Biodiversity applies not only to species, but also to ecological variety at many levels, including genotypes, ecosystem types on the landscape or biogeochemical pathways (Jutro 1993). It is obvious that ecosystem processes and the features that humans desire in systems (including ecosystem services; Ehrlich and Ehrlich 1981) are linked with much broader aspects of diversity. Examples include the genetic diversity of fish stocks, biochemical diversity of microbial transformation pathways, and the structural diversity of habitats. Here, ecological complexity refers to structural diversity of ecological systems in the broadest sense. Ecosystem processes include productivity, nutrient cycling and transformations, and exchange of gases and solutes. Ecosystem processes linked directly to interest of humankind include production and purification of freshwater resources, secondary production of foods and recreation.
Humans are reducing the complexity of the world's freshwater ecosystems. At the largest scale, there is a convergence of ecosystem types and a loss of variety. Worldwide, lakes are eutrophied and polluted, while streams are impounded and channelized. Introductions of aggressive species transform both lentic and lotic waters. There is a reduction in the variety of lakes and streams on the landscape. At the scale of individual systems, changes in community structure are typically the early symptoms of stress, appearing before ecosystem processes change (Schindler 1990; Howarth 1991; Frost et ai 1994). Substantial literature, to be summarized briefly in this chapter, documents strong feedbacks between biotic structure and ecosystem processes. The conservation of biodiversity and ecosystem processes are inseparable in freshwater.
12.2.1 Strong interactions, cascadcs and complementarity
Ecologists have often described sequences of change that occur when perturbations of certain species or groups of species are transmitted through webs of ecological interaction. Organisms that physically or chemically structure habitat in ways that impact other organisms or ecosystem processes have been called "ecosystem engineers" (Jones and Lawton 1994). Freshwater examples include changes in the carbon and hydrologic cycles caused by beavers (Naiman et al. 1988), habitat creation and nutrient fluxes due to macrophytes (Carpenter and Lodge 1986; James and Barko 1991), and unique biogeochemical transformations performed by certain groups of bacteria (Schindler 1990). These interactions are nontrophic. Trophic impacts on other organisms or ecosystem processes were called "cascades" by Paine (1980). Paine also introduced the term "strong interactor" to describe species or groups of species that serve as nodes for transmission of perturbations. In freshwater ecology, trophic cascades have been viewed more narrowly as perturbations transmitted from the top to the bottom of the food web (Carpenter et al. 1985). Transmission of organic production in the reverse direction, upward through food webs, is the province of the trophic dynamic concept (Lindeman 1942). Trophic processes govern production, material cycling and bioaccumulation of contaminants in freshwaters (Thomann 1989; Power 1990a; Carpenter and Kitchell 1993; Madenjian et al. 1994).
The spatial scale of strong ecological interactions is system-wide in the pelagia of lakes (Carpenter and Kitchell 1993). In benthic systems, as in terrestrial systems, the spatial scale of effects appears much more variable. The strength of biotic effects on ecosystem process rates depends on the spatial scale at which they are measured and the succcssional state of the system (Grimm 1992).
Functional complementarity is the capacity of certain taxa or abiotic components of ecosystems to suppress change in process rates when ecosystems are altered or stressed. For example, when several resources are substitutable, stress that removes a particular resource may have little effect if other, substitutable resources remain available (Tilman 1982). Functional complementarity often involves species change. Ecosystem stress leading to shifts in certain taxa is compensated by opposite shifts in other taxa having similar ecosystem functions (Schindler 1990; Howarth 1991; Carpenter et al. 1992b; Frost et al. 1994).
Functional complementarity often depends on taxa that were rare before the ecosystem was perturbed. Consequently, predisturbance information and information from unperturbed ecosystems may not forecast complementary species responses or consequences for ecosystem process rates (Frost et al. 1994). It may be difficult to predict which species will be crucial in stabilizing ecosystem function. (Carpenter and Kitchell 1993). A species' capacity to serve as a strong interactor, or stabilize ecosystem processes through functional complementarity, may depend on other species in the community or the state of the ecosystem when it is perturbed. Further research may clarify general patterns of strong interaction or functional complementarity in freshwaters.
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