Strong interactors have several characteristics that enable them to exert substantial control over ecosystem processes. Some taxa have the ability to track and constrain fast-growing resources. This ability may derive from behavioral capacities, a steep or non-saturating functional response, or a rapid numerical response. Strong interactors typically have broad diets that enable them to prey on and constrain entire functional groups of taxa. Many strong interactors are able to survive periods of low resource availability through dormancy, diet switching or other flexible life-history features. At least some life stages are not strongly influenced by prédation. In freshwater systems, many strong interactors are able to recycle nutrients rapidly and translocate nutrients spatially within lakes or rivers. In some cases, strong interactors physically structure the system, altering habitat for other system components.
Interaction strength, and consequently the magnitude of cascades or compensations, depends on the ecosystem context in which interactions occur. We will illustrate the importance of context using two aspects that strongly affect the nature of species interactions and their impact on ecosystem processes: productivity and disturbance.
Productivity gradients and fishes In temperate lakes, substantial changes to the fish fauna take place along a gradient of primary production driven by phosphorus inputs (Hartmann and Ntimann 1977; Persson et al. 1991). Major changes in the fish communities of many lakes have been associated with cultural eutrophication (Persson 1991). These changes involve shifts from a numerical dominance of salmonids in unproductive lakes, to a dominance of percid fishes in moderately productive systems, and to a dominance of cyprinid fishes in highly productive lakes (Figure 12.la). The dominance of cyprinid fishes in highly productive systems means that eutrophication of lakes has negative effects on both water clarity and the economic value of a lake's fish community, Salmonids and percids arc more valuable commercially than cyprinids (Brinska 1991). These patterns occur mainly in European lakes. Studies of North American lakes suggest a similar pattern, except that centrarchids. naturally absent in Europe, also increase monotonically with productivity as do cyprinids (Oglcsby et al. 1987) (Figure 12.lb). This situation illustrates a case where ecosystem processes interact in a complex fashion with biodiversity. Information on fish community shifts with changing production is less detailed for other continents.
A number of fish species are involved in documented changes in the fish community structure with productivity in European lakes. However, two fishes, roach (Rutilus rutilus) and perch (Perca ftuviatilis), are the strong interactors in these systems. Fundamental differences in the comparative autecology of perch and roach illustrate how species shifts can drive basic changes in ecosystem processes.
Roach are efficient zooplanktivores, able to suppress cladoceran and
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