Patch and local dynamics describe processes that operate at distinct spatial scales, but they are rarely coupled together. Patch processes include dispersal and extinction, while local processes can include resource consumption and competition. Importantly, the distribution and abundance of species depends jointly on processes that occur at both scales. In disease ecology for example, the distribution and abundance of pathogens depends both on their transmission among hosts (i.e., patches), and their exploitation of resources within individual hosts (i.e., local dynamics). Here, we present a hierarchical disease model that couples both scales together. Two analytical models serve as important baselines. First, the ‘superinfection’ model describes patch dynamics for two pathogens coexisting among hosts via a competition-virulence tradeoff. Second, a local competition module (two competitors and one resource) describes the dynamics within each host/patch. We explicitly link patch and local dynamics by making virulence a function of within-host resources. Critically, local (i.e., within-host) dynamics do not necessarily go to equilibrium before transmission or host death (patch processes). With simulations, we ask how slowing the pace of within-host dynamics alters the prevalence of the two pathogens, their coexistence among and within hosts, and the abundance of pathogens and resources in individual hosts.
Results/Conclusions
Slowing the pace of local dynamics dramatically alters both the prevalence of infections and abundances of pathogens within hosts. When local dynamics are fast relative to transmission, results mirror the analytical predictions. Specifically, each pathogen depletes resources to its R* in the host patches that it infects. Furthermore, each pathogen infects the proportion of hosts predicted by the ‘superinfection’ patch model. However, slowing the pace of local dynamics causes both the distribution and abundance of pathogens to deviate from these baselines. Several general patterns emerge. First, slower local dynamics always release resources and lower the abundance of pathogens within hosts. Second, slower dynamics always increase the infection prevalence of the superior competitor, and this effect intensifies with pathogen virulence. This increase in prevalence is consistent with the effect of decreasing virulence in the analytical superinfection model. Third, slower dynamics can either increase or decrease infection prevalence of the inferior competitor. Fourth, slower dynamics allow coinfections (i.e., coexistence within as well as among hosts), because resources in hosts infected by the superior competitor sometimes remain above the R* of the inferior competitor. This disease example highlights the rich insights that can be gained by coupling patch and non-equilibrial local dynamics.