Interspecific interference and the functional response of four species of carnivorous stoneflies

Summary 1. A previous study compared the functional responses to their prey and intraspecific interference in mature larvae ofPerlodes microcephalus,Isoperla grammatica,Dinocras cephalotesandPerla 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.Baetislarvae 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 forPerlodes,slightly higher forIsoperla,and much higher forDinocrasandPerla. 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 forPerlodes,and also forDinocrasandPerlain competition withIsoperla. The decrease inDinocrasandPerlawas similar to that for intraspecific interference. 4. The decrease in attack rate was described by a convex curve forPerlodeswith the other three species and forDinocras/PerlawithIsoperla,but by a concave curve (negative power function) forIsoperlacompeting with the other three species, and for bothDinocrasandPerlain competition withPerlodes. 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 forPerlodesin all contests and forDinocrasorPerlacompeting withIsoperla,whilst interspecific interference dominated forIsoperlain all contests and forDinocrasandPerlacompeting withPerlodes. Both types of interference were applicable to competition betweenDinocrasandPerla.Isoperlawas the least, andPerlodesthe most, aggressive of the four species withDinocrasandPerlaintermediate.