Limits to Homeostasis

While ecological systems have some inherent homeo-static ability, there is a limit to the magnitude of the perturbation that they can compensate for. Most complex systems have a homeostatic plateau or range of perturbation sizes that the system is able to accommodate (Figure 3). Beyond this range, the system can no longer maintain homeostasis and state changes in the system are expected to occur. While for an organism the inability to maintain homeostasis results in illness or death, for ecological systems the illness or death of an ecosystem is not well defined. However, the lack of homeostasis can result in major changes in system properties (e.g., nutrient and energy flux, species richness, net primary productivity, abundance of organisms, and standing biomass) and reorganization of the structure of the system (e.g., changes in food web structure, shifts to a new habitat type or vegetation structure), causing the ecological system to shift into an alternative state from that previously documented.

What controls the size of the homeostatic plateau is not well understood. Perturbations that occur within the homeostatic plateau for some ecosystems may cause other

Homeostatic plateau

Homeostatic plateau

- 0 + Deviation from average environmental conditions

Figure 3 The homeostatic plateau (solid black line) is a range of perturbation strengths over which the system is able to maintain homeostasis. Outside this range (gray shaded region), changes in system properties may occur quickly and be hard to predict because of the complex nature of ecological systems.

ecosystems to lose homeostasis. Even within a single ecosystem, not all system properties may exhibit homeostasis in response to the same perturbation. For example, a community may exhibit homeostasis in energy use from time period 1 to time period 2 but differ in the total abundance or biomass of organisms - this is because the metabolic rate of an organism is highly related to the body size of that organism and a large number of small organisms can flux the same amount of energy as a smaller number of large organisms. The complexity of responses both within and across ecosystems has posed a challenge for understanding the generality and importance of homeostasis.

One of the obvious limitations to homeostasis is the range of niche characteristics exhibited by species available to an ecosystem. As mentioned above, for compensatory dynamics to occur at least one species must be present that benefits when the environmental conditions change. Obviously, if no species are available with the niche characteristics that allow it to survive, reproduce, and increase in abundance when other species are declining, then no compensatory dynamics will occur and system properties will not be maintained. Two characteristics have been suggested to be related to the homeostatic ability of any particular community: biodiversity and immigration rate.

Recent research in community ecology has focused on the role of biodiversity in maintaining stable ecosystems. Obviously, there is expected to be some relationship between the species richness of a community or ecosystem and the diversity of niche characteristics present in that system - though the nature of this relationship is not well understood. While biodiversity may be important because it increases the probability that the community contains species with important niche characteristics, it does not guarantee that compensatory dynamics will occur and that a community will be able to maintain homeostasis. What is probably more important is how broadly species cover various niche axes. Species can have narrow niches (i.e., their required conditions cover only a small range of the potential niche axis) or they may have very broad niches. A species-rich system comprised of many species with narrow niches may cover just as much of the environmental range of conditions as a species-poor system comprised of species with broad niches (Figure 4). If the range of niche characteristics across all species is broad enough to respond to most perturbations, then system properties may very well be homeostatic despite the number of species present. Conversely, species-rich systems with species closely packed along an important niche axis may still be vulnerable to environmental change. The relationship between species richness and niche breadth is not well understood, but better understanding of this relationship may help explain why species-richness studies show a mixed relationship between the number of species and the long-term stability of system properties.

Figure 4 The relationship between biodiversity and niche packing may be very complex. This diagram shows how hypothetically two communities with different species richness might still cover the same range of niche characteristics. Each curve shows the fitness of a single species along some portion of the niche axis. The top panel shows a diverse community where the species overlap considerably along this niche axis and have very narrow niches (i.e., cover only a small fraction of the entire niche axis). This results in a tightly packed community. The bottom panel shows a less diverse community. However, each species has a very broad niche (i.e., covers a larger section of the niche axis) and overlap between individual species is less. This results in the same coverage of this niche axis between the species-rich and the species-poor communities.

Environmental niche axis Low-► High

Figure 4 The relationship between biodiversity and niche packing may be very complex. This diagram shows how hypothetically two communities with different species richness might still cover the same range of niche characteristics. Each curve shows the fitness of a single species along some portion of the niche axis. The top panel shows a diverse community where the species overlap considerably along this niche axis and have very narrow niches (i.e., cover only a small fraction of the entire niche axis). This results in a tightly packed community. The bottom panel shows a less diverse community. However, each species has a very broad niche (i.e., covers a larger section of the niche axis) and overlap between individual species is less. This results in the same coverage of this niche axis between the species-rich and the species-poor communities.

In addition to species richness and niche packing, high dispersal rate from a regional species pool may also explain why some systems are better at maintaining homeostasis. Dynamics in closed systems (i.e., systems without immigration) are restricted by what species are already coexisting in the community. If changes in environmental conditions are detrimental to all of the resident species then system properties will change. However, in open systems if conditions are detrimental to all resident species then new species with appropriate niche characteristics can colonize from the regional pool. Colonizing species can then compensate for declines in resident species, maintaining system homeostasis. This interaction between local and regional scales, where the region supplies potential colonists and local system filters out species based on matches between niche characteristics and the environment, is sometimes referred to as species sorting. Species sorting is a central idea in the emerging area of metacommunities - an area of research specifically studying how local environment, niche characteristics of species, and dispersal from a regional pool of species interact to affect the structure and dynamics of communities.

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