Browsing (herbivory)

Browse

The plant material eaten is known as browse and is naturally taken straight from the plant, though owners of livestock such as goats and deer may cut twigs or branches for feeding to their stock. In temperate regions, owners take browse before leaf fall, then dry and store it as a winter feed supplement. In time of drought, herdsmen may cut branches from beyond the reach of their stock, as forage at ground level. In the tropical regions, where population pressure leads owners to resort to this more often, there is a danger of permanent depletion of the supply. Animals in captivity may be fed browse as a replacement for their wild food sources; in the case of pandas, the browse may consist of bunches of banana leaves, bamboo shoots, slender pine, spruce, fir and willow branches, straw and native grasses.

If the population of browsers grows too high, all of the browse that they can reach may be devoured. The resulting level below which few or no leaves are found is known as the browse line. If over-browsing continues for too long, the ability of the ecosystem's trees to reproduce may be impaired, as young plants cannot survive long enough to grow too tall for browsers to reach.

Bark peeling

Related to browsing is bark peeling. Particularly in forestry this is a problem as the trees may then get diseased and also die. Even if they do not die the trees may suffer fungal infections that affect the wood value. With the rise in deer population in some places, such as Austria, the search for food means that the bark peeling reaches economically serious levels in some forest concerns. There is a difference in mode over the seasons, with winter bark peeling being more gnawing of the trees due to the trees being frozen to some extent. Summer bark peeling, with softer barks manifests more often as long strips pulled off the trees.

Certain tree species are more palatable than others, and certain deer are more prone to carry out bark peeling than others. There is also the gender aspect of male deer scratching with their antlers on trees.

Overbrowsing

Overbrowsing occurs when overpopulated or densely concentrated herbivores exert extreme pressure on plants, reducing the resource carrying capacity and altering the ecological functions of their habitat (Côté et al. 2004; Pojar et al. 1980). Examples of overbrowsing herbivores around the world include koalas in Southern Australia, introduced mammals in New Zealand, and cervids in forests of North America and Europe (Côté et al. 2004; Wardle et al. 2001; Whisson et al. 2016).

Overview

Deer exclosures (fenced off areas) are used to determine the ecological impacts of cervids, allowing scientists to compare flora, fauna, and soil in areas inside and outside of exclosures (Jordan 1967; Wardle et al. 2001). Changes in plant communities in response to herbivory reflect the differential palatability of plants to the overabundant herbivore as well as the variable ability of plants to tolerate high levels of browsing (Côté et al. 2004). Compositional and structural changes in forest vegetation can have cascading effects on the entire ecosystem including impacts on soil quality and stability, micro- and macro-invertebrates, small mammals, songbirds, and perhaps even large predators (Côté et al. 2004; Flowerdew and Ellwood 2001; Pojar et al. 1980; Wardle et al. 2001).

Causes

There are several causes of overabundant herbivores and subsequent overbrowsing. Herbivores are introduced to landscapes in which native plants have not evolved to withstand browsing, and predators have not adapted to hunt the invading species (Pojar et al. 1980). In other cases, populations of herbivores exceed historic levels due to reduced hunting or predation pressure. For exampled, carnivores declined in North America throughout the last century and hunting regulations became stricter, contributing to increased cervid populations across North America (Chollet and Martin 2013). Also, landscape changes due to human development, such as in agriculture and forestry, can create fragmented forest patches between which deer travel, browsing in early successional habitat at the periphery (Alverson et al. 1988). Agricultural fields and young sylvicultural stands provide deer with high quality food leading to overabundance and increased browsing pressure on forest understory plants (Côté et al 2004).

Impacts on Plants

Overbrowsing impacts plants at individual, population, and community levels. The negative effects of browsing are greater among intolerant species, such as members of the genus Trillium, which have all photosynthetic tissues and reproductive organs at the apex of a singular stem (Augustine and Decalesta 2003). This means that a deer may eat all the reproductive and photosynthetic tissues at once, reducing the plant’s height, photosynthetic capabilities, and reproductive output (Augustine and Decalesta 2003). This is one example of how overbrowsing can lead to the loss of reproductive individuals in a population, and a lack of recruitment of young plants. Plants also differ in their palatability to herbivores. At high densities of herbivores, plants that are highly selected as browse may be missing small and large individuals from the population (Augustine and Decalesta 2003). At the community level, intense browsing by deer in forests leads to reductions in the abundance of palatable understory herbaceous shrubs, and increases in graminoid and bryophyte abundance which are released from competition for light (Augustine and Decalesta 2003; Chollet et al. 2013a; Pojar et al. 1980).

Impacts on Other Animals

Overbrowsing can change near-ground forest structure, plant species composition, vegetation density, and leaf litter, with consequences for other forest-dwelling animals (Côté et al 2004). Many species of ground-dwelling invertebrates rely on near-ground vegetation cover and leaf litter layers for habitat; these invertebrates may be lost from areas with intense browsing (Stewart 2001). Further, preferential selection of certain plant species by herbivores can impact invertebrates closely associated with those plants (Stewart 2001). Migratory forest-dwelling songbirds depend on dense understory vegetation for nesting and foraging habitat and reductions in understory plant biomass caused by deer can lead to declines in forest songbird populations (Côté et al 2004; Jirinec et al. 2017). Finally, loss of understory plant diversity associated with ungulate overbrowsing can impact small mammals that rely on this vegetation for cover and food (Flowerdew and Ellwood 2001).

Management and Recovery

Overbrowsing can lead plant communities towards alternate equilibrium states which are only reversible if herbivore numbers are greatly reduced for a sufficient period, and actions are taken to restore the original plant communities (Côté et al. 2004). Management to reduce deer populations takes a three-pronged approach: (1) large areas of contiguous old forest with closed canopies are set aside, (2) predator populations are increased, and (3) hunting of the overabundant herbivore is increased (Côté et al. 2004). Refugia in the form of windthrow mounds, rocky outcrops, or horizontal logs elevated above the forest floor can provide plants with substrate protected from browsing by cervids (Chollet et al. 2013b; Kain et al. 2011; Long et al. 1998). These refugia can contain a proportion of the plant community that would exist without browsing pressure, and may differ significantly from the flora found in nearby browsed areas (Chollet et al. 2013b). If management efforts were to reduce cervid populations in the landscape, these refugia could serve as a model for understory recovery in the surrounding plant community (Chollet et al. 2013b).