Competition mediated by parasites: biological and theoretical progress.

387 W ith his usual foresight, J.B.S. Haldane argued nearly 50 years ago that 'a non specific parasite is a powerful competitive weapon' 1. One species can be a superior competitor simply by harbouring and transmitting a pathogen to a more vulnerable species. This is 'apparent' competition, which is mediated via a shared pathogen, and contrasts with 're-source' competition, in which there can be rivalry over resources, such as food or nesting sites. Competition is generally accepted as a major force influencing biodiversity , so it is likely that apparent competition could play a significant role in shaping community structure. Interest in apparent competition has blossomed over the past ten years 2 , but our understanding has recently advanced with some clear experimental studies and theoretical developments. On the biological side, there has been a wealth of examples 2 that have provided evidence of competition through a shared pathogen and highlighted the asymmetry in the competition with clear winners and losers. However, the majority of these examples have been descriptive and have often failed to disentangle the effects of resource competition from those of apparent competition. A recent experimental study 4 has separated these two forms of competition and shown apparent competition in action. The theory of parasite-mediated competition has centred on the deterministic 'susceptible–infectious' (SI) models that have been applied so successfully to the dynamics of single host– pathogen systems, such as measles 3. However, transferring this theory from one host species to multiple host species has been hampered by problems in identifying the equi-libria (caused by the intractability of the algebra). Further theoretical developments have only been resolved through an indirect route. A recent laboratory study provides an excellent illustrative example of how the presence of a second host species can destabilize a parasite–host relationship and result in the elimination of one species 4 (Fig. 1). The system used was a simple ichneumonid parasitoid (Venturia canescens) and two caterpillar hosts (Plodia interpunctella and Ephestia kuehniella); the parasitoid lays her eggs inside the caterpillar host and the larval, parasitic stage of the parasitoid kills the caterpillar and emerges as an adult. When experimental chambers contained just a single host species plus the para-sitoid then both persisted and exhibited damped oscillations tending towards a stable equilibrium. However, when the system was run with the para-sitoid and two host species, the parasitoid had a greater impact on the species with the …