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A sequence of changes in the species composition of a community is called succession. Events such as forest fires, floods, and storms may devastate a community, but it is now thought that such events are a natural part of the dynamics of a community.
In Yellowstone National Park, an enormous fire destroyed nearly 800,000 acres of forest in 1988. Up to that time, fires were extinguished before they could do large-scale damage. As a result, large areas of forest remained, composed of old, flammable timber. The natural cycle of burning and regrowth was prevented by human fears of the devastation that fire can bring.
At the time of the Yellowstone fire, it wasn’t widely accepted that fires are a natural component of community dynamics. Resistance to fire, and the ability to benefit from its aftermath, are now known to be important in ecosystems that are prone to fires.
Certain tree species are adapted to use fire to help them propagate. The cones of the lodgepole pine, which is the most important coniferous tree in Yellowstone, remain closed until they're exposed to extreme heat. Once the cones open, the seeds are spread over a wide area.
After the fire is over and the ground has cooled, as many as 400 lodgepole seedlings per acre may appear. And a carpet of herbaceous plants develops, gettting nutrients from the rich ash left by the fire. As these plants and trees grow, animals eat the shrubs, and the forest regenerates.
Some communities are more sensitive to disturbances than others. Desert ecosystems in the American southwest have been seriously disrupted by recreational vehicles and desert bikes. Plant life that has taken hundreds of years to develop in the harsh, dry desert conditions cannot survive the punishment that wetland plants might quickly recover from.
Although off road vehicles are major culprits in destroying sensitive habitats, you should always be careful not to destroy any natural habitat – even walking off a hiking trail can needlessly trample and kill plant life.
In fact, few communities are completely stable, maintaining the same composition of species over a long period of time. Predators may eat prey, and certain food supplies may increase or decrease.
These patterns affect the populations of organisms that depend on the food source. Some organisms may move away to colonize other ecosystems. Ecologists favor this nonequilibrial model of community ecology, instead of earlier ideas suggesting that communities move toward a “climax” community.
Suppose a plant like this senna species is subject to attack by the herbivorous caterpillars of a certain butterfly. Through mutation and natural selection, the plant now produces a chemical that is toxic to the caterpillar. The plant then radiates into new communities because it's protected against caterpillar attack.
But the butterfly may also develop mechanisms that enable its caterpillar to detoxify the defensive chemicals produced by the plant. If this happens, then the butterfly can also move into the communities colonized by the plant.
In this example of coevolution, one type of organism develops a defense and then radiates into new communities. The other organism develops a defense against the first organism’s defense and has a burst of radiation. This cycle continues in a kind of “arms race,” resulting in two species closely associated with each other.
Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education