The effects of hydrological disturbance on the densities of macroinvertebrate predators and their prey in a coastal stream

1. Environmental Stress Models (ESMs) predict that abiotic disturbance or harshness will differentially affect predators and prey. Consumer Stress Models (CSMs) predict that consumers will be relatively more inhibited by disturbance than prey, and therefore predator impacts will be reduced. Conversely, Prey Stress Models (PSMs) predict that prey will be more adversely affected and consequently predator impacts will increase in disturbed habitats. This study compared the relative tolerances of lotic invertebrate predators and their prey to hydrological disturbance in an Australian coastal stream to test the initial predictions of ESMs.
2. Macroinvertebrates were sampled with a suction sampler at monthly intervals and immediately following four high flow events at five sites on the Cumberland River, in south-west Victoria, Australia. Various statistical procedures were used to compare the relative resistance and resilience of predatory and prey taxa to each high flow event.
3. The relative resistances of seven predator and nine prey taxa to four floods over a 12-month period were highly variable between floods and between runs within the same flood. Prey taxa appeared to be more resilient than predators to the largest flood event, but there were no differences in the resilience of predators and prey following smaller floods. If disturbance tolerance is determined by resistance and resilience, then there was no consistent pattern of differential tolerance to floods among invertebrate predators and prey in this system.
4. The variability in the relative tolerances of taxa to different disturbance events makes general predictions about the effects of disturbance on the community-wide impact of predation extremely difficult.     

A capture-rate model of net-spinning caddisfly communities

Summary Empirical research suggests that net-spinning caddisflies require two basic resources, suspended particulate foods, and the currents which deliver them. I present a theoretical model of caddisfly communities based on quantitative differences in the capture rate produced by different catchnet designs. It assumes that catchnet architecture reflects a tradeoff between water filtration rate (flux through the net) and capture efficiency (the proportion of suspended items retained), and that the marginal resource concentration required by species with different catchnet morphologies should reflect the product of these parameters. The model hypothesizes a) that downstream changes in the physical morphology of the stream channel cause a shift in the relative importance of population limitations imposed by food and current-substrate availability, b) that the interaction of these physical changes with the filtering biota results in a seston resource gradient, and c) that the distribution of each taxon along this resource gradient reflects a marginal resource requirement determined by the functional morphology of its catchnet.     

Phenotypic plasticity of larval retreat design in a net-spinning caddisfly

Larval Macrostemum carolina caddisflies construct silken catchnetswithin protective retreats, often on submerged trees and branches(i.e., snags). In the Savannah River, M. carolina larvae constructthree distinct retreats that differ in the configuration ofthe water entrance hole: (1) at the end of a silken tube, (2)flush with the top of the retreat, and (3) backed by a 180-degreesilken backstop. To identify the proximate mechanism mediatingthis retreat polymorphism, we removed larvae of known phenotypefrom their original retreats and brought them into the laboratory,allowing them to construct new retreats. We found that (1) larvaecan construct more than one type of retreat, so variation inthis behavior is not under strict genetic control; (2) larvaedo not preferentially reconstruct their original retreat design,so these alternative behaviors apparently exhibit little heritability;and (3) larvae primarily construct each phenotype in a particularmicrohabitat (i.e., "tube" and "backstop" retreats are principallyconstructed on the downstream half of the snag, and "flush"retreats on the upstream–bottom quadrant). Therefore,the retreat polymorphism in M. carolina is phenotypically plasticand is controlled by microhabitat location or a correlated environmentalvariable. Most net-spinning caddisflies construct their netsin fairly specific microhabitats. However, behavioral plasticityallows M. carolina larvae to construct retreats all around asnag, thereby reducing potentially intense competition for spacewith other net-spinning caddisflies. Consequently, this maybe the ultimate reason this polymorphism evolved.     

Isolation of polymorphic microsatellite loci in the net‐spinning caddisfly,Polycentropus flavomaculatus (Polycentropodidae)

We have isolated eight polymorphic loci for the caddisfly, Polycentropus flavomaculatus. The application of these markers will allow us to define the genetic population structure of this species and, with complementary ecological work, will enable quantification of its dispersal ability and colonization potential.     

Molecular genetic markers provide no evidence for reproductive isolation among retreat building phenotypes of the net-spinning caddisfly Macrostemum carolina

Larvae of the stream-dwelling, filter-feeding caddisfly Macrostemum carolina construct silken catchnets within protective retreats. In the Savannah River, M. carolina individuals make three different retreats, each with a distinct water entrance hole: (i) at the end of a silken tube; (ii) with a approximately 180 degrees silken backstop; and (iii) flush with the top of the retreat. To resolve whether these different retreats represent alternative behavioural phenotypes within a single panmictic population or fixed phenotypes within three genetically distinct populations or species, we compared the allele frequencies at three polymorphic nuclear loci (allozyme electrophoresis for Gpi, Mpi and Pgm) and the mitochondrial DNA (mtDNA) haplotype frequencies among individuals displaying the three retreat morphs. We also calculated pairwise exact tests of population differentiation using the allozyme and mtDNA allele frequencies. No significant genetic differentiation was detected among caddisflies exhibiting the different retreat morphs. Therefore, these morphs apparently represent a single panmictic population in the Savannah River. Consequently, additional study is required to assess whether this retreat polymorphism is a phenotypically plastic trait under conditional control, or is mediated by alternative alleles at a Mendelian gene or genes (or a combination of the two).     

Chronic effects of low insecticide concentrations on freshwater caddisfly larvae

The insecticide load in surface waters does not ordinarily reach concentrations acutely toxic to aquatic fauna. The effects of the low insecticide concentrations typical of natural habitats are still not clear, for they often appear only after relatively long exposure times. To test such a situation, the insecticides lindane and parathion were introduced into a static-with-renewal outdoor aquaria system at concentrations about four and five orders of magnitude lower than their respective 96-h LC₅₀s, and their chronic (about 90 days) effects on the survival rate of freshwater caddisfly larvae were observed. The emergence and hence survival rate of Limnephilus lunatus Curtis was significantly reduced by lindane at 0.1 ng l^–1, a value nearly five orders of magnitude lower than the 96-h LC₅₀. Parathion, with acute and subacute toxicity similar to that of lindane, did not significantly alter the emergence rate of this species. In contrast, this substance did produce a significant reduction in emergence rate of the closely related species Limnephilus bipunctatus Curtis at 1 ng l^–1, even though this species was significantly less susceptible than L. lunatus to parathion at high concentrations. We conclude that chronic insecticide exposure can be hazardous to freshwater macroinvertebrates even at unexpectedly low concentrations. The low-concentration effects may depend on both species and substance and therefore cannot be predicted from toxicity data at higher concentrations.     

The influence of microhabitat on availability of drifting invertebrate prey to a net-spinning caddisfly

SUMMARY. 1. Invertebrate drift was sampled at both a rockface and a deep pebble riffle site in streams draining both a clearcut and a forested catchment.
2. A sampler was designed to separate the bottom 2 cm of flow, encompassing the effective range of caddisfly (Hydropsychidae: Trichoptera) catchnets. from upper flow.
3. No significant difference in drift density (numbers per cubie metre) was seen between sites within each stream. However, numbers per square centimetre intake area per day at the rockface sites were 4 times higher in the clearcut and 10 times higher in the forested stream than at the pebble-riffle site.
4. Rockface habitat which had highest drift availability was also the site of maximum secondary production of the predaceous collector- filterer Parapsyche cardis in both streams studied.
5. Increased sediment load in the clearcut stream may influence the efficiency of utilization of invertebrate drift by collector-filterers.     

一种毛翅目昆虫的生态治理研究

针对生活在水流速度较快的湖南省柘溪水电站一带河床大量发生的原双栖纹石蛾(Amphipsyche proluta)来说采用一般的防治措施都很难付诸实施,或者只能是治标,不能治本。经1997年至2000年的调查研究和试验分析得知人为去掉河底大型石块,加深河水深度,减缓流速,以破坏适宜于该石蛾幼虫生存的生态环境,引进生态致死因子,是从根本上杜绝该石蛾发生的较为理想的办法。     

Chronic effects of short-term contamination with the pyrethroid insecticide fenvalerate on the caddisfly Limnephilus lunatus

Larvae of the caddisfly Limnephilus lunatus Curtis were exposed to the pyrethroid fenvalerate for one hour and transferred to artificial outdoor stream systems, where their survival rate and emergence were monitored over 84 days. Lethal effects of the pyrethroid almost always appeared shortly after the short-term contamination, but chronic influences were also observed. At a concentration of only 0.01 µg l^–1 a slight increase in mortality over the entire period of observation, as well as a delay in development and hence in emergence occurred. Such reactions under field conditions may impair the survival of this species, and sublethal effects of brief pyrethroid contamination, previously largely ignored, can have important consequences for aquatic macroinvertebrates.     

Effects of group behavior in the predatory raid on damselfish nests by the false cleanerfish Aspidontus taeniatus

The false cleanerfish Aspidontus taeniatus, which resembles the bluestreak cleaner wrasse Labroides dimidiatus, is one of the best-known examples of mimicry in vertebrates. This mimicry system has been viewed as an aggressive mimicry to bite fish fins. However, recent field studies have reported that large individuals of the false cleanerfish often form groups and jointly raid damselfish nests to eat eggs that are guarded by their parents. The benefits of group behavior have been reported in a variety of animals. In the case of false cleanerfish, parental defense of territorial damselfishes is the main factor that constrains the availability of nutritionally valuable food resources. Here, we conducted field observations on the reefs of Okinawa, and found that the false cleanerfish formed groups of 2–12 individuals when they raided breeding nests of 13 species of damselfishes (Pomacentridae) and one species of triggerfish (Balistidae). Since the cleaner wrasse does not form such groups, the feeding groups of the false cleanerfish are assumed to reduce the effectiveness of mimicry. However, our results showed that the group behavior has two effects: a dilution effect, which reduces the risk of being attacked by egg-guarding fish, and an increase in foraging efficiency. We conclude that the false cleanerfish need to form foraging groups during egg-eating because the mimicry has no effect on parental damselfishes.     

Effect of elevated temperature on osmotic and ionic regulation in a salt-tolerant caddisfly from Death Valley,California

The effects of temperature (8–10 or 20°C) on regulation of haemolymph osmotic and ionic concentrations were investigated over a range of salinities (0–25‰) in fifth-instar larvae of the Death Valley caddisfly Limnephilus assimilis. At low temperatures, levels of chloride and sodium in the haemolymph are regulated over a wide range of salinities corresponding to the salinities at which larvae occur in nature and at which they can complete development into adults. In contrast, haemolymph osmolality is constant at low salinities (<14‰) but approaches conformity with the medium at higher salinities. High temperature reduces the larva's ability to maintain low chloride concentrations in its haemolymph and also leads to a reduction in haemolymph osmotic pressure; thus, at high temperatures ions account for more of the haemolymph osmotic concentration than at low temperatures. These data suggest that the absence of larvae from thermal pools and from all Death Valley waters in summer can be explained by the effects of high water temperatures on hydromineral regulation.     

Effect of carnivory on larvae and adults of a detritivorous caddisfly, Lepidostoma complicatum: a laboratory experiment

Lepidostomatid caddisfly larvae are typical detritivores, but they occasionally eat small dead animals. A laboratory feeding experiment was conducted with Lepidostoma complicatum (Kobayashi) larvae using two different feeding treatments: leaves or leaves and dead chironomids. L. complicatum larvae showed significantly higher growth rates and adult emergence weight and a significantly earlier emergence for the leaves and dead chironomids treatment than for the leaves alone treatment. However, the adult emergence rate was not different between the two feeding treatments. Thus, it is apparent that ingestion of dead chironomids by detritivorous L. complicatum larvae positively influences larval growth rates, adult emergence weight, and larval development time.     

Genetic differentiation of a European caddisfly: past and present gene flow among fragmented larval habitats

We describe the genetic structure of a freshwater insect species and interpret it in terms of present-day population dynamics and possible postglacial colonization history. The sampling regime represented a large area of the species range in northwest Europe, particularly focusing on Britain, a region relatively neglected in molecular population genetic studies. Plectrocnemia conspersa generally showed low levels of genetic variation across the sampled populations (Nei's D = 0.0138) and subdivision was unrelated to the pattern of the drainage network. However, the results do suggest that populations across the region are not at equilibrium and that British populations still show effects of the recolonization of the species following the last glacial maximum. Levels of genetic diversity were lower in Britain than in mainland Europe. Two-dimensional scaling showed genetic differentiation between major regions and the pattern of genetic diversity indicates a more recent origin of populations in the north and west of the area compared with the south and east. We argue that, despite the highly fragmented larval habitat, dispersal over tens of kilometres is frequent. Over longer distances, however, P. conspersa does still show evidence of founder effects and postglacial range expansion into Britain.     

Scale-dependent effects of windthrow disturbance on forest arthropod communities

The effect of disturbance on local communities may operate within the context of the spatial landscape. We examined the scale-dependent effects of windthrow disturbance caused by a large typhoon on three arthropod communities in a temperate forest of Japan. Canopy arthropods were collected by beating foliage, forest-floor arthropods were collected by sweeping the vegetation, and flying arthropods were collected in Malaise traps. To assess the “functional spatial scale” at which arthropods responded to tree-fall disturbance, the gap rate was quantified at different spatial scales by sequentially enlarging the radius of a circular landscape sector in 10-m increments from 10 to 500 m. We then analyzed the responses of order richness and abundance to the gap rate for each arthropod community. The spatial scale of the significant best-fitting model, which was selected from the models fitted to the gap rate at stepwise spatial scales, was regarded as the arthropod-specific functional spatial scale. Arthropod order richness was not dependent on the gap rate. In contrast, arthropod order abundance depended significantly on the gap rate in many orders, but varied in the response direction and functional spatial scale. These order-specific, scale-dependent responses to tree-fall gaps could complicate interactions among organisms, leading to complex community organization. An understanding of the spatial processes that link the use of space by organisms with the spatial scale at which ecological processes are experienced is required to elucidate the responses of populations, communities, and biotic interactions to disturbances in a spatial landscape context.     

The influence of different spatial‐scale variables on caddisfly assemblages in Flemish lowland streams

1. Patterns in species assemblages are the result of the combined influence of processes acting on different spatial scales. Various studies describe the distribution of macroinvertebrate communities and their relationship with environmental factors at different geographical scales, but only a few of these studies concentrate on Western European lowlands. 2. Using Flanders as representative for the densely populated Western‐European lowlands, the specific aims of this study are: (i) to identify the different trichopteran species assemblages and to characterise them biologically using indicator species; (ii) to determine which environmental gradients most influence the observed species assemblages; and (iii) to analyse the relative importance of different spatial scale variables in constraining the Trichoptera distributions. 3. Assessment of the main environmental gradients suggested that the absence of Trichoptera from certain locations was mainly due to elevated nutrient concentrations and lower oxygen contents, confirming their sensitivity to anthropogenic disturbance. 4. Five Trichoptera species assemblages were distinguished based on Bray–Curtis dissimilarity coefficients. These assemblages did not differ significantly in species richness, but a shift in stream zonation preference was observed. In the ordination analysis 11 variables that were selected using a stepwise model building function manifested themselves as upstream–downstream and size‐related gradients. The Trichoptera assemblages in lowland streams thus appear to follow a longitudinal succession pattern that corresponds with the species‐specific preferences. 5. Partitioning the variance over the different spatial scales indicated that the reach‐scale variables were far more important in explaining the variation in species composition. The study design, which limited the minimum–maximum range of catchment‐scale characteristics, however, may have led to an overestimation of the impact of the local‐scale variables.     

Physical constraints on the foraging ecology of a predatory snail

We studied the effects of aerial exposure and high summer temperatures on the southern oyster drill ( Stramonita haemastoma ), feeding on the American oyster, Crassostrea virginica . In the laboratory, oyster drill feeding rates and growth were highest at 25 and 30°C, some mortality occurred at 35°C, all snails died at 40 and 45°C, and the 28-day LC 50 was 35.7°C. In a second experiment where both water temperature (25 vs . 33°C) and aerial exposure were varied, only simulated tidal exposure lowered oyster drill feeding and growth rates. In field cage experiments, oyster drills had reduced feeding rates and growth at intertidal sites, but snail growth rates increased in late summer with warmer water temperature. We therefore conclude that aerial exposure, not high temperature, is the major factor limiting oyster drill feeding and growth in intertidal oyster reefs. Field experiments with partial cages also suggested that ambient predation rates were much higher at a subtidal than at a nearby intertidal site. Because southern oyster drills have depressed feeding, growth, and possibly lower fitness in intertidal oyster reefs during the summer, this reduced predation risk may provide a refuge for intertidal oysters.     

Diet and growth of a leaf-shredding caddisfly in southern Appalachian streams of contrasting disturbance history

Diet and growth of leaf-shredding caddisfly larvae, Pycnopsyche spp.,were examined in streams draining a reference catchment and a 16-year-oldclear-cut (disturbed) catchment at Coweeta Hydrologic Laboratory insouthwestern North Carolina, USA. The objective was to explain why shredderproduction is higher in the disturbed streams despite the larvae having lessfood (i.e., leaves) available. We predicted larvae would grow faster onfast-decaying leaf material representative of the disturbed streams. Larvaeconsumed mostly leaf detritus in three streams draining each catchment overthree seasons (fall, winter, and spring), which showed larvae did notconsume higher quality foods (e.g., algae and animal material) in disturbedstreams. When fed 2-month-old conditioned black birch (Betula lenta L.) (afast-decaying leaf species) and white oak (Quercus alba L.) (a slow-decayingleaf species) leaves in the laboratory, larvae grew significantly faster onthe birch leaves. However, when larvae were fed the same leaf types after3-months conditioning, larvae grew significantly faster on oak leaves. Afield growth experiment conducted for 42 d using mixed-species leaf dietsrepresentative of each catchment and initially conditioned for 2 monthsfound that Pycnopsyche grew significantly better on the diet representativeof the reference catchment. The reference diet contained more oak leaveswhich apparently became a more acceptable food as the experiment proceeded.High shredder production in the disturbed streams could not be explained byhigh Pycnopsyche growth rates on fast-decaying leaves. Instead, larvae grewbetter on leaves that were apparently conditioned optimally regardless ofconditioning rate.     

Female‐biased size dimorphism in a diapausing caddisfly,Mesophylax aspersus: effect of fecundity and natural and sexual selection

1. The effect of mating success, female fecundity and survival probability associated with intra‐sex variation in body size was studied in Mesophylax aspersus, a caddisfly species with female‐biased sexual size dimorphism, which inhabits temporary streams and aestivates in caves. Adults of this species do not feed and females have to mature eggs during aestivation. 2. Thus, females of larger size should have a fitness advantage because they can harbour more energy reserves that could influence fecundity and probability of survival until reproduction. In contrast, males of smaller size might have competitive advantages over others in mating success. 3. These hypotheses were tested by comparing the sex ratio and body size of individuals captured before and after the aestivation period. The associations between body size and female fecundity, and between mating success and body size of males, were explored under laboratory conditions. 4. During the aestivation period, the sex ratio changed from 1 : 1 to male biased (4 : 1), and a directional selection on body size was detected for females but not for males. Moreover, larger clutches were laid by females of larger size. Finally, differences in mating success between small and large males were not detected. These results suggest that natural selection (i.e. the differential mortality of females associated with body size) together with possible fecundity advantages, are important factors responsible of the sexual size dimorphism of M. aspersus. 5. These results highlight the importance of taking into account mechanisms other than those traditionally used to explain sexual dimorphism. Natural selection acting on sources of variation, such as survival, may be as important as fecundity and sexual selection in driving the evolution of sexual size dimorphism.     

Effects of predatory risk and resource renewal on the timing of foraging activity in a gerbil community

The foraging decisions of animals are often influenced by risk of predation and by the renewal of resources. For example, seed-eating gerbils on sand dunes in the Negev Desert of Israel prefer to forage in the bush microhabitat and during darker hours due to risk of predation. Also, daily renewal of seed resource patches and timing of nightly foraging activity in a depleting environment play important roles in species coexistence. We examined how these factors influence the timing of gerbil foraging by quantifying foraging activity in seed resource patches that we experimentally renewed hourly during the night. As in previous work, gerbils showed strong preference for the safe bush microhabitat and foraged less in response to high levels of illumination from natural moon light and from artificial sources. We demonstrate here for the first time that gerbils also responded to temporal and spatial heterogeneity in predatory risk through their timing of activity over the course of each night. Typically, gerbils concentrated their activity early in the night, but this changed with moon phase and in response to added illumination. These results can be understood in terms of the nature of patch exploitation by gerbils and the role played by the marginal value of energy in determining the cost of predation. They further show the dynamic nature of gerbil foraging decisions, with animals altering foraging efforts in response to time, microhabital, moon phase, illumination, and resource availability.