Ecological Succession 1
Ecological Succession is The observed process of change in the species structure of an ecological community over time. The community begins with relatively few pioneering plants and animals and develops through increasing complexity until it becomes stable or self-perpetuating as a climax community. Succession occurs in all natural environments. Each environment has a particular name that expresses the nature of their ecological succession: Primary and Secondary 2
Ecological Succession 1st year Horseweed dominant; crabgrass, pigweed 2nd year Asters dominant; crabgrass 3rd to 18th year Grass scrub community; broomsedge grass, pines coming in during this stage 19th to 30th year Young pine forest 30th to 70th year Mature pine forest; Understory of young hardwoods 70th to 100th year Pine to hardwood transition 100th year plus Climax oakhickory forest 3
There are two main types of Ecological Succession Primary Succession: The process of creating life in an area where no life previously existed. Secondary Succession: The process of restabilization that follows a disturbance in an area where life has formed an ecosystem. 4
Examples of Succession Primary Secondary 5
Primary Succession The development of an ecosystem in an area that has never had a community living within it occurs by a process called PRIMARY SUCCESSION. An example of an area in which a community has never lived before, would be a new lava or rock from a volcano that makes a new island. 6
Primary Succession 7
Secondary Succession SECONDARY SUCCESSION begins in habitats where communities were entirely or partially destroyed by some kind of damaging event. When an existing community has been cleared by a disturbance such as a fire, tornado, etc...and the soil remains intact, the area begins to return to its natural community. Because these habitats previously supported life, secondary succession, unlike primary succession, begins on substrates that already bear soil. In addition, the soil contains a native seed bank. Since the soil is already in place, secondary succession can take place five to ten times faster than primary succession. 8
The Circle of Life in Secondary Succession 9
Why Does Ecological Succession Occur? Because it is the process of life for plants, soil and other living organisms. Because organisms alter soil structure, chemistry, and microclimates, the species composition of ecological communities constantly changes over time. Succession will continue until the environment reaches it s final stage the Climax Community. 10
The Climax Community A climax community is a mature, stable community that is the final stage of ecological succession. In an ecosystem with a climax community, the conditions continue to be suitable for all the members of the community. Any particular region has its own set of climax species, which are the plants that are best adapted for the area and will persist after succession has finished, until another disturbance clears the area. 11
These are Climax Communities 12
Two main physical factors determine the nature of the community that develops in an area. These are temperature and the amount of rainfall. If we place the amount of rainfall on a graph s x axis, from 0-10, 10-20,and 20-30+ inches and the temperature along the y axis from hot, moderate, to cold, the various types of ecosystems will fit into the graph based on the conditions that they require. Temperature Cold Cold desert Tundra Taiga Moderate Temperate forest Grassland Deciduous forest Hot Hot desert Savanna Tropical forest Rainfall (inches) 0-10 10-20 20-30+ 13
A summary of changes that occur during succession: Pioneer species colonize a bare or disturbed site. Soil building. Changes in the physical environment occur (e.g., light, moisture). New species of plants displace existing plants because their seedlings are better able to become established in the changed environment. Newly arriving species alter the physical conditions, often in ways that enable other species to become established. Animals come in with or after the plants they need to survive. Eventually a climax community that is more or less stable will become established and have the ability to reproduce itself. Disturbances will start the process of succession again. 14
Threats to Succession The grasses that move in as pioneer species are often thought of as weeds. The subsequent growth of shrubs are considered undesirable "brush". 15
Without these intermediate stages, the disturbed habitat can't return to a natural forest. The fragility and stability of the ecological community are dependent upon several factors. For example, in temperate forests, if the shrubs are not allowed to grow, insect pests begin to feed on young trees instead. This has happened in many places where trees are replanted after an area is clear-cut. Large quantities of pesticides are then brought in, polluting the soil and water and altering the natural ecosystem even further. 16
How Do Humans Affect Ecological Succession? Clearing the land for the garden and preparing the soil for planting represents a major external event that radically re-structures and disrupts a previously stabilized ecosystem. The disturbed ecosystem will immediately begin a process of ecological succession. Plant species that adapt to the sunny conditions and the broken soil will rapidly invade the site and will become quickly and densely established. These invading plants are what we call "weeds in which we consider a nuisance or invaders. 17
A gardener's only course of action is to spend a great deal of time and energy weeding the garden or using chemicals to infringe upon the weeds and the ecosystem around it. The farmers and gardeners who are growing our foods incur an immense cost in terms of time, fuel, herbicides and pesticides that humans pay every growing season because of the force of ecological succession. 18
Does Ecological Succession Ever Stop? We must recognize that any ecosystem, no matter how inherently stable and persistent, could be subject to massive external disruptive forces (like fires and storms) that could re-set and re-trigger the success ional process. As long as these random and potentially catastrophic events are possible, it is not absolutely accurate to say that succession has stopped. 19
Also, over long periods of time ( geological time ) the climate conditions and other fundamental aspects of an ecosystem change. These geological time scale changes are not observable in our ecological time, but their fundamental existence and historical reality cannot be disputed. No ecosystem, then, has existed or will exist unchanged or unchanging over a geological time scale. 20