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Disease and Community Structure: The Importance of Host Self-Regulation in a Host-Host-Pathogen Model

A model is examined of two hosts sharing a directly transmitted pathogen where for the first time both hosts are also subject to self-regulation. The model provides insights into the manner in which self- and pathogen regulation combine in determining the coexistence or otherwise of hosts and the structure of biological communities generally. A number of outcomes are possible: coexistence of both hosts with the pathogen, coexistence of both hosts without the pathogen, the predictable elimination of one host by the other, which then remains with the pathogen, and the elimination of either host by the other, contingent on initial densities. However, whereas without self-regulation coexistence of both hosts with their shared pathogen can only be achieved by the partitioning of enemy-free space, with self-regulation there is a wider range of paths, including coexistence mediated by the hosts' resources (analogous to "exploiter-mediated coexistence"), coexistence mediated by interspecific transmission, and coexistence arising when elements of these other mechanisms are combined. Also, a more affected, "regulated-alone" host can eliminate a less affected, "nonregulated-alone" host, but not vice versa; whereas without self-regulation only the opposite outcome is possible. The incorporation of self-regulation adds a necessary element of realism.