Disturbances Essay ⋆ Environment Essay Examples ⋆ EssayEmpire

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A disturbance event perturbs ecosystems, driving ecological patterns and processes outside their normal range of variability. Disturbances may alter species richness, population structure, net primary production, and nutrient flows. These changes may be temporary or long term, depending on both disturbance type and ecosystem characteristics. Determining whether ecological changes fall within normal ecosystem variability requires a judgment call. When is drought severe enough to be a disturbance? When does insect herbivory grade into insect outbreak?

Anthropogenic disturbances are caused by people, and include fire, oil spills, livestock grazing, pollution, logging, and fishing. Natural disturbances are caused by climatic, geologic or biological change. Examples of climatic disturbances are drought, hurricanes, and windstorms. Geologic events include earthquakes, volcanic eruptions, and landslides. Pest or pathogen outbreaks, mass marine mortality, and algal blooms are all biological disturbances. Disturbance impacts also interrelate. A forest suffering from drought is more susceptible to insect outbreak. Oysters in Chesapeake Bay declined due to a combination of storms, overharvest, pollution, and disease. Determining whether a disturbance is anthropogenic or natural is complicated. Evidence that humans have for millennia influenced ecosystems from African savannahs to Amazonian rainforests draws into question any strict divide between human and “natural” drivers of disturbance. The potential for anthropogenic climate change gives disturbances such as glacial movement and hurricanes ambiguous origins.

Ecologists further differentiate endogenous from exogenous disturbances. Endogenous disturbances are caused by internal ecological changes, such as treefalls that open up gaps and alter light, temperature, and moisture regimes. Treefalls immediately impact understory growth, litter decomposition, and ecosystem productivity. They have long-term impacts on microtopography, soil organic matter, and forest structure. Exogenous disturbances are those events that originate outside an ecosystem.

Disturbances are characterized by their intensity, severity, frequency, timing, and geography. Intensity is the energy a disturbance releases per unit time and area. Hurricanes are more intense than mild breezes. Disturbance intensity depends on ecosystem conditions and external forces. For example, fire intensity depends on fuel mass as well as wind speed. Severity is the magnitude of ecological change a disturbance causes. A flood that forever alters a river channel is a severe disturbance.

Frequency is how often a disturbance reoccurs, and is often inversely proportional to intensity. Low-intensity boreal forest fires occur frequently, whereas high-intensity tropical forest fires occur infrequently. Disturbance timing is important. A severe frost during plant budbreak or wildlife birth season will have a greater impact on survival than at other times. Disturbance geography includes its size, shape, and adjacent ecosystems. Whereas a single treefall may impact less than 2.5 acres (about one hectare) of forest, the hurricane of 1938 flattened forests throughout southern New England. The shape of an area impacted depends on site elevation, aspect, and species composition. Proximity and connectivity to an undisturbed area affect species repopulation.

Assumptions and Reality

Historically, ecologists believed that ecosystems possessed a natural equilibrium, and that variations from this equilibrium reflect an imbalance of nature. This perspective originated with Frederick Clements’s theory of ecological succession, which posits that vegetation composition changes through time until reaching a climax community. Ecologists predicted that stable, unchanging conditions were necessary to develop biodiverse ecosystems such as Amazonian rain forests. Disturbances were thought to throw ecosystems off their trajectory toward climax equilibrium and reduce biodiversity potential. Natural resource management incorporated this assumption. U.S. national parks had policies to suppress both natural and anthropogenic fire, which backfired. Fire suppression contributed to fuel buildup and conflagrations such as that in Yellowstone National Park in 1988.

In recent decades, ecologists recognize that disturbances are integral to a healthy ecosystem. Disturbances alter ecosystem structure and rearrange nutrients and energy sources and sinks. Joseph Connell’s intermediate disturbance hypothesis proposes that a medium level of disturbances actually increases biodiversity. Coral reefs were once thought to be uniform ecosystems that required a steady environmental state to develop their diversity. Instead, coral reefs are a mosaic of patches that reflect different disturbance events and recovery stages across the landscape.

Analyzing how disturbances impact ecosystem health involves measuring not variations from an equilibrium, but rather ecosystem resistance and resilience. Resistance refers to the relative capacity of a system to return quickly to previous or original conditions after a disturbance. The thick bark of Douglas fir increases its resistance to fire. Debris dams increases a stream’s resistance to floods. Resilience is the ability to quickly return to normal conditions after a disturbance. Mountain ash resprouts quickly after fires. The rapid life cycles of stream invertebrates promote population resilience. Postdisturbance recovery may be along a different successional pathway, yet still within normal ecosystem parameters.

Disturbances can push an ecosystem past a threshold to a new state from which it is difficult to return. Once shifts in temperature, nutrients, and consumer species allow algae to dominate coral reefs, corals have a hard time regaining a foothold. Cheatgrass (Bromus tectorum) is an exotic that now holds a near monoculture in western U.S. ecosystems because it has outcompeted native vegetation for moisture and altered fire frequency. Native plants are not likely to grow back without significant human intervention.

Ecosystems have disturbance regimes, namely the spatial and temporal scales of disturbance and recovery. Species adapt to disturbance regimes over evolutionary time. Some species even depend on disturbances. Many pines require fire to open up cones and release seeds. Today, resource managers try to manage within disturbance regimes, rather than fighting them. Techniques such as controlled burns mimic natural fire regimes. This approach is part of ecosystem management, which manages for ecosystem health rather than an idealized balance of nature.

Bibliography:

M.M.M. Crawford, ed., Disturbance and Recovery in Arctic Lands (Spriger-Verlag, 2002);
Carl F. Jordan, Amazonian Rain Forests: Ecosystem Disturbance and Recovery (Spriger-Verlag, 1986);
Lawrence Walker and Roger del Moral, Primary Succession and Ecosystem Rehabilitation (Cambridge University Press, 2003).