PRIORITY EFFECTS AND SPECIES COEXISTENCE - EXPERIMENTS WITH FUNGAL-BREEDING DROSOPHILA

1. Priority experiments were carried out using the fungal-breeding spe cies Drosophila phalerata and D. subobscura and the mushroom Agaricus bispora forma albida. 2. Field experiments showed that an oviposition window exists for these species and that priority in increments of 1 d ay are suitable for laboratory experiments. 3. Priority (arriving firs t) had a clear effect upon three components of 'fitness'. When a speci es arrived late it had lower survival, smaller size and longer develop mental time. 4. A priority model [based upon the 'aggregation model' o f Atkinson & Shorrocks (1981)] showed that traditional priority (both species arrive on average together, but with a range of priorities) do es not significantly contribute to coexistence. 5. When the average ar rival time is moved so that more fugitive situations are modelled, the n priority can have a marked effect upon coexistence. 6. Both 'priorit y' and 'fugitive' situations are dominated by realistic amounts of int raspecific aggregation. This has implications for metapopulation dynam ics.