Spatial Distribution of Chironomid Larvae (Diptera) on Leaves of the Bulrush in a Chalk Stream

SUMMARY (1) Small samples of leaves of the bulrush, Schoenoplectus lacustris were collected from a chalk stream from January 1976 to January 1977 and were used to determine the spatial distribution patterns of individuals, and the spatial profiles of mean density along the leaves, of the larval stages of eight species of chironomid. (2) The mean density of six species increased distally and that of two species showed little variation along submerged sections of leaves. Intraspecific temporal differences in gradient in mean density along leaves were often more pronounced than interspecific differences at a given time. (3) The density of the larvae of all species combined was influenced by the amount of detritus in many samples but not by interspecific interactions between chironomids or invertebrate predators. (4) On leaves in a laboratory stream, characteristic patterns of longitudinal gradients in mean density were established within 2 h and were stable for up to 5 days. Larvae, whose species and instar could not be distinguished, moved towards the distal end of leaves with little epiphytic algae/detritus and moved towards the basal end of leaves with dense epiphytic material. Current speed had no effect on the patterns. (5) The larvae were highly selective in their choice of a site and this resulted in specific patterns of individual spatial distribution which were density dependent. (6) Taylor's and Iwao's spatial distribution models both fitted the field data. Eight species were aggregated except at very low densities and aggregation increased most rapidly with increasing density for Cricotopus bicinctus and C. fuscus, least rapidly for Synorthocladius semivirens, and was independent of density of C. sylvestris. (7) The power, b, from Taylor's model rarely differed between individual instars and the total larvae for each species, and this suggests that the instars behaved similarly in selecting sites. SUMMARY (1) Small samples of leaves of the bulrush, Schoenoplectus lacustris were collected from a chalk stream from January 1976 to January 1977 and were used to determine the spatial distribution patterns of individuals, and the spatial profiles of mean density along the leaves, of the larval stages of eight species of chironomid. (2) The mean density of six species increased distally and that of two species showed little variation along submerged sections of leaves. Intraspecific temporal differences in gradient in mean density along leaves were often more pronounced than interspecific differences at a given time. (3) The density of the larvae of all species combined was influenced by the amount of detritus in many samples but not by interspecific interactions between chironomids or invertebrate predators. (4) On leaves in a laboratory stream, characteristic patterns of longitudinal gradients in mean density were established within 2 h and were stable for up to 5 days. Larvae, whose species and instar could not be distinguished, moved towards the distal end of leaves with little epiphytic algae/detritus and moved towards the basal end of leaves with dense epiphytic material. Current speed had no effect on the patterns. (5) The larvae were highly selective in their choice of a site and this resulted in specific patterns of individual spatial distribution which were density dependent. (6) Taylor's and Iwao's spatial distribution models both fitted the field data. Eight species were aggregated except at very low densities and aggregation increased most rapidly with increasing density for Cricotopus bicinctus and C. fuscus, least rapidly for Synorthocladius semivirens, and was independent of density of C. sylvestris. (7) The power, b, from Taylor's model rarely differed between individual instars and the total larvae for each species, and this suggests that the instars behaved similarly in selecting sites. SUMMARY (1) Small samples of leaves of the bulrush, Schoenoplectus lacustris were collected from a chalk stream from January 1976 to January 1977 and were used to determine the spatial distribution patterns of individuals, and the spatial profiles of mean density along the leaves, of the larval stages of eight species of chironomid. (2) The mean density of six species increased distally and that of two species showed little variation along submerged sections of leaves. Intraspecific temporal differences in gradient in mean density along leaves were often more pronounced than interspecific differences at a given time. (3) The density of the larvae of all species combined was influenced by the amount of detritus in many samples but not by interspecific interactions between chironomids or invertebrate predators. (4) On leaves in a laboratory stream, characteristic patterns of longitudinal gradients in mean density were established within 2 h and were stable for up to 5 days. Larvae, whose species and instar could not be distinguished, moved towards the distal end of leaves with little epiphytic algae/detritus and moved towards the basal end of leaves with dense epiphytic material. Current speed had no effect on the patterns. (5) The larvae were highly selective in their choice of a site and this resulted in specific patterns of individual spatial distribution which were density dependent. (6) Taylor's and Iwao's spatial distribution models both fitted the field data. Eight species were aggregated except at very low densities and aggregation increased most rapidly with increasing density for Cricotopus bicinctus and C. fuscus, least rapidly for Synorthocladius semivirens, and was independent of density of C. sylvestris. (7) The power, b, from Taylor's model rarely differed between individual instars and the total larvae for each species, and this suggests that the instars behaved similarly in selecting sites. SUMMARY (1) Small samples of leaves of the bulrush, Schoenoplectus lacustris were collected from a chalk stream from January 1976 to January 1977 and were used to determine the spatial distribution patterns of individuals, and the spatial profiles of mean density along the leaves, of the larval stages of eight species of chironomid. (2) The mean density of six species increased distally and that of two species showed little variation along submerged sections of leaves. Intraspecific temporal differences in gradient in mean density along leaves were often more pronounced than interspecific differences at a given time. (3) The density of the larvae of all species combined was influenced by the amount of detritus in many samples but not by interspecific interactions between chironomids or invertebrate predators. (4) On leaves in a laboratory stream, characteristic patterns of longitudinal gradients in mean density were established within 2 h and were stable for up to 5 days. Larvae, whose species and instar could not be distinguished, moved towards the distal end of leaves with little epiphytic algae/detritus and moved towards the basal end of leaves with dense epiphytic material. Current speed had no effect on the patterns. (5) The larvae were highly selective in their choice of a site and this resulted in specific patterns of individual spatial distribution which were density dependent. (6) Taylor's and Iwao's spatial distribution models both fitted the field data. Eight species were aggregated except at very low densities and aggregation increased most rapidly with increasing density for Cricotopus bicinctus and C. fuscus, least rapidly for Synorthocladius semivirens, and was independent of density of C. sylvestris. (7) The power, b, from Taylor's model rarely differed between individual instars and the total larvae for each species, and this suggests that the instars behaved similarly in selecting sites.