COMPLEX COMPETITIVE INTERACTIONS IN 4 SPECIES OF LUCILIA BLOWFLIES

1. The effects of intraspecific and interspecific larval competition o n larval survival, adult size, adult longevity and fecundity were quan tified in four species of coexisting Lucilia blowflies: illustris, sil varum, sericata and caesar. 2. There was a general negative effect of increasing density on larval and adult survival, size and fecundity. A dditionally, complex species-specific and frequency-dependent response s were identified, which were not expected in these biologically and m orphologically closely similar species. 3. Lucilia illustris, the nume rically dominant species in the natural community, was a superior comp etitor to L.silvarum at intermediate densities but an inferior competi tor at high density. Such nonlinear responses may be related to differ ences in the life histories and larval behaviour of the species (bigge r eggs and more contest-type outcome of competition in L.silvarum). 4. We parameterized a model of interspecific competition on a subdivided resource in an attempt to reconcile the conflicting results on larval competitive abilities and the abundances of the species in the field. Using laboratory and field-estimated parameter values the model predi cted coexistence of L.illustris and L.silvarum and the observed numeri cal dominance of the former species. The average densities of flies in the field are limited to relatively low levels, apparently preventing L.silvarum (the superior competitor at high density) from dominating and excluding L.illustris.