Fecundity-immunity trade-off curvature modulates host evolutionary dynamics, pathogen propagation, and host abundance

In most species, life history trade-offs generate conflicts between biological functions because of resource allocation constraints. During pathogenic infections, hosts can deploy various immune defenses, but these responses usually carry fitness costs, most often expressed as reduced fertility. This fecundity–immunity trade off has been widely studied for its evolutionary implications, yet its effects on other emergent properties —such as host abundance and pathogen spread— remain poorly understood. Here, we develop an SIR model incorporating three immune strategies —reduced host susceptibility, increased host recovery, and reduced pathogen virulence— and show that the evolutionary impact of the fecundity–immunity trade off depends on both its shape and the specific immune mechanism under selection. For reduced susceptibility and increased recovery, disease spread and host abundance evolve in opposite directions along evolutionary trajectories, such that when disease spread increases, host abundance decreases, and vice versa . For reduced virulence, however, this relationship is more complex and does not follow a simple pattern. Across all three mechanisms, trade offs with low cost intensity, whether concave or convex, favor the joint evolution of high fecundity and high immunity. In contrast, concave or convex trade offs incurring high cost consistently select for high fecundity at the expense of immunity. Trade offs with moderate cost intensity generate two alternative outcomes: one strategy with high fecundity and low immunity, and another combining low fecundity with enhanced immunity. Together, these findings underscore the importance of accounting for the specific immune mechanism evolving and the associated cost intensity when predicting the eco-evolutionary consequences of fecundity–immunity trade-offs.