Forget the slope: Tradeoff intercept drives evolution and densities during epidemics
Eco-evolutionary models of disease systems with tradeoffs between key disease and life history traits often consider complex biology and non-linear tradeoffs. But theory that focuses on the concavity or convexity of a tradeoff can de-emphasize fundamental aspects of the tradeoff. A simple model incorporating a linear tradeoff, however, provides a great deal of fundamental insight on under-developed theory. Inspired by the Daphnia-Metschnikowiasystem, we consider a population of multiple host genotypes that falls along a linear fecundity-susceptibility tradeoff, an enemy population, and a population of the host’s resource, e.g. algae. Though the slope of such tradeoffs are normally considered, the fecundity-axis intercept determines whether or not the environment is essentially “good” or “harsh” for hosts. This determines whether selection will favor risky, fecund hosts or conservative, resistant hosts and how host evolution will shape this eco-evolutionary system. The critical intercept, at which system behavior changes, depends on key parameter values. Further, we investigate how inclusion of more complex life history, such as an infected class, modifies these results. In the simplest model, analytical investigation yields clear predictions. In more complex models analytical investigation is coupled with simulations.
The intercept of the tradeoff –a typically ignored feature- drives the system behavior. Slope, which reflects the cost of resistance, is far less important. Even if resistance is nearly free, a small intercept dictates that hosts will evolve increased susceptibility. A large intercept means hosts can tolerate a “harsh” environment and will degrade the environment until it is harsh, favoring conservative, resistant hosts. A small intercept means hosts can’t survive a “harsh” environment and the system will develop a “good” environment, favoring risky, fecund hosts. Thus, the intercept of the fecundity-susceptibility tradeoff vitally shapes system behavior. Ecological parameters, such as algal carrying capacity, determine the value of the critical intercept. In this way, differences in these parameters lead to qualitatively different host evolution and resultant biomass partitioning. If carrying capacity increases, host evolution can shift from fecund-favoring to resistant-favoring, leading to increased host density, decreased resource density, and decreased enemy density. Such effects are unique to the eco-evolutionary interplay captured by this model and are not predicted by ecological modeling alone. Many of our theoretical results are robust to life history complexity while a few are qualitatively altered. These predictions are well-posed for empirical testing.