Browsing (herbivory)

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White-tailed deer browsing on leaves in Enderby, British Columbia Deer-Eating.jpg
White-tailed deer browsing on leaves in Enderby, British Columbia

Browsing is a type of herbivory in which a herbivore (or, more narrowly defined, a folivore) feeds on leaves, soft shoots, or fruits of high-growing, generally woody plants such as shrubs. [1] This is contrasted with grazing, usually associated with animals feeding on grass or other lower vegetations. Alternatively, grazers are animals eating mainly grass, and browsers are animals eating mainly non-grasses, which include both woody and herbaceous dicots. In either case, an example of this dichotomy are goats (which are primarily browsers) and sheep (which are primarily grazers). [2]

Contents

Browse

Browsing chital in Nagarhole (Karnataka, India) Axis axis (Nagarhole, 2010).jpg
Browsing chital in Nagarhole (Karnataka, India)

The plant material eaten is known as browse [3] and is in nature taken directly from the plant, though owners of livestock such as goats and deer may cut twigs or branches for feeding to their stock. [4] 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. [5]

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. [6] 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. [7]

Overbrowsing

Control fence to assess the impact of browsing by ungulates - outside the fencing, there is a lack of natural forest regeneration Weisergatter.png
Control fence to assess the impact of browsing by ungulates outside the fencing, there is a lack of natural forest regeneration

Overbrowsing occurs when overpopulated or densely-concentrated herbivores exert extreme pressure on plants, reducing the carrying capacity and altering the ecological functions of their habitat. [8] [9] [10] 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. [9] [11] [12]

Overview

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. [11] [13] 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. [9] The heights of plants preferred by herbivores can give indications of the local and regional herbivore density. [14] 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. [9] [15] [10] [11]

Causes

Browsing grey rhebok Grey Rhebok 2017 08 22 5546.jpg
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. [10] 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. [16] 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. [17] Agricultural fields and young silvicultural stands provide deer with high quality food leading to overabundance and increased browsing pressure on forest understory plants. [9]

Impacts on plants

Young Alaska moose browsing on alders Youngbullantlers.JPG
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. [18] 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. [18] 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. [18] 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. [18] [19] [10]

Browsing Pressure and Plant Palatability

The intensity of browsing pressure often varies depending on the palatability of plant species to herbivores. Some plant species may be heavily browsed due to their high palatability, while others may be avoided or less affected. [20] [21]

Effects on Plant Reproduction

Browsing can affect plant reproduction by reducing the availability of leaves for photosynthesis and flowers for pollination. Overbrowsing can lead to a decrease in seed production, hinder the recruitment of new individuals and alter the genetic diversity of plant population. [22] [23] [24]

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. [9] 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. [25] Further, preferential selection of certain plant species by herbivores can impact invertebrates closely associated with those plants. [25] 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. [9] [26] Finally, loss of understory plant diversity associated with ungulate overbrowsing can impact small mammals that rely on this vegetation for cover and food. [15]

Management and recovery

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. [9] 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. [9] Encouragement of tree recovery by promoting seed sources of native trees is an important aspect of managing recovery from overbrowsing. [27] 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. [28] [29] [30] 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. [28] 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. [28]

See also

Related Research Articles

<span class="mw-page-title-main">Herbivore</span> Organism that eats mostly or exclusively plant material

A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthparts adapted to rasping or grinding. Horses and other herbivores have wide flat teeth that are adapted to grinding grass, tree bark, and other tough plant material.

<span class="mw-page-title-main">Edge effects</span> Ecological concept

In ecology, edge effects are changes in population or community structures that occur at the boundary of two or more habitats. Areas with small habitat fragments exhibit especially pronounced edge effects that may extend throughout the range. As the edge effects increase, the boundary habitat allows for greater biodiversity.

<span class="mw-page-title-main">Himalayan tahr</span> Species of even-toed ungulate

The Himalayan tahr is a large even-toed ungulate native to the Himalayas in southern Tibet, northern India, western Bhutan and Nepal. It is listed as Near Threatened on the IUCN Red List, as the population is declining due to hunting and habitat loss.

<span class="mw-page-title-main">Keystone species</span> Species with a large effect on its environment

A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance, a concept introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and numbers of various other species in the community. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether. Some keystone species, such as the wolf, are also apex predators.

<span class="mw-page-title-main">White-tailed deer</span> Species of hooved mammal

The white-tailed deer, also known commonly as the whitetail and the Virginia deer, is a medium-sized species of deer native to North America, Central America, and South America as far south as Peru and Bolivia, where it predominately inhabits high mountain terrains of the Andes. It has also been introduced to New Zealand, all the Greater Antilles in the Caribbean, and some countries in Europe, such as the Czech Republic, Finland, France, Germany, Romania and Serbia. In the Americas, it is the most widely distributed wild ungulate.

<span class="mw-page-title-main">Kelp forest</span> Underwater areas highly dense with kelp

Kelp forests are underwater areas with a high density of kelp, which covers a large part of the world's coastlines. Smaller areas of anchored kelp are called kelp beds. They are recognized as one of the most productive and dynamic ecosystems on Earth. Although algal kelp forest combined with coral reefs only cover 0.1% of Earth's total surface, they account for 0.9% of global primary productivity. Kelp forests occur worldwide throughout temperate and polar coastal oceans. In 2007, kelp forests were also discovered in tropical waters near Ecuador.

<span class="mw-page-title-main">Fire ecology</span> Study of fire in ecosystems

Fire ecology is a scientific discipline concerned with the effects of fire on natural ecosystems. Many ecosystems, particularly prairie, savanna, chaparral and coniferous forests, have evolved with fire as an essential contributor to habitat vitality and renewal. Many plant species in fire-affected environments use fire to germinate, establish, or to reproduce. Wildfire suppression not only endangers these species, but also the animals that depend upon them.

<i>Malus sylvestris</i> Species of the genus Malus

Malus sylvestris, the European crab apple, also known as the European wild apple or simply the crab apple, is a species of the genus Malus, native to Europe and western Asia. Its scientific name means "forest apple".

Trophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when a trophic level in a food web is suppressed. For example, a top-down cascade will occur if predators are effective enough in predation to reduce the abundance, or alter the behavior of their prey, thereby releasing the next lower trophic level from predation.

<span class="mw-page-title-main">Conservation grazing</span> Use of animals to graze areas like nature reserves to maintain habitats

Conservation grazing or targeted grazing is the use of semi-feral or domesticated grazing livestock to maintain and increase the biodiversity of natural or semi-natural grasslands, heathlands, wood pasture, wetlands and many other habitats. Conservation grazing is generally less intensive than practices such as prescribed burning, but still needs to be managed to ensure that overgrazing does not occur. The practice has proven to be beneficial in moderation in restoring and maintaining grassland and heathland ecosystems. The optimal level of grazing will depend on the goal of conservation, and different levels of grazing, alongside other conservation practices, can be used to induce the desired results.

Mesograzers are defined as small invertebrate herbivores less than 2.5 cm in length, and can include juveniles of some larger species. The feeding behaviour of these small invertebrate herbivores is what classifies them as mesograzers. They are commonly found abundantly on Microalgae, seagrass beds, giant kelp, and coral reefs globally, since these are their main food sources and habitats. Their foraging behaviour is grazing on the organism they are living on, where there are typically masses reaching tens of thousands of mesograzers per meter of habitat. They experience predation from micro-carnivorous fish that help regulate the population of kelp and other common food sources of mesograzers by controlling the population of mesograzers; consequently, grazing is an important process linking aquatic vegetation to higher trophic level. Mesograzers show important top-down effect on marine communities, depending on the diversity and presence of predators. Mesograzers are typically overlooked in scientific research however their foraging effects have been suggested to have extreme effects on the population of their common food sources. They both positively and negatively affect macroalgal performance and productivity through grazing on algal, or through removing epiphytes. Mesograzers typically exist in spaces lacking enemies by inhabiting, therefore consuming, marine vegetation which are defended against more mobile, larger consumers through chemical defenses.

Prior to the European colonization of the Americas, indigenous peoples used fire to modify the landscape. This influence over the fire regime was part of the environmental cycles and maintenance of wildlife habitats that sustained the cultures and economies of the Indigenous peoples of the Americas. What was initially perceived by colonists as "untouched, pristine" wilderness in North America was the cumulative result of the Indigenous use of fire, creating an mosaic of grasslands and forests across North America, sustained and managed by the peoples indigenous to the landscape.

<span class="mw-page-title-main">Plant ecology</span> The study of effect of the environment on the abundance and distribution of plants

Plant ecology is a subdiscipline of ecology that studies the distribution and abundance of plants, the effects of environmental factors upon the abundance of plants, and the interactions among plants and between plants and other organisms. Examples of these are the distribution of temperate deciduous forests in North America, the effects of drought or flooding upon plant survival, and competition among desert plants for water, or effects of herds of grazing animals upon the composition of grasslands.

<span class="mw-page-title-main">Close to nature forestry</span>

Close to nature forestry is a management approach treating forest as an ecological system (ecosystem) performing multiple functions. It is developing in the peri-alpine coutries of Europe for more than 70 years, based on certain sustainable forest management practices from the late 19th century. Close to nature silviculture tries to achieve the management objectives with minimum necessary human intervention aimed at accelerating the processes that nature would do by itself more slowly. It works with natural populations of trees, ongoing processes and existing structures using cognitive approach, as in the case of so called selection forest or other types of uneven-aged forests. Its theory and practice takes forest as a self regulating ecosystem and manages it as such.

Empty forest is a term coined by Kent H. Redford's article "The Empty Forest" (1992), which was published in BioScience. An "empty forest" refers to an ecosystem that is void of large mammals. Empty forests are characterized by an otherwise excellent habitat, and often have large, fully grown trees, although they lack large mammals as a result of human impact. Empty forests show that human impact can destroy an ecosystem from within as well as from without.

<span class="mw-page-title-main">Woody plant encroachment</span> Vegetation cover change

Woody plant encroachment is a natural phenomenon characterised by the increase in density of woody plants, bushes and shrubs, at the expense of the herbaceous layer, grasses and forbs. It predominantly occurs in grasslands, savannas and woodlands and can cause biome shifts from open grasslands and savannas to closed woodlands. The term bush encroachment refers to the expansion of native plants and not the spread of alien invasive species. It is thus defined by plant density, not species. Bush encroachment is often considered an ecological regime shift and can be a symptom of land degradation. The phenomenon is observed across different ecosystems and with different characteristics and intensities globally.

In biology, overabundant species refers to an excessive number of individuals and occurs when the normal population density has been exceeded. Increase in animal populations is influenced by a variety of factors, some of which include habitat destruction or augmentation by human activity, the introduction of invasive species and the reintroduction of threatened species to protected reserves.

Herbivores' effects on plant diversity vary across environmental changes. Herbivores could increase plant diversity or decrease plant diversity. Loss of plant diversity due to climate change can also affect herbivore and plant community relationships Herbivores are crucial in determining the distribution, abundance, and diversity of plant populations. Research indicates that by consuming large amounts of plant biomass, herbivores can directly reduce the local abundance of plants, thereby affecting the spatial distribution of different plant species. For example, the impact of herbivory is typically more pronounced in grassland species than in woodland forbs, especially in environments that undergo frequent disturbances.

<span class="mw-page-title-main">Wood-pasture hypothesis</span> Ecological theory

The wood-pasture hypothesis is a scientific hypothesis positing that open and semi-open pastures and wood-pastures formed the predominant type of landscape in post-glacial temperate Europe, rather than the common belief of primeval forests. The hypothesis proposes that such a landscape would be formed and maintained by large wild herbivores. Although others, including landscape ecologist Oliver Rackham, had previously expressed similar ideas, it was the Dutch researcher Frans Vera, who, in his 2000 book Grazing Ecology and Forest History, first developed a comprehensive framework for such ideas and formulated them into a theorem. Vera's proposals, although highly controversial, came at a time when the role grazers played in woodlands was increasingly being reconsidered, and are credited for ushering in a period of increased reassessment and interdisciplinary research in European conservation theory and practice. Although Vera largely focused his research on the European situation, his findings could also be applied to other temperate ecological regions worldwide, especially the broadleaved ones.

Oak regeneration failure is a woodland phenomenon whereby insufficient oak (Quercus) seedlings and saplings are recruited into the canopy to replace dead mature oaks. The result is a local decline in oak numbers while other more shade-tolerant trees such as maple, lime, and ash may become more prominent. Oak regeneration failure has been observed across Eastern and Midwestern forests in the United States as well as in Europe.

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