Nocturnal drift of mayfly nymphs as a post-contact antipredator mechanism

1. The predominantly nocturnal constrained drift of stream invertebrates is commonly regarded as a behaviour that avoids encounters with visually foraging fish in the water column. The alternative explanation, that drift peaks are caused by bottom-feeding, nocturnal predators, has rarely been tested.
2. We examined these hypotheses by collecting invertebrate drift in five streams in northern Finland: one with brown trout ( Salmo trutta , a drift-feeding fish), one with alpine bullhead ( Cottus poecilopus , a benthic fish), one with both species, and two fishless streams.
3. Drift by Baetis mayflies was aperiodic or slightly diurnal in both fishless streams on all sampling occasions. In contrast, drift was nocturnal in streams with trout and, to a lesser extent, in the stream with bullhead. Non-dipteran prey drifted mainly nocturnally in all streams with fish, whereas Diptera larvae were less responsive to the presence of fish.
4. In laboratory experiments, bullheads were night-active, causing a much higher frequency of drift by touching Baetis at night than during the day. Thus, increased nocturnal drift may serve to avoid both visual predators (a pre-contact response) and benthic fish (a post-contact response). In streams with bottom-feeding fish, nocturnal drift should be caused by increased drift by night rather than by reduced drift by day.     

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.     

Historical influence of predation pressure on escape by Podarcis lizards in the Balearic Islands

Island tameness (reduced escape behaviour on islands where prey have experienced prolonged relaxation of predation pressure) is known in several taxa, although the relationships between recent predation pressure and escape on islands are poorly known. We investigated escape by numerous populations exposed to differing predation pressure of two sister species of Podarcis lizards in the Balearic Islands. Our main findings are that flight initiation distance was greater in Podarcis pityusensis than Podarcis lilfordi and increased as predation pressure increased in P. pityusensis. Island tameness led to extinction of P. lilfordi on Menorca and Mallorca following anthropogenic introduction of predators; this species is extant only on nearby islets. The lack of relationship between recent predation pressure and flight initiation distance in P. lilfordi indicates that the historically acquired deficit in the ability to adjust escape behaviour to predation pressure still exists. Podarcis pityusensis, which was exposed to greater natural predation pressure before human introduction of predators, survives on Ibiza and Formentera, as well as on islets. Retention of the ability to respond to predation pressure is consistent with our finding that flight initiation distance increases as predation pressure increases among current populations. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Predator detection and avoidance by starlings under differing scenarios of predation risk

Practically all animals must find food while avoiding predators.An individual's perception of predation risk may depend on manyfactors, such as distance to refuge and group size, but it isunclear whether individuals respond to different factors ina similar manner. We tested whether flocks of foraging starlingsresponded in the same way to an increased perception of predationrisk by assessing three factors: (1) neighbor distances, (2)habitat obstruction, and (3) recent exposure to a predator.We found that in all three scenarios of increased risk, starlingsreduced their interscan intervals (food-searching bouts), whichincreased the frequency of their vigilance periods. We thenexamined how one of these factors, habitat obstruction, affectedescape speed by simulating an attack with a model predator.Starlings were slower to respond in visually obstructed habitats(long grass swards) and slower when they had their head downin obstructed habitats than when they had their head down inopen habitats. In addition, reaction times were quicker whenstarlings could employ their peripheral fields of vision. Ourresults demonstrate that different sources of increased riskcan generate similar behavioral responses within a species.The degree of visibility in the physical and social environmentaffects both the actual and perceived risk of predation.     

Mortality by moonlight: predation risk and the snowshoe hare

Optimal behavior theory suggests that prey animals will reduceactivity during intermittent periods when elevated predationrisk outweighs the fitness benefits of activity. Specifically,the predation risk allocation hypothesis predicts that preyactivity should decrease dramatically at times of high predationrisk if there is high temporal variation in predation risk butshould remain relatively uniform when temporal variation inpredation risk is low. To test these predictions we examinedthe seasonably variable response of snowshoe hares to moonlightand predation risk. Unlike studies finding uniform avoidanceof moonlight in small mammals, we find that moonlight avoidanceis seasonal and corresponds to seasonal variation in moonlightintensity. We radio-collared 177 wild snowshoe hares to estimatepredation rates as a measure of risk and used movement distancesfrom a sample of those animals as a measure of activity. Inthe snowy season, 5-day periods around full moons had 2.5 timesmore predation than around new moons, but that ratio of theincreased predation rate was only 1.8 in the snow-free season.There was no significant increase in use of habitats with morehiding cover during full moons. Snowshoe hares' nightly movementdistances decreased during high-risk full-moon periods in thesnowy season but did not change according to moon phase in thesnow-free season. These results are consistent with the predationrisk allocation hypothesis.     

Patch use, apprehension, and vigilance behavior of Nubian Ibex under perceived risk of predation

Foraging theory predicts that animals will sacrifice feedingeffort in order to reduce predation risk. Once a forager choosesa habitat, it must decide how to allocate its foraging effort.Nubian Ibex are diurnal, social, cliff-dwelling herbivores.Many of their characteristics seem to have evolved as responsesto predation risk. In order to assess the effects that perceivedrisk of predation might have on foraging behavior of free-rangingNubian Ibex in the Negev Desert, Israel, we measured giving-updensities (GUDs) in artificial food patches and used them togauge apprehension level. (Apprehension can be defined as areduction in attention devoted to performing an activity asa consequence of reallocating attention to detecting or respondingto predation risk. A forager can also be vigilant. Vigilanceis often defined as time spent scanning the surroundings withthe head up.) We also quantified time budgeting using focalobservation of individual Nubian Ibex. Habitat preferences andpatch selectivity as a measure of apprehension were considered.In particular, we tested the effect of distance from refugeon GUDs, the effect of micropatch structure on selectivity,and the effect of distance from the refuge and group size onNubian Ibex vigilance level and apprehension. Nubian Ibex allocatetheir foraging effort more toward patches closer to the escapeterrain. At the same time, Nubian Ibex are more apprehensiveat intermediate distances from the cliff edge than nearer thecliff, and their use of vigilance increases with distance fromthe cliff edge. These results suggest that Nubian Ibex may switchfrom apprehension to a more extreme behavior of vigilance atgreater distances from the refuge. This study demonstrated theuse of antipredatory behaviors, apprehension, and vigilanceby a forager. Estimating apprehension and vigilance levels ofa forager simultaneously gives a more complete and accuratepicture of how the habitat is perceived by them and combinedwith measurements of GUD allow a more accurate assessment ofhabitat quality.     

Human activities change marine ecosystems by altering predation risk

In ocean ecosystems, many of the changes in predation risk – both increases and decreases – are human‐induced. These changes are occurring at scales ranging from global to local and across variable temporal scales. Indirect, risk‐based effects of human activity are known to be important in structuring some terrestrial ecosystems, but these impacts have largely been neglected in oceans. Here, we synthesize existing literature and data to explore multiple lines of evidence that collectively suggest diverse human activities are changing marine ecosystems, including carbon storage capacity, in myriad ways by altering predation risk. We provide novel, compelling evidence that at least one key human activity, overfishing, can lead to distinct, cascading risk effects in natural ecosystems whose magnitude exceeds that of presumed lethal effects and may account for previously unexplained findings. We further discuss the conservation implications of human‐caused indirect risk effects. Finally, we provide a predictive framework for when human alterations of risk in oceans should lead to cascading effects and outline a prospectus for future research. Given the speed and extent with which human activities are altering marine risk landscapes, it is crucial that conservation and management policy considers the indirect effects of these activities in order to increase the likelihood of success and avoid unfortunate surprises.     

Reproductive investment under fluctuating predation risk: Microtine rodents and small mustelids

Summary We studied the reproductive investment of microtine rodents (bank vole (Clethrionomys glareolus),Microtus epiroticus andMicrotus agrestis) in western Finland under predation risk from small mustelids. During 1984–1992, the yearly mean litter size of overwintered bank voles was smaller at high least weasel and stoat densities than at low densities (close to 3 versus 4–5). In addition, the annual mean litter size of young bank voles was negatively correlated to the least weasel density. In youngM. agrestis voles, the yearly late summer litter size was negatively associated with the autumn density of small mustelids. In the crash phase of the vole cycle (1989 and 1992), we removed small mustelids (mainly least weasels) from four unfenced areas in late April to late May and studied the reproduction of voles in four removal and comparable control areas (each 2–4 km²). Reduction of small mustelids significantly increased the proportion of pregnant bank vole females, but not that of pregnantMicrotus vole females. We conclude that predation risk apparently reduced reproductive investment of free-living bank vole females; these voles appear to trade their current parental investment against future survival and reproductive prospects. Accordingly, the presence of small mustelids (or their scent) may slow down the reproductive rate of voles. As antipredatory behaviours occurred on a large scale, our results add evidence to the hypothesis that crashes in multiannual vole cycles are driven by small mustelid predators.     

Perch selection by singing chaffinches: a better view of surroundings and the risk of predation

Predation risk varies among perches, and the vulnerability ofsinging chaffinches (Fringilla coelebs) might differ dependingon where they perch in a tree. To find out how singing of thechaffinch is associated with antipredatory behavior, I studiedperch selection in mature pine forest and in pine saplings,two habitats differing in the amount of cover for protection from predators. My results show that male chaffinches preferto sing below the canopy of mature pines and in the uppermostparts of sapling pines. Although these are the canopy partsmost exposed to sparrowhawk (Accipiter nisus) attacks, stayingin open sites may be a beneficial strategy for singing chaffinchesbecause it makes it possible to improve antipredatory vigilance.This assumption was supported by the hawk experiments. Afterbeing exposed to the sparrowhawk model, all of the singingbirds selected the conspicuous perch below the canopy of maturepines. The males that perched higher in the canopy before theexperiment moved to the lower canopy, whereas those males thatsang below the canopy did not change their singing perch.     

Mate-choice copying under predation risk in the Trinigadian guppy (Poecilia reticulata)

Although females of numerous species possess genetically-basedpreferences for certain male trails and male preferentiallywith males possessing these traits, recent theoretical and experimentalevidence indicates that they may also copy (imitate) the matechoices of other females under certain circumstances. Such mate-choicecopying is expected to be most prevalent when females have theopportunity to observe the mate choices of others and when matechoice is costly to females. One potential direct fitness costof mate choice is increased individual risk of mortality dueto predation. Here, we investigate for the first time the effectof increasing the apparent risk of predation on the tendencyof females to copy the experimentally staged mate choice ofanother female. Using adult female guppies (Poecilia reticulata)originating from a Trinidadian population that experiences arelatively high fish predation pressure, we first establishthat they possess a preference for the more brighdy coloredof two simultaneously presented males in the absence of bothmate-copying opportunity and an immediate threat of predation.However, most females reversed their initial mate preferencewhen given an opportunity to copy the mate preference of anotherfemale in the absence of predation threat The proportion offemales reversing their preference when given the opportunityto do so was not affected by increasing the apparent risk of(fish) predation. This result may be owing either to femaleguppies tending to copy the mate choice of others whenever theopportunity arises because the benefits of doing so accrue irrespectiveof the ambient risk of predation or to females choosing randomlybetween males with respect to their color pattern in the presenceof the predator irrespective of mate-copying opportunity. Thesetwo explanations for the apparent lack of an effect of predationrisk on mate-choice copying per se are both plausible but unfortunatelycould not be easily distinguished here. It may thus be possible,and interesting, that individual female guppies chose randomlybetween the available males in the presence of the predatorbut otherwise copied the choice of others when given the opportunityto do so.     

Evolutionary drivers of seasonal plumage colours: colour change by moult correlates with sexual selection,predation risk and seasonality across passerines

Some birds undergo seasonal colour change by moulting twice each year, typically alternating between a cryptic, non‐breeding plumage and a conspicuous, breeding plumage (‘seasonal plumage colours’). We test for potential drivers of the evolution of seasonal plumage colours in all passerines (N = 5901 species, c. 60% of all birds). Seasonal plumage colours are uncommon, having appeared on multiple occasions but more frequently lost during evolution. The trait is more common in small, ground‐foraging species with polygynous mating systems, no paternal care and strong sexual dichromatism, suggesting it evolved under strong sexual selection and high predation risk. Seasonal plumage colours are also more common in species predicted to have seasonal breeding schedules, such as migratory birds and those living in seasonal climates. We propose that seasonal plumage colours have evolved to resolve a trade‐off between the effects of natural and sexual selection on colouration, especially in seasonal environments.     

Patch selection by juvenile black surfperch (Embiotocidae) under variable risk: Interactive influence of food quality and structural complexity

The influence of risk on the selection of foraging patches by young-of-year black surfperch, Embiotoca jacksoni Agassiz, was investigated by laboratory and field experiments. These foragers harvest crustacean prey from a variety of benthic algal substrata. In field environments, patch types vary in two ways. First, substrata differ in structural complexity and probably afford different degrees of protection from predators. Second, substratum types vary in prey richness. There was no correlation between structural complexity and prey richness, and either or both factors could be a component of foraging patch value. Each patch is small and individual foragers are simultaneously confronted with arrays of patches encompassing the full range of variation in structure and prey richness. Furthermore, a major predator of young-of-year black surfperch, the kelp bass, Paralabrax clathratus (Girard), is patchily distributed in space and time. Thus similar arrays of patch types can be characterized by different levels of overall risk. Risk to foragers is dependent on light level as well as the presence and density of predators.The interplay between food quality and shelter potential in influencing patch choice was examined under different regimes of risk. Both laboratory and field experiments indicated patch preference was based primarily on food quality. However, the physical structure of a patch did become a component of patch choice as risk increased. The relative value of physical structure under high risk was dependent on the prey richness of a patch; food-poor substrata with high shelter potential remained unfavored even in situations of high risk.     

Interacting effects of predation risk and male and female density on male/female conflicts and mating dynamics of stream water striders

We used a factorial experiment to examine interacting effectsof male density, female density, and sunfish (predation risk)on mating dynamics of the stream water strider (Aquarius remigis).Many of our results corroborated earlier studies on the isolatedeffects of each factor on mating behavior. The effect of eachfactor, however, depended on the other factors. For example,in low density pools, predation risk decreased male generalactivity, male/female harassment rates, mating activity, andmating duration and increased the large male mating advantage.At higher densities, however, water striders apparently enjoyed"safety in numbers" and did not alter their mating dynamicsin response to the presence of predators. Female activity showeda particularly complex response to male density and fish. Whenmales were scarce, fish caused females to reduce their activity.However, when males were abundant, fish increased female activity,probably because fish decreased male activity thus releasingfemales from harassment by males. The three treatment factorsalso had interacting effects on male mating success. In theabsence of fish, when females were scarce, increased male densityresulted in a decrease in mean male mating success; however,when females were abundant, increased male density enhancedmean male mating success. In contrast, in the presence of fish,male density had little effect on male mating success. Manyof the observed mating patterns can be explained by the effectsof ecological and social factors on male/female conflicts; thatis, on male harassment of females and female reluctance to mate.     

Trading safety for food: evidence from gut contents in roach and bleak captured at different distances offshore from their daytime littoral refuge

1. Regular diel habitat shifts in roach were detected by hydro‐acoustics in five moderately eutrophic, stratifying (maximum depth 24–27 m) and approximately circular lakes (of surface area 15, 75, 125, 300 and 900 ha and diameters 250, 600, 1000, 1700 and 2600 m) in north‐eastern Poland in the years 1998–2000, when the lakes were free of smelt and other typical offshore planktivores, and their offshore areas were completely free of fish during the day. 2. The diel change in roach distribution was shown to assume a similar pattern in each lake: fish migrated from a daytime littoral refuge towards the centre of the lake at dusk, and returned to the littoral refuge at dawn. After sunset, fish gradually dispersed offshore until they covered the entire lake area in each of the three smaller lakes. In each of the two larger lakes, only small numbers of fish were seen in the central area at night, implying that the centre of the lake retained high food availability throughout the summer. 3. Inshore–offshore gradients in zooplankton prey density, body size, and numbers of eggs per clutch were weak or undetectable in the two smallest lakes, but strong and persistent in the three larger lakes, with Daphnia densities 5–30 times as high and body length 1.2–1.5 times as great in the central area as inshore. 4. The likely increase in the potential predation risk with distance from the littoral daytime refuge was found to be compensated by increased food gains in those fish which moved offshore at dusk to feed within a short time window, when light intensity was lower to make the risk reduced, but still high enough to see zooplankton prey. The benefit because of increased prey acquisition was greatest in the centre of the largest lake (at 1300 m from the shore), as revealed from gut inspections of roach and bleak trawl‐sampled at different distances from the edge of the reed belt, and seen as a gradual, order‐of‐magnitude increase in the volume of food in the foregut, The food volume against distance‐from‐shore regression was highly significant on each of the four sampling dates in the largest lake, in spite of the wide variability of food volume in individual fish.