Maintenance of Homeostasis in Ecological Systems

Homeostasis is maintained through negative feedbacks. As changes in the environment push a system property from its equilibrium, negative feedbacks counteract the

Change in environment

Change in environment

Change in system 0 property

Homeostasis Ecosystem

Figure 1 Changes in environment can have large effects or almost no effect on system properties, depending on the operation of homeostatic mechanisms. The top panel shows a hypothetical change in environment through time. The panel starts at zero change in the environment then exhibits a strong shift away and return to average conditions. The bottom panel shows the hypothetical response of two different ecosystems to this change in environment. Ecosystem 1 (the solid line) shows a large change in the system property that mirrors the environmental change. In contrast, ecosystem 2 (the dashed line) shows a minimal response in the system property that is quickly returned to normal. Ecosystem 2 is demonstrating an ability to maintain homeostasis in that property.

Change in system 0 property

Figure 1 Changes in environment can have large effects or almost no effect on system properties, depending on the operation of homeostatic mechanisms. The top panel shows a hypothetical change in environment through time. The panel starts at zero change in the environment then exhibits a strong shift away and return to average conditions. The bottom panel shows the hypothetical response of two different ecosystems to this change in environment. Ecosystem 1 (the solid line) shows a large change in the system property that mirrors the environmental change. In contrast, ecosystem 2 (the dashed line) shows a minimal response in the system property that is quickly returned to normal. Ecosystem 2 is demonstrating an ability to maintain homeostasis in that property.

direction of change. Without a negative feedback to counteract the force exerted by the perturbation, the system property would no longer exhibit stability through time but would directly reflect fluctuations and directional shifts in the environment (Figure 1). Because negative feedbacks work to counteract changes from the steady state, they are a stabilizing force on system properties and a key mechanism for homeostasis.

Maintenance of homeostasis is fairly straightforward in well-integrated systems such as organisms, but is more problematic for loosely coordinated systems such as ecosystems. In physiology, set points reflect optimal operational conditions for the body. Because the ability of the body to function outside of a narrow range of conditions declines, it is obvious why these set points exist. Control of the set point is often coordinated by organs that monitor the system property and implement the negative feedback response to counteract changes. In ecological systems such as communities or ecosystems, there is no centralized control on the functioning of the system. There is no organ to monitor changes and there is no reason to believe that the ecosystem is actively managing its system properties for optimal functioning. Therefore, a central question in studying homeostasis in ecosystems is what type of negative feedbacks can occur in complex systems with potentially thousands of individual components interacting nonlinearly with no central control system?

Homeostatic Mechanisms

Because of the decentralized nature of ecological systems like populations, communities, and ecosystems, negative feedbacks operate more diffusely than in physiology. As mentioned before, there is no central processing unit to implement or coordinate a negative feedback. Instead, negative feedbacks often emerge from interactions among species and individuals, and between species and individuals and their environment. In biology, negative feedbacks that stabilize system properties are often referred to as homeostatic mechanisms.

Stabilizing effects of limiting resources

A common homeostatic mechanism controlling many system properties is an interaction between consumers and a resource. Interactions between predators and prey are widely cited as a classic example of how consumer/ resource dynamics can result in a negative feedback that stabilizes the system. In a simple predator-prey system -for example, a fox and its rabbit prey - initial increases in prey populations result in increased resource availability for their predator. Because consuming prey results in increased reproduction and survival in the predator population, increases in the prey population initially provides impetus for the predator population to increase. As predator abundance increases, the demand for prey also increases and the increase in death rate for the prey causes declines in prey populations. With a decrease in the resource base there is not enough prey to support the high predator abundance, the death rate in the predator population increases, and eventually the predator population declines. Thus the initial increases in predator abundance are counteracted by a negative feedback induced by the limiting resource of the prey. While this is a very simplified example of a single limiting resource (the prey) and a single consumer species (the predator), the concept easily translates into more complex situations. For example, in a community of desert rodents that all feed upon seeds produced by a diverse annual plant community, increases in consumption by any species or individual decreases the seeds available to other species or individuals, thus resulting in overall stability in resource consumption by the rodent community.

The above examples are special cases of a general process that occurs across many systems. When food resources (e.g., prey items (animal or plant), nitrogen, phosphorus, and water) are limiting, they impose strong stabilizing constraints on the dynamics of the community. Increases in resource usage by any component of the system must be matched by declines in usage by other components because resources are finite. This balance

between increases and decreases in species populations is often referred to as species compensation or compensatory dynamics. The finite nature of the resources provides the negative feedback to counteract increases in consumption by the community or ecosystem overall. Because resources are used by individuals for growth, maintenance, reproduction, and survival, the resource constraints influence birth, death, and net production. Therefore, resource constraints are not just important for stabilizing overall consumption but are also important in stabilizing important system-level properties such as total abundance, standing biomass, and biomass production.

Additional stabilization of system properties can sometimes occur when more than one limiting (or potentially limiting) resource is operating in an ecological system. Changes in environment, either natural or anthropogenic can result in increases in resource levels, resulting in rapid changes in state properties as the system equilibrates to the new constraint. However, response of the system to the release from the original constraint can be slowed down or even reversed if there is a secondary constraint on the system. Occasionally, two resources may be co-limiting in an ecosystem; for example, both nitrogen and phosphorous are sometimes found to limit plant productivity in some ecosystems. In these cases the system may be prevented from responding to an increase in one resource unless both limiting resources change. More commonly, one resource is the primary limiting factor in which case the system will respond initially to increases in that resource. However, with the primary resource constraint lifted, another resource often becomes a constraint on the system. For example, in desert ecosystems plants are primarily limited by water availability but when the water limitation is released nitrogen can become a limiting resource constraining plant response. Multiple resource constraints can lead to discrepancies between short-term and long-term responses to change as initial changes are slowed or even reversed by the implementation of new constraints.

Compensatory dynamics

Compensatory dynamics are an important homeostatic mechanism. Compensatory dynamics occur when increases in the abundance, biomass, or energy use of some species are matched by decreases in other species. While resource constraints create the conditions for negative feedbacks to occur in ecological systems, compensatory dynamics are often necessary for maintaining a steady state between resource availability and resource use when environmental conditions other than the limiting resource change. Without compensatory dynamics, strong fluctuations in only a few dominant species would result in large changes in system properties, regardless of the presence of constraints on the system (Figure 2). In essence, compensatory dynamics allow the ecosystem to fully utilize resources and

No compensatory dynamics Compensatory dynamics

Perturbation Perturbation start end

Perturbation Perturbation start end

Time I Time

Figure 2 Compensatory dynamics among species are important for the ability of an ecological system to maintain homeostasis. A hypothetical ecosystem responds to an environmental perturbation with a discrete start and end in time. The bold black line shows the total abundance of organisms in that system through time. The thin black line shows the abundance of species 1 through time and the dashed line shows the abundance of species 2 through time. Species 1 responds positively to this perturbation, increasing in abundance. Without a compensatory response by species 2 (left panel), in this case a decrease in abundance, total abundance in the system increases during the perturbation and returns to its previous state after the perturbation ends. However, if compensatory dynamics occur (right panel) and species 2 decreases in abundance as species 1 increases, the total abundance of the system shows no response to the environmental perturbation.

create the condition where resources are able to limit the ecosystem. The constant operation of the constraint on the ecosystem creates the scenario where the negative feedbacks discussed above can operate and dampen changes in the system properties.

Changes in environmental conditions can have important implications for the ability of a system to maintain homeostasis because of their effects on individual species. These environmental conditions, sometimes referred to as modulators in ecosystem ecology, are typically physical and chemical components of the ecosystem that affect the activity of organisms (e.g., pH, salinity, temperature, soil structure, and physical structure of vegetation). In other words, these are conditions that affect how an organism interacts with its environment and with other species. For example, species that are good competitors and able to obtain large amounts of resource under neutral pH may decline when pH becomes more acidic or basic. Activities affected by modulators often include the ability of an organism to obtain resources and/or reproduce, and may sometimes directly cause the death of the organism. While modulators may change the environmental milieu of a system, it is important to note they do not necessarily affect the overall availability of a limiting resource. In addition to abiotic conditions, other species can also affect the activity of an organism. Pathogens, predators, or new resource species can all have important impacts on the overall environmental conditions of a system. Because of their effect on organisms, modulators can negatively affect species in an ecosystem and result in population declines. Declines in populations result in increased resource availability in the system because (1) modulators do not affect resource availability and (2) fewer individuals in the system results in less overall use of resources - that is, resources are being underutilized. To utilize these freed resources, a species needs the appropriate traits, or niche characteristics, to survive and forage in that environment. Because species differ in their niche characteristics, conditions that are detrimental for some species also tend to be beneficial to other species. This creates the potential for declines in some species to be compensated for by increases in other species. Therefore these differences in niche characteristics between species, especially those niche characteristics related to modulators, are critically important for compensatory dynamics to occur.

There is a great deal of empirical evidence demonstrating the importance of niche differences among species in driving compensatory dynamics and maintaining homeosta-sis. For example, experimental manipulations of pH in lakes result in changes in the composition of species, with species preferring lower pH values increasing and species preferring neutral or high pH declining. Similar compositional changes have been documented for a variety of taxa (e.g., perennial plants, zooplankton, phytoplankton, fish, mammalian mega-herbivores, rodents, and annual plants) in a variety of ecosystems (e.g., deserts, temperate forests, grasslands, tundras, lakes, and oceans) for a variety of modulators (e.g., pH, habitat, temperature, light availability, and predation pressure). Despite the fact that in many of these cases large changes in environmental conditions occur, compensatory dynamics among species were able to maintain homeostasis of system-level properties (e.g., species richness, biomass, CO2 flux, energy flow, and total abundance).

There have been recent suggestions that compensatory dynamics could result not from differences among species in niche characteristics but from stochastic processes. Key to this view of compensatory dynamics are (1) the idea that death and birth are both stochastic processes, (2) that resources released by the death of an individual are allocated to an individual new to the system, and (3) that the species identity of the new individual is randomly determined. In this stochastic view of species dynamics, which species gain or lose resources does not depend on any niche characteristics of the species involved. This idea is often referred to as a 'neutral theory', with the term 'neutral' referring to the fact that any changes in species composition are due to neutral processes (e.g., stochastic birth, death, and dispersal rates) that have nothing to with the environment or the niche characteristics of the species involved. Because 'neutral theory' has been so recently proposed there are not yet many empirical studies explicitly testing the ability of neutral changes to maintain homeostasis. However, given the empirical support for niche-driven compensatory dynamics, it is highly probable that if stochastic processes play an important role in homeostatic mechanisms, they are likely to be important mainly in systems where environmental stability is high and therefore not selecting for some species over others.

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  • giorgia
    How do koala maintain homeostasis and equilibrium?
    6 years ago
  • jackson milne
    How can nature control population in any ecological system?
    6 years ago
  • Julia
    How does the ecosystem "aquarium" it maintain homeo?
    6 years ago
  • isumbras
    How do koalas maintain homeostasis?
    6 years ago
  • Tabitha
    How do ecosystems maintain homeostasis?
    6 years ago
  • giancarlo
    How Homeostasis is maintained in ecosystem?
    6 years ago
  • aatifa
    How do ecosystems achieve homeostasis?
    6 years ago
  • Girma
    What are the ways to maintain homeostasis in ecosystem?
    6 years ago
  • mary
    What happen to a population if homeostasis in the ecosystem will not be maintain?
    3 years ago
  • franziska
    How ecological homeostasis is achieved in an ecosystem?
    2 years ago
  • maximilian
    How doea ecosystems response to homeostasis?
    2 years ago
  • eric bosch
    How do homeostasis respond to the ecology?
    2 years ago
  • matthew bigelow
    What is the important os homeostasis ecology?
    2 years ago
  • quintina
    Why is homeostasis important in ecosystem?
    2 years ago
  • leon
    Why ecological homeostasis is important?
    2 years ago
  • geneva brewer
    How do predator prey relationships maintain homeostasis in ecosystems?
    1 year ago
  • Bildad
    How does homeostasis balance itself in the ecosystem?
    1 year ago
  • ernesto
    How ecosystem maintain its homeostatic equilibrium?
    1 year ago
  • Atte M
    7 months ago
    How does temperature affect ecological homeostasis?
    6 months ago
  • betty
    How is diversity in ecosystem important to community homeostasis?
    2 months ago

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