Competitive Networks and Community Structure: A Simulation Study

We have conducted a series of simulations to determine the effects of competitive networks and hierarchies on species abundance patterns. We have restricted our attention to situations involving spatial competition among sessile, colonial invertebrates in the absence of disturbance. Variables in this model include overgrowth ranking patterns, recruitment rates, growth rates, over- growth rates, substratum size (i.e., size of a spatial array), and species number. In terms of species richness, complex networks > simple networks > hierarchies. A large range in recruitment rates greatly reduced species richness in simple network simulations due to the decreased frequency of encounters between the lowest- and highest-ranked species forming the network loops. Reduction in substratum size had a similar effect on species richness and also reduced the predictability of species recruitment. Increases in species number (size of available pool) reduced species richness in simple network simulations, again due to reduced encounter frequencies, but also increased species richness in complex network simulations. A large range in overgrowth rates drastically reduced species rich- ness in complex network simulations. The spatial array in these simulations was not always dominated by the species with the highest overgrowth rate, indicating the importance of "position effects' as well as "growth effects." It is suggested that network situations should strongly favor the selection for mechanisms which inhibit rates of overgrowth and which allow lower ranked species to find favorable locations (i.e., refuges) such as small substrata or sites adjacent to higher ranked species which they can overgrow.

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