February 2, 2009

Effects of Frog Declines on Bears in the Sierra Nevada

A few years ago, Dave Graber (Chief Scientist, Pacific Southwest Region of the National Park Service) asked me a question that I've never been able to get out of my head. Dave, who studied black bears in the Sierra Nevada for many years, asked me if there was a connection between the dramatic decline of mountain yellow-legged frogs in the 1970s and the coincident increase in the number of bears frequenting campgrounds and mountain towns. As an example, he mentioned that black bears were essentially absent from the eastern Sierra until the 1970s, and today the town of Mammoth Lakes alone is home to dozens of dumpster-diving ursines. I'm afraid my answer was little more than "we know that bears feed on frogs so the potential exists for frog declines to affect bears". Unfortunately we'll never know the answer to Dave's question because mountain yellow-legged frogs no longer play the important ecological role they once did as the most abundant vertebrate in the Sierra Nevada.

But what do we know about this frog-bear interaction? Over the years we've made numerous observations of bears feeding on frogs. Those I made in a fishless basin in Kings Canyon National Park in 1997 still haunt me today. This unnamed basin has never been stocked with fish and in 1997 virtually every lake was full of mountain yellow-legged frogs and tadpoles. Frog counts from that 1997 visit were 11,330 frogs and 107,750 tadpoles. During that time it was rare to walk around a lake without stumbling across numerous bear shits. In a shallow cove of one lake, tadpoles aggregated by the thousands to take advantage of the warm water found there every afternoon. Bear tracks ringed the cove shoreline and frequently veered into the lake where tadpoles were particularly dense. The only way to make sense of these observations was if bears were feeding on the abundant tadpoles. The frog population
in this basin crashed about seven years ago due to chytridiomycosis, and during recent visits bear sign seems much scarcer than it was 12 years ago.

I've witnessed something similar in Humphreys Basin following fish removal and subsequent frog recovery. In 1997, Marmot Lake contained lots of small trout and a few mountain yellow-legged frogs. Following fish removal, the frog population exploded and more than 3,000 frogs exist at the lake today. As the numbers of frogs increased so did my observations of bear sign which had been nonexistent during all of my previous years spent at this lake. Marmot Lake doesn't seem like typical bear habitat given its very high elevation but it is now not uncommon to see bear tracks all around the lake shoreline. I'm guessing that the motivation for such shoreline ambulations are the little frog snacks found there.

Given the historic abundance of mountain yellow-legged frogs in the Sierra Nevada I suspect that frogs were often included in bear diets. And maybe, just maybe, the decline of frogs has driven bears into campgrounds and towns in search of alternate food sources. Maybe someday those backcountry lakes will once again contain an abundance of frogs sufficient to interest a hungry bear.

So, as I've said many times before, although the frog restoration issue is often couched simplistically as "fish versus frogs", the issue is in fact much broader.

Back to The Mountain Yellow-legged Frog Site.


  1. Interesting. I suspect frog decline had little or no influence on bears moving to the east side. The 70s & 80s paralleled increased emphasis by NPS (and not USFS or towns) to secure food from bears. The USFS started emphasizing food storage by the mid 80s or so and canisters (very effective) started about then as well. That's when I remember bears really starting to move onto the east side (Onion Valley). Garbage cans over there were not bear proof. USFS didn't do that until the 90s, so then the bears moved into towns where they had not yet bear proofed trash (and still mostly don't).

    Bears will always move to where the easiest food is, of course.... .

    Happy to hear the bears are showing up as frogs reappear. Still, I wonder how much of their diet was frogs. Under natural food conditions, they didn't go up that high all that often, in my experience. So I wonder if they'd just make an occasional foray, hang out for a week of frogs, then back down to better food??

    I've never seen bear sign at the 11,200 ft. frog ponds on the north side of forester, yet bears used to visit the camp there all the time.

    Have there been bears reappearing at LeConte, Dusy or 60 Lakes?

    Great, as always.


  2. Hi George. I also wonder how much of the average bear's diet was made up of frogs. If the frogs occurred historically only at high elevations I would suspect that the dietary contribution made up of frogs would be relatively small. But historically mountain yellow-legged frogs were abundant at low elevations as well as at high elevations, but they've long since been eliminated from most of those low-elevation sites. In the central and northern Sierra, including Yosemite NP, there are lots of lakes at those lower elevations (7,000-9,500') where bears are relatively common. If frogs were an important food item anywhere in the Sierra, I'd bet it was in those lower elevation lakes.

    In addition to the observations I described for Humphreys Basin, evidence of bears foraging on frogs has been seen in Sixty Lake Basin and LeConte Basin. As you noted, these are both basins where fish eradications have resulted in increases in frog populations.

  3. Yes, of course, you're right. I forget that they commonly occurred at lower elevations. This is why you're so highly paid... . Even as long as I've been around, they were pretty much gone from those lower elevation lakes. I remember them only from about 10,000' and above from the early 70s.

    The one time I saw a bear foraging (?) for frogs was at the small lakes NW of Dumbell (close to 11,000'). He was just going through the shallows whacking the water trying to throw them up onto the bank. Extremely cool! Also, I'd assume there was a break in training for cubs or, at least, not a major opportunity to learn that behavior, so they're probably figuring it out as they go.

    Still, the frog population crash at the lower elevation lakes would coincide with the huge increase in backpackers and subsequent availability of food. I was in Little Yosemite Valley in '65 and an obviously experienced bear got our food then. More than makes up for food loss of froggies in their diet, I would think. As such, I think it's still safe to say that the Great Bear Diaspora wasn't secondary to loss of frogs in their diet.

    Seems like you could, though, come up with a very loose guess of what percent frogs played in their diet. Observe them foraging at a few lakes (a bit time intensive... but you've got people out there) then figure out how many lakes historically had frogs at those lower elevations. Crude, but it would be really interesting.

    Oh, and Center Basin -- I know where there's a great bear bed not far from the current habitat restoration there.You and Danny should coordinate to try to get more detailed observations. Might be extremely useful supporting data for EA's.


  4. Well, after some more conversations with Roland and Graber and a couple of other rangers, I'll re-re-re revise my thinking and post it for the metaverse... .

    I now think it is possible the frog population crash may have played a role in bears moving to the east side of the Sierra. I asked Graber about it yesterday and he is now a little skeptical because bears rarely went above 9,500 feet. However, Roland's right that there's a bunch of lakes in Yosemite that are lower that bears might have used. And, although we don't today see bears utilizing many of the lakes that do have frogs, I think that's because, as Roland suggested to me, they only forage where there are literally thousands of frogs -- a more efficient return on their efforts. There are only a handful (paw full??) of lakes like that nowadays.

    So I'm wondering if the sequence was:
    1) bears utilizing frogs pre-1970 or so;

    2) bears switch to people's food, who came on the scene in huge numbers by the 70s;

    3) then, when backpacker food was made fairly secure by steel boxes and canisters in the mid 80s, bears expand to the east side in significant numbers. There was no more backpacker food AND there were no longer frogs in sufficient densities to go back to (also, of course, they were habituated to human's food source by then).

    Whatever it is, Roland's main point is that it's more than frogs -- it's an entire ecosystem.