The community is an assemblage of species populations that occur together in space and time. Community ecology seeks to understand the manner in which groupings of species are distributed in nature, and how they are influenced by their abiotic environment and by species interactions.
We begin by explaining how the structure of communities can be measured and described, in terms of species composition, species richness, diversity, equitability (evenness) and rank-abundance diagrams.
Figure 16.22 (left) Temporal changes in species composition and relative abundance of microcosms composed of a specific mix of protists and metazoans. The change is expressed in ordination plots based on a procedure called detrended correspondence analysis (DCA). (Recall that ordination is a mathematical treatment that allows communities to be organized on a graph so that those that are most similar in species composition and relative abundance appear closest together, whilst communities that differ greatly in the relative importance of a similar set of species, or that possess quite different species, appear far apart.) Data points are the mean ordination scores on different days in the experiment (from day 5 to day 35). The letter D indicates periods of drought disturbance, and the letter M, mosquito disturbance. (a-e) The results of the control and disturbances imposed in different sequences. (After Fukami, 2001.)
The assessment of community patterns in space has progressed from subjective 'gradient analysis' to objective mathematical approaches ('classification' and 'ordination') that permit relationships between community composition and abiotic factors to be systematically explored. We note that most communities are not delimited by sharp boundaries, where one group of species is abruptly replaced by another. Moreover a given species that occurs in one predictable association is also quite likely to occur with another group of species under different conditions elsewhere.
Just as the relative importance of species varies in space, so their patterns of abundance may change with time. A particular species can occur where it is capable of reaching a location, appropriate conditions and resources exist, and competitors, predators and parasites do not preclude it. A temporal sequence in the appearance and disappearance of species therefore requires that conditions, resources and/or the influence of enemies themselves vary with time. We emphasize and explain patterns of community change that follow a disturbance. Sometimes these patterns are predictable (succession; dominance control), in other cases highly stochastic (founder control).
Although we can discern and often explain patterns in community composition in space and in time, it is often more meaningful to consider space and time together. The patch dynamics concept of communities views the landscape as patchy, with patches being disturbed and recolonized by individuals of various species. Implicit in this view are critical roles for disturbance as a reset mechanism, and of migration between habitat patches. The community dynamics of patchy landscapes are strongly influenced by the frequency of gap formation and the sizes and shapes of these gaps in relation to the colonization and competitive properties of the species concerned.
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