Community structure represents the natural and unpredictable patterns as well as relationships, which exist in a network of interacting species. Community structures are complex and cannot be simply understood by constructing a food web. Rather community structure takes into account the prey and predator dynamics of a community, competition between species, the niches which organisms possess, the genetic basis of a community and the overall ecosystem.
Community structure is a characteristic which is constantly in flux. At times it may be stable and at other times it might be weaker or stronger than normal. This is all dependent on resource availability, environmental conditions and the existing relationships between species.
For example, the presence of keystone species can help maintain healthy community structures. Keystone species are organisms which have a disproportionately large influence on a community and assist in keeping other species population sizes in check. In aquatic ecosystems, sea stars are considered keystone species since they prey on sea urchins, shellfish and mussels, thus keeping their population sizes from exploding. Furthermore, ecosystem engineers are another type of keystone species. Beavers are ecosystem engineers because they create dams transforming streams into ponds.
At certain times, depending on the strength of a community, the coexistence of species which occupy similar niches and require the same resources is possible. This can occur when there is heterogeneity for resources. For example, if two species of birds eat seeds from the same tree, but one species prefers smaller seeds found at the top of the tree and the other species prefers larger seeds found at the bottom of the tree, both species can co-exist. Additionally, coexistence can occur when a community is rich in resources.
Evidently, community structure is critical to the survival of species and the overall functioning of entire ecosystems. Understanding the different features which influence community structures is inextricably linked to the study of ecology.
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