1. A previous study compared the functional responses to their prey and intraspecific interference in mature larvae of Perlodes microcephalus, Isoperla grammatica, Dinocras cephalotes and Perla bipunctata. The present study extends this work by assessing interspecific interference between pairs of these species in equal numbers (one, two or three larvae per species) to provide total predator densities of two, four or six larvae. Baetis larvae as prey were replaced as they were eaten, and their density per predator was varied between 20 and 200 larvae.
2. The number of prey eaten by each competing species increased curvilinearly with prey density, the relationship being well described by a Type II model. Of the two constants in the model, handling time varied considerably between species, mean values being shortest for Perlodes, slightly higher for Isoperla, and much higher for Dinocras and Perla. It was not affected significantly either by predator density or the identity of the competing species.
3. Attack rate also varied between species and decreased with predator density. This decrease was slight for Perlodes, and also for Dinocras and Perla in competition with Isoperla. The decrease in Dinocras and Perla was similar to that for intraspecific interference.
4. The decrease in attack rate was described by a convex curve for Perlodes with the other three species and for Dinocras/Perla with Isoperla, but by a concave curve (negative power function) for Isoperla competing with the other three species, and for both Dinocras and Perla in competition with Perlodes. Prey consumption also decreased with predator density, the severity of competition with different species reflecting that for attack rate.
5. A comparison with previous results for intraspecific interference showed that the latter was dominant for Perlodes in all contests and for Dinocras or Perla competing with Isoperla, whilst interspecific interference dominated for Isoperla in all contests and for Dinocras and Perla competing with Perlodes. Both types of interference were applicable to competition between Dinocras and Perla. Isoperla was the least, and Perlodes the most, aggressive of the four species with Dinocras and Perla intermediate.