9.3: Herbivory

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Herbivory is a kind of predation in which the prey is a plant. Upon detecting herbivory, the plant may pump toxins into the leaf to dissuade the herbivore. In response, the herbivore may quickly chew out a pattern to isolate part of the leaf from the toxins and then dine in relative salubrity (Figure \(\PageIndex{1}\), left).

Multitudes of leaf cutter ants can be formidable herbivores (Figure \(\PageIndex{1}\), middle), cutting luggable-sized pieces of leaves to take back to their nests. The ants do not eat the leaves, but chew them further, feed them to fungus, and then eat the fungus—creating an ant–fungus mutualism, with ants being predators on the trees.

At a larger scale, multitudes of bison were formidable herbivores of the prairie (Figure \(\PageIndex{1}\), right). The native tallgrass prairies were wildflower gardens, with a few dozen species of grasses but hundreds of species of ﬂowers—said to bloom a different color each week.

Figure \(\PageIndex{2}\). *Beaver–aspen herbivory. Or is it mutualism?*

Beavers fell whole aspen trees (Figure \(\PageIndex{2}\), left) to make their dams and lodges (middle and right), and also for food. This sounds like herbivory. At the same time, however, they girdle and kill other species of trees that they do not use as food, clearing the way for new aspen. This is mutualism.

The photo on the right in Figure \(\PageIndex{2}\) shows a pond colonized by beaver after an absence of more than a century. Though it is midsummer, a few of the trees are defoliated— including one giant bur oak—because beaver have chewed off the cambium layer all around the trees, girdling and killing them. Judiciously followed, this practice would keep the forest in an early successional stage, a condition which favors staple aspen.