The strength of the fecundity-immunity trade-off modulates host evolutionary dynamics, pathogen propagation, and host abundance

In most species, life history trade-offs generate conflicts between several biological functions because of resource allocation constraints. If under a pathogenic infection, the host can trigger some defense mechanisms against the pathogen, there is usually a fitness cost associated in terms of fecundity reduction. This pattern, known as the fecundity-immunity trade-off, has received much attention, but its implications on host eco-evolutionary dynamics, and particularly on emergent properties such as host abundance and pathogen propagation remain poorly understood. Here, we use an SIR model that incorporates three distinct host immune mechanisms, which are a reduction in pathogen transmission, an increase in host recovery, and a reduction in pathogen-associated host death. Our results indicate that the effect of a fecundity-immunity trade-off depends not only on its strength, but also on the particular immune mechanism considered. We found that, in all three immune mechanisms, for a weak trade-off, the host evolves simultaneously toward high fecundity and high immunity, while for a strong trade-off, high fecundity is selected at the expense of immunity. Intermediate trade-off results in two distinct alternative evolutionary strategies; high fecundity and low immunity, and low fecundity and high immunity. Moreover, we show that disease propagation and host abundance usually show opposite dynamics along the evolutionary trajectories. We here believe that the incorporation of this trade-off in further analysis of host-pathogen interactions may help to better understand the complexity of the ecological and evolutionary dynamics in such systems.