Spatial constraints masking community assembly rules: A simulation study

The effect of competition on species coexistence is usually strongly modified by other factors especially in nonequilibrium systems of sessile organisms with limited availability of propagules. As a consequence, competition-based assembly rules (even if their existence seems to be unambiguously detected) would result in incomplete understanding of the coexistence of species in plant communities. J. Bastow Wilson suggested measuring variance deficit in the number of co-occurring species as a means to detect niche limitation in a community. The method provides a relatively simple and quick ''snap-shot'' analysis of a community. However, it has been questioned whether niche limitation is the only factor which might account for variance deficit. The paper presents a spatially explicit simulation experiment in which artificial communities are produced by pre-defined rules for competitive interactions. Then we examine whether these rules can be detected by a proposed method for pattern analysis. Two limiting cases are simulated: (A) all the species share the same niche, and (B) all the species have different niches. The difference between these cases in the variance of species numbers is examined. Using the simulation results, some basic spatial constraints upon species assembly are emphasized. It is argued that the assumptions of Wilson's approach confine its applicability to species-saturated equilibrium communities. The study of assembly rules in dynamically changing, spatially structured communities requires the consideration of a set of coenological characteristics and the use of careful spatio-temporal scaling to detect their patterns. The use of spatially explicit individual-based models to study the mechanisms and constraints limiting species coexistence at different scales is suggested.