Ecosystem Homeostasis

E. Odum (1969) presented a number of testable hypotheses concerning ecosystem capacity to develop and maintain homeostasis, in terms of energy flow and biogeochemical cycling, during succession. Although subsequent research has shown that many of the predicted trends are not observed, at least in some ecosystems, Odum's hypotheses focused debate on ecosystems as cybernetic systems. Engelberg and Boyarsky (1979) argued that ecosystems do not possess the critical goal-directed communication and low-cost/large-effect feedback systems required of cybernetic systems. Although ecosystems can be shown to possess these properties of cybernetic ecosystems, as described later in this section, this debate cannot be resolved until ecosystem ecologists reach consensus on a definition and measurable criteria of stability and demonstrate that potential homeostatic mechanisms, such as biodiversity and insects (see later in this chapter), function to reduce variability in ecosystem conditions.

Although discussion of ecosystem goals appears to be teleological, nonteleo-logical goals can be identified (e.g., maximizing distance from thermodynamic ground; see B. Patten 1995, a requisite for all life). Stabilizing ecosystem conditions obviously would reduce exposure of individuals and populations to extreme, and potentially lethal, departures from normal conditions. Furthermore, stable population sizes would prevent extreme fluctuations in abundances that would jeopardize stability of other variables. Hence, environmental heterogeneity might select for individual traits that contribute to stability of the ecosystem.

The argument that ecosystems do not possess centralized mechanisms for communicating departure in system condition and initiating responses (e.g., Engelberg and Boyarsky 1979) ignores the pervasive communication network in ecosystems (see Chapters 2,3, and 8). However, the importance of volatile chemicals for communicating resource conditions among species has been recognized relatively recently (Baldwin and Schultz 1983, Rhoades 1983, Sticher et al. 1997, Turlings et al. 1990, Zeringue 1987). The airstream carries a blend of volatile chemicals, produced by the various members of the community, that advertises the abundance, distribution, and condition of various organisms within the community. Changes in the chemical composition of the local atmosphere indicate changes in the relative abundance and suitability of hosts or the presence and proximity of competitors and predators. Sensitivity among organisms to the chemical composition of the atmosphere or water column may provide a global information network that communicates conditions for a variety of populations and initiates feedback responses.

Feedback loops are the primary mechanisms for maintaining ecosystem stability, regulating abundances and interaction strengths (W. Carson and Root 2000, de Ruiter et al. 1995, B. Patten and Odum 1981, Polis et al. 1997a, b, 1998). The combination of bottom-up (resource availability), top-down (predation), and lateral (competitive) interactions generally represent negative feedback, stabilizing food webs by reducing the probability that populations increase to levels that threaten their resources (and, thereby, other species supported by those resources). Mutualistic interactions and other positive feedbacks reduce the probability of population decline to extinction thresholds. Although positive feedback often is viewed as destabilizing, such feedback may be most important when populations are small and likely is limited by negative feedbacks as populations grow beyond threshold sizes (Ulanowicz 1995). Such compensatory interactions may maintain ecosystem properties within relatively narrow ranges, despite spatial and temporal variation in abiotic conditions (Kratz et al. 1995, Ulanowicz 1995). Omnivory increases ecosystem stability, perhaps by increasing the number of linkages subject to feedback (Fagan 1997). Ecological succession represents one mechanism for recovery of ecosystem properties following disturbance-induced departures from nominal conditions.

The concept of self-regulation does not require efficient feedback by all ecosystems or ecosystem components. Just as some organisms (recognized as cybernetic systems) have greater homeostatic ability than do others (e.g., homeotherms vs. heterotherms), some ecosystems demonstrate greater homeo-static ability than do others (J.Webster et al. 1975). Frequently disturbed ecosystems may be reestablished by relatively random assemblages of opportunistic colonists and select genes for rapid exploitation and dispersal. Their short duration provides little opportunity for repeated interaction that could lead to stabilizing cooperation (cf. Axelrod and Hamilton 1981). Some species increase variability or promote disturbance (e.g., brittle or flammable species; e.g., easily toppled Cecropia and flammable Eucalyptus). Insect outbreaks increase variation in some ecosystem parameters (Romme et al. 1986), often in ways that promote regeneration of resources (e.g., Schowalter et al. 1981a). Despite this, relatively stable environments, such as tropical rainforests, might not select for stabilizing interactions. However, stable environmental conditions should favor consistent species interactions and the evolution of reciprocal cooperation, such as demonstrated by a diversity of mutualistic interactions in tropical forests. Selection for stabilizing interactions should be greatest in ecosystems characterized by intermediate levels of environmental variation. Interactions that reduce such variation would contribute to individual fitnesses.

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