What is Herbivory?

Without plants, life on earth would be impossible. Through photosynthesis, plants convert sun energy into carbohydrates, making that energy available for consumption by animals. Thus, plants are the foundation of terrestrial food webs.

Herbivory happens when an animal eats a plant. Just as you don’t like it when an insect nibbles on you, plants generally don’t “want” to be nibbled on (with some notable exceptions, e.g. fruit has evolved to provide a tasty treat for animals that help to disperse seeds). From the plant’s point of view, most herbivory can be seen as parasitism or small-scale predation.

Plant defenses can be morphological -- i.e. spines, hairs, or very hard/thick tissue. Plants also produce chemicals that deter herbivores by tasting bad, smelling bad, and/or being toxic. These defensive chemicals are called secondary metabolites -- secondary because they are not directly necessary for basic physiological processes (chemicals directly related to basic physiological processes are primary metabolites). If you’ve ever experienced a poison ivy rash, you’ve experienced urushiol, an irritating chemical produced as an anti-herbivory defense. If you’ve ever enjoyed warm oatmeal cookies, you’ve experienced cinnamon, which is the ground bark of a Cinnamomum tree. The bark contains cinnamaldehyde and many other compounds that insects find distasteful. In fact, most of the spices and herbs in our kitchens are plant secondary metabolites. The bitter tang of mustard and horseradish comes from glucosinolates. The heat of a jalapeno is from capsaicin. The pleasant buzz of coffee comes from caffeine, an alkaloid found in Coffea seeds (coffee beans) and Camellia leaves (tea leaves). In small quantities these compounds bring zest to our cooking. But all would be distasteful in large doses, and all deter herbivores.

Herbivory can result from many kinds of vertebrate and invertebrate animals -- here in upstate NY we’d include numerous insects, slugs and snails, squirrels, chipmunks, rabbits, and deer, just to name a few. We will focus primarily on invertebrate herbivory. Invertebrates tend to graze on a leaf, leaving behind holes (internal damage) and/or jagged edges (marginal damage). These damaged areas reduce the total area available for photosynthesis. They also open the plant to infection and require it to seal the wounds, which takes energy.

Plant responses to herbivory can be complex. Herbivory may stimulate plants to increase production of secondary metabolites (“manning the barricades”, as it were). It can also stimulate extra above-ground growth, as the plant attempts to compensate for the leaf area lost to herbivory. This extra growth may leave less energy available for reproduction or below-ground root growth. Plants generally compete intensely for space and access to sunlight. Herbivory can reduce a plant’s competitive ability, thus reducing its overall fitness. Understanding how herbivory varies among plants and among habitats is very important if we want to understand how terrestrial ecosystems function.