The influence of predatory fish on mayfly drift: extrapolating from experiments to nature

1. A knowledge of how individual behaviour affects populations in nature is needed to understand many ecologically important processes, such as the dispersal of larval insects in streams. The influence of chemical cues from drift‐feeding fish on the drift dispersal of mayflies has been documented in small experimental channels (i.e. < 3 m), but their influence on dispersal in natural systems (e.g. 30 m stream reaches) is unclear. 2. Using surveys in 10 Rocky Mountain streams in Western Colorado we examined whether the effects of predatory brook trout (Salvelinus fontinalis) on mayfly drift, that were apparent in stream‐side channels, could also be detected in natural streams. 3. In channel experiments, the drift of Baetis bicaudatus (Baetidae) was more responsive to variation in the concentration of chemical cues from brook trout than that of another mayfly, Epeorus deceptivus (Heptageniidae). The rate of brook trout predation on drifting mayflies of both species in a 2‐m long observation tank was higher during the day (60–75%) but still measurable at night (5–10%). Epeorus individuals released into the water column were more vulnerable to trout predation by both day and night than were Baetis larvae treated similarly. 4. Drift of all mayfly taxa in five fishless streams was aperiodic, whereas their drift was nocturnal in five trout streams. The propensity of mayflies to drift was decreased during the day and increased during the night in trout streams compared with fishless streams. In contrast to the channel experiments, fish biomass and density did not alter the nocturnal nature nor magnitude of mayfly drift in natural streams. 5. In combination, these results indicate that mayflies respond to subtle differences in concentration of fish cues in experimental channels. However, temporal and spatial variation in fish cues available to mayflies in natural streams may have obscured our ability to detect responses at larger scales.     

Loss of genetic diversity in the North American mayfly Ephemerella invaria associated with deforestation of headwater streams

1. Terrestrial dispersal by aquatic insects increases population connectivity in some stream species by allowing individuals to move outside the structure of the stream network. In addition, individual survival and reproductive success (as well as dispersal) are tightly linked to the quality of the terrestrial habitat. 2. In historically forested catchments, deforestation and altered land use have the potential to interfere with mayfly dispersal or mating behaviours by degrading the quality of the terrestrial matrix among headwater streams. We hypothesised that loss of tree cover in first‐order catchments would be associated with an increase in population substructure and a decrease in genetic diversity of mayfly populations. 3. To test this hypothesis, we investigated spatial patterns of genetic variation in the common mayfly Ephemerella invaria across a gradient of deforestation in the central piedmont region of eastern United States. Intraspecific genetic diversity and population substructure were estimated from data obtained using fluorescent amplified fragment length polymorphism (AFLP) markers. 4. We found that mayfly populations had low population substructure within headwater stream networks and that genetic diversity was strongly negatively correlated with mean deforestation of the first‐order catchments. The large‐scale pattern of population substructure followed a pattern of isolation by distance (IBD) in which genetic differentiation increases with geographical distance, but assignment tests placed a few individuals into populations 300 km away from the collection site. 5. Our results show that loss of genetic diversity in this widespread aquatic insect species is co‐occurring with deforestation of headwater streams. 6. Most arguments supporting protection of headwater streams in the United States have centred on the role of these streams as hydrological and biogeochemical conduits to downstream waters. Our work suggests that headwater stream land use, and specifically tree cover, may have a role in the maintenance of regional genetic diversity in some common aquatic insect species.     

Small‐scale patterns of genetic variation in a headwater specialist mayfly: No influence of selective forest harvesting on diversity

Terrestrial environments allow the adults of some aquatic insects to disperse between headwater streams, which may be important for maintaining population connectivity and persistence. Winged adult stages of aquatic insects are particularly sensitive to degradation of terrestrial habitat, relying on it for food, reproduction and dispersal. In this study we examined the genetic pattern of the Australian mayfly Ulmerophlebia sp. AV2, in north‐eastern New South Wales, and compared the genetic diversity in forested and partially deforested sub‐catchments. Our hypotheses were (i) patterns of mitochondrial DNA (mtDNA) variation in the Leptophlebiidae mayfly Ulmerophlebia sp. AV2 show a pattern of structuring that reflects low or widespread dispersal along the stream network and across catchments; and (ii) genetic diversity will be lower in partially deforested sub‐catchments compared to forested sub‐catchments. We found gene flow was not restricted among headwater streams within sub‐catchments but was restricted at distances >15 km. Genetic diversity was high (mean haplotype diversity >0.85) in both control and harvested sub‐catchments. Instead, a historical signature of population expansion was detected which is consistent with findings for other aquatic insect taxa of eastern Australia. Our results suggest that the selective harvesting management strategy, including the use of riparian buffer zones, within these sub‐catchments does not appear to restrict dispersal between streams or erode diversity within streams for Ulmerophlebia sp. AV2. Selective harvesting therefore appears to have minimal impacts on terrestrial/aquatic links in the life cycle of this insect.     

Riparian ground beetles (Coeloptera, Carabidae) preying on aquatic invertebrates: a feeding strategy in alpine floodplains

The food and feeding habits of riparian ground beetles were studied in four alpine floodplains (Bavaria, Germany): a 5th-order stream (the Isar) and three 3rd-order streams. The riparian fauna along the streams mainly consists of predaceous species. Riparian ground beetle densities were much higher along the Isar than along the small streams. Aquatic invertebrates composed 89% of the potential prey for carnivorous terrestrial insects along the Isar. Besides aquatic organisms washed ashore, stoneflies emerging on land are of considerable importance as potential prey for terrestrial predators. In contrast, only 34% of the potential prey organisms collected along the small streams were of aquatic origin. Food abundance was 9 times higher in the shore region of the Isar compared to the small streams. Surface drift in the Isar, a potentially important food source for riparian organisms, was about 10⁶ organisms and exuviae per meter stream width in 24 h. The drift density in the Isar was 59 times higher than that in a small stream. Terrestrial organisms provided only 3% of the drifting particles in the Isar, but 50% in the small stream. Gut content analysis reveals, that riparian ground beetles in the Isar floodplain mainly feed on aquatic organisms washed ashore or emerging on land. While small Bembidion species prefer chironomids (larvae and adults) the larger species Nebria picicornis feeds on emerging stoneflies, terrestrial riparian organisms and aquatic organisms accumulating along the shoreline. The prey of riparian ground beetles in the floodplain of the three small streams mainly consists of terrestrial species some of which may have been washed ashore. Received: 2 September 1996 / Accepted: 26 February 1997     

Artificial light as a disturbance to light‐naïve streams

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Drift and settlement of stream insects in a complex hydraulic environment

1. The hydraulic and geomorphic characteristics of stream patches are often associated with distinctive assemblages or densities of stream invertebrates, and it is routinely presumed that these patterns reflect primarily species‐specific habitat requirements. An alternative hypothesis is that such patterns may be influenced by constraints on movement, such as the results of departure and settlement processes. We describe a manipulative experiment that examined how the hydraulic environments created by topographic bedforms influenced the drift behaviour and potential settlement sites for two species of mayfly (Baetis rhodani and Ecdyonurus torrentis). These species are common in the drift and often co‐occur in streams, but differ in their small‐scale distribution patterns, body shape and movement behaviour. 2. Flume experiments were carried out to determine how the hydraulic environments conditioned by a step bedform influence the behaviour of mayflies in the drift (swimming, posturing, tumbling), and the consequences of those behaviours (drift distance and time), compared to drift over a plane bed. The ramped step in the flume mimicked step bedforms that are common in coarse‐grained, high‐gradient streams. In contrast to the plane bed, a zone of recirculating flow was created downstream of the step, above which flow was faster and more turbulent. Uniform flows are used in most flume studies of drift; our approach is novel in recreating a complex hydraulic environment characteristic of stream channels. 3. Both species had some behavioural control over drift, and drift distances and times were shorter for live larvae than for dead larvae over the plane bed. The step had no impact on drift time or distance for live Baetis, but dead larvae were trapped in the flow separation eddy and drift time increased accordingly. Some Ecdyonurus also became trapped in the eddy, but live larvae drifted farther than dead larvae, and farther over the step than the plane bed. 4. Whilst in the drift, larvae altered their behaviour according to the ambient hydraulic environment, but in a species‐specific manner. Over the plane bed, Baetis had occasional swimming bursts, but primarily postured (maintained a stable body orientation), whereas Ecdyonurus spent roughly equal time posturing and swimming. In the more turbulent flows generated by the step, Baetis spent proportionately more time swimming, whereas Ecdyonurus spent more time posturing and often tumbling as body orientation became unstable. 5. In a high‐gradient stream, Baetis was more abundant close to steps than in plane bed patches with less complex flow, whereas the opposite pattern held for Ecdyonurus. Thus, the small‐scale distribution patterns of these species within streams correspond to their drift behaviours and ability to access various hydraulic patch types in our flume. These results are consistent with the hypothesis that constraints on movement and settlement may be important driver of distribution patterns within streams.     

Stream drift as a consequence of disturbance by invertebrate predators

Summary We carried out an experimental field study in a Swedish stream in order to determine whether mobile predators enhance the drift of stream insects. We increased the density of nymphs of the predaceous perlid stonefly, Dinocras cephalotes, in an experimental section of a stream up to densities in another more densely populated part of the same stream. The drift of several benthic species increased significantly compared to a control section where D. cephalotes were rare. Experiments carried out in September showed a strongly elevated drift response in nymphs of the mayfly Baetis rhodani only, whereas May experiments resulted in increased drift in B. rhodani as well as the amphipod Gammarus pulex, the stonefly Leuctra fusca, chironomids, and the total number of drifting animals. In September, we found that the drift response of Baetis rhodani to predator disturbance was dependent on the size of mayfly nymphs; small nymphs appeared in greater numbers in the drift nets than did large nymphs. A subsequent laboratory analysis of drift lengths of B. rhodani nymphs supported the hypothesis that small nymphs travel in the drift for longer than do large nymphs, particularly in darkness. We suggest that morphological constraints in vision or swimming performance, or both, cause small nymphs to drift longer. In May, size-dependent drift was less obvious, probably because the size of the nymphs was considerably greater than in September.     

广东横石水河流域溪流大型底栖动物漂流的昼夜节律

比较了广东横石水河流域二条相邻的3级溪流大型底栖动物的漂流种类组成及昼夜节律,其中一条溪流受广东大宝山矿外排的酸性矿山废水严重污染,水体pH值仅为3.45且重金属严重超标,而另一条为相邻的清洁溪流.结果表明：清洁溪流中大型底栖动物的数量和种类远比受酸性矿山废水污染的溪流丰富.在清洁溪流中共采获漂流底栖动物6 871头,隶属10目52类群,其中水生昆虫的数量和种类占绝对优势 (99.5%).但总漂流密度占优势的类群（相对多度超过5%）不多,主要集中在以下几种水生昆虫：七鳃假二翅蜉 (28.5%)、宜兴似动蜉 (13.8%)、短脉纹石蛾(13.2%)、白背锯形蜉（7.5%)、摇蚊科(6.5%) 和肖扁泥甲(5.0%).蜉蝣目种类和数量最多,占全部漂流底栖动物总个体数的65%,其中又以四节蜉科居多,占蜉蝣目总个体数的63%.毛翅目昆虫的数量（18%）仅次于蜉蝣目.大型底栖动物的漂流表现出明显的昼夜节律,漂流主要在夜间进行,未发现有日漂者.漂流密度高峰出现在21:00和2:00,漂流密度分别为（70.3±10.8）和（289.0±124.6） 头·100 m^-3.大多数优势种类漂流高峰出现的时段略有不同,但有些优势种类（如摇蚊科和肖扁泥甲）并未表现出明显的漂流昼夜差异.在受酸性矿山废水污染的溪流中,漂流动物只有1种嗜酸性的多足摇蚊,其漂流活动也在夜间进行,并有3个明显的漂流高峰,分别出现在19:00、0:00和4:00,最高漂流密度仅为（6.7±5.2)头·100m^-3.说明酸性矿山废水不仅降低了溪流中漂流底栖动物的物种多样性和数量,也改变了其漂流模式.     

Importance of Terrestrial Arthropods as Subsidies in Lowland Neotropical Rain Forest Stream Ecosystems

The importance of terrestrial arthropods has been documented in temperate stream ecosystems, but little is known about the magnitude of these inputs in tropical streams. Terrestrial arthropods falling from the canopy of tropical forests may be an important subsidy to tropical stream food webs and could also represent an important flux of nitrogen (N) and phosphorus (P) in nutrient‐poor headwater streams. We quantified input rates of terrestrial insects in eight streams draining lowland tropical wet forest in Costa Rica. In two focal headwater streams, we also measured capture efficiency by the fish assemblage and quantified terrestrially derived N‐ and P‐excretion relative to stream nutrient uptake rates. Average input rates of terrestrial insects ranged from 5 to 41 mg dry mass/m²/d, exceeding previous measurements of aquatic invertebrate secondary production in these study streams, and were relatively consistent year‐round, in contrast to values reported in temperate streams. Terrestrial insects accounted for half of the diet of the dominant fish species, Priapicthys annectens. Although terrestrially derived fish excretion was found to be a small flux relative to measured nutrient uptake rates in the focal streams, the efficient capture and processing of terrestrial arthropods by fish made these nutrients available to the local stream ecosystem. This aquatic‐terrestrial linkage is likely being decoupled by deforestation in many tropical regions, with largely unknown but potentially important ecological consequences.     

Eating local: influences of habitat on the diet of little brown bats (Myotis lucifugus)

We employ molecular methods to profile the diet of the little brown bat, Myotis lucifugus, and describe spatial and temporal changes in diet over their maternity season. We identified 61 prey species of insects and 5 species of arachnid. The largest proportion of prey (～32%) were identified as species of the mass-emerging Ephemeroptera (mayfly) genus Caenis. Bats roosting in agricultural settings had lower dietary richness than those occupying a roost located on a forest fragment in a conservation area. We detected temporal fluctuations in diet over the maternity season. Dipteran (fly) species dominated the diet early in the season, replaced later by species of mayfly. Because our methodology provides species-level identification of prey, we were able to isolate environmental indicator species in the diet and draw conclusions about the location and type of their foraging habitat and the health of these aquatic systems. The species detected suggested that the bats use variable habitats; members of one agricultural roost foraged on insects originating in rivers or streams while those in another agricultural roost and the forest roost fed on insects from pond or lake environments. All source water for prey was of fair to good quality, though no species detected are intolerant of pollution thus the habitat cannot be classified as pristine. Our study outlines a model system to investigate the abiotic and biotic interactions between habitat factors through this simple food chain to the top predator.     

An enigmatic heteronemertean from the Gulf of Mexico

The effects of acid (HNO3) on drift and survival of benthic invertebrate communities were assessed in stream microcosms over a 7-day exposure period. Communities were obtained from the Cache la Poudre River, Colorado, using artificial substrates colonized in the stream for 30 days and then transferred to stream microcosms. Streams receiving the highest acid concentration (pH 4.0) contained significantly fewer individuals ( F = 378.42, p < 0.0001) and taxa ( F = 7.8, p = 0.0123) at the end of the experiment compared to the other two treatments (pH 5.5, 6.5) and the control (pH 7.4). Reduced macroinvertebrate abundance resulted primarily from reduced abundance of mayflies (Ephemeroptera) which were particularly sensitive. Comparisons of Plecoptera, Trichoptera, and Diptera abundances showed no statistically significant differences among treatments. Analysis of invertebrate drift samples, collected after 2, 6, 18, and 42 h exposure, revealed that percent drift in the most acidic streams was nine times that of control streams. Ephemeroptera was the only aquatic insect order to exhibit a significant drift response, and timing and magnitude of responses varied among mayfly taxa. Differences in sensitivity to acid among aquatic insect orders observed in our experimental streams were similar to those reported from field studies in other regions. Effects of acid on drift and survival of benthic invertebrate communities were also similar to effects of heavy metals, one of the primary water quality concerns in the Rocky Mountain region. These results suggest a general pattern of responses to chemical stressors in benthic communities from Rocky Mountain streams. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of the North Atlantic Oscillation on growth and phenology of stream insects

Climatic variation associated with the North Atlantic Oscillation (NAO) influences terrestrial and marine ecosystems, but its effects on river and stream ecosystems are less well known. The influence of the NAO on the growth of stream insects was examined using long-term empirical data on the sizes of mayfly and stonefly nymphs and on water temperature data. Models of egg development and nymphal growth in relation to temperature were used to predict the effect of the NAO on phenology. The study was based in two upland streams in mid-Wales UK that varied in the extent of plantation forestry in their catchments. Winter stream temperatures at both sites were positively related to the winter NAO index, being warmer in positive phases and colder in negative phases. The observed mean size and the simulated developmental period of mayfly nymphs were significantly related to the winter NAO index, with nymphs growing faster in positive phases of the NAO, but the growth of stonefly nymphs was not related to the NAO. This may have been due to the semivoltine stonefly lifecycle, but stonefly nymph growth is also generally less dependent on temperature. There were significant differences in growth rates of both species between streams, with nymphs growing more slowly in the forested stream that was consistently cooler than the open stream. Predicted emergence dates for adult mayflies varied by nearly two months between years, depending on the phase of the NAO. Variation in growth and phenology of stream insects associated with the NAO may influence temporal fluctuations in the composition and dynamics of stream communities.     

Reduction of predation risk under the cover of darkness: Avoidance responses of mayfly larvae to a benthic fish

Summary Mayfly larvae of Paraleptophlebia heteronea (McDunnough) had two antipredator responses to a nocturnal fish predator (Rhinichthys cataractae (Valenciennes)): flight into the drift and retreat into interstitial crevices. Drift rates of Paraleptophlebia abruptly increased by 30 fold when fish were actively foraging in the laboratory streams but, even before fish were removed, drift began returning to control levels because larvae settled to the substrate and moved to areas of low risk beneath stones. This drifting response was used as an immediate escape behavior which likely decreases risk of capture from predators which forage actively at night. Surprisingly, drift most often occurred before contact between predator and prey, and we suggest that in darkness this mayfly may use hydrodynamic pressure waves for predator detection, rather than chemical cues, since fish forage in an upstream direction. Although drifting may represent a cost to mayfly larvae in terms of relocation to a new foraging area with unknown food resources, the immediate mortality risk probably out-weighs the importance of staying within a profitable food patch because larvae can survive starvation for at least 2 d. In addition to drifting, mayflies retreated from upper, exposed substrate surfaces to concealed interstitial crevices immediately after a predator encounter, or subsequent to resettlement on the substrate after predator-induced drift. A latency period was associated with this response and mayflies remained in these concealed locations for at least 3 h after dace foraging ceased. Because this mayfly feeds at night and food levels are significantly lower in field refugia under stones, relative to exposed stone surfaces, predator avoidance activity may limit foraging time and, ultimately, reduce the food intake of this stream mayfly.     

Linking stream habitats and spider distribution: spatial variations in trophic transfer across a forest–stream boundary

In headwater streams, many aquatic insects rely on terrestrial detritus, while their emergence from streams often subsidizes riparian generalist predators. However, spatial variations in such reciprocal trophic linkages remain poorly understood. The present study, conducted in a northern Japanese stream and the surrounding forest, showed that pool–riffle structure brought about heterogeneous distributions of detritus deposits and benthic aquatic insects. The resulting variations in aquatic insect emergence influenced the distributions of riparian web-building spiders. Pools with slow current stored greater amounts of detritus than riffles, allowing more benthic aquatic insects to develop in pools. The greater larval biomass in pools and greater tendency for riffle insects to drift into pools at metamorphosis resulted in an emergence rate of aquatic insects from pools that was some four to five times greater than from riffles. In the riparian forest, web-building spiders (Tetragnathidae and Linyphiidae) were distributed in accordance with the emergence rates of aquatic insects, upon which both spider groups heavily depended. Consequently, the riparian strips bordering pools had a density of tetragnathid spiders that was twice as high as that of the riparian strips adjacent to riffles. Moreover, although limitations of vegetation structure prevented the aggregation of linyphiid spiders around pools, linyphiid density normalized by shrub density was higher in habitats adjacent to pools than those adjacent to riffles. The results indicated that stream geomorphology, which affects the storage of terrestrial organic material and the export of such material to riparian forests via aquatic insect emergence, plays a role in determining the strength of terrestrial–aquatic linkages in headwater ecosystems.     

Effects of trout on the diel periodicity of drifting in baetid mayflies

Some benthic invertebrates in streams make frequent, short journeys downstream in the water column (=drifting). In most streams there are larger numbers of invertebrates in the drift at night than during the day. We tested the hypothesis that nocturnal drifting is a response to avoid predation from fish that feed in the water column during the day. We surveyed diel patterns of drifting by nymphs of the mayfly Baetis coelestis in several streams containing (n=5) and lacking (n=7) populations of rainbow trout, Oncorhynchus mykiss. Drifting was more nocturnal in the presence of trout (85% of daily drift occurred at night) than in their absence (50% of daily drift occurred at night). This shift in periodicity is due to reduced daytime drifting in streams with trout, because at a given nighttime drift density, the daytime drift density of B. coelestis was lower in streams occupied by trout than in troutless streams. Large size classes of B. coelestis were underrepresented in the daytime drift in trout streams compared to nighttime drift in trout streams, and to both day and night drift in troutless streams. Differences in daytime drift density between streams with and without trout were the result of differences in mayfly drift behaviour among streams because predation rates by trout were too low to significantly reduce densities of drifting B. coelestis. We tested for rapid (over 3 days) phenotypic responses to trout presence by adding trout in cages to three of the troutless streams. Nighttime drifting was unaffected by the addition of trout, but daytime drift densities were reduced by 28% below cages containing trout relative to control cages (lacking trout) placed upstream. Drift responses were measured 15 m downstream of the cages suggesting that mayflies detected trout using chemical cues. Overall, these data support the hypothesis that infrequent daytime drifting is an avoidance response to fish that feed in the water column during the day. Avoidance is more pronounced in large individuals and is, at least partially, a phenotypic response mediated by chemical cues.     

Factors affecting the distribution and abundance of Cloeon and Caenis (Ephemeroptera) larvae in a tropical impounded river, Nigeria

Impoundment of rivers affects the mayfly (Ephemeroptera) fauna inhabiting such water bodies, especially with respect to their distribution and abundance. A two‐year study of the mayfly fauna and some of the physicochemical parameters of the Opa stream–reservoir revealed that there are two mayfly genera inhabiting it, Cloeon and Caenis. The number of Cloeon larvae collected was 10,930 while the number of Caenis larvae was 450. It was observed that although both genera occurred at all the sampling stations, their numbers were reduced in the stream below the dam due to increased water current velocity. There were significant differences in the abundance of Cloeon among the stations, but none for Caenis. Submerged aquatic plants and water current velocity were found to be the major factors responsible for the significant differences. There were significant correlations between the number of Cloeon larvae and pH as well as between dissolved oxygen concentration and Caenis. These findings are discussed with reference to inter‐specific differences in patterns of response to environmental parameters. A species‐specific approach is suggested for studies on the strategies that enable mayfly species maintain their populations in stressed and unstable aquatic ecosystems.     

Food partitioning between coexisting Atlantic salmon and brook trout in the Sainte‐Marguerite River ecosystem, Quebec

Food resource partitioning between similar‐sized, sympatric Atlantic salmon Salmo salar and brook trout Salvelinus fontinalis was examined as a possible mechanism enabling their coexistence in a stream (Allaire) of the Sainte‐Marguerite River ecosystem, Quebec, Canada. Fish stomach contents and invertebrate drift were collected concurrently during three diel cycles in August to September 1996. The food and feeding habits of an allopatric brook trout population in a nearby stream (Epinette) were studied for comparison. The diel feeding rhythms of the two coexisting fish species were similar. The composition of their diet, however, showed significant differences. Atlantic salmon predominantly (60–90%) fed on aquatic insects, mainly Ephemeroptera (35–60% of the diet). The brook trout mostly (50–80%) fed upon the allochthonous terrestrial insects (mainly adults of Coleoptera, Hymenoptera and Diptera) which comprised 5–40% of the stream drift. The allopatric brook trout fed opportunistically on the more abundant aquatic insects and terrestrial insects rarely formed 25% of its diet. The allopatric trout fed nearly twice as much as the sympatric brook trout during a day. The results suggest that the differences in feeding by brook trout in the two streams (with and without Atlantic salmon) are the result of inter‐specific interaction with Atlantic salmon and are not related to the differences in food availability between the two streams. Food resource partitioning between Atlantic salmon and brook trout may be viewed as an adaptive response resulting in a greater exploitation of available resources and coexistence.     

The lateral extent of the subsidy from an upland stream to riparian lycosid spiders

Adult aquatic insects emerging from streams can subsidize riparian food webs, but little is known of the spatial extent of these subsidies. Stable isotope (¹⁵N) enrichment of aquatic insects, principally a species of stonefly (Plecoptera: Leuctridae), emerging from an upland stream was used to trace the subsidy from the stream ecosystem to riparian spiders (Lycosidae). The downstream profile of spider δ¹⁵N correlated closely with that of adult stoneflies, indicating that they were deriving nutrition from aquatic sources. The contribution of adult aquatic insects to spider diets was determined using a two-source mixing model. Adult aquatic insects made up over 40% of spider diets adjacent to the stream, but <1% at 20 m from the stream. Enrichment of riparian spiders declined exponentially with distance from the stream channel. Aquatic-terrestrial subsidies were spatially restricted, but locally important, to riparian lycosid spiders at the study site.     

Night — day drift patterns and the size of larvae of two aquatic insects

A test of Allan's (1978) hypothesis about differential drift abundance of mayflies of the genus Baetis between night and day, and the size of larvae was performed at a mountain stream in Idaho. Palisades Creek, Idaho, contains a different species of mayfly, B. tricaudatus, and vertebrate predator, Salmo clarki, than Cement Creek, Colorado (B. bicaudatus, and brook trout, Salvelinus fontinalis). Consequently it was not known if B. tricaudatus would exhibit a similar pattern as its congener in Cement Creek, with large instars tending to avoid daylight drift, as found by Allan (1978). However, similar results were observed in the present study. It appears that the earlier hypothesis may have generality for geographically distinct streams with a different vertebrate predator and mayfly prey. The existence of a similar pattern for chironomid larvae was also tested, however, no such pattern existed. This discrepency between taxa may be due to differential predation, or to inherent differences in drift abilities.     

Long-term effects of local disturbance history on mobile stream invertebrates

The patchy distribution of benthic invertebrates in streams and rivers is an important and widely researched phenomenon. Previous studies on reasons for this patchiness have neglected the potential role of local disturbance history, probably because most lotic invertebrates are mobile and any effect of disturbance history was thought to be short-lived. Here we demonstrate for a New Zealand gravel-bed stream that local disturbance history can have long-term effects on the distribution of highly mobile stream invertebrates. Buried scour chains (100 at each of three 20-m sites within a 350-m reach) indicated that a spate with a return period of 5 months caused a mosaic of bed patches with different stabilities. More than 2 months after the spate, we took random, quantitative samples at each site from five patches that had experienced 4 cm or more of scour during the spate, from five patches with 4 cm or more of fill, and from five stable patches. Density of the dominant invertebrate taxon, the highly mobile mayfly Deleatidium spp., and densities of another three of the seven most common taxa differed significantly between patch stability categories. Larvae of Deleatidium, the black fly Austrosimulium spp. and the dipteran Eriopterini were most abundant in fill patches, whereas Isopoda were most abundant in scour patches. Total invertebrate densities and densities of six common taxa also differed between sites, although these were only 95–120 m apart. These results show that local disturbance history can have long-term effects on lotic invertebrates and be an important cause of invertebrate patchiness. The observed effects might have been even stronger had we sampled sooner after the spate or after a large flood. Disturbance history may influence invertebrates both directly (through dislodgement or mortality) and indirectly, through effects on the spatial distribution of their resources. Our results suggest that the role of disturbance in structuring animal communities dominated by mobile species may be more important than previously thought. Received: 25 January 2000 / Accepted: 14 April 2000