ASSEMBLY RULES: DETERMINISM vs. RANDOMNESS IN THE FORMATION COMMUNITIES

Abstract

<p>Elucidating the mechanisms structuring communities has been a challenge for community ecology since its beginnings. One theory argues that assembly rules structure communities by means of deterministic mechanisms arising from biological interactions. Another view maintains that patterns seen in community composition and species abundance result from stochastic processes such as migration and extinction. The dilemma has yet not been resolved unambiguously. The main issue is that communities shaped by deterministic mechanisms can produce stochastic patterns via priority effects. The main goal of this study was to determine whether assembly rules structure communities. My strategy was to minimize priority effects by controlling timing of colonization. To do this I used a null community by combining communities of 17 rock pools. This null community was later divided among experimental communities. I conducted three experiments: (1) Experimental communities were exposed to the same external conditions. (2) Communities were exposed to different environments, disturbance, dispersal and habitat heterogeneity. (3) Replicated null communities were connected to allow inter-replicate dispersal. After 4 months, communities (experiment 1) formed alternative states, suggesting the lack of assembly rules control in community structure. The second experiment showed that adding factors results in more alternative states. The increasing number of alternative states among replicate communities indicates that diversified environment and migration are needed to reproduce qualitative patterns observed in nature. The last experiment (3) showed that patterns observed among connected replicate communities resemble patterns that emerged in the presence of biological interactions in unconnected communities. Similarity of patterns between connected and unconnected groups of communities suggest that local biological interactions can be sufficient to structure communities to a considerable degree. Nevertheless, the regional processes appear necessary in their role of supplying species for local communities.</p>