Competition in multispecies native communities promotes coexistence with and without invasion

Background/Question/Methods

Increased species diversity increases the overall productivity and function of ecosystems; thus understanding the processes that promote species rich systems is crucial.  Ecological processes such as herbivory and disturbance often promote diversity in plant communities through disruption of competitive dominance.  However, diverse systems appear to persist in the absence of such non-equilibrium factors, which warrants some curiosity about the effects of other processes such as competition.  Conceptual theory based on two species models and pairwise experimentation has long established interspecific competition as a process that results in competitive exclusion, not coexistence.  This has generated a great deal of theory for how diversity is maintained despite competition.  However, other experimental and theoretical studies have shown that non-transitive interactions among competitors have the potential to maintain coexistence through indirect interactions, suggesting that in complex, multi-species communities competition may have profoundly different effects on coexistence than that predicted from two species interactions.   We used novel designs for field experiments in which communities were built to vary in species composition and complexity in both native and invaded systems.

Results/Conclusions

Native species had strong effects on each other in pairwise experiments, and the invasive Centaurea stoebe had exceptionally strong effects on natives.  But when natives competed with each other in complex multi-species communities net competitive effects were much weaker.  In invaded multi-species communities natives had even weaker net competitive effects, but the net effects of C. stoebe remained strong.  In no multi-species community did we find non-transitive interactions.  We also found that the summed competitive effect of native species on each other decreased as the number of interactions increased (p=0.008) and shifted from significant to non-significant as communities change from native to invaded (p=0.025, p=0.971).  Our results suggest that as communities increase in species complexity and the number of simultaneous interspecific interactions, the species-specificity of the effect and the interaction intensity that native species exert on each other decreases dramatically.  Also, competitive interactions in native communities may promote coexistence without non-transitive interactions.