Dispersal and spatial heterogeneity allow coexistence between enemies and protective mutualists

Area of Science:

• Ecology
• Evolutionary Biology
• Microbial Ecology

Background:

• Protective mutualisms, where symbionts shield hosts from harm, are widespread but their evolutionary origins remain poorly understood.
• Existing hypotheses suggest protective mutualists may arise from more pathogenic variants with differing competitive abilities and host fitness impacts.

Purpose of the Study:

• To model the ecological conditions favoring the emergence and spread of protective mutualists from pathogenic ancestors.
• To investigate the role of environmental productivity, initial densities, and spatial structure in the evolution of protective mutualisms.

Main Methods:

• Mathematical modeling to analyze the conditions for invasion and exclusion of protective mutualist variants.
• Numerical simulations in an explicitly spatial system to explore regional coexistence and competitive exclusion.

Main Results:

• Protective mutualists generally cannot invade from rarity or exclude more pathogenic strains without specific conditions.
• Increased environmental productivity and exceeding initial density thresholds enhance the likelihood of mutualist invasion.
• Spatial structure facilitates regional coexistence, and in some cases, allows mutualist populations to exclude pathogenic strains.

Conclusions:

• The evolution of protective mutualisms is constrained by ecological factors, often requiring spatial structure and specific life-history traits.
• Life-history differences, such as vertical transmission or direct host benefits, may be crucial for the establishment of protective mutualists over pathogenic forms.