Another Disease-caused Frog Conservation Crisis

The mountain chicken, Leptodactylus fallax, is one of the world's largest frogs, with adults sometimes weighing up to one kilogram (>2 lbs). This frog was once common on some Caribbean islands but has been decimated by the amphibian chytrid fungus (Bd). In recent years the frogs were found only on the island of Montserrat, an island that until recently was free of Bd. During the last few weeks, these frog populations have suffered severe Bd outbreaks that have killed many hundreds of animals. To ensure that these frogs are not driven to extinction frogs have been flown to three zoos in Europe where they will be used to start a captive breeding population. For more information, check out this story: http://www.wildlifeextra.com/go/news/mountain-chickens472.html#cr

The challenges posed by Bd to the conservation of the world's amphibians would have been unthinkable only a few years ago. This stuff gets more challenging by the day.

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Trout Impacts in California's Trinity Alps

When it comes to impacts of introduced trout on aquatic ecosystems, the Sierra Nevada is by far the best-studied such ecosystem in the world. However, an increasing number of studies are now being conducted elsewhere and results from this research make it clear that broad impacts are a common consequence of trout introductions into montane habitats. In recent years, a research group at the Forest Service Redwood Sciences Laboratory in Arcata has published several studies showing that introduced trout have severe impacts on a wide range of aquatic amphibians native to mountains in the Klamath bioregion, including the Cascades frog, Pacific treefrog, and long-toed salamander.

In a study just published in Freshwater Biology, Karen Pope and colleagues extend this past research by focusing on impacts to lake-dwelling invertebrates. Using a replicated whole-lake experiment conducted in the historically fishless Trinity Alps, they described changes over a three-year period in the emergence of aquatic insects from lakes in which (1) trout had never been introduced, (2) stocking of nonnative trout continued during the study, (3) stocking was suspended, and (4) stocking was halted and trout populations were removed using gill nets. Trout removal caused rapid increases in aquatic insect biomass over the three-year study period. Insect emergence was low in both the stocked lakes and stocking-suspension lakes because trout densities remained relatively high in lakes assigned to these treatments. Therefore, trout introduced into these naturally fishless lakes caused significant reductions in the biomass of native invertebrates and these taxa recovered quickly following trout removal.

The focus of the study by Pope and colleagues on insect emergence helps to emphasize a key point which is that trout impacts on lake-dwelling invertebrate communities aren't likely to be restricted to the lakes themselves. Adult forms of aquatic insects emerge from lakes and become available to a wide range of terrestrial predators and scavengers, including reptiles, birds, bats, and ants. Therefore, the reductions in insect emergence due to trout introductions could impact terrestrial species and the functioning of terrestrial ecosystems. Our increasing understanding of the connections between aquatic ecosystems and the adjacent terrestrial ecosystems is a fascinating area of study and provides a more complete picture of how pervasive the effects of trout introductions can be.

For additional information on the impacts of introduced trout on amphibians in the Klamath bioregion, check out the following papers:

Welsh, H. H., K. L. Pope, and D. Boiano. 2006. Sub-alpine amphibian distributions related to species palatability to non-native salmonids in the Klamath mountains of northern California. Diversity and Distributions 12:298-309 [link].

Pope, K. L. 2008. Assessing changes in amphibian population dynamics following experimental manipulations of introduced fish. Conservation Biology 22:1572-1581 [link].

Pope, K. L., J. M. Garwood, H. H. Welsh Jr, and S. P. Lawler. 2008. Evidence of indirect impacts of introduced trout on native amphibians via facilitation of a shared predator. Biological Conservation 141:1321-1331 [link].

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Trout Impacts on Yosemite Streams

Most research on the impacts of nonnative trout on aquatic habitats in the Sierra Nevada have focused on lakes, in part because these are the habitats favored by the declining mountain yellow-legged frog. This research has shown dramatic changes to vertebrate and invertebrate species composition caused by trout predation on the larger, more conspicuous taxa. Trout also alter nutrient cycling in these lake habitats as a consequence of the changes in species composition.

A study by Dave Herbst and colleagues, just published in Freshwater Biology, shows very clearly that impacts of introduced trout on Sierran streams are similar to those documented for lakes. Herbst et al. compared the invertebrate communities, algal cover, and algal biomass in 21 paired streams in Yosemite National Park, one stream of each pair containing introduced trout and the other member of the pair remaining in a natural fishless condition. Densities of 10 out of 50 common invertebrate taxa were significantly reduced in the trout-containing compared to the troutless streams, and these taxa tended to be conspicuous forms whose native habitats are primarily at high elevation above the original range of trout.

Reductions in species richness in the trout-containing streams caused significant increases in algal cover and biomass compared to levels in troutless streams.
Increases in algae are likely a consequence of grazing invertebrates being reduced in the presence of trout and consequent reductions in herbivory. These indirect effects (trout reduce the density of grazing insects, reduction in grazers causes an increase in algal biomass) are in agreement with numerous other studies conducted on the effects of trout on streams all around the world.

The findings of this study serve as yet another example of the considerable impacts of trout introductions on Sierra Nevada aquatic ecosystems. Given the almost complete absence of watersheds anywhere in the Sierra Nevada that haven't been stocked with trout, it is clear that we need to think about how to restore at least some entire watersheds to their historic fishless condition. That is no small undertaking.

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Amphibian Decline Movie - Sadness and Hope

I watched the film, "Frogs: The Thin Green Line", last night. Filmmaker Allison Argo and her team did an amazing job of bringing the amphibian decline issue to a general audience. Despite how dire the situation is I was left with a feeling of hope that collectively we just might make a difference. As usual, time will tell.... The film can now be viewed online on the PBS web site.

As I mentioned recently on The Mountain Yellow-legged Frog Site, the April issue of Fly Rod & Reel magazine has an article by renowned conservation writer, Ted Williams, on the lawsuit over fish stocking in California that was recently won by the Center for Biological Diversity and the Pacific Rivers Council. This is far and away the best article written so far on this issue. I've been hoping that this article would soon be posted on the Fly Rod & Reel web site, but that hasn't happened yet. So, here is a scanned copy of the article (PDF). Additional details on this topic are available on the California Department of Fish and Game web site and in my 4/18/08 and 11/21/08 Frog Blog posts.

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