Community structure and the interplay between interspecific infection and competition.

Our motivation is the need to understand how two different interactions between species-shared infection and interspecific competition-combine to determine community structure. We introduce a proto-typical model of two hosts sharing a pathogen and also competing directly. We discuss forces of infection, forces of competition and invasion criteria and their relevance to long-term outcomes and community structure. To understand their interplay, we consider first purely competitive and second purely infective interactions. We then investigate our full model to establish how the two forces combine and how the combination and related invasion criteria determine community structure. The forces of infection and competition do not merely add; there is a synergetic resistance to invasion. Using generalised invasion criteria and subsidiary conditions for the feasibility and stability of uninfected coexistence, we classify long-term outcomes. We distinguish two main routes to three-species coexistence. In the first, two host species, each of which would not alone support the pathogen, support it jointly if interspecific competition is relatively weak, interspecific infection strong. In the second, at least one host species would alone support the pathogen and both are invadable by the other, but subsidiary conditions yield two cases. In one, infected coexistence results when the two hosts would coexist stably purely competitively and at sufficiently high densities to support the pathogen jointly. Thus coexistence is promoted by weak interspecific competition but there is a tension between weak interspecific infection favouring invadability and strong interspecific infection promoting pathogen survival. In the other, infected coexistence results when the two hosts would not coexist in the absence of the pathogen. This pathogen-mediated host coexistence is expected where there is strong intraspecific infection (lowering densities) and weak interspecific infection (favouring invadibility) as necessary. Results are compared with previous work and apparent competition and resource- and transmission-mediated coexistence are discussed.