Effects Of Predation Risk Research Articles

Foraging competition, vigilance and group size in two species of gregarious antelope

In gregarious species, rates of foraging behaviour are often positively related to group size while there is simultaneously a negative relation between group size and vigilance. Although the mechanisms underlying these behavioural patterns are still incompletely understood, decreased predation risk or increased foraging competition in larger groups have been put forward as two possible explanations. Since most empirical tests of these two hypotheses have used manipulative experiments, they have mainly been limited to small-bodied species or to animals in captivity. Here we suggest a time-budget model to test for the causal effects of predation risk and foraging competition that does not necessitate manipulative approaches. We used this method on two species of gregarious antelope, blesbok (Damaliscus pygargus phillipsi) and impala (Aepyceros melampus). We suggest that increased foraging competition in large groups drives the negative relationship between vigilance and group size in these species. This study shows that observational data can be used to test explicit hypotheses on species that are logistically unsuitable for manipulative experiments, and also render support for hypotheses suggesting that factors unrelated to predation risk are involved in shaping the differences in vigilance rates over different group sizes in gregarious species.

TRAIT‐MEDIATED EFFECTS ON FLOWERS: ARTIFICIAL SPIDERS DECEIVE POLLINATORS AND DECREASE PLANT FITNESS

Although predators can affect foraging behaviors of floral visitors, rarely is it known if these top-down effects of predators may cascade to plant fitness through trait-mediated interactions. In this study we manipulated artificial crab spiders on flowers of Rubus rosifolius to test the effects of predation risk on flower-visiting insects and strength of trait-mediated indirect effects to plant fitness. In addition, we tested which predator traits (e.g., forelimbs, abdomen) are recognized and avoided by pollinators. Total visitation rate was higher for control flowers than for flowers with an artificial crab spider. In addition, flowers with a sphere (simulating a spider abdomen) were more frequently visited than those with forelimbs or the entire spider model. Furthermore, the presence of artificial spiders decreased individual seed set by 42% and fruit biomass by 50%. Our findings indicate that pollinators, mostly bees, recognize and avoid flowers with predation risk; forelimbs seem to be the predator trait recognized and avoided by hymenopterans. Additionally, predator avoidance by pollinators resulted in pollen limitation, thereby affecting some components of plant fitness (fruit biomass and seed number). Because most pollinator species that recognized predation risk visited many other plant species, trait-mediated indirect effects of spiders cascading down to plant fitness may be a common phenomenon in the Atlantic rainforest ecosystem.

Behaviour and resource use of two competing vole species under shared predation risk

Indirect interaction between two competing species via a shared predator may be an important determinant of population and community dynamics. We studied the effect of predation risk imposed by the least weasel Mustela nivalis nivalis on space use, foraging and activity of two competing vole species, the grey-sided vole Myodes rufocanus, and the bank vole Myodes glareolus. The experiment was conducted in a large indoor arena, consisting of microhabitat structures providing food, shelter, trees for refuge and separated areas with high and low predation risk. Voles were followed for 5 days: 2 days before, 1 day during and 2 days after the presence of weasel. Our results suggest an effect of weasel presence on the vole community. Voles of both species shifted their activity from risky to less risky areas, climbed trees more often and were less active. Seed consumption was not affected by weasel presence. The time spent in the risky and less risky area did not differ between species, but bank voles spent more time in trees than grey-sided voles. Males of both species were more exposed to predation risk than females, i.e. generally spent more time in the risky area. Proportion of time spent in the risky area, the use of area, trees and food stations were sex dependent. Activity and use of trees were species dependent. We found no evidence for despotic distribution between our two species, although bank voles seemed to be more affected by coexistence, since they lost weight during the experiment. Based on our results we conclude that predator response was largely similar between species, while the sex-specific responses dominated. Besides a stronger escape response in the bank vole, the strongest individual differences were sex specific, i.e. males were more prone to take risks in space use and activity.

Within-group spatial position and vigilance: a role also for competition? The case of impalas (Aepyceros melampus) with a controlled food supply

Theory predicts that individuals at the periphery of a group should be at higher risk than their more central conspecifics since they would be the first to be encountered by an approaching terrestrial predator. As a result, it is expected that peripheral individuals display higher vigilance levels. However, the role of conspecifics in this “edge effect” may have been previously overlooked, and taking into account the possible role of within-group competition is needed. Vigilance behavior in relation to within-group spatial position was studied in impalas (Aepyceros melampus) feeding on standardized patches. We also controlled for food distribution in order to accurately define a “central” as opposed to a “peripheral” position. Our data clearly supported an edge effect, with peripheral individuals spending more time vigilant than their central conspecifics. Data on social interactions suggest that it was easier for a foraging individual to defend its feeding patch with its head lowered, and that more interactions occurred at the center of the group. Together, these results indicate that central foragers may reduce their vigilance rates in response to increased competition. Disentangling how the effects of competition and predation risk contribute to the edge effect requires further investigations.

Seasonal variation in patch use in a tropical African environment

We used giving‐up densities (GUD) to study patch use decisions of small granivorous passerines throughout the year. We measured GUDs continuously in four sites for a period of 9–10 months per year during 2004 and 2005 in a savannah area in Jos, central Nigeria. The study thus covered a period from the middle of the dry season, through the wet season to the beginning of the next dry season in each year. We placed experimental food patches in both open areas and in cover to investigate possible effects of predation risk and thermal hazard on the foraging behavior of the birds. We found a difference in GUDs between the microhabitats, with a consistently lower GUD in cover throughout the year and for the two years. During both years GUDs followed a pattern coinciding with the seasonal change in local seed availability. An initial decline in GUDs late in the dry season was followed by a steady increase during and after the rains. A similar trend in GUDs observed for both years supports the conclusion that GUDs measure the feeding birds’ assessment of environmental quality, possibly in combination with other factors changing predictably during the year. We conclude that food abundance may act with other environmental and ecological factors to affect foraging decisions throughout the year.

Minnows and the selfish herd: effects of predation risk on shoaling behaviour are dependent on habitat complexity

Hamilton's theory of the selfish herd suggests that gregarious behaviour is a strategy by which individuals seek cover behind other group members to reduce the risk of predation. Therefore, it can be predicted that individuals will have less reason to seek cover within a group if the habitat itself provides physical structure, and therefore cover. We tested this prediction with European minnows, Phoxinus phoxinus, in a large, seminatural stream channel using a fully factorial design in which habitat complexity and predation risk were manipulated independently. We recorded shoal size, rates of movement and association preferences for familiar fish. Consistent with predictions, European minnows responded to the presence of predatory pike, Esox lucius, by forming larger shoals, but this effect was observed only in structurally simple habitats. Fish also responded to increased predation risk by reducing their rates of movement, and this was particularly pronounced in complex habitats. Association preferences for familiar fish were unaffected by predation risk in simple habitats. Together, these results suggest that when at risk from predation, European minnows shoal to obtain shelter, but only when there is insufficient physical structure available. Moreover, the results show the extent to which physical habitat complexity can affect the expression of gregarious behaviour.

A review of lethal and non-lethal effects of predators on adult marine turtles

Although predation is recognized as an important factor for sea turtles at early life history stages, the possible influences of predators on adult sea turtles have largely been overlooked. Indeed, because predation rates on adult turtles generally are low, it often is assumed that predators do not influence turtle behavior, affect population sizes, or structure sea turtle impacts on marine communities. Yet, sharks, killer whales and crocodiles may take adult turtles, and no sea turtle species is invulnerable. Furthermore, as long-lived animals, adult sea turtles are predicted to invest in anti-predator behavior even when predation risk is relatively low to avoid the reproductive costs of an early death. Several studies suggest that predators may influence behavior and habitat use of adult turtles. For example, in a pristine ecosystem in Australia green turtles, especially those in good body condition, avoid profitable feeding areas in order to be safe from tiger sharks. Such predation-sensitive foraging could lead to populations being partially regulated by synergistic effects of predation risk and food abundance. Indeed, observed rapid rebounds of sea turtle populations may have been facilitated by reductions in shark populations. Finally, if sea turtles modify their foraging habitat use in response to predators, the spatiotemporal pattern of turtle effects on their communities could be shaped by predators. Historical reconstructions of past turtle populations and conservation strategies aimed at increasing turtle populations must consider the possible effects of predators, including non-lethal ones. Future work should focus on gaining a broader understanding of the effects of predators across a range of sea turtle taxa and habitats.

Non‐lethal effects of predation in birds

Predators can affect individual fitness and population and community processes through lethal effects (direct consumption or ‘density’ effects), where prey is consumed, or through non‐lethal effects (trait‐mediated effects or interactions), where behavioural compensation to predation risk occurs, such as animals avoiding areas of high predation risk. Studies of invertebrates, fish and amphibians have shown that non‐lethal effects may be larger than lethal effects in determining the behaviour, condition, density and distribution of animals over a range of trophic levels. Although non‐lethal effects have been well described in the behavioural ecology of birds (and also mammals) within the context of anti‐predation behaviour, their role relative to lethal effects is probably underestimated. Birds show many behavioural and physiological changes to reduce direct mortality from predation and these are likely to have negative effects on other aspects of their fitness and population dynamics, as well as affecting the ecology of their own prey and their predators. As a consequence, the effects of predation in birds are best measured by trade‐offs between maximizing instantaneous survival in the presence of predators and acquiring or maintaining resources for long‐term survival or reproduction. Because avoiding predation imposes foraging costs, and foraging behaviour is relatively easy to measure in birds, the foraging–predation risk trade‐off is probably an effective framework for understanding the importance of non‐lethal effects, and so the population and community effects of predation risk in birds and other animals. Using a trade‐off approach allows us to predict better how changes in predator density will impact on population and community dynamics, and how animals perceive and respond to predation risk, when non‐lethal effects decouple the relationship between predator density and direct mortality rate. The trade‐off approach also allows us to identify where predation risk is structuring communities because of avoidance of predators, even when this results in no observable direct mortality rate.

The Effects of Predation Risk, Food Abundance, and Population Size on Group Size of Brown‐Headed Cowbirds (Molothrus ater)

AbstractSocial and ecological conditions can influence flock formation (e.g. number of flocks, flock size, etc.) depending on the degree of social attraction of a species. We studied group formation in brown‐headed cowbirds (Molothrus ater) over short time periods (30 min) in two semi‐natural experiments conducted under controlled conditions. First, we determined the shape of the relationship between intake rate and flock size by manipulating group size in a single enclosure. Second, we assessed the role of population size, food abundance, and predation risk, and their interactions, in flock size formation in a system of four enclosures (two with and two without food) connected to a central refuge patch. In the first experiment, we found that pecking rates peaked at intermediate flock sizes (three to six individuals), which was influenced by greater availability of foraging time and more aggressive interactions in large groups. In the second experiment, flock sizes in the patches with food increased with population size likely due to the benefits of patch exploitation in groups. Flock size decreased after predator attack probably because refuge availability reduced perceived predation risk more than flocking in larger groups. Food abundance had minor effects, varying flock sizes between the two patches with food, under high food availability conditions when population size was high, probably due to social cohesion effects. Our results suggest that: (1) this species has an inverted‐U food intake–group size relationship with a range of intake‐maximizing flock sizes rather than a single peak, (2) the presence of a near refuge modifies the expected benefits of group patch exploitation under high predation risk, and (3) an increase in population size would more likely be translated into rapid increases in the size of the flocks rather than in more new flocks.

Experimental support for the cost–benefit model of lizard thermoregulation: the effects of predation risk and food supply

Huey and Slatkin's (Q Rev Biol 51:363-384, 1976) cost-benefit model of lizard thermoregulation predicts variation in thermoregulatory strategies (from active thermoregulation to thermoconformity) with respect to the costs and benefits of the thermoregulatory behaviour and the thermal quality of the environment. Although this framework has been widely employed in correlative field studies, experimental tests aiming to evaluate the model are scarce. We conducted laboratory experiments to see whether the common lizard Zootoca vivipara, an active and effective thermoregulator in the field, can alter its thermoregulatory behaviour in response to differences in perceived predation risk and food supply in a constant thermal environment. Predation risk and food supply were represented by chemical cues of a sympatric snake predator and the lizards' food in the laboratory, respectively. We also compared males and postpartum females, which have different preferred or "target" body temperatures. Both sexes thermoregulated actively in all treatments. We detected sex-specific differences in the way lizards adjusted their accuracy of thermoregulation to the treatments: males were less accurate in the predation treatment, while no such effects were detected in females. Neither sex reacted to the food treatment. With regard to the two main types of thermoregulatory behaviour (activity and microhabitat selection), the treatments had no significant effects. However, postpartum females were more active than males in all treatments. Our results further stress that increasing physiological performance by active thermoregulation has high priority in lizard behaviour, but also shows that lizards can indeed shift their accuracy of thermoregulation in response to costs with possible immediate negative fitness effects (i.e. predation-caused mortality).

The effect of predation risk on an acarine system

Current theory on trait-mediated interactions in tri-trophic food chains shows that antipredator behaviour of the middle species can cause similar indirect effects giving rise to trophic cascades as mediated by density changes. In this article, the effect of predation risk in a tri-trophic food chain (bean plant–two-spotted spider mite prey–predatory mite, Phytoseiulus persimilis) is measured, both on plants (changes in leaf damage) and spider mites (changes in egg numbers, mortality rate and dispersal behaviour), under two risk scenarios. In the predator risk treatment a predatory mite was caged in an experimental cell that was placed above a leaf disc with spider mites. This prevented the predator from subduing prey while the predator was perceived by spider mites as a potential threat. In the predator cues treatment a predatory mite was introduced to the leaf disc for 24 h before placing the spider mites on the disc. Compared to control without predators, after four days we observed in both risk treatments a nonsignificant increase in plant damage per spider mite and a significant decline in spider mite fecundity. No significant effect of predation risk on spider mite mortality was observed. We also showed that the above effects are not uniformly distributed in time. For example, the spider mite fecundity and dispersal rate in the predator cues treatment were significantly different from the control only the first day. We also found that under predator cues treatment, walking activity of the spider mites increased significantly.

Effects of predation risk on mating behavior of the Kanzawa spider mite

In the Kanzawa spider mite, Tetranychus kanzawai, adult males locate pre-reproductive quiescent females and engage in precopulatory mate guarding. We found that re-reproductive quiescent females preferred to be near veins, rather than other leaf parts, and moreover, adult males spent more time along the vein than on other parts. Consequently, T. kanzawai males found more quiescent females along veins than those on other parts. However, the predatory mite Neoseiulus womersleyi also found more quiescent T. kanzawai females along veins than those on other parts. Moreover, N. womersleyi found more guarding males than solitary males of T. kanzawai. Thus, we experimentally examined the effects of predation risk on the mating behavior of T. kanzawai. The presence of N. womersleyi reduced T. kanzawai female preference for vein vicinity as a quiescent site. Although the predation risk of guarding T. kanzawai males was lower than that of solitary males after detection by predators, the presence of N. womersleyi also reduced the proportion of guarding T. kanzawai males. These results suggest that the possible benefits of preferring vein vicinity as quiescent sites by T. kanzawai females is outweighed by predation risk in the presence of predators, and that the risk of detection by predators would be more important for T. kanzawai males than the risk of being preyed upon.

Foraging under the risk of predation in desert grassland whiptail lizards (Aspidoscelis uniparens)

The whiptail lizard Aspidoscelis uniparens searches for fossorial prey using a series of moves and pauses punctuated by bouts of digging. We examined the effect of predation risk on foraging A. uniparens in outdoor enclosures, observing their behavior in the presence and absence of the predatory lizard Gambelia wislizenii. Predator presence led to changes in activity patterns and foraging behavior. When predators were present, A. uniparens reduced both the proportion of time active and time moving, shifted activity periods, reduced their movement rate, and changed the duration of moves. There were no apparent changes in digging effort, but the likelihood of successfully digging for food decreased when a predator was present.

Tradeoffs involved in site selection and foraging in a wolf spider: effects of substrate structure and predation risk

Understanding how animals weigh habitat features, exposure to predators and access to resources is important to determining their life history and distribution across the landscape. For example, when predators accumulate in structurally complex habitats, they face an environment with different competitive interactions, foraging opportunities and predatory risks. The wolf spider Pardosa milvina inhabits the soil surface of highly disturbed habitats such as agricultural fields throughout eastern North America. Pardosa displays effective antipredator behavior in the presence of chemical cues produced by a larger coexisting wolf spider, Hogna helluo. We used those cues to simulate predation risk in laboratory and field experiments designed to test the effects of habitat substrate and predation risk on site selection and prey consumption of Pardosa. In general, Pardosa preferred more complex substrates over bare dirt but those preferences were eliminated or reversed when cues from Hogna were present. Feeding trials revealed that substrate alone had few effects on Pardosa prey consumption, which we measured by documenting the change in the abdomen width. Although the presence of Hogna cues reduced prey consumption overall in field feeding trials, the negative effect of predation risk on prey consumption was only observed in grass and bare dirt substrates in the laboratory. We also found that prey capture was negatively affected by habitat complexity for both spider species but that same complexity offered Pardosa protection from predation by Hogna. This study provides insight into how two predator species interact to balance site selection and feeding in order to avoid predation. Shifts in foraging and distributional patterns of predators can have profound implications for their role in the food web.

Tradeoffs involved in site selection and foraging in a wolf spider: effects of substrate structure and predation risk

Tradeoffs involved in site selection and foraging in a wolf spider: effects of substrate structure and predation risk

Avoidance of predator chemical cues by bats: an experimental assessment

[Predation places selection pressure on the behaviour of organisms, but the effect of predation risk on bat behaviour has gone largely untested. Using a Y-maze, we tested the hypothesis that female big brown bats ( Eptesicus fuscus ) avoid olfactory cues from known predators — raccoons ( Procyon lotor ) and black rat snakes ( Elaphe obsoleta ). These chemical cues are most likely encountered by bats during the assessment of potential roost sites, so avoidance of predator cues could affect roost selection. We found that big brown bats do not avoid cues from raccoon urine and there was a non-significant trend for bats to choose the arm with black rat snake cues. In a second experiment, bats were exposed to olfactory and potentially auditory cues of live black rat snakes. Bats tended to avoid the arm with live snakes, although results were non-significant. This suggests olfactory cues may not be representative of live snakes, or that auditory cues may be more important than olfactory cues for predator recognition. During late spring, female bats often avoid torpor for thermoregulatory or reproductive reasons. Therefore, bats may be alert to the approach of predators and be less responsive to chemical cues compared to other times of the year., Predation places selection pressure on the behaviour of organisms, but the effect of predation risk on bat behaviour has gone largely untested. Using a Y-maze, we tested the hypothesis that female big brown bats ( Eptesicus fuscus ) avoid olfactory cues from known predators — raccoons ( Procyon lotor ) and black rat snakes ( Elaphe obsoleta ). These chemical cues are most likely encountered by bats during the assessment of potential roost sites, so avoidance of predator cues could affect roost selection. We found that big brown bats do not avoid cues from raccoon urine and there was a non-significant trend for bats to choose the arm with black rat snake cues. In a second experiment, bats were exposed to olfactory and potentially auditory cues of live black rat snakes. Bats tended to avoid the arm with live snakes, although results were non-significant. This suggests olfactory cues may not be representative of live snakes, or that auditory cues may be more important than olfactory cues for predator recognition. During late spring, female bats often avoid torpor for thermoregulatory or reproductive reasons. Therefore, bats may be alert to the approach of predators and be less responsive to chemical cues compared to other times of the year.]

Effects of Predation Risk on Vertical Habitat Use and Foraging of Pardosa milvina

AbstractAnimals face the risk of predation while engaging in regular activities, such as foraging, mate‐seeking, and reproducing. In order to avoid predation, prey can modify behavior to prevent capture. Pardosa milvina may climb in response to chemotactile cues of Hogna helluo, a larger cooccurring wolf spider, to avoid predation. The purpose of this study was to test the effects of the location of predator cues on the climbing response of P. milvina and to test how this antipredator behavior affected foraging success. In experimental arenas, when cues were on the bottom of the containers, P. milvina moved upward, and when cues were on the walls, individuals moved downward. These results suggest that P. milvina respond to H. helluo cues with general avoidance and do not automatically climb in response to the cues. As H. helluo spend most of their time on the ground, P. milvina may avoid predation by spending more time climbing in areas with H. helluo cues. The presence of predator cues significantly decreased foraging by P. milvina. But within the predator cue treatments, climbing ability had no effect on foraging, possibly due to the short height of the feeding arenas. Future studies are needed to determine if climbing by P. milvina in response to cues of H. helluo has direct and indirect negative effects on herbivores in the field.

Foraging by fearful frugivores: combined effect of fruit ripening and predation risk

Summary 1 Plant defensive compounds and predation risk are main determinants of herbivore foraging, though empirical studies have seldom measured the combined effects of these two factors. By considering the interaction between the herb Helleborus foetidus and its main fruit and seed predator, the Wood Mouse Apodemus sylvaticus, we evaluated whether the defensive role against seed predators of compounds present in H. foetidus unripe fruits holds across a micro-landscape that differs in foraging costs (i.e. predation risk). 2 First, we used standardized food patches that simulated fruiting H. foetidus plants to ascertain fruit preferences of captive mice. Then, by means of field experiments, we assessed the combined effects of fruit ripening and predation risk on foraging by free-ranging mice. 3 Captive mice avoided plants with unripe fruit and avoided consuming unripe fruits within a particular plant. Free-ranging mice also avoided unripe fruits in safe microhabitats (rocky substrate), but not in risky microhabitats (bare ground) where few fruits were consumed. This unexpected result may be driven by predation risk experienced by mice foraging on H. foetidus fruits, and/or plant defensive compounds acting in a dose-dependent manner. 4 Frugivorous mice responded to both chemical defences present in unripe H. foetidus fruits as well as to predation cost though such response was sequential. Plant defence compounds appeared to play a part in mouse foraging only after mice selected low predation risk microhabitats. 5 Our study indicates that both digestive and ecological factors influence foraging decisions, which in turn affects pressures exerted by herbivores on plant populations.

The effects of predation risk from crab spiders on bee foraging behavior

Recent studies have suggested that top–down effects of predation on plant–pollinator interactions may not be, as previously thought, rare and/or weak. In this paper, we explore the effects of crab spiders (Araneae: Thomisidae) on the behavior of 2 species of bee (Hymenoptera: Apidae) foraging for nectar and pollen on 3 different plant species in central Portugal. In 2 experiments, we found that the eusocial bee Apis mellifera was significantly less likely to inspect and accept a flower or inflorescence if it harbored a spider. In contrast, we found no such effects of spiders on the behavior of the solitary bee Eucera notata. Further experiments showed that the effects of environmental cues associated with predators on flower visitation by A. mellifera were detectable even when no spider was present at the moment a flower was encountered. Such indirect effects were only identified, however, in bees foraging on 1 of 2 plant species studied. In a final experiment, A. mellifera was shown to respond negatively to the presence of the corpses of conspecifics glued to flowers. This suggests that prey corpses left exposed on petals or bracts by spiders provide an obvious cue that bees can use to avoid predators. These results add to a growing body of evidence that plant–pollinator interactions are not immune to the effects of predation and suggest that the strength of such effects vary both between and within species.

Delayed effects of larval predation risk and food quality on anuran juvenile performance

Metamorphosis can disrupt the correlation structure between juvenile and adult traits, thus allowing relatively independent evolution in contrasting environments. We used a multiple experimental approach to investigate how diet quality and larval predation risk affected the rates of growth and development in painted frogs (Discoglossus galganoi), and how these changes influence post-metamorphic performance. A high-energy diet entailed growth advantages only if predation risk did not constrain energy acquisition, whereas diet quality affected primarily the extension of the larval period. Predation risk influenced juvenile shape, most likely via the effects on growth and differentiation rates. Juvenile frogs emerging from predator environments had shorter legs and longer bodies than those from the nonpredator tanks. However, these morphological changes did not translate into differences in relative jumping performance. Neither size-adjusted lipid storage nor fluctuating asymmetry was significantly influenced by food quality or predation risk. Our data suggest that the post-metamorphic costs of predator avoidance during the larval phase are mostly a consequence of small size at metamorphosis.