Stream Ecology and Threats to Ecological Integrity

Ecology is the study of interactions among living organisms and their environment. In every ecosystem, species

Table 1 Examples of ecological stream restoration of different scales

Pool/riffle/reach

Segment/basin

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 (landscale scale)

Table 2 Major human source of nitrogen and phosphorus

Source

Main forms (D = dissolved, P=particulate)

Transport mechanisms

Agriculture Animal (waste)

Cropland (fertilizer, sediment) Urban/suburban

Sewage disposal/septic systems (effluent, sludge)

Lawns/golf courses (fertilizer) Construction/land clearing (sediment) Automobiles (fossil fuel combustion)

Industrial

Liquid/solid waste discharge (multiple) Atmospheric discharge (multiple)

NH NO

Runoff, groundwater, direct animal defication Runoff, groundwater

Direct discharge, groundwater

Runoff, groundwater Runoff

Dry deposition, rain, snow

Direct discharge, runoff Dry deposition, rain, snow composition and abundance at every level, from primary producers to top consumers to decomposers, are regulated by environmental (abiotic) conditions. Compared to most other aquatic ecosystems, stream environments are more dynamic and characterized by nonequilibrium conditions. 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 components 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 variables that may alter these interactions and produce unintended results from restoration activities, and unaccounted for interactions may unintentionally degrade ecosystem integrity further.

The major physical factors that regulate stream ecosystems include hydrology (the daily and seasonal pattern of a stream's discharge), geomorphology (development and subsequent changes of a channel's physical dimensions over time), temperature, and light availability. Chemical components 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). 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 populations and whole community dynamics both during stable environmental conditions and after disturbances.

Biological succession is a fundamental process in ecological 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 (primary 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, ^-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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