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Indirect Drivers
Indirect drivers include economic activity, technology, changes in human populations and cultural or socio-political factors. Human population growth is one easily observable indirect driver. As the world population doubled between 1969 and 2009, ecosystems in developing countries slowly grew more taxed by the people who utilized them. While newer technologies have yielded more efficient means by which to utilize natural resources, the shift toward efficiency has typically not kept up with the increase in demand, and these indirect drivers have led humans toward direct involvement in ecological change.
Direct Drivers
Climate change, pollution, habitat change, climate change and invasive species are all examples of direct drivers. The introduction of a foreign organism into an environment where it has no natural predators can devastate the food chain, causing die-offs and the potential for extinction among indigenous species. Warmer temperatures may lengthen the lifespan or breeding cycle of parasites that were traditionally kept in check by seasonal temperatures, increasing their consumption of the organisms on which they feed. The expansion of human settlements causes habitat changes, and an increase in the need for resources triggers overexploitation. When these factors interfere with the existing balance between organisms within an ecosystem, vital forms of life can be diminished while harmful ones flourish, and the entire ecosystem becomes endangered as a result.
Intentional and Unintentional Change
It could be argued that humans were once unaware of the impact their habits had on ecosystems. In modern times, however, the effects of pollution, fertilizer use, lumber harvesting and land development are difficult to miss. While some humans continue to enact harmful intentional change on the environment, others seek to intervene on the ecology's behalf. The science of geoengineering is an attempt to restore damaged ecosystems by intentionally altering their environments. For example, in an effort to reduce global warming's effect on ecosystems, scientists have examined the feasibility of seeding clouds over the ocean, hoping to whiten them and reflect more sunlight back into space. A similar theory that seeks to reflect heat away from the Earth involves placing small particles high in the atmosphere, where it is hoped that they will reflect some sunlight to space.
Temporary and Permanent Change
Ecosystems are suited to recover naturally from the effects of some drivers. One season of unusual weather, for example, may temporarily damage part of an ecosystem, but it will regain balance over time. Small man-made disturbances also often create only temporary change, as diminished populations of the affected species gradually return to their predisturbance levels. The length of time natural recovery takes may be great, depending on the cause and manner of the disturbance. Brazil's Caatinga forest, for example, may take a century or more to recover from slash-and-burn agricultural development, while areas where the forest was cleared by bulldozers may take a thousand years or longer to recover. When an ecosystem is physically transformed, or when indigenous species are completely removed, natural processes alone will not restore it.