The integration of facilitation into the neutral theory of community assembly

Facilitation has been recognized as an important biotic interaction driving community structure during the recent two decades, such as the integration of facilitation into competition-based community assembly theories, such as the neutral theory of community assembly, is now an emergent task for ecologists. We used an individual-based spatially explicit model to evaluate the impacts of facilitation on the diversity and structure of the neutral community. We found that facilitation promotes neutral community species coexistence and diversity by two kinds of mechanisms, one related to the abiotic environment and the other to biotic interactions. First, facilitation increased species abundances by ameliorating the abiotic environmental conditions, thus reducing species stochastic extinctions at harsh environmental conditions. Second, facilitation played a role as a stabilizing mechanism through “heterospecific density dependence”, resulting in a negative relationship between species fitness and relative abundance. On the other hand, facilitation also has a counteracting negative impact on neutral community diversity through inhibiting colonization of new species from the meta-community by amplifying the “dilution effects”. This negative impact of facilitation overwhelmed its positive impact which caused the reduction of neutral community diversity from low to intermediate environmental severity levels. By comparing the neutral community with the niche-structured community, our results indicated that the negative effect of facilitation on community diversity seems to be a general phenomenon which needs further explorations.