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Nature of Communities Essay

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At the beginning of the 20th century there was much debate about the nature of communities. The driving question was whether the community was a self-organized system of co-occurring species or simply a haphazard collection of populations with minimal functional integration (Verhoef, 2010). Krebs (1972) described a community as an assemblage of populations of living organisms in a prescribed area or habitat. However, according to Wright (1984), the working definitions of community can be divided into two basic categories: organismic or individualistic. The organismic approach contends that communities have discrete boundaries and that the sum of the species in an area behaves as organism with both structure and function.

In contrast, the individualistic concept regards communities as collections of species requiring similar environmental conditions (Wright, 1984).

A)Organismic versus individualistic distribution

Solomon (2005) stated that the nature of communities is discussed based on two traditional views which are Clements’s organismic model and Gleason’s individualistic model. The organismic model views community as a superorganism that goes through certain stages of development (succession) toward adulthood (climax).

In this view, biological interactions are primarily responsible for species composition, and organisms are highly interdependent. In contrast, according to individualistic model, abiotic environmental factors are the primary determinants of species composition in a community, and organisms are largely interdependent on each other.

According to organismic concept it is expected that an entire community or biome will respond as a unit and to relocate as climatic conditions change. Pleistocene biome migration in response to multiple glaciations, the accordian effect, is a classic example of this model (Wright, 1984). In contrast, Wright (1984) further explained that the individualist expects each species experiencing similar climatic changes to respond independently and thus, the community composition of an area to change via both immigration and emigration of some individual taxa while others remain in the area. Communities are not stable under this model but recognize in response to changing local conditions.

According to Clements’ organismic hypothesis, species that typically occupy the same communities should always occur together. Thus, their distributions along the gradient would be clustered in discrete groups with sharp boundaries between groups (Russell et al., 2011).

In the 1920s, ecologists; Frederic Clements and Henry A. Gleason developed two extreme hypotheses about the nature of ecological communities (Russell et al., 2011). Clements championed an interactive (organismic) view describing communities as “superorganism” assemblages of species bound together by complex population interactions. According to this view, each species in a community requires interactions with a set of ecologically different species, just as every cell in an organism requires services that other types of cells provide.

In contrast, Gleason proposed an alternative, individualistic view of ecological communities. He believed that population interactions do not always determine species composition. Instead, a community is just an assemblage of species that are individually adapted to similar environmental conditions.According to Gleason’s hypothesis, communities do not achieve equilibrium; rather, they constantly change in response to disturbance and environmental variation. According to Gleason’s individualistic hypothesis, each species is distributed over the section of an environmental gradient to which it is adapted. Different species would have unique distributions, and species composition would change continuously along the gradient. In other words, communities would not be separated by sharp boundaries.

B)Stochastic Versus Equilibrium Schools

The stochastic school believes that most communities exist in a state of equilibrium, where competitive exclusion principle is prevented by periodic population reductions and environmental fluctuations (Crawley, 1997). More generally, stochastic effects can cause a population to shift from one type of dynamic behavior to another (Turchin, 2003). In addition, stochastic school maintains that physical and temporal factors are dominant influences of community composition. This view argues that species abundance varies and is largely determined by differential responses to unpredictable environmental changes (Levin, 2009).

In contrast, the equilibrium explanations assume that community composition represents the stable outcome of interspecific interactions (set of species abundances reached when the rates of change in population is zero) and also assume that the community will return to an equilibrium after those populations are perturbed (Verhoef, 2010). For instance, the traditional equilibrium model assumes that the probability of an individual fish larva surviving to reproduce is limited in a density-dependent manner by the abundance of the adult fish. Alternatively, stochastic model predicts that recruitment to the adult phase is independent of the density of the adults (Chapman et al., 1999).

Equilibrium model states that species richness is entirely determined by ongoing immigration and extinction (Kricher, 2011). Therefore, equilibrium model can be said to be deterministic process which is important in shaping community structure through competition and predation on native species over short temporal scales (Thorp et al., 2008). For example, Chapman et al., (1999), stated that coral reefs communities are at equilibrium showing precise resource partitioning in response to the competition between the various fish species.

However, in contrast, the community may also be more susceptible to stochastic processes. For example, the number of fish species on coral reefs is kept high largely by stochastic processes. According to Naiman et al., (2001), stochastic processes are unpredictable and operate in a relatively density-independent fashion. This is the opposite of the traditional, equilibrium hypothesis which emphasizes density dependent competition between species.

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