University Koblenz-Landau Ithaca, New York, Germany
Abstract: The richness and composition of co-occurring species can exhibit dramatic variation across space and time. Community assembly provides a conceptual framework for understanding the processes generating and maintaining this variation. In the absence of disturbances, community assembly in given locality may converge to a unique configuration of species independent of the order of species arrivals i.e. a unique end state. Alternatively, community assembly may converge to different end states depending on the order of species' arrivals, or may never converge to any end state due to species composition constantly changing in response to species arrivals. The goal of our work is to understand the relative frequency of these outcomes of community assembly in nature. To shed insight into this issue, we analyze 35 empirically parameterized Lotka-Volterra models of 3 to 13 interacting species, mostly at the same trophic level. Our analysis makes use of Hofbauer and Schreiber's invasion graphs that characterize all possible transitions between communities due to single or multiple species invasions from the species pool. We use these invasion graphs to identify whether there are single or multiple invasion resistant configurations of coexisting species (i.e. end states of community assembly), and whether there are cycles in community assembly (i.e. a sequence of invasions from one community back to itself). Our preliminary analysis reveals that cycles are rare (occurring in about 5% of the models) and typically cryptic i.e. only occur in early phases of community assembly. Species pools are approximately equally likely to have only a single end state of community assembly (54% of the models) or multiple end states (46% of the models). On average, the number of end states increases linearly with the number of species in the species pool; approximately one new end state arises for each pair of additional species in the species pool. For species pools with one end state, the end state typically consists (90% of models) of the entire species pool. In contrast, for species pools with multiple end states, the end states, on average, includes about 45% of species from the species pool. In conclusion, this study suggests that while cycles are rare, species composition of natural communities often may depend on the order and timing of species arrivals.
