Chapter 17 Community Stability
	Harned Hall 301
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Above: A typical roadside scene in the Atacama Desert. Does the uniformity and simplicity of the desert indicate that it is a stable system?

Sections

• What is Community stability?
  • Components of stability
• Difficulties in determining stability
• Diversity - stability
  • Connectance
  • McNaughton's Resistance
• Non equilibrium Communities
  • Intermediate disturbance hypothesis
• Terms

What is Community stability?

Stability is often linked to the idea of an equilibrium

• Something that is stable is enduring, never changing.
  • English songbird communities studied for 18 years changed little in terms of richness or evenness.
• Is this a model of stability for all situations.
  • Coral reef communities are always species rich, but some of the species present often change from year to year, even though the coral reef persists over long periods.
• An equilibrium is a situation that is at some level unchanging, but it need not imply that nothing is happening
  • Static equilibria are unchanging situations (like the English songbird communities)
  • Dynamic equilibria are also unchanging but the lack of change is the product of offsetting changes so that the net change is zero (like the coral reef fish)
  • Stable biological communities might arise from a lack of change or a may result from offsetting changes as new species displace resident species or fill niches left empty by local extinction
• Difficult to define stability in an ecological context
  • Is stability constant numbers through time or persistence without regard for constant numbers?
    • Is that of each species, or for the total, or for groups of species that are involved with particular community processes?
    • Are cycling populations (like lynx-hare numbers) stable, even though the numbers constantly change
  • Are parts of the Atacama desert the most stable systems as there are places there with no plants at all and this never changes!!
• Stability may be a property ascribed to communities by us and have no meaning in terms of biology
  • If community (or ecosystem) stability is an important ecological phenomenon and can be measured, then stability must have a biological effect such as (this list is not meant to be exhaustive):
    • altering the mean or variation in abiotic factors or in a biotic resource (such as food)
    • changing the course of evolution such that:
      • individual species chances of extinction are reduced
      • the probability that speciation will occur is altered
  • Problem of the importance of stability in nature is not confined to basic research, but is also a problem in applied ecology
    • When a human-caused perturbation (oil spill, severe forest fire, strip mining) occurs:
      • What will happen to the biological system affected (resistance - see below)?
      • Will a disturbed community tend to return to its previous (pre-disturbance) state?
      • If it will, how long will it take before the system returns to pre-disturbance state (resilience - see below)?
        • How can we tell that it has returned and can we predict that it will?
    • Stiling textbook uses Exxon Valdez oil spill as an example of such a disturbance that raised these questions in the popular media and in front of Congress

Components of stability

• Resistance
  • How much can you perturb a community (removing some individuals, adding new individuals, altering amount of resource, changing an abiotic factor) before it changes (loses or gains species)
• Resilience
  • Once one perturbs a community, how long before it returns to its pre-perturbation state (this is the elastic resiliance of the system)?
  • How much can one perturb a community and still have it return to its pre-perturbation state (this is the amplitude resilience of a system)?

Difficulties in determining stability in natural and experimental systems

• One must be able to determine the equilibrium point for the community
  • May require years of "pre-perturbation" data
  • some communities may never be at an unchanging equilibrium (e. g. cycling predator-prey systems)
• Often perturbations come too quickly to see a return to equilibrium from one disturbance before another occurs
  • this effect is similar to the situation in the chapter on population regulation where abiotic factors are so stressful that a population never reaches its carrying capacity, although the carrying capacity exists
• Difficult to determine the equilibrium state if there are multiple alternative stabile states, such that, once a community is perturbed enough it will move toward a different equilibrium (and so will seem to be unstable unless one observes long enough to determine what the new equilibrium is).

Diversity - stability relationship

Ecologists have long been interested to know if the diversity of a system is linked to its stability

• Common sense approach often cites the greater possibility of compensation in diverse communities as a possible reason for greater stability in diverse systems
  • No matter what community is studied, each species does something
    • Removal of a species will leave a gap
  • If another species increases its activity (exploits the unused resource, alters its preferred habitat, etc.) to fill the gap, then the system has compensated for the loss of a species
  • It is common sense that compensation is more likely when there are more species present
  • Difficulties with this argument are:
    • not all disturbances involve loss of a species (what about when one species has reached unusually high numbers?)
    • in multi-trophic level communities, not all species can be considered as potential candidates for compensation
    • in multi-trophic level communities, loss of one species can have ripple effects at higher and lower trophic levels and compensation may be impossible
• Elton argued that diversity was linked to stability
  • Less diverse island communities more susceptible to invasion
  • Simple agricultural communities subject to pest outbreaks (although some argue that this is not due to ecological processes but because the organisms on crops have often not had enough time to coevolve with their host plants)
  • Tropical rainforests less likely to have outbreaks of herbivores or parasites (although some have argued that this is not so but the perception that this is so results from our ignorance of tropical rainforests)
• Linkage implies cause-effect, but which is the cause and which the effect?
  • More stable systems might offer greater opportunities for specialization by species, which might lead to more speciation as resources become more finely divided and thus to greater diversity (stability begets diversity)
  • More diverse systems might be more resilient to perturbation than systems with fewer species (diversity begets stability)
• May produced mathematical models in which greater diversity lead to less stability
  • Introduced the concept of Connectance

    • most often applied to communities of organisms that exploit one another as food
      • this is a food web, where species are connected by who eats whom.
    • connectance measures the proportion of links between species in a system compared to the total number of links possible
      • a system in which all species eat lots of other species (= generalist feeders) has high connectivity
      • a system in which all are species eat only a few other species (= specialist feeders} would have lower connectivity, even if the same number of species were found in both communities
    
    • Stable communities satisfy the inequality above, where beta is the average interaction strength between species, S is the number of species and C is connectance
  • As species were added to communities, May found that the community lost stability unless there was less connectance or interactions became weaker (see inequality above)
    • This made sense to many because complex machinery often is more prone to breakdown (at least common sense says so, but today's cars are both more complex and longer-lasting) and complex social organizations are often held to be more prone to breakdown (the lean-and-mean business model)
  • Heavily criticized because communities were randomly constructed (made the math easier to assume this), but we know biological comminutes have internal rules (you don't find grass-eaters where there is no grass, but in May's model you could)
• McNaughton observed the effect of cattle grazing on East African grassland communities of different diversity
  • found that more diversity communities compensated for the biomass lost to the cattle but the less diverse communities did not (or did so to a lesser extent), although the cattle ate the same total biomass in all situations
  • developed a measure of resistance to change based on measuring the biomass of each species before and after a disturbance (like a herbivore or a drought)

• where R is resistance, n = number of species in the community, delta p is the change in biomass or abundance of the ith species before and after the disturbance (j is the index for before and after here)
• McNaughton found that the resistance measure was positively related to diversity (as measured by the Shannon index) for the drought data he used
• this has been criticized as the outcome of chance rather than an emergent property of community diversity
• more species means that there is more likely to be a species able to flourish under the disturbed conditions - this effect of diversity has been labeled the portfolio effect

Non equilibrium Communities and Disturbance

• Based on idea that simpler, less diverse communities are found in stressful environments
  • stress is caused in this model by disturbance - an event that changes the environment of the entire community in a meaningful way (could be edaphic or biotic disturbance)
• Stress leads to the inability of the community to diversify
  • Non equilibrium communities are found in the high stress environments
    • Fewer species, room for new immigrant species, populations not near carrying capacity so density independent population regulation is prevalent
  • Equilibrium communities are found in the low-stress communities
    • More species, no room for new immigrant species, populations near carrying capacity so density dependent population regulation is prevalent
• Intermediate disturbance hypothesis (similar to the Ecosystem Exploitation Hypothesis in chapter notes from chapter 13 on population regulation
  • low levels of disturbance (very stable communities) allow competitive exclusion of some species and the loss of some species due to predators
  • high stress habitats lead to low population numbers and the greater extinction rate in high stress environment leads to simpler, less diverse communities
  • Intermediate levels of disturbance (stress) prevent competitive exclusion by reducing population sizes such that the resources are never limiting, but do not reduce the population sizes so low that species go extinct by chance

Terms

Stability, equilibrium, dynamic equilibrium. static equilibrium, perturbation, disturbance, Resistance, Resilience, elastic resilience, amplitude resilience, Diversity - Stability Linkage, compensation, Food webs, connectance, resistance, portfolio effect, Non equilibrium communities, Equilibrium communities, Intermediate disturbance hypothesis, microcosm

Last updated October 20, 2006