May 9, 2008

Garter Snakes - The Anaconda of the Frog World

In last week's post, I wrote about the relative lack of food for mountain yellow-legged frogs at high elevation sites, and some of the tactics they use to fill their bellies despite this scarcity. Despite this disadvantage of high elevation sites compared to lower elevation sites where food is more plentiful, the high elevation habitats also have a distinct advantage - a virtual lack of predators, especially garter snakes.

Three species of garter snakes inhabit the range of the mountain yellow-legged frog, each in a very specific habitat niche and all with a strong affinity for water and for amphibian prey. The Valley garter snake (Thamnophis sirtalis fitchi) is found at elevations up to about 5,000'. The Sierra garter snake (Thamnophis couchi) is found at elevations of 5,000' to 8,000', and the mountain garter snake (Thamnophis elegans elegans - photo above) is found at elevations of 8,000' to 11,000'. These snakes congregate at water bodies containing amphibians, and mountain yellow-legged frogs are a major component of their diet.

Habitats above 11,000' are generally free of snakes, and it is in these highest elevation lakes that the densities of mountain yellow-legged frogs reach their peak. Densities of three frogs per foot of lake shoreline are not uncommon, and at such sites tadpoles by the thousands often aggregate in shallow, warm waters. These densities are a remarkable sight to behold when all other life forms are so scarce, and are in large part a consequence of a lack of predators. Not only are garter snakes absent, but so too are invertebrate predators such as large beetles that prey on tadpoles. Bears and coyotes might come by on occasion and eat a few frogs but their visits are relatively rare. And so, frogs have these
highest elevation alpine lakes largely to themselves.

Mountain yellow-legged frogs have evolved some interesting behaviors to reduce the risk of predation. For example, tadpoles reduce their swimming activity when confronted by the smell of a garter snake. In other amphibian species, the presence of predator scents causes tadpoles to develop a more streamlined body shape and a more powerful tail. It remains to be seen whether mountain yellow-legged frogs show similar developmental responses to predators.
Given that mountain yellow-legged frogs don't move much during their lives, I've often wondered whether frogs from low-elevation habitats have evolved anti-predator behaviors that are absent in frogs from the highest elevations.

Given that mountain yellow-legged frogs have coexisted with predators for millenia, you can bet that they have a host of tricks that we have not yet discovered to reduce their chances of become someone else's meal.

Back to The Mountain Yellow-legged Frog Site.

1 comment:

  1. A couple notes: In the Palisade basin by brother and I came across a small pond at 11,300 feet with lots of yellow-legged frogs and tadpoles. We walked around the pond and came across 4 garter snakes actively involved in predation. Quite a sight to watch.