Photo by D. Sillman
(Click on the following link to listen to an audio version of this blog … Ecological Succession
The essay below was part of the “Virtual Nature Trail.” As I wrote last week, Deborah and I decided to let the Virtual Trail and our hiking website (“Between Stones and Trees”) disappear along with their expiring Penn State server. Over the coming months, I will re-publish some the essays, hiking guides and species pages from these two wonderful websites. Enjoy!
Ecological Succession
What is “ecological succession”?
“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”.
Photo by D. Sillman
Why does “ecological succession” occur?
Every species has a set of environmental conditions under which it will grow and reproduce most optimally. In a given ecosystem, and under that ecosystem’s set of environmental conditions, those species that can grow the most efficiently and produce the most viable offspring will become the most abundant organisms. As long as the ecosystem’s set of environmental conditions remains constant, those species optimally adapted to those conditions will flourish. The fundamental “engine” of succession, the basic cause of ecosystem change, is the impact that established species have upon their own environments. A consequence of living is the sometimes subtle and sometimes overt alteration of one’s own environment. The original environment may have been optimal for the first species of plant or animal, but the newly altered environment is often optimal for some other species of plant or animal. Under the changed conditions of the environment, the previously dominant species may fail and another species may become ascendant.
Ecological succession may also occur when the conditions of an environment suddenly and drastically change. A forest fire, wind storm or human activities like agriculture all greatly alter the conditions of an environment. These massive forces may also destroy species and thus alter the dynamics of the ecological community triggering a scramble for dominance among the species still present.
Photo by D. Sillman
Are there examples of “ecological succession” on the Nature Trail?
Succession is one of the major themes of our Nature Trail. It is possible to observe both the on-going process of succession and the consequences of past succession events at almost any point along the trail. The rise and the decline of numerous species within our various communities illustrates both of the types of motive forces of succession: the impact of an established species to change a site’s environmental conditions, and the impact of large external forces to suddenly alter the environmental nature of a site. Both of these forces necessarily select for new species to become ascendant and possibly dominant within the ecosystem.
Some specific examples of observable succession include:
- 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 of the forest floor and the 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)
- 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!
- The succession “garden” plot. This plot was established in April, 2000. 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 in the plot 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.
Photo by D. Sillman
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” blog next week!), 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, human efforts, fuel, herbicides and pesticides that humans pay every growing season because of the force of ecological succession.
Photo by D. Sillman
Does ecological succession ever stop?
There is a concept in ecological succession called the “climax” community. The climax community represents a stable end product of the successional sequence. In the climate and landscape region of the Nature Trail, this climax community is the “Oak-Poplar Forest” subdivision of the Deciduous Forest Biome. An established Oak-Poplar Forest will maintain itself for a very long period of time. Its apparent species structure and composition will not appreciably change over observable time. To this degree, we could say that ecological succession has “stopped”. We must recognize, however, 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 successional process. As long as these random and potentially catastrophic events are possible, it is not absolutely accurate to say that succession has stopped. 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.