The spread of Hydramoeba infections in mixed hydra species systems.

The host-pathogen system, hydra and Hydramoeba hydroxena, was used as an experimental analog to explore properties of epizootics in mixed host species systems, and to determine the contribution of each host species to a measure of overall host community resistance. Epizootics of hydramoeba in systems of two host species were characterized by processes of enhancement and buffering of the infection where one host species is more resistant than the other. More complex host communities were constructed from four hydra species by varying the proportions of each. Community size and number of species were kept constant. Instantaneous infection rates and time infection reached a specific level were judged measures of community resistance or resilience. A Community Resistance Index(R c ) was devised that associated an independent measure of the resistance of each component species in the system with the corresponding abundance of the species in the system. Experimental results indicated significant differences in infection rates and infection level time among the synthetic communities. R c was found predictive of the behavior of the systems under pathogen stress and was considered a realistic measure of community resistance to stress. First order host species interactions were present during the infection process and lead to more complex enhancement processes involving transfer and accumulation of pathogen material. These interactions, however, were not of sufficient magnitude to render the positive R c -community resistance regression insignificant.