Ecology/Community succession and stability

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Chapter 5. Community Succession and Stability

A community can be defined simply as the assemblage of the various special populations occupying the same place at the same time. (See chapter 4 for more details). Therefore, as defined by the Encyclopedia of Earth, community ecology is the branch of ecology that is how interactions between and among species and the abiotic environment affect community structure, including species richness, species diversity, and patterns of species abundance. (See chapter 7 for more details pertaining to species richness and species diversity). One of the most prominent community ecologists is the late Charles Elton. Recently, community ecologists have recognized that the world's flora and fauna are disappearing at alarming rates; such a discovery has led to an increase in investigations researching community stability.

Community Stability
Community stability can be defined as the ability of a community to defy change or rebound from change. The concept of rebounding from change is known as resilience. General resilience is a measure of stability that assumes system stability increases as return time to equilibrium/non-equilibrium solution decreases after perturbation/ disturbance. A rapid response means that a system recoils quickly back to its state of equilibrium. The concept of defying changing change is known as resistance. Resistance is a measure of the degree to which a variable changes after a perturbation/ disturbance However, measuring stability in a system can be a difficult task. In 1955, Charles Elton proposed a theory that stated a positive stability-diversity relationship was a result of the stability of aggregate community measures like the total biomass. There is much controversy on the definition of stability and how it is measured.

Stability Diversity Hypothesis
The stability-diversity hypothesis states that the more diverse a community is, the more stable and productive the community is. This hypothesis was formed from the basis that more stable and productive communities can use their resources better and more efficiently as compared to communities of less diversity. There are correlations with stability measures like constancy of community production and community resilience to species loss and also alterations to the relative abundance of different species. Early research has overlooked the possibility that unexpected fluctuations in the abundance of species may be more important than species interactions when it comes to creating stability-diversity associations.

There are a couple of arguments for and against this hypothesis. An argument for the hypothesis would be that pest outbreaks occur more often on cultivated (disturbed) land. The argument against this statement is that there is no co-evolutionary history with the pests that reside there.

Another statement that can be used to support the hypothesis is that tropical rain forests (which are more diverse) have fewer pest outbreaks than the temperate forests. The argument against this statement is that there is less known about tropical forest as compared to temperate forest, which are heavily studied.

Several experimental tests of the stability-diversity hypothesis have been done. An experiment by Nelson Hairston using bacteria and Paramecium in a two tiered trophic system showed that the extinction rate would decrease with an increase of bacteria species, this result had an increased stability and supported the hypothesis. He also found that the extinction rate increased when there was more Paramecium species present, this result showed an decrease in stability and rejected the hypothesis.

Another experiment to research the stability-diversity hypothesis was done by Tilman. He did a field experiment carried out over eleven years and found that variation in community biomass decreases with increased species richness. But when it came to year to year variation the biomass of a particular species increased with increasing species richness. These results concluded that diversity increases community stability but not population stability. The shortcoming of Tilman's experiment was that he only observed one trophic tier.


 * Read Ecological effects of biodiversity

Disturbance
Disturbance can be defined as a change in environmental conditions that affects the stability of an ecosystem. It is any distinct event which ultimately results in the loss of biomass (Huston 1994) and mortality of organisms. According to Sousa disturbances cause patchiness in natural systems. Disturbance renew limiting resources and allow species to coexist. These patches allow for migration between them increasing diversity which ultimately prevents local species from extinction. In areas of local species the absence of disturbance results in a decline in species diversity. These recurrent patchy disturbances are characteristics of most natural systems. Examples include fires in terrestrial plant communities, hurricanes and wind throws in forests, storms over coral reefs, burrowing activity of badgers in prairie grasslands, drift logs which batter the marine intertidal, damage caused by grazing elephants in East Africia, and many more. As can be seen via the disturbance link, wildfires and anthropogenic effects, such as deforestation, are examples of ecological disturbances. Disturbances, such as fire, often initiate succession. It should be noted that succession can be managed. For example, the most important disturbance of Mediterranean forests might possibly be forest fires. Mediterranean forests are a key factor in bio-geochemical cycles, in protection of soil from erosion, regulating water runoff in watersheds, and stabilizing slopes. Therefore, in the recent decade, research and political efforts have been geared towards fire-fighting and returning scorched areas to their pre-fire conditions. The connection between community stability and disturbance is one that has been questioned many times. The intermediate disturbance hypothesis suggests that in a community, the highest diversity is seen in communities with intermediate disturbance. And since an increase in diversity equals an increase stability, this hypothesis shows that communities with an intermediate amount of disturbance tend to be the most stable.

The Intermediate Disturbance Hypothesis takes into part both benthic and pelagic abilities. The benthic success includes adult survival and reproduction, and the pelagic success involves larval settlement. The amount of coexistence that a dominant species can tolerate determines how many species can be supported. This hypothesis also states that further settlement by other species is maintained at equilibrium by the balance between settlement, productivity, and mortality but is independent of adult body size. Disturbance increases free space, but settlement decreases it. The three factors that determine the fate of an organism in a habitat are equilibrium free space, disturbance, and settlement. Sufficient settlement with little disturbances yields a habitat that has a stable population.

Problems Determining Stability
In order to determine a community's stability, one must be able to determine the equilibrium point for that community. This may require years of observation. In some cases/communities, the equilibrium point can not be determined because the community is never at an unchanging equilibrium; an example of such a situation is the predator and prey systems. Another problem in determining community stability is in areas that have undergone disturbance but return to equilibrium too quickly. This happens in areas affected by abiotic factors that are under so much stress that they never reach their carrying capacity. If there are alternative stable states, it is sometimes difficult to determine the stability of a population. As a result the community will move towards another equilibrium never returning to its original state.

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Charles Elton
Charles Elton was one of the most influential ecologists in history. Born in England in 1900, he studied at Liverpool College and New College, Oxford. He spent most of his time at Oxford. One of his most notable works was the book Animal Ecology (1927). This was the first book to give the basic principles of modern animal ecology. Throughout his career he conducted studies on many different species. He published many books and articles many of which were in the Journal of Animal Ecology which he founded and edited for nearly 20 years. He also received numerous awards for all his work and was a member of many different scientific clubs. He is recognized as having done more than anyone else to explain the complex interrelationships of different animal communities and their effect on their habitat. Elton died in 1991 in Oxford, England.