1. The loss of genetic variability within a population of a species of a given area can reduce its flexibility to adjust to environmental change and narrow the options for adjustments to climate change, for example, as well as for rehabilitating specific habitats.
2. The addition or deletion of a species can have profound effects on the capacity of an ecosystem to provide services. We are beginning to develop the potential to predict which species these will be. They are those with unique traits within an ecosystem for fixing nitrogen, capturing water, emitting trace gases, causing disturbance and so forth. We can predict the consequences of their removal or addition a priori. Although the success of an alien species in a new habitat may be difficult to forecast, its impact on ecosystem functioning upon establishment can be predicted based on whether the new species utilizes or produces a unique resource.
Certain species, without readily recognized traits, when deleted can have profound effects on ecosystem functioning. These are so-called "keystone" species and at present, due to our lack of a general theory, their potential effects on removal can only be assessed by direct experimentation.
3. Recent studies are confirming the proposition that the capacity of ecosystems to resist changing environmental conditions, as well as to rebound from unusual climatic or biotic events, is related positively to species numbers.
4. The simplification of ecosystems in order to produce greater yield of individual products comes at the cost of the loss of ecosystem stability and of such free services as controlled nutrient delivery and pest control, which thus needs to be subsidized by the use of fertilizers and pesticides.
5. Certain ecosystems, such as those found in arid regions and on islands, appear particularly vulnerable to human disruptions and hence alteration of their functioning. These sensitive systems all have low representation of key functional types (organisms that share a common role).
6. Fragmentation and disturbance of ecosystems and landscapes have profound effects on the services provided, since they result in shifting the balance of the kinds of species present-from large, long-lived species to small, short-lived ones. These shifts result in the reduction of the capacity of these systems to store nutrients, sequester carbon and provide pest protection, among other things. Ecosystems, and the services they provide, must be considered in a total landscape context, and in some cases even on an intercontinental basis.
7. We have been more successful in simplifying than in reconstructing ecosystems. Our lack of success in ecosystem restoration suggests the need for great caution in reducing biodiversity through management practices because of the potential loss of goods and services over the long run. As society exerts ever greater control and management of the ecosystems of the world, great care must be taken to ensure their sustain-ability, which is in large part due to the buffering capacity provided by biotic complexity.
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