Trout have been nearly ubiquitous in the Sierras since before World War II. Read books from that period, such as "Waters of the Golden Trout Country" and "Yosemite and Kings Canyon Trout" (1946 & 1947 by Charles McDermand), which document both broad distribution and much healthier populations of native and non-native trout in the Sierras, yet yellow-legged frogs apparently thrived in the face of that evil onslaught until just recently. Why do researchers not explain the disconnect between the broad stocking of trout, as well as prior much higher populations of trout in the waters they inhabit, and the much later (by many decades) decline of the yellow-legged frog? It's a glaring oversight in the overly simplistic cause-and-effect theory that trout are a primary cause of the frogs' decline.
There are other holes in the "trout as cause" theory. According to one of the primary researchers on the decline of the frogs, Roland A. Knapp of the Sierra Nevada Aquatic Research Laboratory:
"The combination of data from Wallis (1952) and Botti (1977) suggests that approximately 34% of Yosemite National Park lakes still contain fish (Elliot and Loughlin 1992)."
Is it reasonable to conclude that the water-bound residents of 1/3 of an area's lakes are responsible for a species' decline across the entire area?
Let's agree for a moment with D. Knapp's theory that introduced trout are a primary cause of frog decline, and see what he says about another area receiving attention, Kings Canyon National Park:
Wouldn't we all know a lot more about frog decline if we studied what's threatening them in the 83% of that basin's lakes that don't contain trout?"However, a considerable amount of inter-drainage variation in the relative proportions of fish-containing versus fishless lakes is apparent from a second survey by Bradford et al. (1994a). This survey included 104 lakes in a particularly remote portion of Kings Canyon National Park, and trout were only found in 17%."
I do believe that trout have some impact on frogs, where trout are found, but surely native predators such as garter snakes, birds, and coyotes are much more mobile and ubiquitous threats across the frogs' range. Some combination of environmental causes is tipping the scales and denying these predators their froggie dinners across the frogs' entire range. Focusing on trout, found in a minority of Sierra lakes (and native to many of those lakes), only sucks research dollars as well as researcher and volunteer resources away from the greater underlying issues.
So why, you might ask, would a researcher do this? In parks and wilderness areas, researchers can get grants to study and remove non-native species. The process begs for an easy scapegoat. By highlighting only the trout "issue," bioligists can continue to get paid to spend summers camping in beautiful remote areas of the Sierras, gill-netting and removing trout (which according to the numbers they provide affects somewhere in the range of only 17-46% of Sierra lakes). Nice work if you can get it. One biologist has made a tidy living from this theory for over a decade. Doesn't he have to release supporting "findings" to reinforce the valuable expenditure of last year's grant, and to set the stage to get awarded next summer's grant funds? It appears to be a treadmill that's hard to get off of, and at some point, perhaps even when a theory is first offered, a researcher's professional reputation may get caught up in justifying and furthering the theory.
With so much drama and momentum built up around that theory, now even the lack of a quick rebound of frogs in the trout-cleared lakes must be blamed on the removed trout (they damaged the ecosystem so thoroughly). Now the problem HAS to be the trout. Now more than ever.
A much more balanced view of frog threats is presented at sites such as: http://eces.org/archive/ec/ecosystems/a ... ml#sources
The problems facing amphibians are global and multi-faceted. The fungus killing yellow-legged frogs in common in soil, not some new exotic invasion, and it is a documented factor in amphibian declines and extinctions as far afield as Australia and South America.
What else could be involved? Naming global warming and UV radiation as contributing factors seems like a bit of a cop out: they're too trendy and they're a convenient shift of blame and response away from local action. Deformities in frogs may be linked to viruses. What are we going to do, vaccinate amphibians? The decline of mountain yellow-legged frogs may be linked to the use of pesticides in the Central Valley. See the work by Gary Freel (spelling?) of the USGS. Pesticide use issues would require entering the arena of the EPA registration process - a real "can of worms" especially under the present administration. If you were successful providing information justifying the need to ban or more closely control some chemical applications, you would be assailed on the economics. What would be the effect on agricultural output? On jobs?
Unfortunately too few Sierra lakes have trout to have their removal make much of a difference either way to the frogs. Through the siphoning off of valuable research dollars some underlying decline factors may go relatively unexplored and unaddressed. We may see another decade of trout-targeting papers (how the trout destroyed lakes long term, even when they're no longer in habiting those waters) before that horse is thoroughly beaten to death.
My bet is on mercury. Rain arrives on California's shores with 300% the estimated pre-industrial level of mercury, and due to interaction with ozone once it starts inland contains 44% more mercury when it falls inland. Methylmercury is toxic to tadpoles at concentrations of .05ppm (Chang, et al 1973), can build up in vertebrates to concentrations many thousands of times greater than mercury levels in the surrounding water, and can be a greater problem in clear bodies of water with low dissolved organic matter (Gorski 2006).
Effects of methylmercury chloride on Rana pipiens tadpoles
http://www.sciencedirect.com/science?_o ... 55f3157c63
Mercury From China Rains Down on California
http://www.evesgarden.org/archives/2002 ... California
Study shows link between clear lakes and contaminated fish
Up to 83% of the mercury load to the Great Lakes comes from atmospheric deposition (see Shannon and Voldner, 1995).