Browsing (herbivory)

Moose 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.[10][12] 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.[8] 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.[8][13][9][10]

Browsing grey rhebok

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.[9] In other cases, populations of herbivores exceed historic levels due to reduced hunting or predation pressure. For example, carnivores declined in North America throughout the 19th century and hunting regulations became stricter, contributing to increased cervid populations across North America.[14] Also, landscape changes due to human development, such as in agriculture and forestry, can produce fragmented forest patches between which deer travel, browsing in early successional habitat at the periphery.[15] Agricultural fields and young silvicultural stands provide deer with high quality food leading to overabundance and increased browsing pressure on forest understory plants.[8]

Young Alaska moose browsing on alders

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.[16] 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.[16] 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.[16] 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.[16][17][9]

Overbrowsing can change near-ground forest structure, plant species composition, vegetation density, and leaf litter, with consequences for other forest-dwelling animals.[8] 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.[18] Further, preferential selection of certain plant species by herbivores can impact invertebrates closely associated with those plants.[18] Migratory forest-dwelling songbirds depend on dense understory vegetation for nesting and foraging habitat; reductions in understory plant biomass caused by deer can lead to declines in forest songbird populations.[8][19] Finally, loss of understory plant diversity associated with ungulate overbrowsing can impact small mammals that rely on this vegetation for cover and food.[13]

Overbrowsing can lead plant communities towards 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.[8] Management to reduce deer populations has a three-method 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.[8] 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.[20][21][22] 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.[20] 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.[20]