Variation In Predation Pressure Research Articles

Context-dependent insect predation pressure on an avian ectoparasite.

Context dependence arises when ecological relationships vary with the conditions under which they are observed. Context dependence of interactions involving parasites is poorly known, even if it is key to understanding host-parasite relationships and food web dynamics. This paper investigates to which extent predation pressure on an avian ectoparasite (Carnus hemapterus) is context-dependent. Based on a predator-exclusion experiment, predation pressure on C. hemapterus pupae in the host's nest for 3 years, and its variation between habitat types are quantified. Variation in precipitation and normalized difference vegetation index (NDVI) is also explored as a likely cause of context dependency. We hypothesize that predation pressure should fluctuate with such surrogates of food availability, so that inter-annual and intra-annual differences may emerge. The number of nests with significant reduction of pupae varied widely among years ranging from 24% to 75%. However, average pupae reduction in nests where a significant reduction occurred did not vary between years. No differences in predation rates between habitat types were detected. Precipitation and NDVI varied widely between years and NDVI was consistently lower around nests on cliffs than around nests on trees and farmhouses. Parallels were found between variation in predation pressure and precipitation/NDVI at a wide scale (highest predation the driest year, and much lower the 2 rainier ones), but not at the nest scale. This paper shows clear context-dependent insect predation pressure on an ectoparasite under natural conditions, and that such interaction changes in signs rather than magnitude between years. The causes for these variations require longer-term studies and/or well-designed, large-scale experiments.

Predation and parasitism as determinants of animal personalities.

Within the same population, proactive (i.e. bolder, more exploratory, active and aggressive) and reactive (i.e. more timid, less exploratory, less active and more passive) individuals could be hypothetically maintained due a trade-off between foraging and vigilance behaviours, provided that both phenotypes differ in their state (e.g. metabolic rates, body condition or energetic needs). Yet, recent findings indicate that among-individual variation in intrinsic state can explain only a small proportion of variation in behaviour, meaning that other mechanisms, such as the presence of trophically transmitted parasites, might contribute to maintaining inter-individual behavioural differences. Empirical evidence, indeed, suggests strong relationships between certain animal personality traits and parasitic load within host populations. However, the direction of causation between these traits remains unclear: are different behaviours in infected hosts in contrast to uninfected ones the result of manipulation by parasites to increase host predation, or are some personalities inherently more susceptible to infection than others? To better understand the role of parasites in shaping behavioural differences within host populations and examine to what extent parasite manipulation and/or intrinsic differences in parasite susceptibility contribute to maintaining behavioural differences, we used a simulation approach and analysed the change in the frequencies of proactive and reactive individuals over time under different predation and starvation scenarios, when individual phenotype either affected a host's risk of infection or not. We found that in the absence of parasites, predation pressure strongly affected the expression of host personality, but the trade-off between foraging and vigilance behaviours alone could not explain the maintenance of inter-individual behavioural differences without temporal variation in predation pressure. By contrast, in the presence of parasites, the two host phenotypes could coexist within populations even when individuals experienced no temporal variations in predation risk, but only when proactive and reactive hosts were equally susceptible to parasitism. Our findings, thus, indicate that parasites can play an important role in maintaining genetic diversity in their host populations in addition to generating behavioural differences though manipulation.

The relationships between toad behaviour, antipredator defences, and spatial and sexual variation in predation pressure.

BackgroundAnimal behaviour is under strong selection. Selection on behaviour, however, might not act in isolation from other fitness-related traits. Since predators represent outstanding selective forces, animal behaviour could covary with antipredator defences, such that individuals better suited against predators could afford facing the costs of riskier behaviours. Moreover, not all individuals undergo equivalent degrees of predation pressure, which can vary across sexes or habitats. Individuals under lower predation pressure might also exhibit riskier behaviours.MethodsIn this work, I tested these hypotheses on natterjack toads (Epidalea calamita). Specifically, I gauged activity time, exploratory behaviour and boldness in standard laboratory conditions, and assessed whether they correlated with body size and antipredator strategies, namely sprint speed, parotoid gland area and parotoid gland colour contrast. Additionally, I compared these traits between sexes and individuals from an agrosystem and pine grove, since there is evidence that males and agrosystem individuals are subjected to greater predation pressure.ResultsSprint speed as well as parotoid gland contrast and size appeared unrelated to the behavioural traits studied. In turn, body mass was negatively related to activity time, boldness and exploration. This trend is consistent with the fact that larger toads could be more detectable to their predators, which are mostly gape unconstrained and could easily consume them. As predicted, females exhibited riskier behaviours. Nonetheless, agrosystem toads did not differ from pine grove toads in the behavioural traits measured, despite being under stronger predation pressure.

Repeatability of alert and flight initiation distances in king penguins: Effects of colony, approach speed, and weather

AbstractAlert distance (AD) and flight initiation distance (FID) are popular measures used to explore the reaction of prey to approaching predators, and thus the economics underlying optimal escape strategies. AD likely mirrors the effort invested into vigilance, while FID provides an estimate of the perceived risk of an approaching threat. Although individual variation in AD and FID is influenced by environmental factors such as variation in predation pressure and human disturbance, the repeatabilities of these traits (especially AD), and therefore their designation as personality traits, remain under‐investigated. Here, we studied the repeatability of AD, FID, and the decision to flee in a flightless, ground‐breeding seabird, the king penguin (Aptenodytes patagonicus). A single experimenter approached three times over three consecutive days 47 incubating king penguins from two different colonies with varying human disturbance levels. We explored the effects of weather, time of day, and approach speed of the experimenter on these behaviors. Weather had an effect on all three behaviors. In warm, sunny weather AD increased. In rainy, windy weather birds were more likely to flee, but waited longer before initiating flight. Faster approach speeds between AD and FID increased FID. Weather conditions and speed of approach affected repeatability estimates, highlighting the need to consider external sources of variation when refining such estimates. FID and the decision to flee were significantly and moderately repeatable (r = 0.26 and 0.57, respectively), while AD was not. There was no evidence of behavioral habituation or sensitization in either colony.

Predation risk in relation to brain size in alternative prey of pygmy owls varies depending on the abundance of main prey.

Large brains in prey may select for adoption of anti-predator behavior that facilitates escape. Prey species with relatively large brains have been shown to be less likely to fall prey to predators. This results in the prediction that individuals that have been captured by predators on average should have smaller brains than sympatric conspecifics. We exploited the fact that Eurasian pygmy owls Glaucidium passerinum hoard small mammals and birds in cavities and nest-boxes for over-winter survival, allowing for comparison of the phenotype of prey with that of live conspecifics. In Northern Europe, main prey of pygmy owls are voles of the genera Myodes and Microtus, while forest birds and shrews are the most important alternative prey. Large fluctuations (amplitude 100-200-fold) in vole populations induce rapid numerical responses of pygmy owls to main prey populations, which in turn results in varying predation pressure on small birds. We found, weighed and measured 153 birds in food-stores of pygmy owls and mist-netted, weighed and measured 333 live birds of 12 species in central-western Finland during two autumns with low (2017) and high (2018) pygmy owl predation risk. In two autumns, individuals with large brains were captured later compared to individuals with small brains, consistent with the hypothesis that such individuals survived for longer. Avian prey of pygmy owls had smaller heads than live birds in autumn 2018 when predation risk by pygmy owls was high. This difference in head size was not significant in 2017 when predation risk by pygmy owls was reduced. Finally, avian survivors were in better body condition than avian prey individuals. These findings are consistent with the hypothesis that pygmy owls differentially prey on birds in poor condition with small brains. These findings are consistent with the hypothesis that predation risk imposed by pygmy owls on small birds in boreal forests varies depending on the abundance of the main prey (voles).

Keeping predators out: testing fences to reduce livestock depredation at night-time corrals

AbstractLivestock depredation by large carnivores is a global conservation challenge, and mitigation measures to reduce livestock losses are crucial for the coexistence of large carnivores and people. Various measures are employed to reduce livestock depredation but their effectiveness has rarely been tested. In this study, we tested the effectiveness of tall fences to reduce livestock losses to snow leopards Panthera uncia and wolves Canis lupus at night-time corrals at the winter camps of livestock herders in the Tost Mountains in southern Mongolia. Self-reported livestock losses at the fenced corrals were reduced from a mean loss of 3.9 goats and sheep per family and winter prior to the study to zero losses in the two winters of the study. In contrast, self-reported livestock losses in winter pastures, and during the rest of the year, when herders used different camps, remained high, which indicates that livestock losses were reduced because of the fences, not because of temporal variation in predation pressure. Herder attitudes towards snow leopards were positive and remained positive during the study, whereas attitudes towards wolves, which attacked livestock also in summer when herders moved out on the steppes, were negative and worsened during the study. This study showed that tall fences can be very effective at reducing night-time losses at corrals and we conclude that fences can be an important tool for snow leopard conservation and for facilitating the coexistence of snow leopards and people.

Age and sex differences in numerical responses, dietary shifts, and total responses of a generalist predator to population dynamics of main prey

Fluctuations in the abundance of main prey species might shape animal communities, by inducing numerical responses and dietary shifts in predators. Whether numerical responses and dietary shifts differ among individuals of different age and sex has so far gained little attention. These differences could affect how much predators consume main and alternative prey, thus causing variation in predation pressure on main and alternative prey species. We studied the effect of fluctuating main prey abundance (voles) in autumn on the age and sex composition of a food-hoarding population of Eurasian pygmy owls Glaucidium passerinum (327 individuals), and on the species composition of their food stores in western Finland during 2003–2017 (629 food stores). Numbers of yearlings (< 1-year old) of both sexes and adult (+ 1-year old) females increased with increasing vole abundance. During low vole abundance, adult owls stored more small birds and less small mammals than yearlings. Females stored more small mammals than males and showed a tendency to store less birds. The amount of consumed birds (the most important alternative prey), and in particular of crested, willow, great, and blue tits, increased with low vole densities. Our results show that numerical, functional, and total responses of pygmy owls, and probably also other vertebrate predators, to the availability of the main prey in winter are shaped by the age and sex composition of the predator population, which both show large spatio-temporal variation in boreal forests.

Brain size affects responsiveness in mating behaviour to variation in predation pressure and sex ratio.

Despite ongoing advances in sexual selection theory, the evolution of mating decisions remains enigmatic. Cognitive processes often require simultaneous processing of multiple sources of information from environmental and social cues. However, little experimental data exist on how cognitive ability affects such fitness-associated aspects of behaviour. Using advanced tracking techniques, we studied mating behaviours of guppies artificially selected for divergence in relative brain size, with known differences in cognitive ability, when predation threat and sex ratio was varied. In females, we found a general increase in copulation behaviour in when the sex ratio was female biased, but only large-brained females responded with greater willingness to copulate under a low predation threat. In males, we found that small-brained individuals courted more intensively and displayed more aggressive behaviours than large-brained individuals. However, there were no differences in female response to males with different brain size. These results provide further evidence of a role for female brain size in optimal decision-making in a mating context. In addition, our results indicate that brain size may affect mating display skill in male guppies. We suggest that it is important to consider the association between brain size, cognitive ability and sexual behaviour when studying how morphological and behavioural traits evolve in wild populations.

Wild and laboratory exposure to cues of predation risk increases relative brain mass in male guppies

Abstract There is considerable diversity in brain size within and among species, and substantial dispute over the causes, consequences and importance of this variation. Comparative and developmental studies are essential in addressing this controversy. Predation pressure has been proposed as a major force shaping brain, behaviour and life history. The Trinidadian guppy, Poecilia reticulata, shows dramatic variation in predation pressure across populations. We compared the brain mass of guppies from high and low predation populations collected in the wild. Male but not female guppies exposed to high predation possessed heavier brains for their body size compared to fish from low predation populations. The brain is a plastic organ, so it is possible that the population differences we observed were partly due to developmental responses rather than evolved differences. In a follow‐up study, we raised guppies under cues of predation risk or in a control condition. Male guppies exposed to predator cues early in life had heavier brains relative to their body size than control males, while females showed no significant effect of treatment. Collectively our results suggest that male guppies exposed to predation invest more in neural tissue, and that these differences are at least partly driven by plastic responses. A plain language summary is available for this article.

Developmental plasticity of the stress response in female but not in male guppies.

To survive, animals must respond appropriately to stress. Stress responses are costly, so early-life experiences with potential stressors could adaptively tailor adult stress responses to local conditions. However, how multiple stressors influence the development of the stress response remains unclear, as is the role of sex. Trinidadian guppies (Poecilia reticulata) are small fish with extensive life-history differences between the sexes and population variation in predation pressure and social density. We investigated how sex and early-life experience influence hormonal stress responses by manipulating conspecific density and perceived predation risk during development. In adults, we sampled cortisol twice to measure initial release and change over time in response to a recurring stressor. The sexes differed considerably in their physiological stress response. Males released more cortisol for their body mass than females and did not reduce cortisol release over time. By contrast, all females, except those reared at high density together with predation cues, reduced cortisol release over time. Cortisol responses of males were thus less dynamic in response to current circumstances and early-life experiences than females, consistent with life-history differences between the sexes. Our study underscores the importance of early-life experiences, interacting ecological factors and sex differences in the organization of the stress response.

Predation drives morphological convergence in the Gambusia panuco species group among lotic and lentic habitats.

Fish morphology is often constrained by a trade-off between optimizing steady vs. unsteady swimming performance due to opposing effects of caudal peduncle size. Lotic environments tend to select for steady swimming performance, leading to smaller caudal peduncles, whereas predators tend to select for unsteady swimming performance, leading to larger caudal peduncles. However, it is unclear which aspect of performance should be optimized across heterogeneous flow and predation environments and how this heterogeneity may affect parallel phenotypic evolution. We investigated this question among four Gambusia species in north-eastern Mexico, specifically the riverine G.panuco, the spring endemics G.alvarezi and G.hurtadoi, and a fourth species, G.marshi, found in a variety of habitats with varying predation pressure in the Cuatro Ciénegas Basin and Río Salado de Nadadores. We employed a geometric morphometric analysis to examine how body shapes of both male and female fish differ among species and habitats and with piscivore presence. We found that high-predation and low-predation species diverged morphologically, with G.marshi exhibiting a variable, intermediate body shape. Within G.marshi, body morphology converged in high-predation environments regardless of flow velocity, and fish from high-predation sites had larger relative caudal peduncle areas. However, we found that G.marshi from low-predation environments diverged in morphology between sub-basins of Cuatro Ciénegas, indicating other differences among these basins that merit further study. Our results suggest that a morphological trade-off promotes parallel evolution of body shape in fishes colonizing high-predation environments and that changing predation pressure can strongly impact morphological evolution in these species.

Does spatial variation in predation pressure modulate selection for aposematism?

It is widely believed that aposematic signals should be conspicuous, but in nature, they vary from highly conspicuous to near cryptic. Current theory, including the honest signal or trade‐off hypotheses of the toxicity–conspicuousness relationship, cannot explain why adequately toxic species vary substantially in their conspicuousness. Through a study of similarly toxic Danainae (Nymphalidae) butterflies and their mimics that vary remarkably in their conspicuousness, we show that the benefits of conspicuousness vary along a gradient of predation pressure. Highly conspicuous butterflies experienced lower avian attack rates when background predation pressure was low, but attack rates increased rapidly as background predation pressure increased. Conversely, the least conspicuous butterflies experienced higher attack rates at low predation pressures, but at high predation pressures, they appeared to benefit from crypsis. Attack rates of intermediately conspicuous butterflies remained moderate and constant along the predation pressure gradient. Mimics had a similar pattern but higher attack rates than their models and mimics tended to imitate the signal of less attacked model species along the predation pressure gradient. Predation pressure modulated signal fitness provides a possible mechanism for the maintenance of variation in conspicuousness of aposematic signals, as well as the initial survival of conspicuous signals in cryptic populations in the process of aposematic signal evolution, and an alternative explanation for the evolutionary gain and loss of mimicry.

Comparing the non-lethal and lethal effects of predation risk ‬on goldfish anti-predatory behavior

Little egrets (Egretta garzetta) and common goldfish (Carassius auratus) interacted in experimental theaters that challenge them with a behavioral game. We studied the behavioral tactics of both players. The experimental theaters consist of three equally spaced pools, each with a shelter in its center. The fish can take shelter in a safe but foodless habitat, or swim exposed in the open that contains food. The egrets can move among the pools to catch the exposed fish. We investigated the importance of non-lethal effects versus lethal effects on predator–prey interactions. We created a variance in predation pressure by keeping the number of egrets fixed but varying the number of pools of the experimental theater between 1 and 3 pools. In all treatments, even when the egret was present, individual goldfish emerged from protected cover occasionally, exposing at least their heads and sometimes their entire bodies in apparent disregard for the possibly lethal consequences. We assumed that this behavior stems from the fish's constant need to collect information about its surroundings. The fish responded appropriately to the variations in predation pressure by changing their activity level outside the cover, i.e., the fish drastically and significantly reduced their exposure outside the cover, as well as the rate of peeping, as predation pressure increased. The results demonstrate the importance role of non-lethal effects, and how they drive the behavior of prey in response to predation risk, which in turn, drives the action of the predator in an asymmetric two-player game of stealth and fear.

A simple predation pressure index for modeling changes in natural mortality: Application to Gulf of Maine northern shrimp stock assessment

This paper describes a method for incorporating varying predation pressure in stock assessment that does not require estimation of consumption rates or absolute consumption by predators. The method is applied to assessment of northern shrimp Pandalus borealis, an important forage species in the North Atlantic Ocean and the target of major fisheries. A predation pressure index (PPI) was developed using data collected during resource surveys in Gulf of Maine northern shrimp habitat areas during 1968–2013. Predators were identified based on the percent frequency of occurrence (PFO) of Pandalid shrimp in their diets. The PPI for each year was the weighted sum of the biomass indices of 21 identified predators, where the weights were the time-series average PFO for each predator. The PPI thus reflected the effects of both the biomass of each predator and its importance as a predator of shrimp. The PPI time series was used to scale an assumed average natural mortality rate (M) that replaced a constant M assumption in the stock assessment model. Use of the PPI-scaled M improved the overall fit of the model and reduced a retrospective pattern in the constant M model by nearly 60%. The PPI approach allows time-varying predation to be accounted for in an assessment model without requiring estimates of absolute abundance of predators and their total consumption, quantities which may be unavailable or difficult to estimate accurately.

Predators drive community structure in coral reef fish assemblages

The importance of top‐down effects in structuring ecological communities has been widely debated by ecologists. One way in which to examine these processes is to study the secondary effects of predator removal on communities. This study examined the role of predatory fishes in structuring communities of coral reef fishes, by using a network of marine reserves (the Great Barrier Reef Marine Park) as a natural experiment. We hypothesized that reefs with high densities of piscivores (marine reserves) would have distinct fish communities from those where piscivores have been depleted through fishing, due to variation in predation pressure. We predicted that predator depletion would result in “prey release”, and a corresponding increase in prey densities along a gradient of fishing intensity, causing a change in the community composition of reef fishes. To address this, fish counts and habitat surveys were conducted at four locations on the Great Barrier Reef. At each location, comparisons were made amongst three marine park zones that varied in their exposure to fishing practices; no‐ take marine reserves, limited fishing areas, and open fishing areas.The density and biomass of predators varied consistently among zones at each location. Furthermore, we found strong evidence for prey release at all four locations, resulting in distinct fish assemblages amongst zones. Reefs open to fishing had much lower densities of piscivores, and higher densities of prey and herbivorous fishes compared to marine reserves. This broad pattern was consistent amongst locations, and persisted at the level of species, trophic groups, families and communities. Habitat characteristics did not vary significantly amongst zones in a consistent manner amongst locations. Although habitat relationships were strong for specialist species such as butterflyfishes, densities of predators were stronger predictors of prey density for most species, and the trophic composition of reef fish communities differed significantly amongst zones at all locations. Results from this study support the concept that top‐down effects can be strong divers of prey populations and influence community structure in highly diverse systems. These data emphasize the vital role of predators, and reinforce the importance of preserving and restoring top‐down trophic interactions in ecological systems.

Latitudinal variation of a defensive symbiosis in the Bugula neritina (Bryozoa) sibling species complex.

Mutualistic relationships are beneficial for both partners and are often studied within a single environment. However, when the range of the partners is large, geographical differences in selective pressure may shift the relationship outcome from positive to negative. The marine bryozoan Bugula neritina is a colonial invertebrate common in temperate waters worldwide. It is the source of bioactive polyketide metabolites, the bryostatins. Evidence suggests that an uncultured vertically transmitted symbiont, “Candidatus Endobugula sertula”, hosted by B. neritina produces the bryostatins, which protect the vulnerable larvae from predation. Studies of B. neritina along the North American Atlantic coast revealed a complex of two morphologically similar sibling species separated by an apparent biogeographic barrier: the Type S sibling species was found below Cape Hatteras, North Carolina, while Type N was found above. Interestingly, the Type N colonies lack “Ca. Endobugula sertula” and, subsequently, defensive bryostatins; their documented northern distribution was consistent with traditional biogeographical paradigms of latitudinal variation in predation pressure. Upon further sampling of B. neritina populations, we found that both host types occur in wider distribution, with Type N colonies living south of Cape Hatteras, and Type S to the north. Distribution of the symbiont, however, was not restricted to Type S hosts. Genetic and microscopic evidence demonstrates the presence of the symbiont in some Type N colonies, and larvae from these colonies are endowed with defensive bryostatins and contain “Ca. Endobugula sertula”. Molecular analysis of the symbiont from Type N colonies suggests an evolutionarily recent acquisition, which is remarkable for a symbiont thought to be transmitted only vertically. Furthermore, most Type S colonies found at higher latitudes lack the symbiont, suggesting that this host-symbiont relationship is more flexible than previously thought. Our data suggest that the symbiont, but not the host, is restricted by biogeographical boundaries.

Predator-mediated interactions between lemmings and shorebirds: A test of the alternative prey hypothesis

The alternative prey hypothesis (APH) suggests that the functional and numerical response of predators to fluctuating rodent populations may drive annual variation in predation pressure on other available prey such as bird eggs. Most studies that have provided evidence supporting the APH in arctic bird populations have been conducted in the eastern hemisphere, and considerably less evidence for APH has emerged from western hemisphere populations. We tested the hypothesis that predation pressure on shorebird nests would increase as lemming abundance decreases due to apparent competition between lemmings and shorebirds via their shared predators in the eastern Canadian High Arctic. Over a period of 5 years on Bylot Island, Nunavut, Canada, we found that lemming abundance had a significant negative effect on predation risk as measured by artificial nests. Survival probabilities of artificial nests were also negatively related to fox abundance but positively associated with the abundance of breeding avian predators, likely due to predator exclusion around avian predator nests. Models of daily nest survival for real nests also indicated that interannual variation in nest survival was best explained by lemming abundance. Combined results from both artificial and real nests indicate that fluctuations in lemming populations likely have an indirect effect on predation pressure on shorebird eggs in the Canadian High Arctic, although mechanisms explaining the observed relationship require further investigation.

EFFECTS OF SEX, ENVIRONMENT, AND CONDITION ON THE MUSKING BEHAVIOR OF SYMPATRIC GARTERSNAKES (THAMNOPHIS SPP.)

Despite an abundance of studies documenting antipredator and defensive behaviors of gartersnakes (genus Thamnophis), few have quantitatively examined musking, a widely utilized antipredator tactic. In this study we quantify musking behaviors in the Terrestrial Gartersnake (Thamnophis elegans) and the Plains Gartersnake (T. radix) when hand-captured at four sites in and near Denver, Colorado, USA. Overall, Plains Gartersnakes musked significantly more often than Terrestrial Gartersnakes. Female Terrestrial Gartersnakes musked more frequently than males, a pattern not evident in the Plains Gartersnake. Additionally, we observed a positive correlation in body condition and musking propensity in Terrestrial Gartersnakes, suggesting resource-dependent behavior in this species. Musking behavior was consistent across variations in predation pressure, environmental conditions, and snake body size, all factors shown to influence other gartersnake defensive behaviors. These results corroborate other research which demonstrates that snake antipredator behaviors are determined by complex interactions of abiotic and biotic factors.

Vertical heterogeneity in predation pressure in a temperate forest canopy

The forest canopy offers a vertical gradient across which variation in predation pressure implies variation in refuge quality for arthropods. Direct and indirect experimental approaches were combined to assess whether canopy strata differ in ability to offer refuge to various arthropod groups. Vertical heterogeneity in impact of avian predators was quantified using exclosure cages in the understory, lower, mid, and upper canopy of a north-temperate deciduous forest near Montreal, Quebec. Bait trials were completed in the same strata to investigate the effects of invertebrate predators. Exclusion of birds yielded higher arthropod densities across all strata, although treatment effects were small for some taxa. Observed gradients in predation pressure were similar for both birds and invertebrate predators; the highest predation pressure was observed in the understory and decreased with height. Our findings support a view of the forest canopy that is heterogeneous with respect to arthropod refuge from natural enemies.

The contribution of an avian top predator to selection in prey species

Natural selection can vary in magnitude, form and direction, yet the causes of selection, and of variation in selection, are poorly understood. We quantified the effect of a key predator (Eurasian sparrowhawks) on selection on fledging body mass in two bird species (blue tits and great tits). By partitioning selection into within- and between-brood components, we were able to separate individual from brood-level effects of fledging mass on predation probability and recruitment. In blue tits, selection on fledging mass by sparrowhawk predation was nonsignificant and could not explain selection to recruitment. In contrast, in great tits, sparrowhawk predation selected for increased fledging mass at the individual level and could explain 73% of individual-level selection on fledging mass to recruitment. Moreover, in great tits, individual-level selection on fledging mass was significantly stronger in years in which sparrowhawks were present compared to years when sparrowhawks were absent. Selection at the brood level was independent of sparrowhawk presence. These results provide compelling evidence that sparrowhawk predation acts as an important causal agent of selection on fledging mass in great tits but not in blue tits. Variation in predation pressure can therefore account for variation in selection, but specific patterns may not be easily generalized across species.