Herbivory is the ecological process where animals consume plants or their products, forming the foundational link between producers and consumers in nearly every ecosystem on Earth. The diversity of plant life, from microscopic algae to colossal trees, has driven the evolution of varied animal life specialized in processing this vegetative food source.
Categorizing Plant Eaters
The immense number of plant-eating species has led scientists to classify herbivores by the breadth of their food choices and their specific feeding behaviors. A primary distinction is drawn between generalists, which consume a wide variety of plant species, and specialists, which have a highly restricted diet. The koala, for example, is a famous specialist, subsisting almost entirely on the leaves of a few species of Eucalyptus. Generalists, by contrast, can switch between many different food sources, giving them an advantage when environmental conditions change.
Functional classifications further refine these categories by describing the specific plant material consumed. Grazers, such as bison and cattle, are adapted to feed mainly on low-lying vegetation like grasses, relying on specialized digestive systems to break down high cellulose content. Browsers, including deer and moose, prefer the leaves, soft shoots, and twigs of woody plants and shrubs, often using nimble lips and tongues to select the most nutritious parts.
Other functional groups focus on reproductive structures, which are high in energy and nutrients. Frugivores specialize in eating fruit, such as fruit bats and many primates, often contributing significantly to seed dispersal. Granivores, like many rodents and birds, consume seeds and nuts, which are concentrated packets of energy and protein. These specializations often involve unique anatomical adaptations, such as specialized gut microbes required to process plant cell walls.
The Specific Parts of Plants That Are Eaten
The nutritional composition of a plant varies dramatically between its different anatomical parts, leading animals to develop distinct strategies for each.
Leaves
Leaves, or foliage, represent the most common food source, often containing a relatively high concentration of protein necessary for photosynthesis. The challenge for herbivores is that this protein is often locked within cell walls composed of tough, indigestible cellulose and lignin. This requires long digestive processes involving symbiotic microbes. Furthermore, leaves can be chemically defended with compounds like tannins, which interfere with an animal’s ability to digest the available protein.
Roots and Tubers
Roots and tubers, the underground storage organs, offer a completely different nutritional profile characterized by high energy content. These structures, like potatoes and cassava roots, are loaded with nonstructural carbohydrates, primarily starch or sugars, which are much easier for most animals to digest than the fibrous components of leaves. They are generally low in crude protein and are often protected by their subterranean location.
Stems and Bark
Stems and bark present the greatest digestive challenge due to their function as structural support and transport, making them exceptionally high in lignin and other tough fibers. Animals like beavers and rabbits will selectively target the inner bark, or phloem and cambium, especially in winter when other foods are scarce. This inner layer contains stored sugars and starch, making it the most nutritious part of the woody structure.
Reproductive Parts
Reproductive parts, such as fruits and seeds, are nutrient-dense but temporary resources. Fruits are rich in simple sugars to encourage consumption and aid in seed dispersal. Seeds are compact, high-fat, and high-protein packets, making them an extremely energy-rich food source.
Plant Defenses Against Herbivores
Plants are not passive food sources and have evolved sophisticated defenses to protect their biomass from being consumed. These defenses fall broadly into physical and chemical categories, creating an ongoing evolutionary arms race with herbivores.
Physical Defenses
Physical defenses include structures that deter or physically damage an animal, such as the sharp, modified branches known as thorns, or the modified leaves called spines found on cacti and roses. Grasses employ a less visible defense by incorporating microscopic silica particles into their tissues. This makes the leaves abrasive and gradually wears down the grinding teeth of grazing animals.
Chemical Defenses
Chemical defenses, known as secondary metabolites, are compounds produced by plants that serve to repel or poison attackers. Alkaloids, a large class of nitrogen-containing compounds, are often bitter and toxic, affecting the nervous systems of animals; examples include caffeine and quinine. Phenolics, which include tannins, act as digestibility reducers by binding to plant proteins and the herbivore’s digestive enzymes. Other compounds, such as cyanogenic glycosides found in cassava, release highly toxic hydrogen cyanide only when the tissue is damaged by chewing.