Communities versus Assemblages

The question of how important self-organization is in ecosystems has long been debated in ecology. Are ecosystems communities of co-adapted species, or are they simply random assemblages.? Some early theorists, such as Clements, believed that the groups of species found together were specialized for living together, whereas others, such as Gleason, stressed the importance of chance and individuals.

The idea of succession concerns the patterns and processes involved in community change, especially after disturbance. A form of self-organization often associated with succession is facilitation. That is, plants and animals present in an area can alter the local environment, thereby facilitating the appearance of populations that replace them. After a fire, for example, a forest will regenerate with herbs and shrubs growing back almost immediately. The first trees to reappear will be 'pioneer' (disturbance) species, which disperse well, grow fast, and can tolerate open, exposed conditions. These trees create shade and leaf litter, which favor slow-growing, shade-tolerant trees.

Recent theoretical work (such as Hubbell's neutral theory of biodiversity and biogeography) emphasizes the role of chance and spatial dynamics in generating ecological patterns. In these models, self-organization is trivial because all individuals and species are effectively identical, and species abundances are driven by random birth, migration, and death processes. Both neutral and self-organizing models have been successful in explaining real relative abundance and species-area curves.

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