Coexistence of macroparasites without direct interactions

Coexistence of macroparasites is studied by extending the infinite-dimensio nal 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 tradeoff between virulence and transmissibility, then coexi stence of two species of parasites is possible, but only when the parameter s of the model fall into a very narrow parameter region. If, on the other h and, one parasite is more virulent and less transmissible, then it will be competitively excluded. This latter result, though expected in terms of com petition theory, is in contrast with what found in the approximate models s o far used for studying interacting macroparasites. The effect of parasite aggregation on coexistence is studied by considering two modifications of t he basic model (clumped infections and host population heterogeneity in pre disposition to infections) that allow for higher aggregation. It appears th at the width of the coexistence region is insensitive to these modification s. (C) 2000 Academic Press.