These rapid shifts occur when an ecological threshold has been crossed, either from a series of small changes or one abrupt change. Take the case of Florida Bay, a shallow lagoon at the southern tip of Florida. For as long as anyone could remember, the clear water bay was dominated by seagrass, a rooted aquatic plant. Then, in the early s, its waters became murky and choked with phytoplankton blooms. A combination of factors pushed the ecosystem over the edge, including nutrient inputs from septic systems, water diversions, the removal of plant-eating animals manatees and drought.
Multiple drivers were also behind the dieback of arid forests, grassland and shrublands in the southwestern United States. During the s, livestock overgrazing, decades of fire suppression, and an increase in climate variability merged to create vulnerable ecosystems. The result: massive insect outbreaks devastated forests and an unprecedented amount of land was consumed by wildfires. In the Pacific Northwest, a change in water allocation led to the decline of two endangered fish species.
Marshes in the Klamath River basin were drained to provide water for nearby agricultural operations. When several years of drought hit in the late s, the ecosystem tipped. Nutrient loading led to a drop in water quality and blue-green algae took over. In , Lost River sucker and shortnose sucker populations collapsed.
Functioning ecosystems provide humans with essential services, such as clean water and protection from disease. Protecting these services requires managing ecological thresholds. Among the most dominating of the abiotic factors are temperature and precipitation; which over an extended period of time represents climate.
Primary succession on bare rock. Rocks erode, sand accumulates, decomposition adds organic matter, and voila! As soil continues to form, larger and woody plants appear.
Volcanic eruption is primary succession. Following the eruption, there is no life in the blast zone. Eventually, however, pioneer species of plants do establish themselves, beginning the course of succession. Primary succession can also occur from man-made structures. So what is secondary succession?
Secondary succession occurs where communities used to exist, but were destroyed by natural or man-made causes. These could be in areas where forests are cut down or destroyed by fire, other natural disasters, and diseases.
Abandoned farmlands also undergo secondary succession. A volcano or other catastrophe will also cause an area to undergo secondary succession. Usually, secondary succession re-establishes the original community much faster than primary succession because soil already exists; nevertheless, it may take a hundred years or more for the climax stage to return, if it ever does. Pioneer species - the first species to colonize an uninhabited area. For plants it begins with lichens and mosses, for animals generally insects.
Succession - the repeated replacement of one species by another over time. It is the result of interspecies competition and is sometimes referred to as serial succession. The entire progression from a pioneer to a climax community is called a sere. Climax community - the final stage of succession. It is a long-lived, self-perpetuating community or ecosystem based on the limiting factors of the environment. Primary succession bare rock succession - succession occurring on an area previously devoid of life.
Lichens, a mutualistic relation between an algae and a fungi, are the pioneer plant community on land. Aquatic succession - phrase used to describe primary succession in bodies of water. Except for the oceans, inland bodies of water are considered to be temporary. Primary succession involves the gradual filling-in of the water body until a "dry" terrestrial environment exists supporting a climax community.
Secondary succession - succession occurring in an area previously inhabited, but set back to an earlier stage. It begins with the destruction of disturbance of the existing ecosystem. This may be the result of natural disaster such as tornadoes, fires, hurricanes, volcanoes, etc. Geographical succession - an expression used to explain the relationship between altitude, latitude, and climax communities.
Ecosystem stability is an important corollary of sustainability. Over time, the structure and function of a healthy ecosystem should remain relatively stable, even in the face of disturbance. If a stress or disturbance does alter the ecosystem is should be able to bounce back quickly.
Resistance - the ability of the ecosystem to continue to function without change when stressed by disturbance. Resilience - the ability of the ecosystem to recover after disturbance. An ecosystem disturbance can be natural or human induced stress.
An example of a natural disturbance is a hurricane or a tornado. An example of a human-induced or anthropogenic disturbance is tillage or pesticide application.
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