Stability and change in ecosystems can be considered in terms of variables such as population size, number and kinds of species, and productivity. Students should know that ecosystems can be reasonably stable over hundreds or thousands of years. If a disaster such as flood or fire occurs, the damaged ecosystem is likely to recover in stages that eventually result in a system similar to the original one. In the long run, however, ecosystems always change when climate changes or when one or more new species appear as a result of migration or local evolution. Students should know that human beings are part of the earth's ecosystems.
Human activities can, deliberately or inadvertently, alter the equilibrium in ecosystems.
Students should know that at times, environmental conditions are such that plants and marine organisms grow faster than decomposers can recycle them back to the environment. Layers of energy-rich organic material have been gradually turned into great coal beds and oil pools by the pressure of the overlying earth. By burning these fossil fuels, people are passing most of the stored energy back into the environment as heat and releasing large amounts of carbon dioxide. Students should know that the amount of life any environment can support is limited by the available energy, water, oxygen, and minerals, and by the ability of ecosystems to recycle the residue of dead organic materials. Human activities and technology can change the flow and reduce the fertility of the land.