The reduction of species diversity is a common result of stream pollution. Sensitive species that cannot tolerate changing conditions are replaced by more tolerant species. This usually leads to a community with lower complexity and reduces energy flow through the system. An example of this occurred worldwide before the use of secondary treatment in municipal wastewater. Below sewage outfalls, streams would commonly be void of dissolved oxygen due to the high biological and chemical oxygen demand from organic matter decomposition. With increasing distance downstream, water conditions steadily improved due to instream biotic and abiotic processes. In the areas that were anoxic or had very low dissolved oxygen immediately below the outfall, sewage fungus, cyanobacteria, and tolerant invertebrates (such as Tubifex) dominate. As conditions improve, these organisms are replaced by less and less tolerant species until the algal, invertebrate, and fish community is similar to that immediately above the outfall point.
Another common change in the biotic community that accompanies human development is the alteration of native species composition. Species are often introduced purposely, such as fish stockings for sport, or accidentally through ship ballast water exchange. Changes in environmental conditions such as temperature or nutrient availability allow non-natives that are better suited to new conditions to displace native species. However, even when basic environmental conditions remain, non-natives may be better competitors for available resources and thus dominate communities once introduced. Reduction or alteration in higher trophic levels such as fish is the most noticeable, but changes in the microbial community occur more rapidly. Although most introduced species do not become established, when they do, non-native species can alter nutrient cycling and retention, change food web linkages, and possibly eliminate vulnerable native species. Ecosystem integrity is compromised when species with important functional roles are displaced without the availability of other similar species to take over the lost role (i.e., there is low functional redundancy within the system).
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