Browsing (herbivory)

Browsing is a specific form of herbivory, which is the process of animals eating plants. In browsing, the animal, known as a browser, feeds on leaves, soft shoots, or fruits. These food sources usually come from high-growing, woody plants like shrubs.

This behavior is distinct from grazing. Grazers primarily consume grasses or other low-growing vegetation. To understand this difference, you can look at goats and sheep. Goats are primarily browsers that prefer non-grasses, including woody plants. Sheep are primarily grazers that focus on grass.

The material that these animals eat is called browse. In nature, animals take this food directly from the living plant. However, humans often manage browse for livestock like goats and deer. In temperate regions, owners collect browse before the leaves fall. They then dry and store it to use as a winter food supplement. During droughts, herdsmen may cut branches that are too high for their animals to reach.

In tropical regions, owners often cut branches due to population pressure. This carries a risk of permanently depleting the local food supply. Even animals in captivity rely on browse. For example, pandas may eat bamboo shoots, willow branches, or banana leaves.

When the population of browsers becomes too high, they may cause overbrowsing. This occurs when herbivores exert extreme pressure on their habitat. Overbrowsing reduces the carrying capacity of the land and alters ecological functions. If browsers eat everything within their reach, they create a visible browse line. This is a level below which almost no leaves can be found on plants.

Overbrowsing can prevent a forest from regenerating. If young plants are eaten before they grow tall, they cannot survive to become mature trees.

Several factors cause herbivore populations to become overabundant. Sometimes, animals are introduced to new landscapes where native plants have not evolved to survive browsing. In these cases, local predators may not have adapted to hunt the new species. In North America, cervids, or animals in the deer family, have seen population increases. This happened because carnivores declined throughout the 19th century and hunting regulations became stricter. Human development also plays a role. Agriculture and forestry create fragmented forest patches. These patches provide deer with high-quality food in early successional habitats at the forest edges.

Overbrowsing affects plants at individual, population, and community levels. It is especially hard on intolerant species like those in the genus Trillium. These plants have all their photosynthetic tissues and reproductive organs at the top of a single stem. A single deer can eat all these parts at once. This reduces the plant's height, its ability to make food, and its ability to reproduce.

At the community level, intense browsing by deer reduces palatable shrubs. This allows graminoids, such as grasses, and bryophytes, such as mosses, to increase because they face less competition for light.

These changes create a cascading effect on the entire ecosystem. When browsing changes the vegetation density and leaf litter, other animals suffer. Many ground-dwelling invertebrates rely on leaf litter and near-ground vegetation for their homes. If these plants disappear, the invertebrates may be lost. Migratory songbirds also depend on dense understory vegetation for nesting and foraging.

Finally, a loss of plant diversity impacts small mammals that need vegetation for food and cover.

Managing overbrowsing requires helping plant communities return to a stable state. This often requires reducing herbivore numbers over a long period. Managers use three main methods: protecting large areas of old forest with closed canopies, increasing predator populations, and increasing hunting.

Another way to help is by promoting seed sources from native trees. Some plants find safety in refugia. These are protected spots like rocky outcrops, windthrow mounds, or horizontal logs. These refugia can serve as a model for how the rest of the forest might recover.