Random patch dynamics of larval Chironomidae (Diptera) in the bed sediments of a gravel stream

• 1 The larval chironomid community of the sediment surface and the hyporheic inters titial was investigated in two longitudinal transects of an alpine gravel stream between September 1984 and August 1985. Eighty larval species and species groups were identified, most of which belonged to the subfamily Orthocladiinae. Of all larval individuals 51.1% inhabited the first 10cm of the bed sediments, and 93.2% occurred between the surface and 40cm depth.
• 2 The spatial species turnover showed marked variations between horizontally adjacent sampling sites in each of the four sediment depth layers. In both transects the species composition showed a significantly lower turnover in the upper 10cm of the bed sediments than in the deeper layers.
• 3 Spatial community stability showed an oscillating pattern between all sampling sites due to density shifts of larvae between depth layers. Temporal differences in resilience (local stability) were significantly and positively related to changes in the cumulative discharge pattern in the gravel brook, thus indicating the apparent ability of the community to recover quickly following disturbances.
• 4 The five abundant species, Corynoneura lobata, Synorthocladius semivirens, Tvetenia calvescens, Micropsectra atrofasciata and Rheotanytarsus nigricauda, exhibited significant differences in their sediment depth distribution, with density maxima shifting between depth layers. Spatial autocorrelations suggest that these larvae form patches between neighbouring sampling sites with varying sizes and inter-patch distances in each of four sediment layers. A simulation test, in which individuals of each species were randomly permuted between microhabitats of each depth layer separately, indicated that the patches might have arisen by chance.
• 5 To evaluate the significance of observed spatial resource overlap values amongst these five chironomid species, neutral models were developed based on 300 randomizations of each possible species pair-wise association of individuals and patches of species. The spatial organization of a larval chironomid assemblage in the stream Oberer Seebach seemed to be governed by coexistence due to random patch formation and dispersal patterns within the interstitial habitats, which reduce the probability of strong competitive interactions.

     

High aquatic niche overlap in the newts Triturus cristatus and T. marmoratus (Amphibia,Urodela)

We studied spatial niche metrics of large-bodied newts (Triturus cristatus and T. marmoratus) in three breeding ponds in western France. Adults and larvae were sampled with underwater funnel traps. Larvae were identified to the species with diagnostic microsatellite DNA markers. The distribution of adult T. cristatus and T. marmoratus across pond regions differed in one out of six cases, no differences were observed between larvae (two ponds studied). Niche overlap and niche breadth indices across resource states defined as pond regions or individual traps were high (Schoener's C: pond regions 0.60–0.98, traps 0.35–0.71; Levins' B: pond regions 0.71–0.98, traps 0.35–0.76). Adults of large-bodied newts significantly differed in resource use from small-bodied newts (T. helveticus). The results are discussed in view of the occurrence of interspecific breeding attempts, and the unpredictable ecological characteristics of newt breeding ponds.     

The effect of invertebrate and vertebrate predators on the foraging movements of Ischnura elegans larvae (Odonata: Zygoptera)

SUMMARY.

• 1 The foraging movements of late instar Ischnura elegans larvae were monitored in laboratory experiments to study the effects of predators on larval feeding behaviour.
• 2 Ischnura larvae are sit-atid-wait, or ambush, foragers, moving occasionally between perches in search of profitable feeding sites. Larval foraging movements, monitored at different densities of Daphnia prey, increased significantly when prey were absent.
• 3 In experiments without prey, larval movement was inhibited by the presence of fish predators, as well as by invertebrate predators (Notonecta glauca), but not by closely related, non-predatory invertebrates (Corixa punctata) or physical disturbance of the water (intermittent air bubbles).
• 4 Further experiments varied Ischnura hunger levels (0–8 days without food) and illumination (light or dark) with and without notonectid predators. Hunger had no consistent effect on penultimate instar behaviour but final instar foraging activity was significantly modified: movements increased after 4 days starvation and decreased again after 8 days. This response was suppressed by the presence of predators. Both larval instars moved significantly less often in the light, even when predators were absent.
• 5 These phenotypically flexible predator-avoidance responses are likely to decrease the risk of predation by both visual and tactile predators. However, predators clearly have an important influence on the feeding niche of Ischnura larvae, and may decrease the overall feeding efficiency, growth rate, and survival of larvae by constraining their movement in search of profitable feeding sites.

     

Wetland isolation facilitates larval mosquito density through the reduction of predators

• 1 Wetlands harbour high biodiversity and offer important ecosystem services, but they are also a habitat for mosquito larvae (Diptera: Culicidae), which are important disease vectors.
• 2 Isolation among remnant, or newly created wetlands and ponds, and their consequent density in the landscape, is a key factor that can influence a variety of food web processes, including effects on mosquitoes which are important prey to many predators.
• 3 We assess the impact of habitat isolation on the density of pond‐breeding mosquitoes (several Anopheles and Culex species) both directly and indirectly through the food web.
• 4 Results from structural equation modelling of survey data shows that larval mosquitoes are denser in ponds that are more isolated from one another, and that this result was primarily driven indirectly by a reduction of larval mosquito predators (e.g. predaceous insects and amphibians). Furthermore, results from a long‐term mesocosm experiment factorially manipulating isolation and predator reduction show that the effect of isolation on mosquito density was eliminated when predators were experimentally reduced.
• 5 It is concluded that metacommunity processes, both directly and indirectly mediated through predators, can play an important role in the local abundance of wetland breeding mosquitoes and possibly the diseases they spread.

     

Reduction of oviposition time and enhanced larval feeding: two potential benefits of aggregative oviposition for the viburnum leaf beetle

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The upstream/downstream movement paradox of lotic invertebrates: quantitative evidence from a Welsh mountain stream

• 1 This study attempts to determine the mechanisms by which the downstream displacement of lotic invertebrates is compensated by the upstream movements of immatures and adults. To this end, submerged and aerial traps were set up at three sites, each 100 m apart on a small mountain stream (Yr Ogof) in North Wales and operated for 12 months.
• 2 At Site 1 (the source), 19.3% of the benthic invertebrates lost as drift were replaced by the upstream movement of aquatic stages. At Site 2, the average upstream compensation was 51.1% (although there was a marked difference between the two traps at the site). At downstream Site 3 the average compensation was 55.4%. The net loss of benthos downstream over 1 year was estimated to be 261920 individuals from Site 1, 41891 from Site 2, and 40470 from Site 3.
• 3 At Sites 1 and 2, the numbers of invertebrates drifting were positively correlated with both current speed and water depth. At Sites 2 and 3, the numbers moving upstream were positively correlated with water temperature. At all three sites, the numbers of animals drifting were positively correlated with the numbers moving upstream, under water.
• 4 In none of the eight common species of stoneffy did females show a strong overall upstream flight preference—however, two species, Leuctra nigra and Nemoura erratica, showed a significant downstream preference.
• 5 As with the stonefly nymphs, most of the caddisfly larvae showed a longitudinal distribution. Agapetus fuscipes larvae were most abundant at Site 1, yet females showed no preferred flight direction—this would seem to be inappropriate for maintaining this larval distribution pattern. However, females caught moving upstream at Site 2 were bigger and contained twice as many eggs, which were also larger, than females moving upstream at Site 1. The flight patterns of the other Trichoptera also seemed inappropriate for maintaining their respective larval distributions. Immature insects moved upstream at certain sites and times, but trends were not consistent. There was no correlation between female flight direction and local wind speed and direction.
• 6 Downstream displacement of the most abundant non-insect species, Gammarus pulex, was quite considerable at all three sites. However, at several times during the year the numbers moving upstream approached or exceeded those drifting. Maintenance of benthic population densities in this species is thought to be through a combination of upstream movement (which was highly correlated with benthic density at Sites 1 and 3) and production of young over a long (10 month) reproductive period. Maintenance of the benthic populations of the immature stages of the majority of insect species in Yr Ogof is thought to be through oviposition by females in the vicinity of their emergence sites.

     

To hatch or not to hatch? Egg hatch response to larval density and to larval contact in a treehole mosquito

Abstract.

• 1 We investigated the effect on egg hatch of exposure to: (1) varying larval density, and (2) larval contact in Aedes triseriatus Say (Diptera: Culicidae). For 2 days in the laboratory we submerged eggs into a treehole water medium containing 0 (control), 4, 12 or 24 larvae that could either contact the eggs directly or were separated from them by a screen. Following treatment, abundance of microorganisms on the egg surfaces, a food source for newly hatched larvae and a proposed hatching stimulus, was assessed by counts made from serial dilutions of samples.
• 2 We discovered a complex hatching response to larval contact and to larval density, and an interaction between these two factors in their effect on microbial growth. Hatching was inhibited in the 0-larva control, even though microorganisms grew abundantly on the eggs. Hatch rate, as well as microbial counts, were high for eggs in direct contact with 4 larvae. As density increased in the larval contact treatment, microorganisms disappeared from the egg surfaces and hatch rate declined.
• 3 When protected from larval grazing, eggs supported numerous microbial colonies irrespective of larval density. In contrast to the contact treatment group, egg hatch increased with increasing larval density. These observations suggest that the combination of microbial growth and a larval factor stimulates hatch. This hatching response may have evolved because both abundant microorganisms and numerous larvae reflect a habitat of good quality.

     

Life history studies of anisocentropus kirramus neboiss (trichoptera: Calamoceratidae) in a tropical Australian rainforest stream

Eggs and larvae of Anisocentropus kirramus were collected from leaf packs in riffles and pools in a small upland rainforest stream in tropical Queensland. Adults were collected in floating emergence traps. Egg masses contained 80–100 eggs. None developed in water at 12–15°C but at 22–25°C larvae hatched in 3–10 days. There were five larval instars and complete development appeared to take several months. Instars II‐V were present in all nine months sampled; instar I was present in all but one month. Adults emerged in all ten months sampled, but there was clear seasonality with peak emergence in the summer. The sex ratio of adults was 1:1.     

Competition between Bufo larvae in a eutrophic pond

Competition between larvae of two anuran species (Bufo bufo and B. calamita) was investigated under field conditions likely to disfavour cell-mediated interference mechanisms. The experiment used triplicated cage treatments in an unshaded farm pond, a poor habitat for the unicellular pathogen Anurofeca richardsi implicated in interference competition between these anurans in sand dune ponds. The farm pond experienced lower maximum temperatures than a nearby dune pond but sustained larger numbers of eukaryotic algae and therefore had higher primary productivity. Survival and growth of B. calamita larvae were inversely related to density in all treatments but interspecific effects were much more severe than intraspecific ones. There was no evidence of A. richardsi in any treatment and competition between the Bufo larvae was therefore intense in the absence of Anurofeca-mediated interference effects. Anuran larvae reduced the standing crop and altered the community composition of algae in the treatment cages but larval growth rates were not simply related to food availability. Algal cell numbers in larval guts, a measure of food acquisition, were however inversely related to tadpole density in both species. Feeding niche overlap was high but decreased as larval density increased. Resource competition was implicated as the most probable major mechanism. Received: 1 September 1999 / Accepted: 20 January 2000     

The life histories of two species of Sialidae (Megaloptera) from Japan

The life histories of two species of Sialidae (Megaloptera) from Japan were investigated, focusing on their larval climbing and emergence conditions and then adult emergence behaviours. At the Rokuman pond, a fence trap and pitfall traps were constructed beside the pond to collect final-instar larvae of Sialis yamatoensis and Sialis japonica as they climbed for pupation in the night. In the two species, 48–49% of climbings occurred under wet ground conditions, due to rainfall during or before the climbing night. Approximately 50% of larval climbings occur under dry ground conditions, during continuous sunny or cloudy weather. This is thought to be due to pupation occurring close the edge of the pond. Male and female adult emergences were nearly simultaneous in both species. Pupal periods varied, later final-instar larvae climbings resulted in shorter pupal periods. Ground temperature is thought to have an effect on pupal periods.     

Dissimilar numerical responses of macroinvertebrates to disturbance from drying and predatory sunfish

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Patterns of resource distribution among conspecific larvae in two fruit fly species: Anastrepha fraterculus and Ceratitis capitata (Diptera: Tephritidae)

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Winter feeding leads to a shifted phenology in the browntail moth Euproctis chrysorrhoea on the evergreen strawberry tree Arbutus unedo

• 1 The browntail moth Euproctis chrysorrhoea is a highly polyphagous univoltine forest pest. Although its young larvae usually overwinter in diapause from early autumn to the beginning of spring, winter larval feeding has been reported when this species feeds on the evergreen woody shrub strawberry tree Arbutus unedo.
• 2 The present study investigated life‐history traits of four populations of E. chrysorrhoea feeding on A. unedo, including phenology of the different life stages, larval feeding activity and diapause incidence. By modelling the relationship between larval size and host plant leaf persistence, elevation and mean annual temperature, we also studied larval development in ten populations of this species sampled from a range of geographical locations in Spain, from both A. unedo and deciduous hosts.
• 3 The results obtained revealed that on A. unedo, E. chrysorrhoea phenology has shifted: from October to March, A. unedo larvae doubled their size, whereas, on deciduous Ulmus minor and Quercus faginea, larval size did not change. General linear models demonstrated that such differences were not related to environmental variables. We also found that on A. unedo larval feeding was arrested for 2 months, with this period representing a true diapause.
• 4 The results obtained in the present study suggest that E. chrysorrhoea populations are phenologically adapted to their local host plants, and that the presence of foliage is a key element in the phenological shift reported on A. unedo. These results may have implications with respect to the formation of E. chrysorrhoea host races.

     

Oviposition habitat selection by container‐dwelling mosquitoes: responses to cues of larval and detritus abundances in the field

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Substrate discrimination by larvae of the sessile rotifer Ptygura beauchampi Edmondson

SUMMARY. In an acid bog pond, the sessile rotifer Ptygura beauchampi was limited to the trap door areas of one of three distinct prey capturing organs of the carnivorous hydrophyte Utricularia vulgaris ; four other co-occurring congeneric species ( U. gibba, inflata var. minor, intermedia , and purpurea ) were not colonized. This pattern was not the result of differential capture of larvae by the prey traps. Laboratory settling experiments demonstrated that P. beauchampi larvae select this particular substrate to the exclusion of all others. This selectivity was independent of the presence of captured prey organisms in the traps.
P. beauchampi larvae select younger vestibules (trap door areas) over older ones. Observations of natural populations showed denser colonization of those slightly bigger traps closer to the plant stem. However, no relation was found between vestibular area and adult rotifer density. Selectivity for traps more proximal to the plant system was not demonstrated. In natural populations a distinct distribution of adults within the vestibule was noted. This pattern was statistically different from sites which the larvae colonize in laboratory experiments. Adult distribution is probably a modification of the larval settling pattern by site-dependent survivorship of adults.
Larval settling experiments showed that larvae select uncolonized vestibules over those previously colonized of approximately equal age. Settling outside the vestibule area was highest on traps closest to the plant stem and was related to the density of adults colonizing the vestibule.
The adaptive significance of larval selection of a particular substrate is discussed.     

Chironomidae (Diptera) of Baldwin Lake,Illinois, a cooling reservoir

The chironomid fauna of Baldwin Lake, a closed-cycle cooling reservoir for a power plant, was sampled monthly to semi-monthly for one year at four stations, two in the discharge channel and two in the main basin. Qualitative samples were also taken elsewhere and with multiple-plate samplers. Twenty-four species were collected. Annual mean population density was less than 100 larvae m^–2 in the channel, 1037 M^–2 in the main basin. Three species of Tanypodinae formed over 96 percent of the larval population in the main basin: Tanypus stellatus Coquillett, Procladius bellus (Loew), and Coelotanypus concinnus (Coquillett). The principal species on the multiple-plate samplers were Dicrotendipes nervosus (Staeger), Glyptotendipes lobiferus (Say), and Parachironomus monochromus (Wulp). Temperatures of 35°C or more in the channel virtually eliminated the chironomids, whereas temperatures up to 32°C in the main basin increased the number of generations of T. stellatus from the usual two to four at Station 3 and three at Station 4. Station 3 had almost 1,000 degree-days more heat than Station 4.     

The association between the phytoplankton, Rhopalosolen species (Chlorophyta; Chlorophyceae), and Anopheles gambiae sensu lato (Diptera: Culicidae) larval abundance in western Kenya

Algae are important food resources of the larvae of the African malaria vectors, Anopheles gambiae Giles and Anopheles arabiensis Patton (Anopheles gambiae sensu lato), and other zooplankton, but empirical evidence remains meager about the agal flora in ephemeral water bodies. The animals present in natural aquatic habitats in western Kenya were sampled from July to November 2002 to study abiotic and biotic environmental factors determining A. gambiae sl larval abundance. The five highest concentrations of third and fourth instars and pupae (hereafter referred to as old-stage larvae) were sampled in conjunction with the unicellular epizoic green algae, Rhopalosolen species (Chlorophyta; Chlorophyceae). Canonical correspondence analysis revealed that the presence of Rhopalosolen species was the most important determinant of the animal assemblage. The density of old-stage A. gambiae sl larvae was positively correlated with the presence of Rhopalosolen species, but the density of first and second instars of A. gambiae sl was not. The water bodies with Rhopalosolen sp. yielded larger mosquitoes in spite of the higher density of larvae. We demonstrated that the productivity of water bodies in terms of the larvae of malaria vectors can differ in magnitude depending on the agal flora. We discuss phytoplankton as a regulator of mosquito larval populations.     

The impacts of overwintered green frog larvae on wood frog embryo mortality under a wetter climate

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Habitat and substrate influences on population and production dynamics of a stream caddisfly, Ceraclea ancylus (Leptoceridae)

SUMMARY. Population and production dynamics of Ceraclea ancylus (Vorhies), a leptocerid caddisfly with a univoltine life cycle, five larval instars, and a single cohort population, were examined in Brashears Creek, Kentucky. A systematic sampling approach that included analysis in riffle, pool, and waterwillow bed habitats was used and fourteen sampling transects were taken across Brashears Creek between May 1971 and July 1972. For each of the 70 m² samples taken along a transect, the depth, habitat, and substrate type were recorded. The number of C. ancylus cases and numbers and dry weights of each larval instar were determined. The majority of the C. ancylus population was in the second larval instar in July, the third larval instar in August, the fourth larval instar in September, and the fifth larval instar from October through May. The spring population was 30% of the early instar maximum estimated the previous August. The spatial distribution of C. ancylus reflects a logarithmic pattern with early instar larvae more highly clumped than the late larval instar and pupal populations. Initial standing stock estimates were less than 1% of the standing stock biomass. A production estimate of 9.9 mg/m²·year (dry weight) was determined using the instantaneous growth method, Allen's Curve method, and the Hynes method of estimating secondary production. Turnover ratios (7.0 annual TR, 5.7 cohort TR) were higher than estimates reported as typical for benthic macroinvertebrates because of the low initial to final mean individual weight of the C. ancylus larvae (0.4%). Production estimates calculated for C. ancylus populations in waterwillow bed (4.4 mg/m²·year) and pool (13.7 mg/m²·year) habitats were less than calculated for the riffle population (19.9 mg/m²·year). The annual turnover ratio (10.2) was higher in the pool population than in other habitats because of spring larval migration into the pool from other habitats. A trend of higher production estimates with larger sized substrate particles and increased numbers of particles may reflect the increase in available food sources or attachment sites. The accuracy of production estimates and their applicability to water resource management may be limited by a failure to consider the spatial distributions of the population being sampled and the tendency for sampling procedures to be confined to single habitats.     

Wind‐driven thermocline movements affect the colonisation and growth of Achnanthidium minutissimum,a ubiquitous benthic diatom in lakes

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