Ecology is the study of interactions among living organ isms and their environment. In every ecosystem, species composition and abundance at every level, from primary producers to top consumers to decomposers, are regu lated by environmental (abiotic) conditions. Compared to most other aquatic ecosystems, stream environments are more dynamic and characterized by nonequilibrium con ditions. When compared to small streams, larger streams and rivers are typically more stable in regard to discharge, chemical composition, and community structure. However, smaller streams have been studied much more and can be manipulated more easily and usually at lower cost. Therefore, restoration plans often differ with stream size, even though they may be mitigating the same human activity.
The fundamental ecological interactions among com ponents are very important because knowing how ecosystems are likely to respond to each restoration action is key to the success of a restoration program. Ecological stream restoration depends on understanding the physical, chemical, and biological constraints on developing stream communities that are predictable, and uses these relationships as a starting point for restoration design. There are, however, many uncontrollable vari ables that may alter these interactions and produce unintended results from restoration activities, and unac counted for interactions may unintentionally degrade ecosystem integrity further.
The major physical factors that regulate stream ecosys tems include hydrology (the daily and seasonal pattern of a stream's discharge), geomorphology (development and sub sequent changes of a channel's physical dimensions over time), temperature, and light availability. Chemical compo nents of streams that are often important in restoration projects include nutrients (mainly nitrogen and phosphorus) (Table 2), metals (mercury, lead, copper, cadmium, zinc, selenium, and arsenic), acidity, salinity, pesticides, and organic compounds (DDT, PCB's, PAH's, and ecoestrogens).
Table 1 Examples of ecological stream restoration of different scales
Add/modify instream habit Restore stream sinuosity
Add/remove riparian vegetation Restore connection with floodplain
Remove invasive species Remove flow constrictions (dams, levees)
Reintroduce native species Reduce sediment input (landscape scale)
Remove/reduce pollution point source Reduce nutrient input (landscape scale)
Table 2 Major human source of nitrogen and phosphorus
Main forms (D = dissolved, P = particulate)
Agriculture Animal (waste)
Cropland (fertilizer, sediment) Urban/suburban
Sewage disposal/septic systems (effluent, sludge)
Liquid/solid waste discharge (multiple) Atmospheric discharge (multiple)
Runoff, groundwater, direct animal defication Runoff, groundwater
Direct discharge, groundwater
Runoff, groundwater Runoff
Dry deposition, rain, snow
Direct discharge, runoff Dry deposition, rain, snow
The article Stream Management examines each of the above factors and their relationship to stream ecology in more detail. In addition, it lists major anthropogenic pollution sources and activities associated with these factors, and some basic, direct consequences of component alterations on stream ecosystem structure and function. These physical and chemical components shape the biological component of the ecosystem by regulating individual popu lations and whole community dynamics both during stable environmental conditions and after disturbances.
Biological succession is a fundamental process in eco logical restoration. Succession is the process through which ecosystem biota develop over time, and is regulated by the order and rate that species colonize and grow in a new or disturbed habitat. The successional concept in restoration can encompass either the entire biotic community (pri mary producers to top predators), or individual species within a single group such as fishes or algae. Early succes sional communities (r strategists) are defined by fast reproduction, short life spans, and low competitive ability. Over time, K strategists become the dominant species. These species are defined as having slow reproduction, long life spans, and high competitive ability. In stream ecosystems, succession naturally occurs on a large scale after disturbances such as floods and droughts. Anthropogenic disturbances also lead to a loss in native species richness and abundance (Figure 1 ). A central goal of ecological stream restoration is to return native species, either through direct planting or introductions, or through natural migration and colonization from nearby sources.
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