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Most of the time, when we talk about an ecosystem or habitat, we assume that it is stable and not changing very much; however, this is not always the case in nature. Granted, some habitats may remain relatively unchanged for hundreds or even thousands of years. However, there are others that undergo dramatic changes every few years. The causes of these changes can be natural occurrences, which include fire, floods, volcanic eruptions, tsunamis, and glacial retreat. Sometimes the changes are caused by human activities, such as logging, dam building, and agricultural use. In either case, if the change is severe enough to strip away the existing vegetation or expose a new landscape, species will colonize the disturbed area and then likely be replaced by other species. Over ecological time, the area may experience several transitions in species composition. This process by which the species structure of an ecological community changes over time is called ecological succession.
There are two basic types of ecological succession, which are categorized mainly based on how many nutrients - or lack thereof - are already present in the soil after the disturbance. For instance, primary succession is succession that begins in an area where the soil has not yet formed. Examples of events that precede primary succession would be the formation of a new island by a volcanic eruption or the retreat of a glacier. Secondary succession is succession that begins after an event clears the community but leaves the soil intact. Examples of events that precede secondary succession would be wildfires and deforestation by clearcutting. Because the soil is intact when secondary succession begins, it often progresses much faster than primary succession, and, in fact, certain types of natural events, such as wildfires and floods, can add nutrients to the soil.
A good example of secondary succession can be seen after a fire in a chaparral ecosystem, which is a type of ecosystem characterized by dense, evergreen shrubs; mild, rainy winters; and hot, dry summers. Actually, chaparral is a type of biome, or one of the world's major ecosystem types, that is classified according to its predominant vegetation and climate. The predominant vegetation in a chaparral ecosystem consists of woody evergreen shrubs. Small annual plants are also common but can only grow and survive during the rainy winter months. When the weather becomes dry, the annual plants dry up and die, which leaves only the woody shrubs and the occasional evergreen tree as the only live plants in the summer. Typical animals found in chaparral ecosystems include deer, small rodents and birds, lizards, snakes, and, of course, insects. The soil in chaparral ecosystems is very poor and often very rocky. This is partially due to the fact that the short growing season makes chaparral one of the less productive ecosystems, and most of the biomass produced is retained in the woody shrubs.
The combination of the hot, dry summers and abundance of dry vegetation makes chaparral ecosystems prone to fires. In fact, chaparral plants and animals are typically adapted to periodic fires. Most of the animals retreat to underground burrows that are deep enough to be insulated from the extremely hot fires that burn the dense shrubs that typically contain very flammable oils in their leaves. While the branches and leaves of the shrubs are very flammable and are quickly consumed by the fires, most chaparral shrubs store lots of nutrients in their root crowns that survive underground while the fire burns the exposed parts of the plants.
Anyway, back to secondary succession. In the winter following a chaparral fire, the shrubs resprout from their root crowns. However, the shrubs are slow-growing compared to annual species, so in the winter following a fire, the predominating vegetation will be soft-leaved annual plants that have an unusual amount of nutrients available to them in the form of ash from the burned vegetation as well as an abundance of sunlight without a canopy of bushes covering them.
Not only will the number of thriving annual plants be higher after a fire, but the number of species will also be significantly higher. This is due not only to the fact that there are more resources available but also because several plant species in chaparral biomes are so adapted to the cycle of periodic fires that their seeds are only able to sprout after a fire.
For example, the seeds of a native chaparral wildflower, called whispering bells, require smoke exposure to germinate. These annual plant species are sometimes called fire-followers because they are only seen in the first couple of years following a fire when they are a part of the dominant vegetation in a chaparral ecosystem. Some types of fire-followers require the removal of leaf litter or exposure to direct sunlight to germinate, which is why they're mostly seen after fires.
However, within a few years, the woody shrub species are able to re-establish themselves as the dominant vegetation in a habitat where most plants cannot survive the dry, hot summer and where the soil has now become depleted of most of its nutrients. The number of annual plants and annual plant species that can be found in the habitat declines. A decade after the fire, the chaparral will be back to looking pretty much like it did before the fire. Secondary succession in chaparral habitats is a very quick process, as is succession in grasslands after a fire; however, succession after a forest is clearcut is a succession process that takes place in several steps and may take up to two centuries to complete.
Unlike secondary succession, primary succession is never a fast process. Because primary succession always starts from scratch without a significant amount of soil or nutrients (and certainly without dormant roots or large numbers of seeds waiting for their chance to sprout), it takes a special type of organism to colonize and survive in the new habitat.
Let's use the example of a retreating glacier in Alaska to look at how primary succession occurs. When the glacier retreats from an area, it leaves mostly rocks and nutrient-poor dirt behind. The first organisms that colonize the new habitat and make up the pioneer community are mostly lichens and mosses.
Lichens and mosses require very few nutrients and do not need soil to survive. Instead, they can grow directly on solid rock surfaces where they begin the process of soil formation. After soil is created by the mosses and lichens, grasses and small plants begin to colonize and dominate the habitat. These small plants then begin to be replaced by shrubs and small trees, which in turn give way to alder and cottonwood trees some 50 years after the glacier retreated.
Decaying alder leaves eventually lower the pH of the soil in the alder/cottonwood forest. Once the soil is acidic enough, spruce and hemlock trees, that need acidic soil to survive, move into the habitat and eventually dominate the landscape. In areas of Alaska where glaciers are retreating, the spruce-hemlock forest is the climax community, or the community that can be stably maintained after ecological succession is completed. The entire process from glacier retreat to establishment of a stable spruce-hemlock forest takes about 200 years to complete.
In summary, ecological succession is the process by which the species structure of an ecological community changes over time. Ecological succession occurs after a dramatic change to the landscape either strips the vegetation from the habitat or exposes a new habitat that can be colonized by organisms. Such changes can occur by several means, including fire, floods, volcanic eruptions, tsunamis, glacial retreat, logging, dam building, and agricultural use.
Secondary succession is succession that begins after an event clears the community but leaves the soil intact. A good example of secondary succession can be seen after a fire in a chaparral ecosystem, which is a type of ecosystem characterized by dense, evergreen shrubs; mild, rainy winters; and hot, dry summers. In the winter following a chaparral fire, the predominating vegetation will be soft-leaved annual plants. Not only will the number of thriving annual plants be higher after a fire, but the number of species will also be significantly higher. However, the chaparral shrubs can resprout from their root crowns, so it only takes them a few years to re-establish themselves as the dominant vegetation.
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Back To CourseCLEP Biology: Study Guide & Test Prep
24 chapters | 219 lessons