Ferns produce a toxin in their leaves that discourages most vertebrates from eating them, although dinosaurs ate these plants in large quantities before their extinction. Most insects and animals that eat ferns only eat specific species of fern during specific seasons.
The European woodmouse, or Apodemus sylvaticus, eats only the fertile spores of the European fern Culcita macrocarpa, and only between December and January. The short-tailed bat, Mystacina tuberculata, also eats fern spores. Deer and the eastern cottontail rabbit often eat young bracken ferns.
During the winter and early spring, the Azores Bullfinch, or Pyrrhula murina, eats the high-fat spores of three specific species of fern. In the spring and early summer, the bullfinches switch to three different types of fern. During the spring, the birds sometimes eat the leaves of the ferns as well.
The sawfly and several species of caterpillar eat the bracken fern. The Peruvian fern insect exclusively eats ferns. Other insects that eat ferns include aphids, whiteflys, cutworms, beetles, crickets and grasshoppers.