Many exploiter-victim (e.g., predator-prey or pathogen-host) systems involve multiple exploiter species who exploit the same victim species. In such systems, it is often thought that the exploiters have competitive, i.e., (-,-), interactions because exploitation of an individual victim by one exploiter depletes the pool of victims available to other exploiters. However, previous multi-predator-multi-prey models have shown that noncompetitive, i.e., (-,+) or (+,+), interactions can arise between predators when interspecific competition between prey is strong and asymmetric. The underlying mechanism causing positive correlations between predator abundances is that decreased abundance (e.g., removal) of one predator causes shifts in prey abundances, which due to strong asymmetric prey competition, decreases food quality for the other predator and causes a decline in its abundance. This strongly suggests that positive indirect interactions can also arise between pathogens that share host species who compete for resources, but this has been relatively unexplored. In this talk I explore when competitive versus noncompetitive interactions occur in a two-pathogen-two-host SIR-type model that includes competition for resources between the host species.
Results/Conclusions
I present the conditions under which noncompetitive, i.e., (-,+) or (+,+), indirect interactions arise between the pathogen species at endemic equilibria. In particular, I show when strong interspecific host competition promotes noncompetitive indirect pathogen interactions and how the signs of those indirect interactions depend on the competitive abilities of the infected classes (i.e., when infected hosts are equally competitive versus much weaker competitors than uninfected hosts). I also show that the sign of the indirect interaction depends on the method used to measure the interaction (e.g., methods based on small changes in the mortality of infected individuals versus removal of a pathogen from the system). Finally, I contrast the results for two-host-two-pathogen systems with two-predator-two-prey systems, highlighting how an important mechanistic difference between predator-prey and host-pathogen interactions (i.e., immediately lethal interactions vs. infection) influences the indirect interactions between exploiters that share resources.