Ecological succession may also occur when the conditions of an environment suddenly and drastically change. A forest fires, wind storms, and 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. 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: 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.
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, 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. 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. Mature oak standards are left to shelter the coppice and provide larger timber, originally for house and shipbuilding. Areas of cleared and regenerating woodland combine with mature coppice to give considerable niche diversity.
Agriculture reduces biodiversity. Agricultural communities eg potato fields are simple. This means a pest species has a large amount of food available with few predators. If it is an introduced pest, it may have no predators. This makes a farmers field a very unstable ecological community.
In addition, monocultures areas of only one species are very prone to disease spread, due to the high density of the plants. The natural biodiversity which has built up over millenia helps to ensure stability. Humans upset this balance through clearance of diverse communities and the introduction of exotic species. The ability of a community to recover from total destruction is linked to the diversity of the community, the stability of the soil and the alteration to the environment that occurs during clearance.
Rainforest cleared in small areas and then left can regenerate. Upland tropical areas cleared, farmed and then abandoned cannot. In any well adapted climax community, pollution tends to reduce biodiversity eg freshwater streams. Agricultural landscapes are characterised by their ecological simplicity. The many layered complex ecosystems of the climax forest have been replaced - usually by a single layer of monoculture crop.
Traditional farming methods relying on mixed farming, regular crop rotation and low intensity production encourage a greater biodiversity than modern intensive methods.
Main points Agriculture has major impacts on plant succession because the whole purpose of agriculture is to deflect plant productivity away from non-food crops to food crops - either human food or animal food. Even "natural" areas like the New Forest in Hampshire above have a strong human impact.
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