Donner pass is one of the most butterfly rich areas in North America, over 120 species of butterflies have been seen since the beginning of the study! Ranging between 2000 and 2200 meters, Donner is the second highest elevation site on the transect, and is directly across I-80 from Castle Peak. This route primarily consists of forest edge and meadow, following roads, trails, and train tracks. This site is usually monitored from the Spring through the Fall.