-legged frog habitat in the Sierra Nevada I think of lake basins in the alpine and subalpine zones. But scientists who came before me, including Joseph Grinnell and Richard Zweifel, would have had a very different view. Back in their time, they found abundant mountain yellow-legged frog populations in streams at 6000' in the Ponderosa Pine belt of the western Sierra, in the expansive meadows of the unglaciated Kern Plateau, in the lakes, ponds, and streams in the Lake Tahoe watershed, in streams in Nevada's Carson Range, and in the far northern reaches of the historic range, in streams as low as 2000' on the Plumas National Forest. 

Most of these mountain yellow-legged frog populations are gone now, pushed aside by reservoirs, water diversions, introduced trout, and disease. The people who remember frogs in those places are mostly gone too. Luckily, they left us with field notes containing detailed descriptions of what they saw and collections of frogs in jars on museum shelves. Those relics serve as indelible reminders of the places these frogs once occupied and provide an unchanging reference point against which to judge the current situation and the success of future conservation efforts. 

I was recently assembling a data set of historic frog localities, and it was that data set that reminded me of just how ubiquitous mountain yellow-legged frogs once were. These records provide an amazing glimpse into a nearly-forgotten past. For example, in 1911 Joseph Grinnell and his survey team worked their way up the South Fork Kern River, describing the vertebrate fauna that they encountered and collecting the specimens that now reside at the U.C. Berkeley Museum of Vertebrate Zoology. In Monache Meadow, field notes record the presence of mountain yellow-legged frogs and also note the impacts caused by intensive livestock grazing. As they moved upstream, they saw mountain yellow-legged frogs in abundance in Templeton and Ramshaw Meadows, and at Big Whitney Meadow, at the headwaters of Golden Trout Creek, they observed Swainson's Hawks feeding on the seemingly ubiquitous frogs. At the nearby Rocky Basin Lakes, Grinnell had this to say:

"There are great numbers of apparently full-grown frogs around the shores of the lakes. The conspicuous thing about them is their extreme wariness. They jump into the water and dive quickly into the deepest holes within reach when one is yet fully 10 yards from them. There is a shower of frogs in advance of a person as he walks along the beach. They must have some nimble and persistent enemy."

That is a sight I would have loved to see, but the point of this isn't to romanticize the past. Instead, it is to recognize how quickly we forget what once was and replace it with a view based on our own experiences. Every human generation replaces what the previous generation knew to be true with their own version of reality, producing a series of shifting baselines through time. It is only the field notes and collections of those who came generations ago that remind us of what the world really looked like 100 years past. As someone who has hiked all over the High Sierra studying frogs, I thought I knew this frog well. It took Grinnell's journals to once again remind me of how narrow my own understanding is and of how much we've lost.

Back to The Mountain Yellow-legged Frog Site.

1 comment:

  1. We much appreciate the perspective you give on the role amphibians play in California ecology. Great writing!