FIELD EVIDENCE AND MODEL PREDICTIONS OF BUTTERFLY-MEDIATED APPARENT COMPETITION BETWEEN GENTIAN PLANTS AND RED ANTS

In recent spatial models describing interactions among a myrmecophilou s butterfly Maculinea rebeli, a gentian Gentiana cruciata and two comp eting species of Myrmica ant, we predicted that apparent competition s hould exist between gentians (the food of young M. rebeli caterpillars ) and Myrmica schencki, which supports M. rebeli in its final instar. Here we extend and quantify model predictions about the nature of this phenomenon, and relate them to ecological theory. We predict that: (i ) Within sites supporting the butterfly, fewer M. schencki colonies oc cur in sub-areas containing gentians than in identical habitat lacking this plant. (ii) Where G. cruciata and M. schencki do co-exist, the a nt colonies will be less than half the size of those living >1.5 m fro m gentians; (iii) The turnover of M. schencki colonies will be much gr eater than that of other Myrmica species in nest sites situated within 1.5 m of a gentian. All three predictions were supported in the field on 3-6 sites in two mountain ranges, although the exact strength of t he apparent competition differed from some model predictions. Field da ta were also consistent with predictions about apparent mutualisms bet ween gentians and other ants. We suggest that apparent competition is likely to arise in any system in which a specialist enemy feeds sequen tially on two or more species during its life-cycle, as occurs in many true parasite-host interactions. We also predict that mor: complex pa tterns involving other Myrmica species and G. cruciata occur in our sy stem, with apparent competition existing between them in some sub-area s of a site being balanced by apparent mutualism between them in other sub-areas.