Diel vertical movements by lotic mayfly nymphs under variable predation risk

1. The diel foraging periodicities of two grazing mayfly (Ephemeroptera) nymphs, Heptagenia dalecarlica and Baetis rhodani, under variable fish (European minnow) predation risk were examined in a series of laboratory experiments. 2. Heptagenia dalecarlica were almost exclusively nocturnal in their use of feeding areas on stone tops. There was a sharp increase in the proportion of nymphs out of refuge at nightfall, both in the control and fish-odour treatments. In a treatment with freely moving fish, H. dalecarlica decreased their activity during both the day and night. In feeding trials with three freely foraging minnows, nymphs were completely safe when they had access to refuges beneath stones, whereas almost half the nymphs were consumed when no refuges were available. 3. Baetis rhodani nymphs reduced their use of stone tops when exposed to four caged minnows or a freely moving fish, but this occurred both day and night. In feeding trials, B. rhodani were captured only while in the water column, and their mortality risk was independent of refuge availability. 4. It is suggested that because H. dalecarlica lack efficient post-encounter defences, they must rely on pre-encounter mechanisms to reduce the threat of fish predation. It appears that in order to dwell sympatrically with fish, H. dalecarlica have evolved a coexistence by hiding strategy. In contrast, Baetis are vulnerable to fish attacks only if they enter drift in daylight, and are thus able to remain on stone tops both during the day and at night.     

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.     

Utilization of autochthonous macroinvertebrate drift by a pool fish community in a woodland stream

Qualitative and quantitative drift data were collected simultaneously above and below a pool both before and after the pool was heavily electrofished. These data revealed no significant difference between drift organism densities (#/m³) above or below the pool before or after fish collection. Qualitative and quantitative fish gut analyses suggest that the sunfishes Lepomis megalotis, L. macrochirus and L. cyanellus may be feeding on drifting invertebrates as these organisms comprised 58%, 37% and 35% gut volume, respectively. A design for a long-term drift net apparatus is presented.     

Diel drift of Chironomidae larvae in a pristine Idaho mountain stream

Simultaneous hourly net collections in a meadow and canyon reach of a mountain stream determined diel and spatial abundances of drifting Chironomidae larvae. Sixty-one taxa were identified to the lowest practical level, 52 in the meadow and 41 in the canyon. Orthocladiinae was the most abundant subfamily with 32 taxa and a 24 h mean density of 294 individuals 100 m⁻³ (meadow) and 26 taxa and a mean of 648 individuals 100 m⁻³ (canyon). Chironominae was the second most abundant subfamily. Nonchironomid invertebrates at both sites and total Chironomidae larvae (meadow) were predominantly night-drifting. Parakiefferiella and Psectrocladius were day-drifting (meadow) whereas 8 other chironomid taxa (meadow) and 2 taxa (canyon) were night-drifting. All others were aperiodic or too rare to test periodicity, Stempellinella cf brevis Edwards exhibited catastrophic drift in the canyon only. The different drift patterns between sites is attributed to greater loss of streambed habitat in the canyon compared to the meadow as streamflow decreased. Consequent crowding of chironomid larvae in the canyon caused catastrophic drift or interfered with drift periodicty. This study adds to knowledge of Chironomidae drift and shows influences on drift of hydrologic and geomorphic conditions.     

Predation on mayfly nymph, Baetis rhodani, by native and introduced Gammarus: direct effects and the facilitation of predation by salmonids

1. In a series of laboratory experiments, we assessed the predatory nature of the native Irish amphipod, Gammarus duebeni celticus , and the introduced G. pulex , towards the mayfly nymph Baetis rhodani . We also investigated alterations in microhabitat use and drift behaviour of B . rhodani in the presence of Gammarus , and indirect predatory interactions with juvenile Atlantic salmon, Salmo salar .
2. In trials with single predators and prey, B. rhodani survival was significantly lower when Gammarus were free to interact with nymphs as than when Gammarus were isolated from them. The invader G. pulex reduced the survival of B. rhodani more rapidly than did the native G. d. celticus . Both Gammarus spp. were active predators.
3. In `patch' experiments, B. rhodani survival was significantly lower both when G. pulex and G. d. celticus were present, although the effect of the two Gammarus species did not differ. Again, active predation of nymphs by Gammarus was observed. Significantly more nymphs occurred on the top and sides of a tile, and per capita drifts were significantly higher, when Gammarus were present. Baetis rhodani per capita drift was also significantly higher in the presence of the introduced G. pulex than with the native G. d. celticus .
4.  Gammarus facilitated predation by salmon parr of B. rhodani by significantly increasing fish–nymph encounters on exposed gravel and in the drift. There were no differential effects of the two Gammarus spp. on fish – B . rhodani encounters or consumption.
5. We conclude that Gammarus as a predator can have lethal, nonlethal, direct and indirect effects in freshwaters. We stress the need for recognition of this predatory role when assigning Gammarus spp. to a `Functional Feeding Group'.     

Invertebrate drift and benthic community dynamics in a lowland neotropical stream, Costa Rica

In this study we quantified invertebrate drift and related it to the structure of the benthic community, over a 6–8 month period, in a 4th-order tropical stream in Costa Rica. Relative to reports from similar-sized temperate and tropical streams, drift densities were high (2-fold greater: mean 11.2 m⁻³; range 2.5–25 m⁻³), and benthic insect densities were relatively low (>3-fold lower: mean 890 m⁻²; range 228–1504 m⁻²). Drift was dominated by larval shrimps that represented more than 70% of total drift on any given date; the remaining 30% was composed of 54 insect taxa. Among insects, Simuliidae and Chironomidae (Diptera) and Baetidae, Leptohyphes and Tricorythodes (Ephemeroptera) comprised 24% of total drift. Drift periodicity was strongly nocturnal, with peaks at 18:00 h (sunset) and 03:00 h. Our results, and those of previous experiments in the study stream, suggest that nighttime drift is driven by the presence of predatory diurnal drift-feeding fishes and nocturnal adult shrimps. There were no clear seasonal patterns over both ‘dry’ and wet seasons, suggesting that benthic communities are subject to similar stresses throughout the year, and that populations grow and reproduce continuously. This revised version was published online in July 2006 with corrections to the Cover Date.     

Drift dispersal of mayfly nymphs in the presence of chemical and visual cues from diurnal drift- and nocturnal benthic-foraging fishes

1. The drift of Baetis thermicus nymphs in the presence of chemical, visual and hydrodynamic cues, considered individually and in combination, produced by different predatory fishes was examined experimentally in laboratory streams. Masu salmon ( Oncorhynchus masou ) and freshwater sculpin ( Cottus nozawae ) are typical drift- and benthic-foraging fishes, respectively.
2. Observations of fish swimming in the streams revealed differing diel periodicity between the species; sculpin were nocturnal foragers and salmon diurnal.
3. The drift rate of Baetis by night increased in the presence of chemical cues from sculpin, with other cues having no interactive effects. In contrast, the drift rate increased primarily in the presence of both chemical and, particularly, visual cues from salmon, although no additional effects were found for any non-visual cues. Visual cues could enable Baetis to assess precisely the predation risk from foraging salmon by day, whereas Baetis could not use visual cues to detect sculpin either at night, because of the low light intensity, or during the day, because of the low activity of sculpin at that time.
4. In natural streams, which are often inhabited by several predatory fish employing different modes of foraging, invertebrates may be able to precisely assess the risk and effectively to avoid predators by using cues unique to each.     

Diel drift of Chironomidae in a large lowland river (Central Poland)

The diel drift patterns of Chironomidae larvae were investigated in a seventh order section of the Warta River (Central Poland) over two diel cycles during May 1989. Three nets (mesh size 400 m) were installed in a cross section of the Warta River.The estimated drift density was low, but was comparable to that calculated for other large rivers. Spatio-temporal fluctuations in abundance and composition of macroinvertebrate drift, including Chironomidae, were observed with the highest density of drifting macrobenthos recorded near the depositional bank of this river. The ratio benthosdrift indicated differing propensities for of the older instars of a given chironomid taxon to drift. Orthocladiinae larvae were the most abundant subfamily of Chironomidae in drift but not in benthos, reaching up to 73% of the total drifting chironomid larvae. More taxa but fewer individuals (about 20% of the chironomid larvae collected) belonged to the tribe Chironomini, the dominant group in benthos.A major part of chironomid drift collection may represent behavioural drift because the net mesh size used in the Warta River was insufficient to catch the earliest instars (distributional drift). Both at the family and subfamily level chironomid larvae exhibited a distinct nocturnal drift periodicity. Nocturnal periodicity was documented for the dominant species, but due to the low density of many chironomid species, it was impossible to determine their diel drift pattern. Some Chironomidae appeared to be aperiodic.     

Upstream-downstream movements of aquatic invertebrates in a Rocky Mountain stream

Simultaneous collections of drift and organisms moving either upstream or downstream in association with the substrate were made using a specially designed sampler. Samples were taken in a diel series along a transect across the study riffle of a Colorado foothills stream on six dates over an annual cycle. In addition to longitudinal movements, taxonomic composition and diel periodicity were evaluated. The insect-dominated fauna showed a net downstream displacement. Only the caddisflies Helicopsyche borealis and Hesperophylax occidentalis exhibited net upstream movement, primarily a result of low drift frequencies. The taxonomic composition of moving invertebrates differed from that of the benthos. Drift resembled downstream moving substrate-associated invertebrates in composition, but differed from that of the upstream directed fauna. Taxa collectively exhibited four types of diel patterns: 1) similar downstream (drift and substrate-associated movements) patterns, which generally differed from the upstream pattern; 2) similar benthic (upstream and downstream) patterns, which differed from that of drift; 3) aperiodic patterns; and 4) independent patterns for each type of directional movement. Analysis of size classes based on head capsule width for the mayfly Baetis tricaudatus showed significantly smaller size in stationary individuals compared with moving individuals in the population and revealed that nymphs moving during the day were smaller than those moving at night.     

Spatial and temporal patterns of invertebrate drift in streams draining a Neotropical landscape

1. Invertebrate drift in streams draining a tropical landscape in Costa Rica was studied to assess differences in assemblage composition above and below a major gradient break in geomorphic landform and to assess temporal patterns of drift in lowland reaches below the gradient break. The gradient break (～50 m a.s.l.) is the point at which the foothills of the Costa Rican Cordillera Central (piedmont) merge with the Caribbean coastal plain (lowlands).
2. Spatial patterns were assessed along two streams by sampling drift over 24 h once a month for 3 months in both the piedmont (90 m a.s.l.) and lowlands (30 m a.s.l.). Temporal patterns of drift were assessed through monthly diel sampling of three lowland sites over 8–10 months, encompassing both ‘dry’ (<400 mm precipitation per month, November to May) and wet (July to October) seasons.
3. Drift composition was insect dominated in piedmont sites and larval shrimp dominated in the lowlands. Percent similarity of assemblages between piedmont and lowland sites was low (range 26–43%) because of high larval shrimp densities in lowland versus piedmont sites.
4. Drift densities were higher during night than day, with peaks at sunset on all dates and at all sites. Diel patterns in drift agree with previous observations for the study area and support the ‘risk of predation’ hypothesis.
5. Analysis of monthly patterns in lowland sites showed high variability in drift densities; however, all major taxa were found every month. Overall, there was a trend for high invertebrate densities during the ‘dry’ season, but these trends were not significant.
6. Observed changes in drift composition support the concept of river zonation, which predicts a change in community composition along the stream continuum due to geomorphic features. Drift at lowland sites below the gradient break was dominated by shrimps, which are linked to marine environments via their migratory behaviour.     

Relationship between the drift of macroinvertebrates and the activity of brown trout in a small stream

Brown trout Salmo trutta were most active in a small stream at night, dusk and dawn when drift rate was highest, but correlations between hourly drift rates and the trout's activity varied substantially between individuals, between different dates for a single individual, and between different periods of the daily cycle. On some occasions, the trout were responsive to the total drift rate, either at night or during the day, and on others to the largest drifting organisms only (terrestrial organisms, adults of Ephemeroptera, Diptera and Trichoptera). The study supports the idea that trout adapt their activity pattern to the abundance of drifting prey, either as generalists towards any organism, or as specialists towards the largest ones.     

Life-history responses of a mayfly to seasonal constraints and predation risk

Abstract. 1. An experiment was conducted in the laboratory to examine the effects of photoperiod and predation risk on life-history variation in the mayfly Ephemerella subvaria .
2. Both photoperiod and predation risk affected age at maturity significantly but neither factor affected size at maturity. Mayflies perceiving themselves to be late in the growing season matured in fewer days than those perceiving themselves to be early in the growing season. The presence of predators delayed mayfly maturity significantly.
3. These results suggest that the large variation in life-history traits observed in aquatic insects may be attributed partially to seasonality but that other biotic and abiotic factors may also underlie variation in these traits.     

Quantifying insect predation with predator exclusion cages: the role of prey antipredator behavior as a source of bias

There are limitations imposed by current methodologies to detect and quantify insect predation. However, there has been relatively little effort to experimentally document the sources of biases associated with the various methodologies. In this study, we examined how predation estimates in the field using predator exclusion cages may be biased when one fails to account for antipredator behavioral responses. To do this, we did the usual comparison of the number of insects missing from plants where predators were allowed access to the number missing from plants where predators were excluded, but also determined how many of the missing insects reacted to predators by dropping from plants and how many were actually preyed upon. Our results provide evidence that estimates of insect mortality in the field are significantly reduced if prey antipredator behavior is taken into account. As it is commonly assumed that prey missing in the field are predated, documenting the incidence of predator‐mediated ‘disappearance’ and capturing insect prey before they escape can provide with a relevant estimate of bias.     

Epigenetic gambling and epigenetic drift as an antagonistic pleiotropic mechanism of aging

Generations of biogerontologists have been puzzled by the marked intraspecific variations in lifespan of their experimental model organisms despite all efforts to control both genotype and environment. The most cogent example comes from life table studies of wild‐type Caenorhabditis elegans when grown in suspension cultures using axenic media. While nuclear and mitochondrial somatic mutations and ‘thermodynamic noise’ likely contribute to such lifespan variegations, I raise an additional hypothetical mechanism, one that may have evolved as a mechanism of phenotypic variation which could have preceded the evolution of meiotic recombination. I suggest that random changes in cellular gene expression (cellular epigenetic gambling or bet hedging) evolved as an adaptive mechanism to ensure survival of members of a group in the face of unpredictable environmental challenges. Once activated, it could lead to progressive epigenetic variegation (epigenetic drift) amongst all members of the group. Thus, while particular patterns of gene expression would be adaptive for a subset of reproductive individuals within a population early in life, once initiated, I predict that continued epigenetic drift will result in variable onsets and patterns of pathophysiology – perhaps yet another example of antagonistic pleiotropic gene action in the genesis of senescent phenotypes. The weakness of this hypothesis is that we do not currently have a plausible molecular mechanism for the putative genetic ‘randomizer’ of epigenetic expression, particularly one whose ‘setting’ may be responsive to the ecology in which a given species evolves. I offer experimental approaches, however, to search for the elusive epigenetic gambler(s).     

Nocturnal hypothermia impairs flight ability in birds: a cost of being cool

Many birds use regulated drops in night-time body temperature (T_b) to conserve energy critical to winter survival. However, a significant degree of hypothermia may limit a bird''s ability to respond to predatory attack. Despite this likely energy–predation trade-off, the behavioural costs of avian hypothermia have yet to be examined. We thus monitored the nocturnal hypothermia of mourning doves (Zenaida macroura) in a laboratory setting in response to food deprivation. Nocturnal flight tests were used to quantify the flight ability of hypothermic doves. Many hypothermic doves (39% of tests) could not fly while carrying a small weight, but could do so after quickly warming up to typical daytime T_b. Doves that were unable to fly during their first test were more hypothermic than those that could fly, with average T_b reductions of 5.3°C and 3.3°C, respectively, but there was no overall indication of a threshold T_b reduction beyond which doves were consistently incapable of flight. These results suggest that energy-saving hypothermia interferes with avian antipredator behaviour via a reduction in flight ability, likely leading to a trade-off between energy-saving hypothermia and the risk of predation.     

Effectiveness of the caudal spine as a defense mechanism in Keratella cochlearis

The temporal distribution of spined and unspined forms of Keratella cochlearis in Cubillas reservoir during an annual cycle, and the stomach contents of Asplanchna girodi, were studied. The results suggest that the length of the caudal spine plays a critical role in the defense against predation by Asplanchna girodi. Short-spined forms showed a higher susceptibility to predation than unspined forms, which may have important consequencies for spine length variation in Keratella cochlearis populations.     

Foraging behaviour of non-breeding Western Sandpipers Calidris mauri as a function of sex, habitat and flocking

Individuals within a population may vary considerably in the way they exploit available food resources. If the sexes differ in the size of their feeding apparatus, there can be differences in foraging behaviour and habitat use, hence one sex may be more susceptible to competition. We examined relationships between sexual dimorphism in bill size and foraging behaviour, and habitat and microhabitat use of non-breeding Western Sandpipers Calidris mauri at Bahía Santa María, northwestern Mexico. Western Sandpipers are sexually dimorphic, with females about 15% longer-billed than males. Males used a higher proportion of pecks, had a lower probing–pecking rate, walked at higher rates, foraged at sites with lower water content, and had greater variation in foraging technique than females. Moreover, males decreased their proportion of pecks and foraged at a higher rate than females when they changed from feeding alone to feeding in flocks, suggesting a greater safety advantage or susceptibility to conspecific interference when flock feeding. We compared behaviour and microhabitat usage in three habitats: brackish flats, mangroves, and cattail marshes. Sex-related differences in foraging behaviour and microhabitat use were consistent among habitats. Birds in brackish flats and mangroves used a higher proportion of pecks, foraged at lower rates and walked at higher rates, and foraged at deeper sites, with a lower proportion of water cover, than those in cattail marshes. Sex-related differences in foraging behaviour and microhabitat should reduce the level of competition between sexes, and may account for sex differences in Western Sandpiper distribution observed between habitats in Bahía Santa María.     

Testing hypotheses about the function of repeated nest abandonment as a life history strategy in a passerine bird

Nest structures are essential for successful reproduction in most bird species. Nest construction costs time and energy, and most bird species typically build one nest per breeding attempt. Some species, however, build more than one nest, and the reason for this behaviour is often unclear. In the Grey Fantail Rhipidura albiscapa, nest abandonment before egg‐laying is very common. Fantails will build up to seven nests within a breeding season, and pairs abandon up to 71% of their nests before egg‐laying. We describe multiple nest‐building behaviour in the Grey Fantail and test four hypotheses explaining nest abandonment in this species: cryptic depredation, destruction of nests during storm events, and two anti‐predatory responses (construction of decoy nests to confuse predators, and increasing concealment to ‘hide’ nests more effectively). We found support for only one hypothesis – that abandonment is related to nest concealment. Abandoned nests were significantly less concealed than nests that received eggs. Most abandoned nests were not completely built and none received eggs, thus ruling out cryptic predation. Nests were not more likely to be abandoned following storm events. The decoy nest hypothesis was refuted as abandoned nests were constructed at any point during the breeding season and some nests were dismantled and the material used to build the subsequent nest. Thus, Grey Fantails are flexible about nest‐site locations during the nest‐building phase and readily abandon nest locations if they are found to have deficient security.