Disturbance
Encyclopedia
In ecology
, a disturbance is a temporary change in average environmental conditions that causes a pronounced change in an ecosystem
. Outside disturbance forces often act quickly and with great effect, sometimes resulting in the removal of large amounts of biomass. Ecological disturbances include fires, flooding, windstorm, insect outbreaks, as well as anthropogenic disturbances such as forest clearing
and the introduction of exotic species
. Disturbances can have profound immediate effects on ecosystems and can, accordingly, greatly alter the natural community
. Because of these and the impacts on populations, these effects can continue for an extended period of time.
. Succession often leads to conditions that will once again predispose an ecosystem to disturbance.
Pine forests in the western North America provide a good example of such a cycle involving insect outbreaks. The mountain pine beetle
(Dendroctonus ponderosae) play an important role in limiting pine trees like lodgepole pine
in forests of western North America. In 2004 the beetles affected more than 90,000 square kilometres. The beetles exist in endemic and epidemic phases. During epidemic phases swarms of beetles kill large numbers of old pines. This mortality creates openings in the forest for new vegetation. Spruce, fir, and younger pines, which are unaffected by the beetles, thrive in canopy openings. Eventually pines grow into the canopy and replace those lost. Younger pines are often able to ward off beetle attacks but, as they grow older, pines become less vigorous and more susceptible to infestation. This cycle of death and re-growth creates a temporal mosaic of pines in the forest. Similar cycles occur in association with other disturbances such as fire and windthrow.
A disturbance changes forests significantly. Afterwards, the forest floor is often littered with dead material. This decaying matter and abundant sunlight promote an abundance of new growth. In the case of forest fires a portion of the nutrients previously held in plant biomass is returned quickly to the soil as biomass burns. Many plants and animals benefit from the conditions created by disturbances.
Some species are particularly suited for exploiting recently disturbed sites. Vegetation with the potential for rapid growth can quickly take advantage of the lack of competition. In the northeastern United States, shade-intolerant trees like pin cherry and aspen
quickly fill in forest gaps created by fire or windthrow (or human disturbance). Silver maple and eastern sycamore are similarly well adapted to floodplains. They are highly tolerant of standing water and will frequently dominate floodplains where other species are periodically wiped out.
Another species which is well adapted to a particular disturbance is the Jack Pine
in boreal forests exposed to crown fires. They, as well as some other pine species, have specialized serotinous cones that only open and disperse seeds with sufficient heat generated by fire. As a result, this species often dominates in areas where competition has been reduced by fire.
Species that are well adapted for exploiting disturbance sites are referred to as pioneers or early successional species. These shade-intolerant species are able to photosynthesize at high rates and as a result grow quickly. Their fast growth is usually balanced by short life spans. Furthermore, although these species often dominate immediately following a disturbance, they are unable to compete with shade-tolerant species later on and replaced by these species through succession.
While plants must deal directly with disturbances, animals are not as heavily affected by them. Most animals can successfully evade fires, and many thrive afterwards on abundant new growth on the forest floor. New conditions support a wider variety of plants, often rich in nutrients compared to pre-disturbance vegetation. The plants in turn support a variety of wildlife, temporarily increasing biological diversity in the forest.
A good example of this role of disturbance is in ponderosa pine
(Pinus ponderosa) forests in the western United States, where surface fires frequently thin existing vegetation allowing for new growth. If fire is suppressed, douglas-fir
(Pesudotsuga menziesii), a shade tolerant species, eventually replaces the pines. Douglas-firs, having dense crowns, severely limit the amount of sunlight reaching the forest floor. Without sufficient light new growth is severely limited. As the diversity of surface plants decreases, animal species that rely on them diminish as well. Fire, in this case, is important not only to the species directly affected but also to many other organisms whose survival depends on those key plants.
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
, a disturbance is a temporary change in average environmental conditions that causes a pronounced change in an ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
. Outside disturbance forces often act quickly and with great effect, sometimes resulting in the removal of large amounts of biomass. Ecological disturbances include fires, flooding, windstorm, insect outbreaks, as well as anthropogenic disturbances such as forest clearing
Deforestation
Deforestation is the removal of a forest or stand of trees where the land is thereafter converted to a nonforest use. Examples of deforestation include conversion of forestland to farms, ranches, or urban use....
and the introduction of exotic species
Introduced species
An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...
. Disturbances can have profound immediate effects on ecosystems and can, accordingly, greatly alter the natural community
Community
The term community has two distinct meanings:*a group of interacting people, possibly living in close proximity, and often refers to a group that shares some common values, and is attributed with social cohesion within a shared geographical location, generally in social units larger than a household...
. Because of these and the impacts on populations, these effects can continue for an extended period of time.
Disturbance conditions
Specific conditions are often required for disturbances. With natural disturbances such as fire and flooding, conditions are influenced mainly by climate, weather, and location. Fire disturbances will only occur in areas where there is low precipitation, some form of ignition (typically lightning), and enough flammable biomass to allow spread. Conditions often occur as part of a cycle and disturbances may be periodic. Other disturbances, such as those caused by humans and invasive species, can occur anywhere and are not necessarily cyclic.Disturbance as part of a cycle
Often, when a disturbance occurs naturally, it provides conditions that favor the success of different species over pre-disturbance organisms. This can be attributed to physical changes in the abiotic conditions of an ecosystem in combination with reduced levels of competition. Because of this, a disturbance can change an ecosystem for significantly longer than the period over which the immediate effects persist. However, in the absence of further disturbance, many ecosystems will trend back toward pre-disturbance conditions. Such alteration, accompanied by changes in the abundance of different species over time, is called ecological successionEcological succession
Ecological succession, is the phenomenon or process by which a community progressively transforms itself until a stable community is formed. It is a fundamental concept in ecology, and refers to more or less predictable and orderly changes in the composition or structure of an ecological community...
. Succession often leads to conditions that will once again predispose an ecosystem to disturbance.
Pine forests in the western North America provide a good example of such a cycle involving insect outbreaks. The mountain pine beetle
Mountain pine beetle
The mountain pine beetle Dendroctonus ponderosae, is a species of bark beetle native to the forests of western North America from Mexico to central British Columbia. It has a hard black exoskeleton, and measures about 5 millimeters, about the size of a grain of rice.Mountain pine beetles inhabit...
(Dendroctonus ponderosae) play an important role in limiting pine trees like lodgepole pine
Lodgepole Pine
Lodgepole Pine, Pinus contorta, also known as Shore Pine, is a common tree in western North America. Like all pines, it is evergreen.-Subspecies:...
in forests of western North America. In 2004 the beetles affected more than 90,000 square kilometres. The beetles exist in endemic and epidemic phases. During epidemic phases swarms of beetles kill large numbers of old pines. This mortality creates openings in the forest for new vegetation. Spruce, fir, and younger pines, which are unaffected by the beetles, thrive in canopy openings. Eventually pines grow into the canopy and replace those lost. Younger pines are often able to ward off beetle attacks but, as they grow older, pines become less vigorous and more susceptible to infestation. This cycle of death and re-growth creates a temporal mosaic of pines in the forest. Similar cycles occur in association with other disturbances such as fire and windthrow.
Species adapted to disturbance
Some species are particularly suited for exploiting recently disturbed sites. Vegetation with the potential for rapid growth can quickly take advantage of the lack of competition. In the northeastern United States, shade-intolerant trees like pin cherry and aspen
Aspen
Populus section Populus, of the Populus genus, includes the aspen trees and the white poplar Populus alba. The five typical aspens are all native to cold regions with cool summers, in the north of the Northern Hemisphere, extending south at high altitudes in the mountains. The White Poplar, by...
quickly fill in forest gaps created by fire or windthrow (or human disturbance). Silver maple and eastern sycamore are similarly well adapted to floodplains. They are highly tolerant of standing water and will frequently dominate floodplains where other species are periodically wiped out.
Another species which is well adapted to a particular disturbance is the Jack Pine
Jack Pine
Jack pine is a North American pine with its native range in Canada east of the Rocky Mountains from Northwest Territories to Nova Scotia, and the northeast of the United States from Minnesota to Maine, with the southernmost part of the range just into northwest Indiana...
in boreal forests exposed to crown fires. They, as well as some other pine species, have specialized serotinous cones that only open and disperse seeds with sufficient heat generated by fire. As a result, this species often dominates in areas where competition has been reduced by fire.
Species that are well adapted for exploiting disturbance sites are referred to as pioneers or early successional species. These shade-intolerant species are able to photosynthesize at high rates and as a result grow quickly. Their fast growth is usually balanced by short life spans. Furthermore, although these species often dominate immediately following a disturbance, they are unable to compete with shade-tolerant species later on and replaced by these species through succession.
While plants must deal directly with disturbances, animals are not as heavily affected by them. Most animals can successfully evade fires, and many thrive afterwards on abundant new growth on the forest floor. New conditions support a wider variety of plants, often rich in nutrients compared to pre-disturbance vegetation. The plants in turn support a variety of wildlife, temporarily increasing biological diversity in the forest.
Importance
Biological diversity is dependent on natural disturbance. The success of a wide range of species from all taxonomic groups is closely tied to natural disturbance events such as fire, flooding, and windstorm. As an example, many shade-intolerant plant species rely on disturbances for successful establishment and to limit competition. Without this perpetual thinning, diversity of forest flora can decline, affecting animals dependent on those plants as well.A good example of this role of disturbance is in ponderosa pine
Ponderosa Pine
Pinus ponderosa, commonly known as the Ponderosa Pine, Bull Pine, Blackjack Pine, or Western Yellow Pine, is a widespread and variable pine native to western North America. It was first described by David Douglas in 1826, from eastern Washington near present-day Spokane...
(Pinus ponderosa) forests in the western United States, where surface fires frequently thin existing vegetation allowing for new growth. If fire is suppressed, douglas-fir
Douglas-fir
Douglas-fir is one of the English common names for evergreen coniferous trees of the genus Pseudotsuga in the family Pinaceae. Other common names include Douglas tree, and Oregon pine. There are five species, two in western North America, one in Mexico, and two in eastern Asia...
(Pesudotsuga menziesii), a shade tolerant species, eventually replaces the pines. Douglas-firs, having dense crowns, severely limit the amount of sunlight reaching the forest floor. Without sufficient light new growth is severely limited. As the diversity of surface plants decreases, animal species that rely on them diminish as well. Fire, in this case, is important not only to the species directly affected but also to many other organisms whose survival depends on those key plants.
See also
- Patch dynamicsPatch dynamicsPatch dynamics is a conceptual approach to ecosystem and habitat analysis that emphasizes dynamics of heterogeneity within a system ....
- Ecological successionEcological successionEcological succession, is the phenomenon or process by which a community progressively transforms itself until a stable community is formed. It is a fundamental concept in ecology, and refers to more or less predictable and orderly changes in the composition or structure of an ecological community...
- Intermediate Disturbance HypothesisIntermediate Disturbance HypothesisThe Intermediate Disturbance Hypothesis states that local species diversity is maximized when ecological disturbance is neither too rare nor too frequent. At low levels of disturbance, more competitive organisms will push subordinate species to extinction and dominate the ecosystem...