Global epistasis in ecosystems arises from resource constraints

Area of Science:

• Ecology
• Microbiology
• Systems Biology

Background:

• Global epistasis describes how biological system state, not composition, influences perturbation effects.
• Observed across scales from proteins to ecosystems, its mechanistic basis in ecology is unclear.
• A simple form shows a linear relationship between perturbation effect and background function/fitness.

Purpose of the Study:

• Propose a mechanistic explanation for global epistasis in microbial communities.
• Investigate the role of shared resource pools in driving global epistasis.
• Extend the framework to multi-species communities and explore deviations from linear dependence.

Main Methods:

• Modelled single-species growth on substitutable resources under nutrient limitation.
• Developed a framework for multi-species communities competing for a single resource.
• Analyzed global epistasis in trophic cascades, considering facilitation and niche partitioning.

Main Results:

• Shared resource pools generate global epistasis in microbial communities.
• The marginal effect of species addition is linear with background community function, determined by resource fraction.
• Global epistasis persists in trophic cascades but is altered by facilitation and niche partitioning.

Conclusions:

• Shared resource competition provides a simple explanation for global epistasis in ecosystems.
• Global epistasis is a null expectation in ecosystems dominated by competition.
• Resource pool dynamics may also explain organismal-level global epistasis.