Mountain yellow-legged frogs play a unique role in aquatic ecosystems. The majority of organisms occupy a single level in a food web throughout their lives. In aquatic systems, these levels include primary producers (algae) that make their own food via photosynthesis, herbivores (primary consumers) that eat algae, predators (secondary consumers) that eat herbivores, and top-level predators (tertiary consumers) that eat other predators. The mountain yellow-legged frog's role in the food web is unique because instead of occupying a single level of the food web, it occupies markedly different levels throughout its life.
Mountain yellow-legged frogs start life as tadpoles that feed on algae. When a tadpole metamorphoses into a frog, it switches from being an herbivore to being a predator. Juvenile and adult mountain yellow-legged frogs feed primarily on the terrestrial stages of herbivorous aquatic insects (Finlay and Vredenburg 2007). However, mountain yellow-legged frogs themselves are also important prey for terrestrial predators, including garter snakes, several species of birds (e.g., Clark's Nutcracker), coyotes, and bears.
The fact that Rana muscosa and Rana sierrae occupy multiple
levels of the food web is critically important because it indicates that
the loss of these frogs will have multiple impacts on the aquatic ecosystem.
For example, the Sierra garter snake (Thamnophis elegans elegans)
is strongly dependent on mountain yellow-legged frogs as prey (Jennings
et al. 1992), and the disappearance of mountain yellow-legged frogs
has therefore led to the subsequent decline of garter snakes (Matthews
et al. 2002). Similar effects on other top-level predators are likely.
In addition, loss of tadpole grazers could dramatically alter nutrient
cycling, algal biomass, and the abundance of herbivorous insects. Many
of these potential impacts are still poorly described, but it is clear
that the loss of mountain yellow-legged frogs as predator, prey, and herbivore
could cause profound changes to aquatic ecosystems.