The impact of invader number on whole community invasions in biomethane-producing communities

Microbes can invade as whole communities, but the ecology of whole community invasions are poorly understood. Here, we investigate how invader frequency affects the composition and function of invaded laboratory methanogenic communities. An invading community was equally successful at establishing itself in a resident community regardless of initial invader frequency, which varied between 0.01 and 10%. Invasion resulted in enhanced biogas production (to the level of the pure invading community), but only when invader frequency was 1% or greater. This inconsistency between invasion success and changes in function can be explained by a lower number of invading taxa (but not individuals) at lower initial invader frequencies, and an important functional role of the taxa that were absent. Our results highlight that whole community invasion ecology cannot simply be extrapolated from our understanding of single species invasions. Moreover, we show that methane production can be enhanced by invading poorly performing reactors with a better performing community at levels that may be practical in industrial settings.