Relating temporal changes in species dispersion, inter-specific associations and trait dispersion to mechanisms underlying plant community assembly

If neutral processes such as migration and speciation can predict the same patterns observed in natural communities traditionally attributed to competition and niche theory, how can the two mechanisms be separated? Traditional theory emphasizes the increasing importance of competition in structuring plant communities along gradients of time-since-disturbance and productivity, and hence succession, whereas neutral processes should follow no clear developmental path. Thus, spatial patterns resulting from neutral processes should have a different temporal 'signature' from patterns arising from traditional competition and niche-based mechanisms during community assembly.;Even though the dune chronosequence exhibited the classic hump-shaped pattern of species richness through succession traditionally explained by competitive mechanisms, there was little evidence that competition and niche-based mechanisms were important in determining spatial organization in this community. Instead, the increasing spatial randomness through the succession is consistent with neutral theory and the 'competitive combining ability' hypothesis, which predict competitive equivalence of species. Furthermore, the spatial randomness suggests that fitness components other than size may be important in determining competitive ability. Given that traditional competition theory is not supported in this community, the hump-shaped pattern of species richness is explained with reference to the species pool hypothesis, which suggests that few species have evolved the characteristics required to compete or tolerate competition in highly fertile, undisturbed habitats due to the historical uncommonness of these habitats.;This study investigates the potential role of 'niche' versus 'neutral' processes on several elements of spatial organization of the plant community along a sand dune chronosequence on Lake Michigan spanning 485 years of primary succession. Temporal changes in species dispersion, species associations, size distributions, and trait dispersion were compared to patterns predicted from the species segregation hypothesis, spatial niche differentiation, size-related competition theory, and the competition/colonization trade-off to infer whether competition was important in defining community structure.