riparian zone structure and function result from geomorphic, hydrologic, topographic, edaphic, and biotic processes occurring within the watershed. Gregory and colleagues defined reaches by channel and valley floor geomorphic features and as having distinct topographic, edaphic, and disturbance regimes. These reaches had distinct longitudinal, lateral, vertical, and temporal characteristics.

Ecological Significance of the Species

For understanding many biological phenomena, such as evolution, genetics, and the growth and development of organisms, the species, defined as genetically related organisms that can produce offspring, is the fundamental unit. As discussed previously, the ecosystem is another fundamental unit governing biological processes. The definition of boundaries of an ecosystem can be designated to facilitate study of particular processes, whereas the definition of a species is more exact. Questions arise as we try to conceptually relate species and ecosystems.

Species, Niches and Ecosystem Function

Aquatic ecologists strive to understand the controlling relationships within an ecosystem. Two relationships that are currently under active research are (1) grazing on microorganisms by microheterotrophs (referred to as "the microbial loop"), and (2) complex, multiple pathways linking primary producers and top predators (sometimes referred to as "top-down or bottom-up" controls). These are not distinct issues, because the nature of the microbial loop in a lake ecosystem could influence the importance of higher trophic-level organisms in regulating the ecosystem. Although questions may be posed at the level of trophic interactions, in order to examine these interactions in detail in any particular aquatic ecosystem, the questions become focused on which species is doing what, how fast, and when?

Question: How do characteristics and habitat requirements of individual species fit together to control ecosystem-scale processes, such as productivity and resilience?

Aquatic ecosystems often contain many species within each trophic level. The magnitude of the challenge presented to the researcher by this diversity varies with trophic group and aquatic environment. For some groups, species-level identification may be very difficult to obtain. Even the knowledge of species may seem to be insufficient because it does not carry with it detailed knowledge of habitat requirement or ecological function.

Individual benthic invertebrate species have been categorized according to their ecological function as scrapers (on periphyton), shredders (of leaves), predators (on other invertebrates), etc. Therefore, a list of the



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