In successional systems, we often separate drivers into stochastic (e.g. dispersal or disturbances) and deterministic (competitive displacement) and ask which is the more important force. Within a site at a single point in time, this may be appropriate, but these two forces act in concert over the long term. Stochastic processes become the context for deterministic processes and vice versa as successional dynamics proceed. To illustrate the interplay between drivers, we explored the temporal dynamics of the Buell-Small Succession Study a long term, permanent plot study of old field succession. From these data we examined the dynamics of non-native species to determine the context dependency between stochastic and deterministic processes. For each species invasion, we selected a time (T1) immediately prior to invasion or very early in invasion and a time after the species had become dominant (T2). We used these data to determine 1) the conditions at T1 linked with successful plot colonization by T2, 2) the conditions at T1 that determine species success (increase in cover by T2) and 3) the impacts of the invasion on plot conditions at T2.
Results/Conclusions
In the liana Lonicera japonica, the stochastic process of dispersal/colonization into plots was controlled by a range of factors. More species rich plots, and those dominated by two herbaceous species, Symphyotrichum pilosum and Luecanthemum vulgare were more likely to be invaded between years 5-15 post abandonment. Subsequent to the invasion, this species impacted the community, with invaded plots losing species richness, having fewer species colonizations at both a community scale and for individual species. Similarly, for the woodland invader Microstegium vimineum, higher local species richness was associated with an increased probability of being invaded. Plots with the most understory cover experienced the greatest increase in M. vimineum, though there were some resident species which appeared to inhibit invasion. Finally, invasion resulted in reduced species richness and associated understory cover. In both of these case studies, the context of invasion was necessary in understanding the impacts. Plots that were more species rich were more likely to be invaded and to experience the invasion-related reductions in local richness. This linkage between stochastic colonization and deterministic impacts is likely pervasive throughout succession, representing a continuing challenge for understanding community dynamics.