Bacteria as ecosystem engineers: Unraveling clues through a novel functional response and tritrophic model

Bacteria play a crucial role in regulating the nutrient cycle of ecosystems, making their abundance essential for the sustainability of these environments. Maintaining a thriving bacterial population is achieved through complex nutrient–bacteria–protozoa interactive dynamics, which can be likened to a tritrophic food chain model. The interaction between bacteria and protozoa is governed by a critical factor known as the bacteria–protozoa functional response, which serves as the foundation for developing this tritrophic model. However, existing functional responses have shown limitations in accurately describing the intricacies of the bacteria–protozoa interaction. One significant drawback is the neglect of bacterial behavioral traits. To address this issue, we consider the concept of cooperation as a group defense mechanism employed by bacteria facilitated through a quorum-sensing communication process. By incorporating the cooperation trait into the functional response, our model offers a more comprehensive understanding of the complex tritrophic food chain dynamics. We evaluate the stability of different equilibrium points, along with Hopf-bifurcation around the coexistence equilibrium point. We find that a balance between strong group defense and moderate cooperation is essential for bacteria sustainability and overall system stability. Our results also elaborately address the effects of the increasing group defense through the bistable equilibrium followed by a branch point and saddle–node bifurcation. Through comprehensive analyses and simulations, we examine the paradox of enrichment in nutrition flow at the community level and explore how nutrient washout controls system stability. This innovation not only enhances our comprehension of ecosystem sustainability but also opens up new avenues for studying the intricate relationships that govern the overall balance of nature.