Coexistence of macroparasites without direct interactions.

Coexistence of macroparasites is studied by extending the infinite-dimensional model considered by Anderson and May (1978, J. Anim. Ecol. 47, 219-247, 249-267) to several species of parasites that are assumed to interact only by causing the death of a common host. An exact invadability condition is found for this model. By studying when mutual invasibility is possible, the region where two parasite species can coexist is found. The result is that, if there is a trade-off between virulence and transmissibility, then coexistence of two species of parasites is possible, but only when the parameters of the model fall into a very narrow parameter region. If, on the other hand, one parasite is more virulent and less transmissible, then it will be competitively excluded. This latter result, though expected in terms of competition theory, is in contrast with what found in the approximate models so far used for studying interacting macroparasites. The effect of parasite aggregation on coexistence is studied by considering two modifications of the basic model (clumped infections and host population heterogeneity in predisposition to infections) that allow for higher aggregation. It appears that the width of the coexistence region is insensitive to these modifications.