The diet of two species of Isoperla (Plecoptera: Perlodidae) in relation to season, site, and sympatry

Abstract

The seasonal change in gut contents of nymphs of Isoperla grammatica and I. di#formis from six streams in southern Sweden was analysed. Both species had ingested a variety of benthic prey and vegetable matter, predominantly diatoms. Some seasonality was evident with high percentages of diatoms in spring in I. grammatica, and in autumn in I. dzfformis. The scope of food was larger in the latter species which contained about equal proportions of vegetable matter, chironomids, mayfly, stonefly, and black fly larvae. In I. grammatica plant matter and chironomids dominated strongly, comprising > 50% of the gut contents on an annual basis, > 90% in spring. While small nymphs of I. dfformis contained a low proportion of animal matter, only gradually increasing with size, the nymphs of I. grammatica were carnivorous from very early instars. Both species switched to a temporarily strong utilization of algae in spring. This differed among sites, and appeared to reflect differences in insolation and thus the availability of algae. There was a significant negative relationship between the mean densities of Isoperla nymphs and the proportion of animal material found in the guts of I. grammatica (R2 = 0.86). Considering the density of I. grammatica alone, the relationship was weaker (R2 = 0.56). A positive correlation between predator and prey size was observed. With chironomid prey the size range increased with predator size. With simuliid prey, however, prey size increased with predator size in such a way that it suggests selection rather than just an expanding prey size range. Correlations were stronger and regression coefficients significantly higher for I. grammatica than for I. dfformis. We suggest that I. grammatica, which ingests a much wider size range of prey might choose prey of optimal sizes more readily than the more synchronously developing I. difformis. Although the life cycles of the two species are staggered, overlap in size distribution indicates that competition for food could be important in spring. However, observed differences in diet should facilitate coexistence. Gut content differences might in turn be accomplished through microhabitat segregation.

Contrasting diel activity and feeding patterns of four species of carnivorous stoneflies

“…2a). Isoperla grammatica therefore appears to be a very ef®cient predator that is feeding for most of the time when it is active (see also Malmqvist et al, 1991). Isoperla grammatica therefore appears to be a very ef®cient predator that is feeding for most of the time when it is active (see also Malmqvist et al, 1991).…”

Life history, growth patterns and feeding habits of two predatory stoneflies,Skwala pusilla(Perlodidae) andKamimuria tibialis(Perlidae) in northern Japan

“…Using the stable isotope method, Lancaster et al (2005) showed that Perla bipunctata (Pictet) (Perlidae) consumes algae but does not assimilate them into their somatic tissues, whereas two perlodids, Perlodes microcephala (Pictet) and Isoperla grammatica (Poda), acquire 40-55% of somatic nitrogen from algae. Furthermore, it is reported that Isoperla grammatica is mainly carnivorous but temporarily switches to an algae-feeder in spring (Malmqvist et al 1991). These results indicate that some perlodids utilise algae as an alternative food resource for their development.…”

Intraguild omnivory in predatory stream insects

“…Empirical evidence indicates that some freshwater detritivores may scavenge animal material (Zhang et al . 2003) or attack other invertebrates (Dick 1992; Kelly, Dick & Montgomery 2002), that predators may consume algae or detritus for short periods (Winterbourn 1974; Siegfried & Knight 1976; Malmqvist, Sjöström & Frick 1991) and that some species may undergo ontogenetic shifts from algivory to carnivory (Feminella & Stewart 1986; Céréghino 2002).…”