Disturbances, Disruption, Succession

Natural Disturbances

Types of natural disturbances include:

• Weather, drought, warming, climate change, windthrow
• herbivore outbreaks (insects and others)
• disturbances that remove organic ground cover or change the canopy, fires and treefalls due to wind and hurricanes
• floods, freshet, lake turnover
• changes in glaciation
• Wildfires
• volcanic eruptions
• insect and disease outbreaks

Impact

Disturbances such as hurricanes, wildfires and floods often have negative impact on the natural and (human) build environment. (Disturbances such as storms and hurricanes can be temporarily destructive to kelp protect and lead to beach erosion. Disturbances can remove grasses protecting sand from wind erosion.

Trees reduce the speed of flood waters, leading to deposition of sediments and reducing water energy that would otherwise cause erosional changes in channel morphology. Removal of trees can lead to erosion of riverbanks and flood plains.

Fire removes live and dead organic material and can raise temperatures to lethal levels. Fires may volatilize large amounts of nitrogen and return nutrients in ash while releasing carbon into the atmosphere.

Floods and landslides remove or add soils, floods can deplete oxygen from tree roots.

Disturbances that eliminate plant biomass increase runoff through removal of ground cover that binds the soil and disrupts the flow of water. Reduction of biomass reduces evaporation and transpiration and leads to destruction in root systems that absorb water.

Disruption can change albedo (reflection of sunlight). Vegetation absorbs light and energy. Snow reflects light and energy. Reduced reflection contributes to higher surface temperatures and surface evaporation.

The impact of disturbance depends on (1) sensitivity of the ecosystem, (2) severity of the disturbance, (3) the properties of the landscape, for example, an isolated area is less resilient, (4) the frequency of disturbances occurring and (5) the types of disturbance occurring in succession.

Organisms have different sensitivity to disturbances. Trees from wet environments generally tolerate periodic flooding. Root depths affects tolerance to frost, fire and drought. Density of species influences spread of fire, pathogens or insect pests.

Disturbance severity measures the magnitude of loss. Severity is the major factor determining the rate and trajectory of vegetation development (succession) after disturbance.

The timing of disturbance may affect the impact of the disturbance. A fire that occurs during a bud outbreak will have more impact on growth. A flood will have more impact when roots are active.

Disturbances often leave islands of undisturbed vegetation. If a disturbance is ongoing the disturbance has a sustained effect.

Adapting to Disturbance

Species are typically adapted to a natural disturbance regime. Adaptations of organisms to the characteristic disturbance regime of their environment often reduces the impacts of disturbance. Disturbance is critical to maintaining the richness of systems. Disturbance shapes the long-term fluctuations in the structure and functioning of ecosystems and therefore their resilience and vulnerability to change.

Changes in species composition and diversity both cause and respond to the changing availability of light, water and nutrients as succession proceeds, leading to characteristic changes in the cycling of carbon, water and nutrients and the associated supply of ecosystem services.

Varying conditions in some zones cause wildlife species to consistently migrate from region to region, locating habitats that are compatible for each season of the year. Other wildlife stay in one zone all year round as they have adapted to the climate in that select region.

Biological legacies are the elements of a pre-disturbance ecosystem that survive to take part in its recovery.

The dividing line between disturbances and normal function is somewhat arbitrary and may be a question of degree. There is a continuum of severity, for example from minor moisture stress to severe drought.

A biological community can create conditions that lead to its own destruction. For example, as trees grow older, they become weak and vulnerable to destruction by insects or diseases. When a biological community grows old and ‘dies’, and another biological community takes its place.

A biological community can be destroyed by natural disturbances or human-generated disruptions’ and replaced by another biological community.

Human Impact

Human activities can have a powerful effect on ecosystems, the ecoservices they provide and the way they change.

Types of Human Impact

People have converted forests, wetlands, grasslands, and other terrestrial ecosystems for their needs, causing a large decline in the number of species worldwide over the past decades. Types of human impact include:

• Habitat loss and degradation occur when thinning, fragmentation and/or destruction of an existing natural habitat eliminates or reduces resources and living space for one or many of the species using the habitat. Species that cannot adapt to the succession or migrate to another ecosystem are eliminated.
  1. Examples of habitat loss are logging that removes trees, damages organisms and changes the canopy and ground cover. Shade is removed from streams. The gravel of stream beds used by fish for spawning is washed out. Replanted forests are often monocultures (consisting of one species of a few).
  2. Farms similarly replace forest, grassland and wetlands with fields. Fertilizer run off from lands changes the ecology of rivers and lakes. Plowing destroys the complex root structures and services provided by native species and diminishes the carbon sequestration of root systems. Pesticides and herbicides have toxic impact on non-target organisms. Abandoned agricultural lands may be depleted or enriched depending on agricultural practices.
  3. Mines replace forest with excavations and tailing ponds. Survey and exploration work from oil and gas production opens up territory for predators. Mining can produce unfavourable environments due to toxicity.
  4. Fish farms attract sea lice, killing wild salmon.
• Overexploitation : Harvesting beyond the capacity for surviving populations to replace their losses often results in some species being depleted to very low number or rendered extinct. The populations of commercially valuable fish decline and are replaced by trash fish or other aquatic animals of little or no commercial value. Desertification is the change from a grassland ecosystem to a desert ecosystem in a region where the climate is suitable for grassland. There can be enough rainfall for grass but overgrazing or irrigation can change the grassland to desert.
• Introduction of invasive species : Transport of species around the world increases the frequency of biological invasions. Invasive species may out compete native species causing the original population to decline.
• Pollution can contribute to biodiversity loss by causing health issues in exposed species. Pollution can kill, harm reproductive functions, and threaten species survival. Nutrient enrichment of waters from agricultural and industrial runoff and from human and livestock sewage has increased algal production. Decomposition depletes oxygen within the water column creating dead zones where anaerobic conditions kill fish and other animals.
• Forest Fires : Human interference with fire disturbance and succession destroys mature ecosystems and transform them to an earlier stage of succession. Fire suppression can leave an understory of shrubs and grasses, leading to more intense fires.
• Climate Change : Human acceleration of climate change can change the makeup of ecosystems at a faster rate than species are able to adapt. These changes may be severe enough to destroy the ecosystem through temperature and precipitation change. As climate-driven stresses become more pronounced and local extinctions occur more frequently, the functional redundancy and biodiversity of the matrix is affected.
• Stream channeling, rerouting, and diking leads to the elimination of wetlands and nurseries for young fish.
• Other examples of human impact include harbours, airports, road construction and the resulting traffic, and urban development (homes, stores, office buildings and factories). Cities replace forests, grasslands and wetlands. Gasses from industry, cars and homes blanket hillsides with chemicals—sometimes denuding them—causing climate changes and altering cycles of carbon, nitrogen, phosphorus, sulfur and water.

People have altered ecosystems more rapidly in recent history. Human impact is so extensive that it usually can’t be equated to disturbances. With human population and development expansion, ecosystem change happens more frequently causing biodiversity loss. The impact is sometimes long term and are usually permanent, causing a large decline in the number of species worldwide over the past decades. Changes are often unexpected and sometimes seriously detrimental to the benefits that people derive from ecosystems. A biological community can be destroyed by human activities and replaced by another biological community.

Human activities such as overgrazing, acid rain and other pollutants have effects on growth and production of leaf areas. Pollutants reduce carbon gain, primarily by reducing leaf area or photosynthetic capacity. Pollutants directly reduce photosynthesis by entering the stomata and damaging the photosynthetic machinery. Nitrogen deposition can stimulate leaf production.

Activities have created large changes in the ecological trajectory of different landscapes, ultimately resulting in ecological succession. Warming temperatures, drought flooding and other changes to climate are additional factors leading to change. The cumulative impact of human activity extends well beyond individual ecosystems, affecting climate and generating new ecosystems. It is often difficult to predicting the impact of a disturbance, thus, a precautionary approach is needed.

The United Nations recommends that:

“…at least 30 per cent of terrestrial, inland water, and of coastal and marine areas, especially areas of particular importance for biodiversity and ecosystem functions and services, are effectively conserved and managed through ecologically representative, well-connected and equitably governed systems of protected areas and other effective area-based conservation measures…”

Succession

A biological community is an interacting group of various species in an environment. Ecological succession is the observed process of change in the variety of species in a community, the progression from immature biological communities to mature and climax communities. Some species in communities might become abundant others might vanish from the ecosystem altogether or shift location The study of ecological succession describes how a biological community changes over time.

Succession occurs after significant disturbances to ecosystem processes. Disturbances lead to succession and ultimately causes ecosystems to change. Disturbances can take many different forms and can vary in intensity and size. These massive forces may destroy species and thus alter the dynamics of the ecological community.

Succession is influenced by environmental factors such as soil type, water regimes, vegetation history, climate and invasive species. Succession is accompanied by changes in the sizes and types of plants leading to a diversity of food and habitat for animals and soil microbes. A biological community can change the physical or biological conditions of a site, making it more favourable for another biological community. One biological community therefore leads to another. The scale of successional dynamic ranges from individual plants to an entire community and from years to centuries.

There are three stages of succession:

Early or Primary Succession

Primary succession occurs when new land is formed or empty, bare landscape is exposed, providing a habitat that can be colonized for the first time. Primary succession can result from disturbances.

‘Pioneer Species’ are the first species to colonize an area during early succession. The organisms most suited to a particular environment, and the ones that can be found in the vicinity, are most likely to be the pioneers. The primary succession stage is referred to as immature because composition of the area is simple, with fewer species.

Smaller species of plants and animals generally grow and reproduce rapidly. Larger plants and animals take more time to grow. The rapidly growing plants and animals populate a site first and the slower ones take over later. For example, if a fire or logging destroys a forest, there will be many species of grass growing on the site within months because grasses grow quickly. Later, shrubs grow over the grasses and after that trees grow over the shrubs.

The nutrient base improves as nutrients are mobilized by microbes and vegetation. Decomposition leads to accumulation of nutrients.

Early succession forests support large populations of deer and other browsers, leading to further accumulation of nutrients. Plants shift from growth to defense against insects. Succession birds, and rodents disperse seeds. Many grasslands are maintained by mammals that prevent succession to forests. The proportion of primary production consumed by herbivores is maximal in early to mid-succession.

Secondary Succession

Secondary succession can happen as a result of disturbances or can happen as a result of progression from primary succession. Facilitation happens when early successional species make the environment more favourable for the growth other species through nitrogen fixation and accumulation of soil organic material. Secondary succession begins on soils that develops after primary succession. Over the long-term herbivores accelerate plant succession by removing early successional species.

Herbivores, insects, and pathogens account for plant mortality during secondary succession. Herbivores enhance nutrient availability by returning available nutrients to the soil. Growth of plants can alter temperature and moisture regimes for decomposition.

Most biological communities are in a continual state of secondary succession.

Climax Community

Ecologists identify a third final stage that is the final stage of ecological succession called a ‘Climax Community’. Over time, as the biological community develops, the succession process becomes more complex. In this stage, there is an accumulation of more species, many of them more specialized with regard to diet and interaction with other plants and animals in the food web. The ecosystem consequently is described as being more ‘mature’.

Disturbance regimes may prevent an ecosystem from reaching its climax community.

Ecosystems or landscapes remain in a steady state if there is no long-term directional trend in their properties. Species in a biological community remain stable, functioning in balance with each other and their environment.

Climax communities do not change to another stage by themselves. Species populations change at a slower rate until a disturbance or disruption from human impact occurs.

Ecological succession can be relatively predictable in ecosystems.

Resilience

(see also Biodiversity Resilience)

Most systems can potentially exist in various states. Moreover, they continually change in unpredictable ways in response to a changing environment.

Ecological resilience is the capacity of an ecosystem to cope with disturbance or stress and return to a stable state. A resilient ecosystem can better withstand shocks and rebuild itself without collapsing into a different state. The ability of ecosystems to adapt to past and continuing change is dependent upon their resilience, that resilience comes largely from the biological diversity of the community.

The concept of ecological resilience is consistent with the notion that ecosystems are complex, dynamic and adaptive systems that are rarely at equilibrium. The resilience of the integrated disturbance-renewal system depends on and functional redundancy on a diversity of species capable of sustaining the characteristic spectrum of ecosystem functions. Resilience and succession following disturbance depends on the severity and type of disturbance or disruption.

When ecosystems become simplified through the loss of component parts or processes, they lose the ability to withstand—and adapt to—disturbances. Ecosystem change can occur suddenly if the resilience that normally buffers change has been reduced. Change becomes more likely when the diversity of species, their abundance in the ecosystem, and regional variability diminishes. After major disturbances, the remaining plants may be too few, have too few meristems (cells capable of cell division) or lack the productive potential to quickly produce the leaf area that could potentially be supported by the climate and soil resources of the site.

Disturbances and disruption may permanently change an ecological zone

Resilience can be enhanced by

• reduction of stresses
• fostering responses
• minimizing interacting stresses
• and assisted migration

Diverse ecosystems are not always more productive or more efficient in using resources than monocultures (farms or tree farms that consist of one species). Monocultures may be just as productive, but they are not as resilient.