Ecosystem Disturbances
A disturbance in an ecosystem is an event caused by physical, chemical, or biological agents, resulting in changes in population size or community composition. Examples of disturbances include hurricanes, tornadoes, air pollution, and forest-clearing. Not every ecosystem disturbance is a disaster. For example, although a low-intensity fire might kill some plant species, it might help fire-adapted species that can use the nutrients released from the dead plants. This causes the net primary productivity to remain the same. This ecosystem would be considered resistant. Resistance of an ecosystem is a measure of how much a disturbance can affect the flows of energy and matter. An ecosystem has a high resistance when a disturbance influences populations and communities, but doesn’t have an effect on the overall flow of energy and matter. When an ecosystem is affected by a disturbance, environmental scientists also look at the resilience of the ecosystem. Resilience is the rate at which an ecosystem returns to its original state after a disturbance. A highly resilient ecosystem returns to its original state relatively rapidly, a less resilient ecosystem does so more slowly.
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Restoration Ecology
Sometimes disturbances are so great that they can eliminate entire ecosytems. This is typically due to human actions. Sometimes scientists can work to change these effects and restore the ecosystem to its original state. This is becoming more common among environmental scientists, leading to a new scientific discipline called restoration ecology. Restoration ecology is the study and implementation of restoring damaged ecosystems. Current restoration projects include the Florida Everglades and the Chesapeake Bay where ecologists are restoring the water flows and nutrient inputs that are closer to historic levels so that the functions of these ecosystems can be restored.
Watershed
A watershed is all of the land in a given landscape that drains into a particular stream, river, lake, or wetland. Studying at the watershed scale is easier than studying biogeochemical cycles on a global scale. One study at the watershed scale included investigating the effects of clear-cutting and subsequent suppression of plant regrowth. The researchers cut down the forest in one watershed and used herbicides to prevent the growth of new vegetation. They used a nearby, untouched watershed as the control. Six months after the cutting, the clear-cut watershed showed increases in stream nitrate concentrations. This information allowed scientists to determine that when tress are no longer present to take up nitrate from the soil, nitrate leaches out of the soil and ends up in the stream that drains the watershed. This experiment showed the importance of plants in the regulation of nutrient cycling as well as the detrimental effects of not allowing new vegetation to grow when a forest is cut.
Intermediate Disturbance Hypothesis
Disturbances are not always bad for ecosystems; in fact sometimes they’re natural. The intermediate disturbance hypothesis is the hypothesis that ecosystems experiencing intermediate levels of disturbance are more diverse than those with high or low disturbance levels. In ecosystems where disturbances are rare extreme competition among species is common. This results in only a few highly competitive species eventually dominating the ecosystem. In ecosystems where disturbances are frequent, species have to have a large enough growth rate to counter the disturbances or else they are likely to go extinct.