Ecological succession Essay
“Ecological succession” is the observed process of change in the species structure of an ecological community over time. Within any community some species may become less abundant over some time interval, or they may even vanish from the ecosystem altogether. Similarly, over some time interval, other species within the community may become more abundant, or new species may even invade into the community from adjacent ecosystems. This observed change over time in what is living in a particular ecosystem is “ecological succession”.
Some specific examples of observable succession include:
1. The growth of hardwood trees (including ash, poplar and oak) within the red pine planting area. The consequence of this hardwood tree growth is the increased shading and subsequent mortality of the sun loving red pines by the shade tolerant hardwood seedlings. The shaded forest floor conditions generated by the pines prohibits the growth of sun-loving pine seedlings and allows the growth of the hardwoods. The consequence of the growth of the hardwoods is the decline and senescence of the pine forest. (Observe the dead pine trees that have fallen. Observe the young hardwoods growing up beneath the still living pines).
2. The raspberry thickets growing in the sun lit forest sections beneath the gaps in the canopy generated by wind-thrown trees. Raspberry plants require sunlight to grow and thrive. Beneath the dense shade canopy particularly of the red pines but also beneath the dense stands of oaks, there is not sufficient sunlight for the raspberry’s survival. However, in any place in which there has been a tree fall the raspberry canes have proliferated into dense thickets. You may observe this successional consequence of macro-ecosystem change within the red pine stand and all along the more open sections of the trail. Within these raspberry thickets, by the way, are dense growths of hardwood seedlings. The raspberry plants are generating a protected “nursery” for these seedlings and are preventing a major browser of tree seedlings (the white tailed deer) from eating and destroying the young trees. By providing these trees a shaded haven in which to grow the raspberry plants are setting up the future tree canopy which will extensively shade the future forest floor and consequently prevent the future growth of more raspberry plants!
3. The succession “garden” plot. This plot was established in April, 2000 (please see the series of photographs on the “Succession Garden Plot” page). The initial plant community that was established within the boundaries of this plot was made up of those species that could tolerate the periodic mowing that “controlled” this “grass” ecosystem. Soon, though, other plant species became established as a consequence of the removal of the stress of mowing. Over time, the increased shading of the soil surface and the increased moisture retention of the undisturbed soil-litter interface allowed an even greater diversity of plants to grow and thrive in the Succession Garden. Eventually, taller, woody plants became established which shaded out the sun-loving weed community. In the coming years we expect tree seedlings to grow up within the Succession Garden and slowly establish a new section of the forest.
How are humans affected by ecological succession? Ecological succession is a force of nature. Ecosystems, because of the internal species dynamics and external forces mentioned above, are in a constant process of change and re-structuring. To appreciate how ecological succession affects humans and also to begin to appreciate the incredible time and monetary cost of ecological succession, one only has to visualize a freshly tilled garden plot. 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 adapted 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”. Now “weeds” have very important ecological roles and functions (see, for example, the “Winter Birds” discussion), but weeds also compete with the garden plants for nutrients, water and physical space. If left unattended, a garden will quickly become a weed patch in which the weakly competitive garden plants are choked out and destroyed by the robustly productive weeds.
A gardener’s only course of action is to spend a great deal of time and energy weeding the garden. This energy input is directly proportional to the “energy” inherent in the force of ecological succession. If you extrapolate this very small scale scenario to all of the agricultural fields and systems on Earth and visualize all of the activities of all of the farmers and gardeners who are growing our foods, you begin to get an idea of the immense cost in terms of time, fuel, herbicides and pesticides that humans pay every growing season because of the force of ecological succession.
Causes of Ecological Succession
Causes of ecological succession are classified under two categories.
1) Biotic factors Interactions among the organisms in a community, as called biotic factors, influence the structure, composition and function of a community. In succession, during period of time a community makes the area less favourable for itself and more favourable for the next serial community.
2) Physiographic factors
Includes physical and chemical factors of the environment such as landslides, erosion, catastrophic factors, etc