OK campers. I wrote Vance Vredenburg and asked if he'd summarize the latest science on frog decline. Vance did his PhD on the Sierra mountain-yellow legged frog decline. He's been working with the 60 Lakes Basin population for over 10 years now and works closely with all the other major Sierra froggy researchers (Knapp, Rachowicz, Bradford...). He's currentlly a post-doc at UC Berkeley Museum of vertebrate zoology.
Dr. Vance sez:
So, here is the scoop in three parts. I have included references so that folks can look these things up for themselves (apologies for the in review papers, but they are just more data that support the statements made):
1. When introduced trout are placed in a lake with a mountain yellow-legged frog (mylf) population, the fish eat the frogs. No one can refute that. As you know, this has been rigorously tested with large-scale field experiments by multiple biologists (Vredenburg, 2004; Knapp, Boiano and Vredenburg, in review). Co-existence between reproducing frogs and introduced fish is extremely rare (<10 documented sites out of 14,000 sites surveyed in SN; Knapp and Matthews, 2001; Vredenburg et al., 2005; Vredenburg et al. in review) and is probably only possible when tadpoles have some refugia from trout predation. The other possibility is that those sites are population sinks supported by immigrants from a nearby fish-free frog population. The trout do not eat the eggs, instead they eat the tadpoles and metamophosed frogs (both juvenile and if the trout are large enough, the adult frogs as well). The tadpoles are the most susceptible stage because they cannot escape the water (Needham and Vestal, 1938; Mullaly and Cunningham 1956, Vredenburg, 2004; Vredenburg et al. 2005).
Overall impact of trout introductions on mylf: It is difficult to say whether trout alone lead to the major decline in mylf, but they are undeniably a major factor in declines of this frog. All indications are that in the late 1970s and early 1980s many mylf populations went extinct. This is about 20 years after industrialized fish introduction began (CDFG fish hatcheries and airplanes dropped fish into all major drainages, not just once, but many times over). The fish were dropped mostly into the larger lakes. This action eliminated the largest mylf populations. The new populations of trout then colonized any areas accessible to them, downstream and upstream until they reached natural fish barriers. The remaining mylf populations were left with smaller ponds and creeks and the geographic structure of populations became more fragmented (and isolated) that before fish introductions (Bradford et al. 1993). Some people have remarked that the delay in declines of frogs compared to fish introductions must mean that fish are not important. We don't have great information on when all of the declines occurred, but we do know that adult mylf live up to 11 years based on toe bone chonology work (Matthews,unpublished data, presented at DAPTF meeting UCBerkeley 2004). So, even if you drop fish into a big lake with a mylf population, adults should be able to survive there for a decade, and maybe longer if smaller satellite populations provide new immigrants. Basically, we would expect a delay between introductions on non-native trout and extictions of mylf.
2.Multiple factors: It is likely that multiple factors are involved in amphibian declines. From around the world there is evidence for a variety of factors that negatively affect amphibian populations in nature (new predators, disease, parasites, pollution, habitat destruction, etc.). In the Sierra Nevada we have several factors at play. We have direct evidence of negative effects on mylf from introduced trout (see above) and disease (Rachowicz et al. in press). We have indirect evidence of pollution (pesticide drift; Davidson, 2004), by indirect evidence I mean very strong correlations between amount of predicted pesticide drift and mylf extinctions. We have one study that tested the UV-B hypothesis, and it found no effect of UV-B on hatching success in mylf (Vredenburg, et al., in review)
3.What factor is responsible? This is difficult to answer, but for now we have three factors implicated in the decline, introduced trout, disease, and air pollution. Certainly any lake or stream with introduced trout could have contained mylf and no longer can support them. With 90% of the habitat in a large part of the SN now occupied by introduced trout (Knapp and Matthews, 2001), this factor must be heavily important. But, on the other hand, population models suggest that mylf should be able to survive pretty well even in small populations without disease and introduced trout (Briggs et al., 2005, and Briggs unpublished). So if fish were the only culprit, small populations should survive for a very long time. Here is where the word synergism is important. Disease, chytridiomycosis, is causing local extinction in some populations of mylf (Rachowicz et al, in press) and it could therefore be the culprit that could finish the job the introduced trout started. The air pollution could be weakening the immune systems of the frogs and thus make them more susceptible to disease, but this has not been tested yet. The earliest know frog with the agent that causes chytridiomycosis (Batrachochytrium dendrobatidis) in the Sierra Nevada is 1961 (Ouellet et al 2005) in a foothill yellow-legged frog outside of Sequoia National Park. In Yosemite the oldest know chytrid positive frog is a Yosemite toad collected in 1977 (Ouellet et al. 2005).
Future studies of museum collections may help us better understand the distribution of chytridiomycosis throughout the Sierra Nevada.
I hope this helps.
Bradford, D. F., F. Tabatabai, and D. M. Graber. 1993. Isolation of remaining populations of the native frog, Rana muscosa, by introduced fishes in Sequoia and Kings Canyon National Parks, California. Conservation Biology 7:882-888.
Davidson, C. 2004. Declining downwind: Amphibian population declines in california and historical pesticide use. Ecological Applications 14:1892-1902.
Knapp, R. A., and K. R. Matthews. 2000. Non-native fish introductions and the decline of the mountain yellow-legged frog from within protected areas. Conservation Biology 14:428-438.
Mullaly, D. P., and J. D. Cunningham. 1956. Ecological realtions of Rana muscosa at high elevations in the Sierra Nevada. Herpetologica 12:189-198.
Needham, P. H., and E. H. Vestal. 1938. Notes on growth of golden trout (Salmo aguabonita) in two High Sierra Lakes. California Department of Fish and Game 24:273-279.
Ouellet, M., I. Mikaelian, B. D. Pauli, J. Rodrigue, and D. M. Green. 2005. Historical evidence of widespread chytrid infection in North American amphibian populations. Conservation Biology 19:1431-1440.
Rachowicz, L. J., R. A. Knapp, J. A. T. Morgan, M. J. Stice, V. T. Vredenburg, J. M. Parker, and C. J. Briggs. 2006. Emerging infectious disease as a proximate cause of amphibian mass mortality in Rana muscosa populations. Ecology in press.
Vredenburg, V. T. 2004. Reversing introduced species effects: Experimental removal of introduced fish leads to rapid recovery of a declining frog. Proceedings of the National Academy of Sciences of the United States of America 101:7646-7650.
Vredenburg, V. T., G. Fellers, and C. Davidson. 2005. The mountain yellow-legged frog Rana muscosa (Camp 1917). in M. Lanoo, editor. Status and conservation of U.S. Amphibians. University of California Press, Berkeley.