Diversity of interaction types and community stability

Background/Question/Methods An ecological community can be viewed as a network of coexisting species connected by interspecific interactions. Community complexity, often measured by species richness and interaction density, is a characterizing feature of real ecological communities, but theory often predicts that more complex community is less likely to be stable. This being so, there arises a question of what maintains the species diversity in the natural, complex communities. Our effort is to resolve this contradiction by taking into account the important diversity component, which has been often overlooked in the recent complexity-stability debate – that is, interaction-type diversity. Community dynamics studies often assume a community with only one interaction type, either an antagonistic, competitive, or mutualistic interaction, leaving the question of what, if any, does diversity of interaction types contribute to community dynamics, or the maintenance of communities. We developed a simple mathematical model of ‘hybrid community’, where all the three interaction types may be involved. We analyzed this model to study the potential effect of mixing different interaction types to community stability and complexity-stability relationship.

Results/Conclusions A hybrid community exhibiting competition and mutualism was relatively unstable compared to other hybrid communities. Our analysis, however, reveals a general pattern that a moderate mixture of any two interaction types stabilizes population dynamics, while a community dominated by a single type of interaction tended to be unstable. A similar pattern was observed in a hybrid community with three interaction types. Stable two-interaction-type hybrid communities were highly destabilised by the addition of a small amount of a third interaction type. The highest stability was attained at a moderate mixing of the three interaction types. Furthermore, interaction-type mixing had a major effect to the relationship between complexity and stability. More specifically, contrary to classical theory, the moderate mixing of different interaction types may give rise to a positive complexity-stability relationship in both two- and three-interaction-type hybrid communities. Those results suggest a novel hypothesis that species diversity and interaction type diversity may be the essential elements of biodiversity that jointly provide a self-maintenance mechanism to complex ecological communities.