The effect of food level on competition in tadpoles - interference mediated by Protothecan algae

1. The development of natterjack toad (Bufo calamita Laurenti) tadpoles was monitored under two levels of food and with or without competitors (Rana temporaria L.) using a replicated pond system. The design prevented physical interactions between species but allowed B. calamita access to R. temporaria faeces. 2. Food supplements accelerated growth with or without competitors. The presence of competitors depressed B. calamita growth, even under conditions of high food. 3. B. calamita raised under high food metamorphosed earlier than those raised under low food, but neither food nor competitors affected survival. 4. The growth-inhibiting alga, Prototheca, was detected in the faeces of both R. temporaria and B. calamita. B. calamita raised with R. temporaria for several weeks produced larger numbers of protothecan cells in their faeces than those raised without competitors. 5. Inferior competitors may be diverted from high quality food resources by the attractive properties of Prototheca in faeces. In such a situation superior competitors may exert a negative effect even under high food levels.