Interaction capacity underpins community diversity

Background/Question/Methods: How patterns in community diversity emerge in nature is a long-standing question in ecology. Theories and experimental studies suggested that community diversity and interspecific interactions are interdependent. Nonlinear, state-dependent interspecific interactions have been shown to influence community diversity, composition and even dynamics, and weak interactions are key to the maintenance of community diversity. However, evidence from complex, high-diversity ecological communities is lacking because of practical challenges in characterizing speciose communities and their interactions. In order to overcome the previous limitations, I took 1197 species, DNA-based, quantitative ecological time series from experimental rice plots using the previously developed quantitative MiSeq method and reconstructed time-varying interaction networks of the high-diversity, complex ecological communities by analyzing the time series using empirical dynamic modeling.

Results/Conclusions: Properties of the reconstructed interaction networks showed intriguing patterns. As community diversity increases, the number of interactions increases exponentially but mean interaction capacity of a community (defined as the sum of interaction strength a single species gives and receives) does not, resulting in decreased and converged interaction strength per link. The implications of these patterns were investigated by a simple mathematical model, based on which I propose the “interaction capacity hypothesis”, namely, that interaction capacity and network connectance underpin community diversity. An increase in interaction capacity increases community diversity because species are able to interact more species in a community. Network connectance may also play an important role because it determines how interaction capacity is divided into each interaction link. Furthermore, total DNA concentrations (an index of total abundance/biomass) and temperature influence interaction capacity and connectance nonlinearly, which can explain the complex relationships between temperature, abundance and community diversity observed in previous studies. The interaction capacity hypothesis enables intuitive, mechanistic explanations of community diversity, and how interaction capacity is determined is a key question in ecology.