Complex sentence: Three endangered species are found proximate to Rockaway Beach: the San Bruno elfin butterfly and the San Francisco garter snake; the Red-legged_Frog is found in quarry; the Red-legged_Frog is a critical food source for the San Francisco Garter Snake.
Split into simpler sentences: Three endangered species are found proximate to Rockaway Beach: the San Bruno elfin butterfly, the San Francisco garter snake and the Red-legged_Frog. These last two are found within the quarry property, the Red-legged_Frog is a critical food source for the San Francisco Garter Snake.