The competition cascade: Indirect facilitation emerges as a key driver of species richness under neutral, niche or individual difference
Indirect effects of competitive interactions are widely acknowledged. It is not known, however, whether the expected net result for species richness of these proximate effects on a whole community is positive (facilitative) or negative (competitive). I present a mathematical model that creates two key predictions: 1) the null expectation for community species richness due to the propagation of pairwise competition, i.e., the competition cascade of indirect effects, is positive; and 2) the relative strength of this positive effect for a community increases with niche overlap, i.e., increasing competition increases community-level species richness due to the propagation. To test these two predictions, I develop a novel spatially-explicit individual-based simulation model of pairwise competition. Noting that competition strength (the 2nd prediction) can vary within and between species, and aware that species pool will affect the prediction, I use the simulation model to build a 4-factor experiment: propagation, interspecific competition, intraspecific competition, and species pool. With this experiment I therefore ask, to what extent niche (non-random species-level differences), neutral (random species-level differences) and individual (random individual-level differences) drivers are responsible and whether these effects occur due to direct or indirect pathways.
Supporting my first prediction, richness increased when including indirect effects, compared to direct effects alone. Supporting the second prediction, I found that this indirect positive effect increased with niche overlap, a result that held irrespective of whether it was interspecific or intraspecific overlap. Furthermore, as the species pool increased, these indirect effects increased, with over 80% of the richness due to propagation (with 256 species); with low species pools, direct effects dominated but both were present (4 sp). These results may provide a partial explanation for some empirical studies showing niche effects appearing to dominate at lower species pools and neutral effects appearing to dominate at higher species pools. Furthermore, these results may add a perpendicular dimension (indirect effects) to the niche-neutral axis of biodiversity drivers. Overall, this work shows that indirect facilitation emerges from a large number of local interactions and contributes to the growing evidence of the importance of positive effects on species richness.