Background/Question/Methods Organisms in microbial communities (such as the human gut microbiome) often participate in byproduct chains: microbe 1 consumes resource A and produces byproduct B, microbe 2 consumes B as a resource and in turn produces byproduct C, and so on. It is a known fact that, by and large, the number of consumers in these chains does not exceed four. Puzzlingly, however, there are very rare but existing instances of byproduct chains involving as many as 15 microbial species. Why are long byproduct chains so atypical when they are clearly biologically feasible? To answer this question in as much generality as possible, but avoid being constrained by human imagination in the kinds of evolutionary solutions organisms may come up with, we created a model environment where microbes could freely evolve within the digital evolution platform Avida. We then ran in silico experiments to see whether these digital organisms would spontaneously diversify to form long byproduct chains. Further, we wanted to see whether common ecological processes such as frequent temporal perturbations (as are the norm with the human gut microbiome) would limit the length of these chains. Results/Conclusions We started with just a single ancestral digital organism consuming one basal resource in a homogeneous environment. Even from such humble beginnings, communities with long byproduct chains comprising of 30 or even more steps readily evolved. In light of this, the question is not why natural byproduct chains occasionally become very long, but what mechanisms keep them short most of the time. Applying frequent temporal disturbance to these communities revealed that this indeed causes a strong and sustained reduction in chain lengths, maintained during and sometimes long after disturbances have once again been turned off. Furthermore, the reason for this chain length reduction is not that organisms high in the chain get knocked to extinction by the disturbance events. Instead, their lineages slowly lose the ability to consume higher-order byproducts, as that ability proves not to be adaptive under highly disturbed circumstances. We conclude that the length of byproduct chains is likely not constrained by evolutionary opportunities, but by omnipresent ecological processes such as temporal disturbance.
