![Contrasts - fire](https://upload.wikimedia.org/wikipedia/commons/3/3d/Contrasts_-_fire.jpg)
In
ecology
Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...
, a disturbance is a temporary change in environmental conditions that causes a pronounced change in an
ecosystem
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
. Disturbances often act quickly and with great effect, to alter the physical structure or arrangement of biotic and
abiotic elements. A disturbance can also occur over a long period of time and can impact the
biodiversity
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
within an ecosystem.
Major ecological disturbances may include
fire
Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction Product (chemistry), products.
At a certain point in the combustion reaction, called the ignition ...
s,
flood
A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...
ing,
storms,
insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...
outbreaks and
trampling
Trampling is the act of walking on something repeatedly by humans or animals.
Trampling on open ground can destroy the above ground parts of many plants and can compact the soil, thereby creating a distinct microenvironment that specific species ...
.
Earthquake
An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
s, various
types of volcanic eruptions,
tsunami
A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explo ...
,
firestorms,
impact event
An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...
s,
climate change
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
, and the devastating effects of
human impact on the environment
Human impact on the environment (or anthropogenic impact) refers to changes to biophysical environments and to ecosystems, biodiversity, and natural resources caused directly or indirectly by humans. Modifying the environment to fit the need ...
(
anthropogenic
Anthropogenic ("human" + "generating") is an adjective that may refer to:
* Anthropogeny, the study of the origins of humanity
Counterintuitively, anthropogenic may also refer to things that have been generated by humans, as follows:
* Human im ...
disturbances) such as
clearcutting
Clearcutting, clearfelling or clearcut logging is a forestry/ logging practice in which most or all trees in an area are uniformly cut down. Along with shelterwood and seed tree harvests, it is used by foresters to create certain types of fore ...
,
forest clearing
Deforestation or forest clearance is the removal of a forest or stand of trees from land that is then converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban use. The most concentrated d ...
and the introduction of
invasive species
An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species ad ...
can be considered major disturbances.
Not only invasive species can have a profound effect on an ecosystem, but also naturally occurring species can cause disturbance by their behavior. Disturbance forces can have profound immediate effects on ecosystems and can, accordingly, greatly alter the
natural community. Because of these and the impacts on populations, disturbance determines the future shifts in dominance, various species successively becoming dominant as their life history characteristics, and associated life-forms, are exhibited over time.
Criteria
Conditions under which natural disturbances occur are influenced mainly by
climate
Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...
, weather, and location.
Natural fire disturbances for example occur more often in areas with a higher incidence of lightning and flammable biomass, such as
longleaf pine ecosystems in the southeastern United States. Conditions often occur as part of a cycle and disturbances may be periodic. Other disturbances, such as those caused by humans, invasive species or impact events, can occur anywhere and are not necessarily cyclic.
Extinction vortices may result in multiple disturbances or a greater frequency of a single disturbance. Immediately after a disturbance there is a pulse of recruitment or regrowth under conditions of little
competition
Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indivi ...
for space or other resources. After the initial pulse, recruitment slows since once an individual plant is established it is very difficult to displace.
Due to the varying forms of disturbance this directly impacts the organisms which will exploit the disturbance and create diversity within an ecosystem.
Because scale-dependent relationships are ubiquitous in ecology, the spatial scale modulates the effect of disturbance on natural communities. For example,
seed dispersal
In Spermatophyte plants, seed dispersal is the movement, spread or transport of seeds away from the parent plant.
Plants have limited mobility and rely upon a variety of dispersal vectors to transport their seeds, including both abiotic vectors, ...
and
herbivory may decrease with distance from the edge of a burn. Consequently, plant communities in the interior areas of large fires respond differently than those in smaller fires. Although disturbance types have varied on ecosystems, spatial scale likely influences ecological interactions and community recovery from all cases because organisms differ in
dispersal and movement capabilities.
Cyclic disturbance
![Kyrill-Schäden-04](https://upload.wikimedia.org/wikipedia/commons/e/e9/Kyrill-Sch%C3%A4den-04.JPG)
Often, when disturbances occur naturally, they provide conditions that favor the success of different species over pre-disturbance organisms. This can be attributed to physical changes in the biotic and abiotic conditions of an ecosystem. Because of this, a disturbance force can change an ecosystem for significantly longer than the period over which the immediate effects persist. With the passage of time following a disturbance, shifts in dominance may occur with ephemeral herbaceous life-forms progressively becoming over topped by taller perennials herbs, shrubs and trees.
However, in the absence of further disturbance forces, many ecosystems trend back toward pre-disturbance conditions. Long lived species and those that can regenerate in the presence of their own adults finally become dominant.
Such alteration, accompanied by changes in the
abundance
Abundance may refer to:
In science and technology
* Abundance (economics), the opposite of scarcities
* Abundance (ecology), the relative representation of a species in a community
* Abundance (programming language), a Forth-like computer prog ...
of different species over time, is called
ecological succession
Ecological succession is the process of change in the species structure of an ecological community over time. The time scale can be decades (for example, after a wildfire) or more or less.
Bacteria allows for the cycling of nutrients such as ca ...
. Succession often leads to conditions that will once again predispose an ecosystem to disturbance.
Pine forests in western North America provide a good example of such a cycle involving insect outbreaks. The
mountain pine beetle (''Dendroctonus ponderosae'') plays 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
Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...
and
epidemic
An epidemic (from Ancient Greek, Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of patients among a given population within an area in a short period of time.
Epidemics ...
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 windstorms.
When multiple disturbance events affect the same location in quick succession, this often results in a "compound disturbance," an event which, due to the combination of forces, creates a new situation which is more than the sum of its parts. For example, windstorms followed by fire can create fire temperatures and durations that are not expected in even severe wildfires, and may have surprising effects on post-fire succession.
Environmental stress
Stress, either physiological, biological or psychological, is an organism's response to a stressor such as an environmental condition. Stress is the body's method of reacting to a condition such as a threat, challenge or physical and psych ...
es can be described as pressure on the environment, with compounding variables such as extreme temperature or precipitation changes—which all play a role in the diversity and succession of an ecosystem. With environmental moderation, diversity increases because of the
intermediate-disturbance effect, decreases because of the
competitive-exclusion effect, increases because of the prevention of competitive exclusion by moderate predation, and decreases because of the local extinction of prey by severe predation.
A reduction in recruitment density reduces the importance of competition for a given level of environmental stress.
Species adapted to disturbance (eurytopy)
A disturbance may change a forest 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
nutrient
A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
s previously held in plant biomass is returned quickly to the soil as biomass burns. Many plants and animals benefit from disturbance conditions.
[Pringle, L. 1979. Natural Fire: Its Ecology in Forests. William Morrow and Company, New York. 27-29.]
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
In ecology, shade tolerance is a plant's ability to tolerate low light levels. The term is also used in horticulture and landscaping, although in this context its use is sometimes imprecise, especially in labeling of plants for sale in commercial ...
trees (trees
stenotopic to shade) like pin cherry and
aspen
Aspen is a common name for certain tree species; some, but not all, are classified by botanists in the section ''Populus'', of the ''Populus'' genus.
Species
These species are called aspens:
*'' Populus adenopoda'' – Chinese aspen (China ...
quickly fill in forest gaps created by fire or windstorm (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.
When a tree is blown over, gaps typically are filled with small herbaceous
seedling
A seedling is a young sporophyte developing out of a plant embryo from a seed. Seedling development starts with germination of the seed. A typical young seedling consists of three main parts: the radicle (embryonic root), the hypocotyl (embry ...
s but, this is not always the case; shoots from the fallen tree can develop and take over the gap. The sprouting ability can have major impacts on the plant population, plant populations that typically would have exploited the tree fall gap get over run and can not compete against the shoots of the fallen tree. Species adaptation to disturbances is species specific but how each organism adapts affects all the species around them.
Another species well adapted to a particular disturbance is the
Jack pine
Jack pine (''Pinus banksiana'') is an eastern North American pine. Its native range in Canada is east of the Rocky Mountains from the Mackenzie River in the Northwest Territories to Cape Breton Island in Nova Scotia, and the north-central and ...
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. However these shifts may not reflect the progressive entry to the community of the taller long-lived forms, but instead, the gradual emergence and dominance of species that may have been present, but inconspicuous directly after the disturbance.
Disturbances have also been shown to be important facilitators of non-native plant invasions.
While plants must deal directly with disturbances, many animals are not as immediately affected by them. Most 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
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') lev ...
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 (''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
The Douglas fir (''Pseudotsuga menziesii'') is an evergreen conifer species in the pine family, Pinaceae. It is native to western North America and is also known as Douglas-fir, Douglas spruce, Oregon pine, and Columbian pine. There are three va ...
(''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.
Diversity is low in harsh environments because of the intolerance of all but opportunistic and highly resistant species to such conditions.
The interplay between disturbance and these biological processes seems to account for a major portion of the organization and spatial patterning of natural communities.
Disturbance variability and species diversity are heavily linked, and as a result require adaptations that help increase plant fitness necessary for survival.
See also
*
Environmental disaster
*
Ecological succession
Ecological succession is the process of change in the species structure of an ecological community over time. The time scale can be decades (for example, after a wildfire) or more or less.
Bacteria allows for the cycling of nutrients such as ca ...
*
Forest dynamics
*
Forest pathology
Forest pathology is the research of both Biotic stress, biotic and Abiotic stress, abiotic maladies affecting the health of a forest ecosystem, primarily Fungal infection in plants, fungal pathogens and their insect vectors. It is a subfield of ...
*
Habitat destruction
Habitat destruction (also termed habitat loss and habitat reduction) is the process by which a natural habitat becomes incapable of supporting its native species. The organisms that previously inhabited the site are displaced or dead, thereby ...
*
Human–wildlife conflict
Human–wildlife conflict (HWC) refers to the negative interactions between human and wild animals, with undesirable consequences both for people and their resources, on the one hand, and wildlife and their habitats on the other (IUCN 2020). HWC ...
*
Intermediate disturbance hypothesis
*
Patch dynamics
Patch dynamics is an ecological perspective that the structure, function, and dynamics of ecological systems can be understood through studying their interactive patches. Patch dynamics, as a term, may also refer to the spatiotemporal changes wi ...
*
Stressor
References
External links
Microdocs: Disturbance
{{Authority control
Landscape ecology
Ecological succession
Ecology terminology
Environmental terminology
Habitat