Cooperation under predation risk: a data-based ESS analysis

Two fish that jointly approach a predator in order to inspect it share the deadly risk of capture depending on the distance between them. Models are developed that seek ESS inspection distances of both single prey and pairs, based on experimental data of the risk that prey (sticklebacks) incur when they approach a predator (pike) to varying distances. Our analysis suggests that an optimal inspection distance can exist for a single fish, and for two equal fish behaving entirely cooperatively so as to maximize the fitness of the pair. Two equal fish inspecting cooperatively should inspect at an equal distance from the predator. The optimal distance is much closer to the predator for cooperative pairs than for single inspectors. However, optimal inspection for two equal fish behaving cooperatively operates across a rather narrow band of conditions relating to the benefits of cooperation. Evolutionarily stable inspection can also exist for two equal fish behaving non-cooperatively such that each acts to make a best reply (in terms of its personal fitness) to its opponent''s strategy. Non-cooperative pairs should also inspect at equal distance from the pike. Unlike the ''single fish'' and ''cooperative'' optima, which are unique inspection distances, there exists a range of ESS inspection distances. If either fish chooses to move to any point in this zone, the best reply of its opponent is to match it (move exactly alongside). Unilateral forward movement in the ''match zone'' may not be possible without some cooperation, but if the pair can ''agree'' to move forward synchronously, maintaining equal distance, inspection will occur at the nearest point in this zone to the predator. This ''near threshold'' is an ESS and is closer to the predator than the single fish optimum: pairs behaving almost selfishly can thus attain greater benefits from inspection by the protection gained from Hamilton''s dilution effect. That pairs should inspect more closely than single fish conforms with empirical findings. Phenotypic asymmetries in costs and benefits between the fish are not yet included in the model.     

Effects of sterile insect releases on a population under predation or parasitism

A continuous-time differential equation model was constructed which describes the population dynamics of a predator prey system in which sterile prey are released in a program designed to eradicate or reduce the prey population. It was found that the dynamics of the system behave quite differently when predators are present. Two conditions were found which have differing implications for the control program. If the predators still exist when the wild prey population declines to extinction, then the SIRM is assisted by the predators, sometimes to a considereble extent. If the predators decline to extinction before the wild prey population goes extinct, then the predators may or may not assist the SIRM depending on the parameters of the system. If the predators do assist the SIRM, then a potentially dangerous situation exists in which an explosion of the prey population could occur after the predators go extinct. Predator polyphagy would probably minimize this danger of an explosion since it would stabilize the predator population.     

Foraging patterns of voles at heterogeneous avian and uniform mustelid predation risk

Temporal variation of antipredatory behavior and a uniform distribution of predation risk over refuges and foraging sites may create foraging patterns different from those anticipated from risk in heterogenous habitats. We studied the temporal variation in foraging behavior of voles exposed to uniform mustelid predation risk and heterogeneous avian predation risk of different levels induced by vegetation types in eight outdoor enclosures (0.25 ha). We manipulated mustelid predation risk with weasel presence or absence and avian predation risk by reducing or providing local cover at experimental food patches. Foraging at food patches was monitored by collecting giving-up densities at artificial food patches, overall activity was automatically monitored, and mortality of voles was monitored by live-trapping and radiotracking. Voles depleted the food to lower levels in the sheltered patches than in the exposed ones. In enclosures with higher avian predation risk caused by lower vegetation height, trays were depleted to lower levels. Unexpectedly, voles foraged in more trays and depleted trays to lower levels in the presence of weasels than in the absence. Weasels match their prey's body size and locomotive abilities and therefore increase predation risk uniformly over both foraging sites and refuge sites that can both be entered by the predator. This reduces the costs of missing opportunities other than foraging. Voles changed their foraging strategy accordingly by specializing on the experimental food patches with predictable returns and probably reduced their foraging in the matrix of natural food source with unpredictable returns and high risk to encounter the weasel. Moreover, after 1 day of weasel presence, voles shifted their main foraging activities to avoid the diurnal weasel. This behavior facilitated bird predation, probably by nocturnal owls, and more voles were killed by birds than by weasels. Food patch use of voles in weasel enclosures increased with time. Voles had to balance the previously missed feeding opportunities by progressively concentrating on artificial food patches.     

Interacting effects of predation risk and food availability on larval anuran behaviour and development

Age and size at metamorphosis are two important fitness components in species with complex life cycles. In anurans, metamorphic traits show remarkable phenotypic plasticity, especially in response to changes in growth conditions. It is also possible that the perception of risk directly determines changes in larval period and the size of metamorphs. This study examines how the perception of predation risk affects the timing of and size at metamorphosis in common frogs (Rana temporaria). I raised tadpoles at two risk levels (fish-conditioned water or unconditioned water) crossed with the availability or lack of food at night (all tadpoles had food available in the day). Tadpoles reacted to chemical cues from predatory fish by decreasing activity. A novel behavioural result was a predation×food interaction effect on refuge use, which also accounted for most of the predator main effect: predation risk only caused increased refuge use in the night-starved treatment. Despite these behavioural modifications, the perception of predation risk did not affect growth rate and mass at metamorphosis in a simple way: the effects of food regime on growth and size at metamorphosis were dependent on the level of predation risk as revealed by significant predation×food interaction effects. Tadpoles who had food withheld at night metamorphosed at the smallest size, suggesting a negative relationship between size at metamorphosis and refuge use. Tadpoles raised in fish-conditioned water had longer larval periods than those in unconditioned water, but these differences were significant only if food was available at night. These results conflict with the hypotheses that tadpoles should reduce their larval period or growth rates (and hence metamorphose at a smaller size) as the risk of predation increases. In contrast to predation risk, food availability strongly affected the length of the larval period: night-starved tadpoles metamorphosed relatively early with or without fish stimulus. Thus, early metamorphosis resulted from periods of low food availability, but not from a heightened ”perceived risk” of predation. This example counters the hypothesis of acceleration of the developmental rate (which shortens the time to metamorphosis) as a mechanism to escape a risky environment. Received: 18 August 1999 / Accepted: 10 January 2000     

Characteristics of African wild dog natal dens selected under different interspecific predation pressures

To successfully reproduce, many carnivorous mammals need access to suitable den sites. Den site selection is often based on fitness related criteria like escape from predators, food availability and shelter from extreme weather conditions. African wild dogs are cooperative breeders that use a den to give birth to their offspring. They often co-exist with lions and spotted hyenas, both of which are known to kill African wild dog pups. Little is known about den site selection by African wild dogs. In this study, we compared vegetation characteristics and distribution of roads and waterholes around den sites and random sites, in areas with high and low lion and spotted hyena densities. In both areas, African wild dogs selected den sites in closed woodland with little visibility, which is likely to reduce detection by predators, increase the likelihood of escape when detected, and might provide shelter from extreme weather conditions. In the high predator density area, African wild dogs seemed to spatially avoid predators by selecting den sites in this type of habitat relatively further away from waterholes and roads. African wild dogs have high energetic costs of gestation. Therefore, even when predation risk is relatively low, they are likely to try to maximise their fitness by choosing a den site in habitat that will provide optimal protection for their offspring, leaving little additional options to respond to a higher predation pressure.     

The scale-dependent impact of wolf predation risk on resource selection by three sympatric ungulates

Resource selection is a fundamental ecological process impacting population dynamics and ecosystem structure. Understanding which factors drive selection is vital for effective species- and landscape-level management. We used resource selection probability functions (RSPFs) to study the influence of two forms of wolf (Canis lupus) predation risk, snow conditions and habitat variables on white-tailed deer (Odocoileus virginianus), elk (Cervus elaphus) and moose (Alces alces) resource selection in central Ontario's mixed forest French River-Burwash ecosystem. Direct predation risk was defined as the frequency of a predator's occurrence across the landscape and indirect predation risk as landscape features associated with a higher risk of predation. Models were developed for two winters, each at two spatial scales, using a combination of GIS-derived and ground-measured data. Ungulate presence was determined from snow track transects in 64 16- and 128 1-km(2) resource units, and direct predation risk from GPS radio collar locations of four adjacent wolf packs. Ungulates did not select resources based on the avoidance of areas of direct predation risk at any scale, and instead exhibited selection patterns that tradeoff predation risk minimization with forage and/or mobility requirements. Elk did not avoid indirect predation risk, while both deer and moose exhibited inconsistent responses to this risk. Direct predation risk was more important to models than indirect predation risk but overall, abiotic topographical factors were most influential. These results indicate that wolf predation risk does not limit ungulate habitat use at the scales investigated and that responses to spatial sources of predation risk are complex, incorporating a variety of anti-predator behaviours. Moose resource selection was influenced less by snow conditions than cover type, particularly selection for dense forest, whereas deer showed the opposite pattern. Temporal and spatial scale influenced resource selection by all ungulate species, underlining the importance of incorporating scale into resource selection studies.     

Risk of predation,hydrodynamic efficiency and their influence on school structure

Synopsis Laboratory studies were conducted on 15 schools of blackchin shiners, Notropis heterodon, to determine if they altered their structure in response to changing environmental demands. The hypothesis tested was that fish schools should sacrifice a flat, hydrodynamically efficient structure in favour of an unobstructed visual field in the presence of a predator by staggering in the vertical plane. Ten schools were exposed for two weeks to a simple environment with only a current. For the next two-week period a predator was added. Five control schools were exposed to the simple environment for both two-week periods. Six of the ten treated schools increased their staggering in the vertical dimension as predicted while none of the control schools changed. This result was suggestive that hydrodynamic advantages were sacrificed. Respirometer experiments indicated these fish were capable of achieving some hydrodynamic benefits from schooling but these benefits may be a function of fish size.     

Personality-dependent dispersal cancelled under predation risk

Dispersal is a fundamental life-history trait for many ecological processes. Recent studies suggest that dispersers, in comparison to residents, display various phenotypic specializations increasing their dispersal inclination or success. Among them, dispersers are believed to be consistently more bold, exploratory, asocial or aggressive than residents. These links between behavioural types and dispersal should vary with the cause of dispersal. However, with the exception of one study, personality-dependent dispersal has not been studied in contrasting environments. Here, we used mosquitofish (Gambusia affinis) to test whether personality-dependent dispersal varies with predation risk, a factor that should induce boldness or sociability-dependent dispersal. Corroborating previous studies, we found that dispersing mosquitofish are less social than non-dispersing fish when there was no predation risk. However, personality-dependent dispersal is negated under predation risk, dispersers having similar personality types to residents. Our results suggest that adaptive dispersal decisions could commonly depend on interactions between phenotypes and ecological contexts.     

Limits to predator regulation of rabbits in Australia: evidence from predator-removal experiments

Summary Predator-prey studies in semi-arid eastern Australia demonstrated that populations of rabbits (Oryctolagus cuniculus) could be regulated by predators. The functional, numerical and total responses of foxes (Vulpes vulpes) to rabbits and the numerical response of feral cats (Felis catus) to rabbits, are described. Measurement of the rabbit component of foxes' stomach contents indicates a Type III functional response. The size of the fox population in summer was dependent on the availability of rabbits over the immediately preceding rabbit breeding season but there appeared to be no density-dependent aggregation of young foxes in areas of surplus food. The total response of foxes, estimated using the short-term numerical response of dispersing foxes, was directly density-dependent for low rabbit densities and inversely density-dependent for high rabbit densities. Two states are possible with this form of total response: a state with low rabbit densities regulated by predators and a state with high rabbit densities which occurs when rabbits escape predator regulation. The boundary between regulation and non-regulation by predators was demonstrated by a predator-removal experiment. In the treated areas, predators were initially culled and rabbits increased to higher densities than in an untreated area where predators were always present. When predators were allowed back into the treated areas, rabbit populations continued to increase and did not decline to the density in the untreated area. This is the critical evidence for a two-state system. When predators were present, rabbits could be maintained at low densities which were in the density-dependent part of the total response curve for foxes. Exceptionally high rabbit recruitment, or artificially reduced predation, could result in rabbits escaping predator-regulation. Under these circumstances, rabbits could move into the inversely density-dependent region of the total response curve for foxes.     

Predator-induced changes in morphology of a prey fish: the effects of food level and temporal frequency of predation risk

In a series of experiments, we investigated the effects of food availability and risk frequency on the dynamics of predator-induced changes in growth and morphology of prey fish using goldfish (Carassius auratus) as our test species. In experiment 1, we fed goldfish high or low food rations and exposed them to either alarm cues from conspecifics, cues from swordtails or a water control. After 60 days, goldfish in the alarm cue treatment significantly increased their body depth and body weight but had smaller body length than goldfish exposed to swordtails cues or water, likely reducing their vulnerability to gape-limited predators. Importantly, food level had an impact on the amplitude of the morphological changes. In experiment 2, goldfish were exposed to two different frequencies of predation cues or a water control for 50 days. The cues were either continued or discontinued from day 51 to 100, and all cues were resumed from day 101 to 150. We found that goldfish exposed to predation cues increased their depth and weight at a faster rate than did the goldfish exposed to water, and of particular significance was the fact that frequency of risk had an effect on the amplitude of the change. When the cues were interrupted, the increase in growth rate parameters was reduced to the level of the goldfish exposed to water. However, when the cues were resumed, the rate increased to match the growth rate of the goldfish that were continuously exposed to the cues. Finally, we staged encounters between goldfish of differing morphologies and yellow perch (Perca flavescens) and found that deep-bodied goldfish had better survival than the shallow-bodied ones. These experiments illustrate the dynamic nature of inducible morphological defences.     

Optimal diving under the risk of predation

Many air-breathing aquatic foragers may be killed by aerial or subsurface predators while recovering oxygen at the surface; yet the influence of predation risk on time allocation during dive cycles is little known in spite of numerous studies on optimal diving. We modeled diving behavior under the risk of predation at the surface. The relationship between time spent at the surface and the risk of death is predicted to influence the optimal surface interval, regardless of whether foragers accumulate energy at a constant rate while at the food patch, deplete food resources over the course of the dive, or must search for food during the dive. When instantaneous predation risk during a single surface interval decreases with time spent at the surface, a diver should increase its surface interval relative to that which maximizes energy intake, thereby increasing dive durations and reducing the number of surfacings per foraging bout. When instantaneous risk over a single surface interval does not change or increases with increasing time at the surface, divers should decrease their surface interval (and consequently their dive duration) relative to that which maximizes energy intake resulting in more dives per foraging bout. The fitness consequences of selecting a suboptimal surface interval vary with the risk function and the way divers harvest energy when at depth. Finally, predation risk during surface intervals should have important consequences for habitat selection and other aspects of the behavioral ecology of air-breathing aquatic organisms.     

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.     

Life-history decisions under predation risk: Importance of a game perspective

We model ontogenetic shifts (e.g. in food or habitat use) during development under predation risk. We ask whether inclusion of state and frequency dependence will provide new insights when compared with game-free life-history theory. We model a simple biological scenario in which a prey animal must switch from a low-predation, low-growth habitat to a high-predation, high-growth habitat. To assess the importance of frequency dependence, we compare the results of four scenarios of increasing complexity: (1) no predation; (2) constant predation; (3) frequency-dependent predation (predation risk diluted at high prey density); and (4) frequency-dependent predation as in (3) but with predators allowed to respond adaptively to prey behaviour. State dependence is included in all scenarios through initial size, assumed to be environmental. A genetic algorithm is used to search for optimal solutions to the scenarios. We find substantially different results in the four different scenarios and suggest a decision tree by which biological systems could be tested to ascertain which scenario is most applicable.     

The effects of turbidity and vegetation on the risk of juvenile salmonids, Oncorhynchus spp., to predation by adult cutthroat trout, O. clarkii

Synopsis During their seaward migration, juvenile salmonids encounter structural and visual cover which varies between and within watersheds. In this study, the effects of two types of cover (turbidity and artificial vegetation) on the predation mortality of juvenile salmonids exposed to fish piscivores was investigated in outdoor concrete ponds. During experiments, adult coastal cutthroat trout, Oncorhynchus clarkii clarkii, were allowed to feed on juvenile salmonid prey — chinook salmon, O. tshawytscha, chum salmon, O. keta, sockeye salmon, O. nerka, and cutthroat trout — in separate trials. Daily instantaneous per capita predation rate was determined for each turbidity and vegetation treatment, within each trial. Mean predation rates varied between 1% and 76% daily. In the presence of cover, mean daily predation rates were 10–75% lower than those in controls (no vegetation and clear water), depending on prey species. Predation rates were significantly lower in the presence of vegetation cover and did not covary with prey size or species. The effects of turbidity were generally not significant and were not additive with the effects of vegetation. However, turbidity appeared to significantly reduce the effectiveness of vegetation as cover for juvenile chinook and sockeye salmon. We suggest that these two forms of cover do not affect risk of predation by fish piscivores to juvenile salmonids via the same mechanism.     

Colour polymorphism and selective predation experiments

The significance of searching-image behaviour by predators in relation to visually polymorphic prey is outlined in relation to its possible role in maintaining these polymorphisms by frequency-dependent selection. A simple experimental design is described whereby innate preferences, selection for crypsis, and searching-image behaviour can be distinguished. Four experimental situations are described ranging from an artificial predator-prey relationship (humans as predators—dyed toothpicks as prey) to a semi-natural one (wild song thrushes as predators—artificial populations of live polymorphic snails as prey). The results suggest that “naive” predators may regularly form searching-images for a frequent prey type.     

Spatial refuge from intraguild predation: implications for prey suppression and trophic cascades

The ability of predators to elicit a trophic cascade with positive impacts on primary productivity may depend on the complexity of the habitat where the players interact. In structurally-simple habitats, trophic interactions among predators, such as intraguild predation, can diminish the cascading effects of a predator community on herbivore suppression and plant biomass. However, complex habitats may provide a spatial refuge for predators from intraguild predation, enhance the collective ability of multiple predator species to limit herbivore populations, and thus increase the overall strength of a trophic cascade on plant productivity. Using the community of terrestrial arthropods inhabiting Atlantic coastal salt marshes, this study examined the impact of predation by an assemblage of predators containing Pardosa wolf spiders, Grammonota web-building spiders, and Tytthus mirid bugs on herbivore populations (Prokelisia planthoppers) and on the biomass of Spartina cordgrass in simple (thatch-free) and complex (thatch-rich) vegetation. We found that complex-structured habitats enhanced planthopper suppression by the predator assemblage because habitats with thatch provided a refuge for predators from intraguild predation including cannibalism. The ultimate result of reduced antagonistic interactions among predator species and increased prey suppression was enhanced conductance of predator effects through the food web to positively impact primary producers. Behavioral observations in the laboratory confirmed that intraguild predation occurred in the simple, thatch-free habitat, and that the encounter and capture rates of intraguild prey by intraguild predators was diminished in the presence of thatch. On the other hand, there was no effect of thatch on the encounter and capture rates of herbivores by predators. The differential impact of thatch on the susceptibility of intraguild and herbivorous prey resulted in enhanced top-down effects in the thatch-rich habitat. Therefore, changes in habitat complexity can enhance trophic cascades by predator communities and positively impact productivity by moderating negative interactions among predators.     

Temporal variation in predation risk: stage-dependency, graded responses and fitness costs in tadpole antipredator defences

Temporal variation in predation risk may be an important determinant of prey antipredator behaviours. According to the risk allocation hypothesis, the strongest antipredator behaviours are expected when periods of high risk are short and infrequent. We tested this prediction in a laboratory experiment where common frog Rana temporaria tadpoles were raised form early larval stages until metamorphosis. We manipulated the time a predatory Aeshna dragonfly larva was present and recorded behavioural responses (activity) of the tadpoles at three different time points during the tadpoles' development. We also investigated how tadpole shape, size and age at metamorphosis were affected by temporal variation in predation risk. We found that during the two first time points activity was always lowest in the constant high-risk situation. However, antipredator response in the two treatments with brief high-risk situation increased as tadpoles developed, and by the third time point, when the tadpoles were close to metamorphosis, activity was as low as in the constant high-risk situation. Exposure to chemical cues of a predation event tended to reduce activity during the first time period, but caused no response later on. Induced morphological changes (deeper tail and shorter relative body length) were graded the response being stronger as the time spent in the proximity of predator increased. Tadpoles in the brief risk and chemical cue treatments showed intermediate responses. Modification of life history was only found in the constant high-risk treatment in which tadpoles had longer larval period and larger metamorphic size. Our results indicate that both behavioural and morphological defences were sensitive to temporal variation in predation risk, but behaviour did not respond in the manner predicted by the risk allocation model. We discuss the roles of concentration of predator chemical cues and prey stage-dependency in determining these responses.