How does toxin production maintain diversity in ecological systems?

Competition promotes survival of the fittest, and one of the strategies bacteria can adopt to attain a higher fitness compared to its competitors is through the production of toxins. Intuitively, the presence of toxins should widen the gap in fitness differences between species and thus driving down diversity. The effects of toxins on diversity become less straightforward when one considers the following factors: (1) the trade-offs in strategies, (2) the cost and (3) the regulation of toxin production. Considering the interplay between these factors, under what regimes does toxin production, if possible, promote diversity? Specifically, by using a coarse-grained model of metabolism which satisfies proteome constraints, we show transitions from low to high diversity, and suggest diversity can be maintained by toxin production through two routes: (i) coexistence at steady state, and (ii) persistence through oscillations or chaos. Counterintuitively, the presence of toxins can equalize fitness differences while the toxin producers can act as keystone species, without which the entire system would simply collapse, and may have a role by either of the routes in sustaining diversity beyond the upper limits as suggested by competitive exclusion principle.