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Organisms living in extreme habitats require costly adaptations to cope with these conditions. Among the suggested potential benefits that trade off these costs is refuge from predation. To study these interactions in extreme environments, samples were taken in the cave Cueva de Villa Luz, Tabasco, Mexico, where more than 32 subterranean springs, some [H.sub.2]S rich, rise from the floor. Hydrogen sulfide gas plus oxygen is absorbed by freshwater, and oxidation forms concentrated sulfuric acid. Snottites, whitish hollow mucous tubes, hang from the ceiling of the cave. Fluid drops from these snottites were recorded as having pH values of 0-3. We report the discovery of a new species of nematode that thrives in the highly acidic environment of the snottite. Micro CT scan of snottites reveals a complex interaction between the acidic snottite, nematodes, and abundant nematode-eating mites. The nematode adaptation to low pH probably protects them against mite predation, for which nematodes are most likely the most important source of carbon in this sulfur-driven ecosystem., Introduction Organisms living in extreme habitats require costly adaptations to cope with these conditions. Among the suggested potential benefits that trade off these costs is refuge from predation (Romero and [...]

Animals using sound for communication emit directional signals, focusing most acoustic energy in one direction. Echolocating bats are listening for soft echoes from insects. Therefore, a directional biosonar sound beam greatly increases detection probability in the forward direction and decreases off-axis echoes. However, high directionality has context-specific disadvantages: at close range the detection space will be vastly reduced, making a broad beam favorable. Hence, a flexible system would be very advantageous. We investigated whether bats can dynamically change directionality of their biosonar during aerial pursuit of insects. We trained five Myotis daubentonii and one Eptesicus serotinus to capture tethered mealworms and recorded their echolocation signals with a multimicrophone array. The results show that the bats broaden the echolocation beam drastically in the terminal phase of prey pursuit. M. daubentonii increased the half-amplitude angle from approximately 40[degrees] to approximately 90[degrees] horizontally and from approximately 45[degrees] to more than 90[degrees] vertically. The increase in beam width is achieved by lowering the frequency by roughly one octave from approximately 55 kHz to approximately 27.5 kHz. The E. serotinus showed beam broadening remarkably similar to that of M, daubentonii. Our results demonstrate dynamic control of beam width in both species. Hence, we propose directionality as an explanation for the frequency decrease observed in the buzz of aerial hawking vespertilionid bats. We predict that future studies will reveal dynamic control of beam width in a broad range of acoustically communicating animals. directionality | echolocation | buzz | frequency | perception doi/ 10.1073/pnas.1006630107

Ecologists have long studied the effect of predators on prey population abundance while evolutionary biologists have measured prey trait evolution in response to predation. Ecological and evolutionary processes were generally thought to occur on different time scales, but recent evidence suggests that evolution may alter the ecological effects of predation over the course of ecological experiments. We used a protozoan and its mosquito-larvae predator, naturally found in the water-filled leaves of pitcher plants, to examine the effect of prey evolution on predator--prey interactions. In experiments conducted over 12 days (approximately 50 prey generations, but less than one predator generation), we measured a decrease in the effect of mosquito larvae predators on protozoa prey populations. In a separate set of experiments, we found that the presence of predators corresponded with evolution of smaller cell size and increased population growth rate. In ecological experiments, two situations commonly occur: strong selection pressure applied by the treatment itself and discrepancies in generation times of associate species. Our results suggest that in either situation, the resulting evolutionary patterns may lead to dramatic and important changes in ecological effect size. Key words: Colpoda; effect size; mosquito larvae; predator; prey; protozoa; rapid evolution; resistance; Sarracenia purpurea; tolerance.

ABSTRACT Predation by drilling gastropods is preservable in the fossil record and thus provides valuable examples for the study of evolutionary concepts that emphasize the importance of biotic interactions. For [...]

Acorn weevils (Coleoptera: Curculionidae) are well-known predators of oak acorns in the eastern hardwood forest region of the United States. We examined the germination percentage of seeds, as well as the physical characteristics of seedlings, originating from both weevil-damaged and sound acorns of red oak (Quercus rubra L.) to determine if weevil depredation impacts seedling vigor. Seeds were grown under greenhouse conditions for a period of 8 weeks. X-ray imaging was used as a nondestructive method for determining seed condition prior to germination. The combined data set showed a reduction in germination percentage from 86% for sound acorns to 26% for those damaged by weevils. Seven seedling metrics were evaluated for differences between seedlings originating from sound and those from weevil-damaged acorns. Of these metrics, the number of leaf flushes, total number of leaves, root collar diameter, shoot length, and dry mass were significantly (P < 0.05) different. Two variables, root length and shoot diameter, were not significantly different. Early allocation of resources to root establishment likely explains these findings. Because of reduced stem height and leaf number, seedlings from weevil-damaged acorns are likely to be less competitive in the forest understory than those from sound acorns. Les charancons des glands (Coleoptera : Curculionidae) sont des predateurs bien connus des glands de chene dans la region de la foret feuillue de l'est des Etats-Unis. Nous avons etudie le pourcentage de germination des graines, ainsi que les caracteristiques physiques des semis, provenant de glands endommages par les charancons et de glands sains de chene rouge (Quercus rubra L.) pour determiner si les ravages causes par les charancons ont un impact sur la vigueur des semis. Les graines ont ete cultivees en serre pendant une periode de huit semaines. L'imagerie par rayons X a ete utilisee comme methode non destructive pour determiner l'etat des graines avant la germination. Le jeu des donnees combines a montre que le pourcentage de germination diminuait de 86 % dans le cas des glands sains a 26 % dans le cas des glands endommages par les charancons. Sept metriques des semis ont ete evaluees pour comparer les semis provenant de glands sains et les semis provenant de glands endommages par les charancons. Parmi ces metriques, le nombre de pousses foliaires, le nombre total de feuilles, le diametre au collet, la longueur de la tige et le poids sec etaient significativement (P < 0,05) differents. Deux variables : la longueur des racines et le diametre de la tige, n'etaient pas significativement differentes. L'allocation precoce de ressources a l'etablissement des racines explique probablement ces resultats. A cause de la hauteur reduite de la tige et du plus petit nombre de feuilles, les semis provenant de glands endommages par les charancons ont des chances d'etre moins competitifs dans le sous-bois des forets que les semis provenant de glands sains. [Traduit par la Redaction], Introduction Oaks (Quercus spp.) constitute a major component of the tree canopy throughout the eastern hardwood forest region and are often regarded as a keystone group. However, over the last [...]

We investigated the effects of fluctuating prey numbers on the foraging strategies and potential mechanisms for coexistence of 2 sympatric predators, coyotes (Canis latrans) and red foxes (Vulpes vulpes), in a heterogeneous environment in northeastern Illinois. We quantified the availability of mammalian and avian prey across different habitats, analyzed scats collected along standardized routes for occurrence of prey items, and estimated the number and biomass of vertebrate prey consumed. Abundances of the most frequently consumed prey (voles, rabbits, and mice) differed significantly over time, especially in relation to an ice storm that occurred during the study. Coyotes and red foxes tended to select voles and rabbits in number, rabbits in biomass, and incorporated greater numbers and biomass of other prey such as deer, pheasants, and sciurids after the ice storm. Log-linear analyses indicated that both coyotes and red foxes exhibited switching behavior, with differential shifts among the number and biomass of alternative prey they consumed. Hence, foraging strategies of coyotes and red foxes appeared to be a combination of prey selectivity and switching behavior. Our study suggests that competition between coyotes and red foxes for similar primary prey species and limited environments to exploit at the urban-rural interface may contribute to displacement of red foxes by coyotes. Key words: Canis latrans, competition avoidance, coyote, Illinois, log-linear analysis, prey selection, prey switching, red fox, Vulpes vulpes

Activity levels and foraging success of ectotherms are dependent both on extrinsic factors (e.g., ambient temperature) and intrinsic factors (e.g., hunger level). We focus here on activity and foraging of sand vipers (Cerastes vipera (L., 1758)) (Squamata: Viperidae) in the northwestern Negev desert of Israel. Viper activity is bounded by a specific thermal range, but within this range, temperature does not affect the distance moved. Movement distances are, however, positively correlated with relative humidity. The capture success of lizards, the main prey of this viper, is strongly related both to movement distances of the snake and to the number of the lizard burrows scanned. Body size and shape have a direct effect on the number of burrows scanned and on the number of lizards captured. The increase in capture success with size is probably due to the consequent increase in the range of prey items the sand viper can subdue or handle. Vipers are larger and have a higher mass to length ratio in one of the three study sites, characterized by higher mean annual rainfall and vegetation cover. Finally, females tend to have a higher mass to length ratio than males, which may provide some space within the female body for reproductive materials such as eggs. Les niveaux d'activite et de recherche de nourriture des ectothermes dependent a la fois de facteurs extrinseques (par ex., la temperature ambiante) et de facteurs intrinseques (par ex., l'intensite de la faim). Nous nous interessons ici a l'activite et la recherche de nourriture de la vipere des sables (Cerastes vipera (L., 1758)) (Squamata : Viperidae) dans le nord-ouest du desert du Neguev en Israel. L'activite de la vipere est restreinte a une gamme specifique de temperatures, mais dans cette gamme, la temperature n'affecte pas la distance des deplacements. Il y a cependant une correlation positive entre la distance des deplacements et l'humidite relative. Il existe une forte relation positive entre le succes de la capture de lezards, les proies principales de cette vipere, et a la fois la distance des deplacements du serpent et le nombre de terriers de lezards examines. La taille et la forme du corps ont un effet direct sur le nombre de terriers examines et le nombre de lezards captures. L'accroissement des captures en fonction de la taille s'explique probablement par l'augmentation reliee a la taille de l'entendue des types de proies que la vipere des sables peut maitriser ou manipuler. Les vipers sont plus grandes et possedent un rapport masse/longueur plus eleve dans l'un des trois sites d'etude, caracterise par des precipitations moyennes annuelles plus elevees et une couverture vegetale plus importante. Enfin, les femelles ont tendance a avoir un rapport masse/longueur plus eleve que celui des males, ce qui peut fournir aux femelles de l'espace a l'interieur du corps pour les produits de la reproduction, tels que les oeufs. [Traduit par la Redaction], Introduction All foraging activities of animals are affected by a variety of extrinsic factors (Bell 1990) like prey density and spatial distribution (e.g., Bond 1980), ambient temperature and humidity (e.g., [...]

We evaluated hypotheses explaining risk of predation by American Black Bear (Ursus americanus) at 418 Yellow-bellied Sapsucker (Sphyrapicus varius) and Hairy Woodpecker (Picoides villosus) nests, on the basis of nestling begging and nest-site habitat features in Algonquin Provincial Park, Ontario. Ninety-three percent of Yellow-bellied Sapsuckers in stands dominated by Sugar Maple (Acersaccharum) nested in Sugar Maple or American Beech (Fagus grandifolia) trees that were dead or in declining health, whereas 86% of Yellow-bellied Sapsuckers in stands dominated by aspen (Populus spp.) nested in Quaking Aspen (P. tremuloides) that were in declining health. Black Bears depredated 17% of 315 nests of Yellow-bellied Sapsuckers in Sugar Maple stands, which accounts for 71% of all Yellow-bellied Sapsucker nest failures. Only 1 (2%) of 46 Hairy Woodpecker nests in the same Sugar Maple stands was depredated by a bear. None of 51 Yellow-bellied Sapsucker nests in aspen stands was depredated. In Sugar Maple stands, daily nest survival of Yellow-bellied Sapsucker nests was lowest when nestling begging calls were loudest and carried the farthest, in more recently harvested stands, and in trees other than American Beech (mostly Sugar Maple). Nest substrates were hardest at Hairy Woodpecker nests, followed by successful Yellow-bellied Sapsucker nests in American Beech and Quaking Aspen; Yellow-bellied Sapsucker nests were softest in stands that had been harvested within the past 30 years. Our study suggests that the risk of predation by American Black Bears at woodpecker nests is a combined function of nestling begging calls, which attract bears to the nest, and nest habitat characteristics, which influence accessibility to the interior of the cavity. Key words: American Black Bear, cavity-nest site selection, nestling begging calls, Sphyrapicus varius, tree harvest, Ursus americanus, Yellow-bellied Sapsucker.

We studied 19 color-banded groups of the cooperatively breeding Superb Fairy-wren (Malurus cyaneus) to determine (1) the contributions of breeding and helper males to antipredator vigilance, (2) whether such vigilance reduces predation risk, and (3) the mechanisms by which it might do so. Time spent as a sentinel (perching and scanning from conspicuous locations within sight of the nest) increased with group size, but successful and depredated nests did not differ significantly in sentinel time, and sentinels did not appear to coordinate their vigilance. Both breeder and helper males acted as sentinels, and both were more vigilant when nests contained nestlings than when they contained eggs. Breeders with helpers spent more time as sentinels than those without helpers. Presence of a sentinel reduced the time feeding adults spent pausing to scan when approaching and leaving the nest. Thus, vigilance could enable a male to detect a predator and decoy it away from the nest site or prevent it from locating the nest by deferring feeding visits of other provisioners, but we could not demonstrate that it reduced nest predation by the major nest predator, the Pied Currawong (Strepera graculina). Key words: antipredator behavior, cooperative breeding, Malurus cyaneus, parental behavior, Superb Fairy-wren.

Although multiple predator effects and trophic cascades have both been demonstrated in a wide variety of ecosystems, ecologists have yet to incorporate these studies into an experimental framework that also manipulates a common and likely important factor, spatial heterogeneity. We manipulated habitat complexity, the presence of two top predators (toadfish and blue crabs), and one intermediate predator (mud crabs) to determine whether habitat complexity influences the strength of multiple predator interactions across multiple trophic levels in experimental oyster reef communities. In the absence of toadfish, blue crabs caused significant mud crab mortality. Despite also directly consuming mud crabs, toadfish indirectly benefited this intermediate predator by decreasing blue crab consumption of mud crabs. Toadfish suppression of mud crab foraging activity, and thus decreased mud crab encounters with blue crabs, is likely responsible for this counterintuitive result. Contrary to previous investigations which suggest that more complex habitats reduce interference interactions among predators, reef complexity strengthened emergent multiple predator effects (MPEs) on mud crabs. The degree to which these MPEs cascaded down to benefit juvenile oysters (basal prey) depended on both habitat complexity and nonconsumptive effects derived from predator--predator interactions. Habitat complexity reduced the foraging efficiency of each crab species individually but released crab interference interactions when together, so that the two crabs collectively consumed more oysters on complex reefs. Regardless of reef complexity, toadfish consistently decreased consumption of oysters by both crab species individually and when together. Therefore, interactions between predator identity and habitat complexity structure trophic cascades on oyster reefs. Furthermore, these cascading effects of multiple predators were largely mediated by nonconsumptive effects in this system. Key words: blue crabs; consumptive effects; density-mediated indirect interactions (DMIIs); habitat complexity; mud crabs; multiple-predator interactions; nonconsumptive effects; oysters; predator avoidance behavior; toadfish; trait-mediated indirect interactions (TMIIs); trophic cascades.

We hypothesized that the foraging behavior of urban birds would be less sensitive to variation in the cost of predation than that of nonurban bird species. We predicted that food-patch utilization by House Sparrows (Passer domesticus), a widespread human commensal, would be less sensitive to variation in predation risk than that of Spanish Sparrows (P. hispaniolensis), its nonurban counterpart. We applied the giving-up density (GUD) method to quantify patch use behavior of the two species with respect to distance from shelter. Our results indicated that Spanish Sparrows perceived a steep gradient of increased predation cost with increasing distance from shelter, while House Sparrows seemed indifferent to distance from shelter. These results support the hypothesis that the cost of predation has less effect on the foraging behavior of urban than nonurban birds, although alternative explanations include differences between species in the degree of neophobia, or variations in their ability to assess predation risk. Keywords: competition, foraging behavior; predation, seed trays, sparrows, urban ecosystem. Planteamos la hipotesis de que el comportamiento de forrajeo de las aves urbanas seria menos sensible a la variacion en el costo de depredacion que el de las especies de aves no urbanas. Predijimos que la utilizacion de parches de alimentos por parte de Passer domesticus, un comensal de los humanos ampliamente distribuido, seria menos sensible a la variacion en el riesgo de depredacion que la de P. hispaniolensis, su contraparte no urbana. Aplicamos el metodo de densidad de abandono para cuantificar el comportamiento de uso de los parches por parte de las dos especies con respecto a la distancia a los refugios. Nuestros resultados indicaron que P. hispaniolensis percibio un gradiente marcado de incremento del costo de la depredacion al aumentar la distancia a los refugios, mientras que P. domesticus parecio indiferente a la distancia al refugio. Estos resultados avalan la hipotesis de que el costo de depredacion tiene menos efecto sobre el comportamiento de forrajeo de las aves urbanas que sobre el de las no urbanas, aunque existen explicaciones alternativas como la existencia de diferencias entre las especies en el grado de neofobia o de variaciones en sus habilidades para evaluar el riesgo de depredacion.

The question of whether deterministic chaos occurs in natural populations has been discussed since the 1970s following the discovery that simple population models can generate chaotic dynamics. Natural populations undergo a diverse mixture of deterministic and stochastic processes that define population dynamics. In most habitats populations are also exposed to changes in biotic and abiotic parameters. Models predict that shifts in ecological parameters may lead to a transition between deterministic chaos, stable equilibria, and limit cycles, yet clear examples from empirical studies are rare. However, such transitions should be considered when discussing the occurrence of chaos in nature because ecological time series are in general short and have large sampling intervals. Here we document short-term transitions in population dynamics to and from chaos in an experimental system. Manipulation of only one experimental parameter (chemostat dilution rate) in a multi-species food web of two bacteria and a bacterivorous ciliate showed that switching between different dynamic behaviors occured with surprising rapidity in the microbial populations. Thus, short periods of chaotic dynamics may easily be overlooked in field observations. Key words: bacteria; chaos; plankton dynamics; predator--prey; Tetrahymena pyriformis; transition in population dynamics.

Cannibalism is ubiquitous in natural communities and has the potential to alter the functional relationship of predator-prey interactions. Although cannibalistic species are frequently subject to predation, the consequences of cannibalism in the prey for predator-prey interactions are poorly understood. Using a dragonfly larvae system, I provide the first experimental evidence that cannibalism in the prey creates behavior- and density-mediated indirect effects that result in nonlinear predator--prey interactions. As a consequence, cannibalism in the prey altered the functional relationship of the predator and its prey and reduced the impact of the predator on prey mortality by 47%. By parameterizing a mechanistic predation model, I show that the nonlethal interaction between cannibals and predators reduced cannibalism rates, which explained almost two times more of the observed mortality reduction than the consumption of cannibals. However, only a model that accounted for both behavioral interactions and the consumption of cannibals could predict ~100% of the observed mortality. Using the mechanistic model, I discuss the long-term effects of cannibalism on community dynamics and how they can differ from effects of simple density-dependent mortality. In general, these results demonstrate the importance of accounting for the trophic structure in cannibalistic populations and the resulting nonlinear interactions to predict predator--prey dynamics. Key words: community dynamics; competition; density-dependent mortality; intraguild predation; multiple predators; ontogenetic niche shift: predator--prey dynamics; risk reduction; stage- or size-structured interactions; trait-mediated indirect interactions; trophic structure.

The adaptive function of cheek pouches in the primate subfamily Cercopithecinae remains unresolved. By analyzing the circumstances of cheek pouch use, we tested two hypotheses for the evolution of cercopithecine cheek pouches proposed in earlier studies: (1) cheek pouches reduce vulnerability to predation, and (2) cheek pouches increase feeding efficiency by reducing competition. We studied two groups of wild blue monkeys (Cercopithecus mitis stuhlmanni) in the Kakamega Forest, Kenya, conducting focal observations of feeding individuals. Monkeys were less exposed while emptying their cheek pouches than filling them, supporting the predation-avoidance hypothesis. We investigated several measures of competitive threat, but only one supported the competition-reduction hypothesis: when the nearest neighbor's rank increased, subjects were more likely to increase than to decrease cheek pouch use. Overall, our findings supported the predation-avoidance hypothesis more strongly than the competition-reduction hypothesis. We suggest that variation in cheek pouch use may reflect differing behavioral strategies used by cercopithecines to mitigate competition and predation, as well as factors such as resource size and distribution, home range size, and travel patterns. KEY WORDS Cercopithecidae; foraging strategies; Kenya; predator avoidance; nearest-neighbor dominance score

We describe the occurrence and behavior of Razorbills (Alca torda) visiting Coats Island, Northern Hudson Bay, an area where sea-ice cover in summer has been much reduced since the mid-1990s. Coats Island is 300 km from the previous mostwesterly breeding site for the species and nearly 2,000 km from the nearest large colony, in Newfoundland and Labrador. Razorbills appeared at Coats Island coincidentally with an increase in the delivery of Capelin (Mallotus villosus) and sand lance (Ammodytes spp.) to Thick-billed Murre (Uria lomvia) nestlings at the same site and disappeared when sand lance also disappeared. Razorbill populations are expanding in eastern North America, and this expansion may partly account for their dispersal. The ability of Razorbills to track changes in a preferred prey item well outside the boundary of their normal range suggests that this species is capable of adapting rapidly to climate change. Key words: Alca torda, Ammodytes, climate change, Hudson Bay, range extension, Razorbill. Nous decrivons la presence et le comportement d'Alca torda frequentant l'ile Coats, dans le nord de la baje d'Hudson, une zone ou la couverture estivale de glace en mera ete fortement reduite depuis le milieu des annees 1990. L'ile Coats se trouve 300 km du site de reproduction de l'espece le plus h l'ouest et pros de 2000 km de la plus proche grosse colonie, a Terre-Neuve-et-Labrador. L'apparition d'A. Torda a l'ile Coats a coincide avec l'augmentation de la distribution de Mallotus villosus et d'Ammodytes spp. aux oisillons de Uria lomvia au meme site. A. torda a disparu du site en meme temps qu'Ammodytes spp. Les populations d'A. torda sont en expansion dans l'est de l'Amerique du Nord et cette expansion peut en partie expliquer leur dispersion. La capacite d'A. tord a suivre les changements d'une proie preferentielle en dehors des limites de sa epartition normale suggere que cette espece est capable de s'adapter rapidement aux changements climatiques.

Natural stress gradients can reduce predation intensity and increase prey abundances. Whether the harsh conditions of anthropogenic habitat degradation can similarly reduce predation intensity and structure community dynamics remains largely unexplored. Oxygen depletion in coastal waters (hypoxia) is a form of degradation that has recently emerged as one of the greatest threats to coastal ecosystems worldwide due to increased rates of eutrophication and climate change. I conducted field experiments and surveys to test whether relaxed predation could explain the paradoxically high abundance of clams that have sustained a fishery in a degraded estuary with chronic hypoxic conditions. Hypoxia reduced predation on all experimental species but enhanced the long-term survivorship of only sufficiently hypoxia-tolerant prey due to periodic extreme conditions. As a consequence, only the harvested quahog clam (Mercenaria mercenaria) thrived in hypoxic areas that were otherwise rendered dead zones with depauperate diversity and low abundances of other species. This suggests that enhanced populations of some key species may be part of a predictable nonlinear community response that sustains ecosystem services and masks overall downward trends of habitat degradation. Key words: biodiversity; bivalve; dead zone, ecosystem service; filtration; fishery; environmental stress models; hypoxia; Mercenaria, Mya, and Mytilus; Narragansett Bay, Rhode Island, USA; nonlinear ecosystem responses; predation refuge.

It is well established that predators can scare as well as consume their prey. In many systems, the fear of being eaten causes trait-mediated cascades whose strength can rival or exceed that of more widely recognized density-mediated cascades transmitted by predators that consume their prey. Despite this progress it is only beginning to be understood how the influence of predation risk is shaped by environmental context and whether it can exert an important influence on ecosystem-level processes. This study used a factorial mesocosm experiment that manipulated basal-resource identity (either barnacles, Semibalanus balanoides, or mussels, Mytilus edulis) to determine how resources modify the influence of predation risk, cascade strength, and the efficiency of energy transfer in two, tritrophic, rocky-shore food chains containing the predatory green crab (Carcinus maenas) and an intermediate consumer (the snail, Nucella lapillus). The effect of predation risk and the strength of trait-mediated cascades (both in absolute and relative terms) were much stronger in the barnacle than in the mussel food chain. Moreover, predation risk strongly diminished the efficiency of energy transfer in the barnacle food chain but had no significant effect in the mussel food chain. The influence of resource identity on indirect-effect strength and energy transfer was likely caused by differences in how each resource shapes the degree of risk perceived by prey. We suggest that our understanding of the connection between trophic dynamics and ecosystem functioning will improve considerably once the effects of predation risk on individual behavior and physiology are considered. Key words: barnacle cf. mussel food chain; consumptive and nonconsumptive effects; density-mediated effects; ecosystem function; energy transfer; mesocosm experiment; predation risk perception; resource identity; trait-mediated effects; tritrophic rocky-shore food chains; trophic cascade.

The prevalence of intraguild predation (IGP) in productive environments has long puzzled ecologists. Theory predicts the exclusion of intraguild prey from such environments, but data consistently defy this expectation. This suggests that coexistence mechanisms at high resource productivity may differ from those at lower productivity. Here I present a mathematical model that investigates multiple coexistence mechanisms. I incorporate two biological features widely observed in IGP communities: intraspecific interference via cannibalism or superparasitism, and temporal refuges arising from differential sensitivities to abiotic variation. I develop predictions based on three aspects of the IG prey--IG predator interaction: mutual invasibility, transient dynamics, and long-term abundances. These predictions specify the conditions under which coexistence mechanisms reinforce vs. deter one another: when a competition--IGP trade-off allows coexistence at intermediate productivity a temporal refuge for the intraguild prey always allows coexistence at high productivity, but intraspecific interference does so only at a net fitness cost to the intraguild predator. Intraspecific interference that benefits the intraguild predator not only reduces tradeoff-mediated coexistence at intermediate productivity, but also undermines the refuge's coexistence-enhancing effect at high productivity. Different mechanism combinations yield characteristic signatures in time series data during transient dynamics. By judicious measurement of parameters and examining time series for critical signatures, one can elucidate the mechanisms that allow IGP to prevail in resource-rich environments. Key words: coexistence; competition--IGP trade-off; intraguild predation (IGP); intraspecific interference; temporal refuge.

We determine if lacustrine salmonids show large-scale patterns of coexistence with the keystone predator northern pike (Esox lucius) and test an approach to predict fish communities using coexistence rules set in the context of three hierarchical filters that a species must pass to be present. The mutually exclusive species distribution patterns that we detected among 1029 lakes were repeatedly verified from results of whole-lake interventions with rotenone and introductions. Essentially, pike did not coexist with self-sustaining salmonid populations in lakes. High connectivity to pike (derived from maps) largely predicts the absence of lacustrine salmonids. Our analysis strongly suggests that pike prevented self-sustaining populations of brown trout (Salmo trutta), Arctic char (Salvelinus alpinus), and brook trout (Salvelinus fontinalis) in lakes. High connectivity to pike resulted in nonsalmonid lake fish communities, most often including both European perch (Perca fluviatilis) and roach (Rutilus rutilus). Our analysis suggests that if pike were not present in many boreal lakes where they now dwell, salmonid fish assemblages would prevail, a sharp contrast from the present pike-driven homogenized state with mainly nonsalmonid fish communities. Nous determinons si les salmonides lacustres possedent des patrons de coexistence a grande echelle avec le brochet du nord (Esox lucius), leur predateur le plus important, et nous testons une methode pour predire les communautes de poissons a l'aide de regles de coexistence qui comprennent trois filtres hierarchiques qu'une espece doit traverser pour etre presente. Les patrons de repartition des especes mutuellement exclusives que nous avons detectes dans les 1029 lacs se verifient a repetition lors d'interventions a la rotenone ou d'introductions a l'echelle des lacs entiers. En somme, les brochets ne se retrouvent pas avec des populations autosuffisantes de salmonides dans les lacs. Une forte connectivite avec le brochet (deduite a partir de cartes geographiques) permet la plupart du temps de predire l'absence de salmonides lacustres. Notre analyse indique clairement que le brochet empeche l'etablissement de populations autosuffisantes de truites brunes (Salmo trutta), d'ombles chevaliers (Salvelinus alpinus) et d'ombles de fontaine (Salvelinus fontinalis) dans les lacs. Une connectivite au brochet produit des communautes de poissons lacustres sans salmonides, mais contenant souvent a la fois la perche (Perca fluviatilis) et le gardon (Rutilus rutilus). Notre analyse laisse croire que si le brochet etait absent de plusieurs lacs boreaux ou il habite presentement, il y aurait predominance de peuplements de poissons salmonides--en contraste marque avec l'etat d'homogeneite actuel des communautes de poissons non salmonides, cause par le brochet. [Traduit par la Redaction], Introduction Understanding the main factors that determine the distribution of species is critical for the effective conservation and management of many ecosystems. For freshwaters, a promising tool for predicting species' [...]

Predation has a fundamental role in aquatic ecosystems, but the relative importance of factors governing prey selection by predators remains controversial. In this study, we contrast five lakes of a subarctic watershed to explore how prey community characteristics affect prey selection and growth rate of the common top predator, brown trout (Salmo trutta). The lakes constitute a distinct gradient of different coregonid prey fish, ranging from monomorphic common whitefish (Coregonus lavaretus) to polymorphic whitefish co-occurring with vendace (Coregonus albula). The brown trout was a morph--species- and size-specific pelagic predator, selecting the small-sized, pelagic whitefish morph or vendace over the benthic whitefish morphs. In all lakes, the average prey size increased with predator size, but small-sized prey were also included in the diet of large predators. The selection of small-sized, pelagic prey fish appeared to be a favourable foraging strategy for the brown trout, yielding higher growth rates and an earlier ontogenetic shift to piscivory. The findings emphasize that piscivory appear to be shaped by the diversity, size-structure, and abundance of available prey in a given community. La predation joue un role fondamental dans les ecosystemes aquatiques, mais l'importance relative des facteurs qui determinent la selection des proies par les predateurs reste un sujet controverse. Dans notre etude, nous comparons cinq lacs d'un bassin versant subarctique afin de determiner comment les caracteristiques de la communaute de proies affectent la selection des proies et le taux de croissance du predateur superieur commun, la truite brune (Salmo trutta). Les lacs forment un gradient marque de proies constituees de divers poissons coregonides, allant de coregones lavarets (Coregonus lavaretus) monomorphes a des coregones lavarets polymorphes cohabitant avec des coregones blancs (Coregonus albula). La truite brune est un predateur pelagique qui choisit ses proies en fonction de la forme--espece et de la taille et qui prefere la forme pelagique de petite taille du lazaret et le coregone blanc au lazaret benthique. Dans tous les lacs, la taille moyenne des proies augmente en fonction de la taille des predateurs, mais les proies de petite taille font aussi partie du regime des grands predateurs. La selection de poissons pelagiques de petite taille comme proies semble etre une strategie benefique chez la truite brune, car elle permet des taux de croissance plus eleves et un passage ontogenique a l'ichtyophagie plus precoce. Ces resultats soulignent le fait que l'ichtyophagie semble etre determinee par la diversite, la structure en taille et l'abondance des proies disponibles dans une communaute donnee. [Traduit par la Redaction], Introduction Predator-prey interactions are essential for the population dynamics of both predator and prey (e.g., Sinclair et al. 2003; Myers et al. 2007; Persson et al. 2007). Most predators are [...]

The functional response of Daphnia, a common pelagic herbivore in lakes, was assessed with a combination of secondary and meta-analyses of published data and new data from an experiment conducted using very low food levels. Secondary analyses of literature data (28 studies, n = 239-393) revealed a significant positive influence of food concentration on Daphnia clearance rate at low food levels, i.e., evidence of an overall Type Ill functional response. This result was not an artifact of including data from Daphnia that were exhausted from prolonged food deprivation (more than three hours at very low food). Meta-analysis of Daphnia clearance rate vs. food concentration across a range of low food concentrations (eight studies) showed a significantly positive slope across studies, which also supports the presence of a Type III response. Congruent with these analyses of published data, the feeding experiment showed clear evidence of a Type Ill functional response for D. pulicaria feeding on Ankistrodesmus falcatus. Food levels at which Daphnia clearance rate declined with decreasing food were near the minimum resource requirement for Daphnia population maintenance at steady state (R*). We suggest that Type III responses are more common than previously believed, perhaps because of the relative paucity of observations at low food levels, and that reduced prey mortality at low phytoplankton densities could be a stabilizing mechanism for Daphnia-phytoplankton systems under resource scarcity. Key words: Daphnia feeding; functional response; phytoplankton; population dynamics; population stability; predator-prey.

Abstract: Native salmonid fishes often face simultaneous threats from habitat fragmentation and invasion by nonnative trout species. Unfortunately, management actions to address one may create or exacerbate the other. A [...]

Abstract: Sea lamprey (Petromyzon marinus) control in North America costs millions of dollars each year, and control measures are guided by assessment of lamprey-induced damage to fisheries. The favored prey [...]

Abstract: Energy and nutrients flow in diverse pathways across heterogeneous landscapes and tightly link the discrete food webs in local habitats. However, parasitism that enhances allochthonous resource input has not [...]

Nest placement affects the risk ofpredation on both eggs and incubating parents and determines the mieroclimate for incubation, two functions that may be in conflict, especially in hot deseas. We studied the roles of microclimate and nest predation on nest site selection by Hoopoe Larks (Alaemon alaudipes) in the Arabian Desea. Hoopoe Larks build nests in three microsites: on the gravel plain away from vegetation, at the base of bushes, or above ground in bushes. Early in the breeding season, 70% of nests were placed on the ground, but as the season progressed, nests in small bushes represented 77% of total nests; nest cover increased from 5% to 21%. Daily survival rate of natural nests was 0.82. Predation on eggs did not differ among nest sites, either for natural nests or in an experiment with artificial nests. Measurements of operative and egg temperatures showed that artificial nests on the gravel plain experience higher temperatures than those under and in bushes. Nest attendance totaled 77% of daytime in nests under bushes and 81% in nests in or on top of bushes, with the larger share of attendance contributed by females. However, during midday, when evaporative water requirements--estimated from temperature profiles at artificial nests--were 10-15-fold higher than during early morning, males and females shared incubation duties almost equally. We hypothesize that Hoopoe Larks favor exposed nest sites to reduce predation risk to themselves as incubating parents, but as the season progresses, they select nest sites with more cover at the base of or within bushes because the thermal environment forces them to do so. Key words: Alaemon alaudipes, desert, egg temperature, Hoopoe Lark, incubation, microclimate, nest predation. La ubicacion de los nidos afecta el riesgo de depredacion de los huevos y de los parentales incubantes, asi como el microclima de la incubacion. Estas dos funciones pueden estar en conflicto, especialmente en desiertos calidos. Estudiamos el papel del microclima y de la depredacion de los nidos sobre la seleccion de sitios de nidificacion por parte de Alaemon alaudipes en el desierto de Arabia. Esta especie construye nidos en tres micrositios: planicies de gravilla lejos de la vegetacion, la base de los arbustos, o arbustos por encima del suelo. En la parte inicial de la epoca reproductiva, el 70% de los nidos estuvieron ubicados en el suelo, pero a medida que transcurrio la estacion, los nidos ubicados en pequenos arbustos representaron el 77% del total y la cobertura de los nidos se incremento del 5% al 21%. La tasa de supervivencia diaria de los nidos fue de 0.82. La depredacion sobre los huevos no difirio entre los sitios de nidificacion para nidos naturales ni para los nidos artificiales de un experimento. Las mediciones de las temperaturas operativas y de los huevos mostraron que los nidos artificiales ubicados en la planicie de gravilla estan expuestos a temperaturas mayores que los nidos ubicados bajo arbustos y sobre estos. Los parentales estuvieron presentes en los nidos durante el 77% del dia en nidos ubicados bajo arbustos y durante el 81% del dia en nidos ubicados en arbustos o encima de estos, y las hembras contribuyeron la mayor parte del tiempo dedicado a atender los nidos. Sin embargo, durante el mediodia, cuando los requerimientos evaporativos de agua estimados a partir de perfiles de temperatura de nidos artificiales fueron 10 a 15 veces mayores que durante la parte inicial de la manana, los machos y las hembras compartieron las tareas de incubacion casi por igual. Proponemos la hipotesis de que A. alaudipes prefiere ubicar los nidos en lugares expuestos para reducir el riesgo de depredacion de los padres, pero a medida que transcurre la estacion, los individuos seleccionan sitios para nidificar con mayor cobertura en la base de arbustos o dentro de estos, debido a que el ambiente termico los obliga a hacerlo.

Population responses of synanthropic species to urbanization may be explained by the resource-matching rule, which postulates that individuals should distribute themselves according to resource availability. According to the resource-matching rule, urban habitats will contain greater densities if they provide better resources than rural habitats. However, because resource availability is density dependent, individuals in urban areas would ultimately achieve fitness levels comparable to, but no better than, individuals in less urban areas. Some ecologists suggest that synanthropic birds may not conform to the resource-matching rule and may instead overmatch (i.e., overexploit) in urban habitats, ultimately leading to lower fitness despite greater resource levels. Using the Northern Cardinal (Cardinalis cardinalis) as a focal species, we evaluated if Cardinal populations in urban and rural habitats were consistent with predictions of consumer resource matching. During 2003-2006 we documented population density, adult body condition, apparent survival, and annual reproductive productivity of Cardinals in riparian forest stands within urban (n = 8 stands) and rural (n = 6 stands) landscapes in Ohio, USA. Density of Cardinals in urban forests was four times that found in more rural forests. Mark-resight data from 147 males and 125 females over four years indicated that apparent survival rates were similar between urban and rural landscapes ([phi] = 0.64, SE = 0.039 for males and [phi] = 0.57, SE = 0.04 for females). Similarly, body condition indices of 168 males, 142 females, and 118 nestlings did not differ significantly between landscapes. Annual reproductive productivity (mean number of fledglings per pair over breeding season) of 294 pairs was comparable for urban (2.4 [+ or -] 0.18 [mean [+ or -] SE] and rural (2.1 [+ or -] 0.18) young birds. Thus, contrary to recent suggestions, we find that high densities of certain synanthropic species in urban landscapes are consistent with expectations of consumer resource matching. Key words: Cardinalis cardinalis; condition; consumer resource matching; ideal free distribution; Northern Cardinal; reproductive productivity; predation; survival; urban habitats.

The rate of species loss is increasing at a global scale, and human-induced extinctions are biased toward predator species. We examined the effects of predator extinctions on a foundation species, the eastern oyster (Crassostrea virginica). We performed a factorial experiment manipulating the presence and abundance of three of the most common predatory crabs, the blue crab (Callinectes sapidus), stone crab (Menippe mercenaria), and mud crab (Panopeus herbstii) in estuaries in the eastern United States. We tested the effects of species richness and identity of predators on juvenile oyster survival, oyster recruitment, and organic matter content of sediment. We also manipulated the density of each of the predators and controlled for the loss of biomass of species by maintaining a constant mass of predators in one set of treatments and simultaneously using an additive design. This design allowed us to test the density dependence of our results and test for functional compensation by other species. The identity of predator species, but not richness, affected oyster populations. The loss of blue crabs, alone or in combination with either of the other species, affected the survival rate of juvenile oysters. Blue crabs and stone crabs both affected oyster recruitment and sediment organic matter negatively. Mud crabs at higher than ambient densities, however, could fulfill some of the functions of blue and stone crabs, suggesting a level of ecological redundancy. Importantly, the strong effects of blue crabs in all processes measured no longer occurred when individuals were present at higher-than-ambient densities. Their role as dominant predator is, therefore, dependent on their density within the system and the density of other species within their guild (e.g., mud crabs). Our findings support the hypothesis that the effects of species loss at higher trophic levels are determined by predator identity and are subject to complex intraguild interactions that are largely density dependent. Understanding the role of biodiversity in ecosystem functioning or addressing practical concerns, such as loss of predators owing to overharvesting, remains complicated because accurate predictions require detailed knowledge of the system and should be drawn from sound experimental evidence, not based on observations or generalized models. Key words: Callinectes sapidus; crab; Crassostrea virginica; ecosystem functioning; extinction; foundation species; identity effect; Menippe mercenaria; oyster; Panopeus herbstii; predator-prey interaction; trophic interaction.

Coinfection of a host by multiple parasite species has important epidemiological and clinical implications. However, the direction and magnitude of effects vary considerably among systems, and, until now, there has been no general framework within which to explain this variation. Community ecology has great potential for application to such problems in biomedicine. Here, metaanalysis of data from 54 experiments on laboratory mice reveals that basic ecological rules govern the outcome of coinfection across a broad spectrum of parasite taxa. Specifically, resource-based ('bottomup') and predator-based ('top-down') control mechanisms combined to determine microparasite population size in helminthcoinfected hosts. Coinfection imposed bottom-up control (resulting in decreased microparasite density) when a helminth that causes anemia was paired with a microparasite species that requires host red blood cells. At the same time, coinfection impaired top-down control of microparasites by the immune system: the greater the helminth-induced suppression of the inflammatory cytokine interferon (IFN)-[gamma] the greater the increase in microparasite density. These results suggest that microparasite population growth will be most explosive when underlying helminths do not impose resource limitations but do strongly modulate IFN-[gamma], responses. Surprisingly simple rules and an ecological framework within which to analyze biomedical data thus emerge from analysis of this dataset. Through such an interdisciplinary lens, predicting the outcome of coinfection may become tractable. cytokine | disease ecology | metaanalysis | predation | resource limitation

Predator-prey communities are ubiquitous in ecology, but introduced predators can drive native species to extinction within island systems, prompting the eradication of such exotics. Ecological theory predicts that elimination of top-introduced predators from islands can lead to the counterintuitive decline of native prey populations through the ecological release of smaller introduced species in a process termed 'mesopredator release.' We show, in accordance with mesopredator release theory and counter to conservation goals for a New Zealand island reserve, that initial eradication of cats on Little Barrier Island led to reduced breeding success of Cook's petrels, which also are vulnerable to predation by a mesopredator, the Pacific rat. The rat's impact on prey productivity varied with elevation within the island. Rat eradication was followed by a rise in petrel productivity, in support of both ecological theory and practical conservation management goals. It appears that interactions among introduced predators, native prey, and environmental gradients can drive counterintuitive and spatially heterogeneous responses to predator eradications from islands. Location-specific, ecosystem-level understanding is essential for predicting the outcomes of such restoration management techniques. habitat gradients | introduced predator eradications | island restoration | trophic cascade | Cook's petrel

Although provisioning of habitat by ecosystem engineers is one of the most common biological interactions, previous studies have mostly focused on facilitative or bottom-up processes. Here we show that engineering effects can indirectly strengthen topdown effects mediated by predator abundance. We conducted a small-scale manipulative field experiment and broad-scale field observations of the plant, web spider, and detrital insect system in forest understory habitats. In the field experiment, artificially increasing architectural elements enhanced the abundance of spiders by providing physical support for web building. Moreover, aerial insects derived from the detrital food web decreased in response to increased spider abundance. As artificial architecture per se did not affect aerial detritivores, these results indicate that ecosystem engineering indirectly strengthens top-down effects mediated by predators. In field observations conducted in 12 cedar forests, path analyses supported the importance of an indirect pathway originating from understory vegetation complexity to spider abundance and to aerial detritivores. The effect size of spiders on detrital insects was similar in the field experiment and in the observations. These results indicate that the engineering effects of plants cascade to detrital insects through web spiders across different scales. Key words: cedar forests; Chiba Prefecture, eastern Japan; detritus food web; habitat modification; indirect effect; predator; web spider.

Although cannibalism is ubiquitous in food webs and frequent in systems where a predator and its prey also share a common resource (intraguild predation, IGP), its impacts on species interactions and the dynamics and structure of communities are still poorly understood. In addition, the few existing studies on cannibalism have generally focused on cannibalism in the top-predator, ignoring that it is frequent at intermediate trophic levels. A set of structured models shows that cannibalism can completely alter the dynamics and structure of three-species IGP systems depending on the trophic position where cannibalism occurs. Contrary to the expectations of simple models, the IG predator can exploit the resources more efficiently when it is cannibalistic, enabling the predator to persist at lower resource densities than the IG prey. Cannibalism in the IG predator can also alter the effect of enrichment, preventing predator-mediated extinction of the IG prey at high productivities predicted by simple models. Cannibalism in the IG prey can reverse the effect of top-down cascades, leading to an increase in the resource with decreasing IG predator density. These predictions are consistent with current data. Overall, cannibalism promotes the coexistence of the IG predator and IG prey. These results indicate that including cannibalism in current models can overcome the discrepancy between theory and empirical data. Thus, we need to measure and account for cannibalistic interactions to reliably predict the structure and dynamics of communities. Key words: cannibalism; community structure and dynamics; competition; effect of enrichment; food web; intraguild predation; intraspecific predation; predator prey interactions; size structure.

Abstract: We examined the effects of predator species on yellow perch (Perca flavescens) from lakes in the Sudbury mining region (Ontario, Canada), where fish communities are just beginning to recover [...]

Growth is a critical ecological trait because it can determine population demography, evolution, and community interactions. Predation risk frequently induces decreased foraging and slow growth in prey. However, such strategies may not always be favored when prey can outgrow a predator's hunting ability. At the same time, a growing gape-limited predator broadens its hunting ability through time by expanding its gape and thereby creates a moving size refuge for susceptible prey. Here, I explore the ramifications of growing gape-limited predators for adaptive prey growth. A discrete demographic model for optimal foraging/growth strategies was derived under the realistic scenario of gape-limited and gape-unconstrained predation threats. Analytic and numerical results demonstrate a novel fitness minimum just above the growth rate of the gape-limited predator. This local fitness minimum separates a slow growth strategy that forages infrequently and accumulates low but constant predation risk from a fast growth strategy that forages frequently and experiences a high early predation risk in return for lower future predation risk and enhanced fecundity. Slow strategies generally were advantageous in communities dominated by gape-unconstrained predators whereas fast strategies were advantageous in gape-limited predator communities. Results were sensitive to the assumed relationships between prey size and fecundity and between prey growth and predation risk. Predator growth increased the parameter space favoring fast prey strategies. The model makes the testable predictions that prey should not grow at the same rate as their gape-limited predator and generally should grow faster than the fastest growing gape-limited predator. By focusing on predator constraints on prey capture, these results integrate the ecological and evolutionary implications of prey growth in diverse predator communities and offer an explanation for empirical growth patterns previously viewed to be anomalies. Key words: demographic models; life history evolution; optimal growth; phenotypic plasticity; predator-prey interactions; prey size refuge.

Longstanding theory in behavioral ecology predicts that prey should evolve decreased foraging rates under high predation threat. However, an alternative perspective suggests that growth into a size refuge from gape-limited predation and the future benefits of large size can outweigh the initial survival costs of intense foraging. Here, I evaluate the relative contributions of selection from a gape-limited predator (Ambystoma opacum) and spatial location to explanations of variation in foraging, growth, and survival in 10 populations of salamander larvae (Ambystoma maculatum). Salamander larvae from populations naturally exposed to intense A. opacum predation risk foraged more actively under common garden conditions. Higher foraging rates were associated with low survival in populations exposed to free-ranging A. opacum larvae. Results demonstrate that risky foraging activity can evolve in high predation-risk habitats when the dominant predators are gape-limited. This finding invites the further exploration of diverse patterns of prey foraging behavior that depends on natural variation in predator size-selectivity. In particular, prey should adopt riskier behaviors under predation threat than expected under existing risk allocation models if foraging effort directly reduces the duration of risk by growth into a size refuge. Moreover, evidence from this study suggests that foraging has evolved over microgeographic scales despite substantial modification by regional gene flow. This interaction between local selection and spatial location suggests a joint role for adaptation and maladaptation in shaping species interactions across natural landscapes, which is a finding with implications for dynamics at the population, community, and metacommunity levels. contemporary evolution | gene flow | growth-foraging tradeoffs | predator-prey interactions | prey size refuge

The diets of introduced predators like nonnative red foxes and feral cats in South San Francisco Bay, U.S.A. were examined by analyzing their stomach contents and stable isotopes of carbon and nitrogen. It revealed their commonly consumed prey and their impact on the native species.

An experiment involving reduction of rodent prey was conducted to examine the relationship between rodent abundance and trappability of brown treesnakes (BTS). It was found that rodent reduction might enhance detection and capture of BTS.

Abstract: I present a mechanistic predation model with explicit representation of predator aggregation for analysing the relationship between mortality rate of prey and their vulnerability (e.g., conspicuousness, escape ability). The [...]

Abstract: We tested whether one generation of state-of-the-art hatchery culture influenced the vulnerability of Chinook salmon (Oncorhynchus tshawytscha) fry to predators. Size-matched hatchery and wild origin spring Chinook salmon fry [...]

A predator's per capita feeding rate on prey, or its functional response, provides a foundation for predator-prey theory. Since 1959, Holling's prey-dependent Type II functional response, a model that is a function of prey abundance only, has served as the basis for a large literature on predator-prey theory. We present statistical evidence from 19 predator-prey systems that three predator-dependent functional responses (Beddington-DeAngelis, Crowley-Martin, and Hassell-Varley), i.e., models that are functions of both prey and predator abundance because of predator interference, can provide better descriptions of predator feeding over a range of predator-prey abundances. No single functional response best describes all of the data sets. Given these functional forms, we suggest use of the Beddington-DeAngelis or Hassell-Varley model when predator feeding rate becomes independent of predator density at high prey density and use of the Crowley-Martin model when predator feeding rate is decreased by higher predator density even when prey density is high. Key words: Beddington-DeAngelis model; Crowley-Martin model; foraging; functional response; Hassell-Varley model; Holling Type II model; interference; predator; prey.

The impact of predators on the population dynamics of pheasants is evaluated and options for management to improve predation are suggested. The involvement of state and federal agencies is required to deliver conservation and agricultural programs to producers.

Periodic fluctuations or cycles in populations of small mammals have been widely studied, but much controversy still exists about their causes. Cycles of voles and lemmings are produced by the integrated effects of intrinsic and extrinsic factors, and the problem is to define accurately how these interact. Spacing behavior is a key component of population regulation in voles and lemmings, and this is illustrated most dramatically by the fence effect. We do not know which mechanisms produce changes in social behavior. Phenotypic changes produced by maternal effects or stress are now believed most likely, but there has been too little work done on genetic effects on spacing and we know almost nothing about kin-related social behavior in voles and lemmings. Both predation and food supply may be the extrinsic factors involved in cyclic population dynamics. Single-factor experiments suggest that food shortage by itself does not seem to be a necessary factor for cycles nor does predation, but the interaction between food and predation could be a key variable in understanding how extrinsic factors affect cycles of voles. Cycles of snowshoe hares are caused by an interaction between predation and food supplies, possibly integrated through risk-sensitive foraging. Spacing behavior is not a component of cycles of hares because snowshoe hares differ from voles and lemmings in having no known form of spacing behavior that can produce social mortality. The short-term cycle of voles and lemmings thus seems to have a different explanation from the long-term cycle of snowshoe hares. In some places, lemmings may be locked in a predator-pit at low density. Experimental exclusion of predators improved survival of adults in a population of collared lemmings, but was not sufficient to allow them to escape the predator-pit because of losses of juveniles. Whether cyclic populations of lemmings also fall into a predator-pit in the low phase remains to be determined. The low phase does not occur in every cycle and it is particularly difficult to explain. Progress in analyzing cyclic fluctuations has been made most rapidly when we define clear alternative hypotheses and carry out experimental manipulations on field populations. Much remains to be done on these small mammals. Key words: cycles, snowshoe hares, Lepus americanus, voles, lemmings, Microtus

Mathematical analysis shows that in a simple apparent competition model with two prey species and a shared predator, the dominant prey species is that which supports the highest predator density. In a more complex model, in which the two prey species also share the same resource, the dominant species decreases the resource to lower levels and increases the predator to higher levels than that of the other species, assuming that they have equal predation rates. When unequal predation rates are allowed, the rule to be applied depends on the relation between the mass-balance constraints of the two prey species.

We present a model to examine how foragers should divide attention among different potential prey types in order to maximize their net rate of energy intake. We define attention as the brain's capacity to process information simultaneously. Our model is based on neuropsychological studies suggesting first, that predators who divide attention among an increasing number of different prey types decrease their ability to detect any given type, and second, that this decrease is larger when prey items are more difficult to detect. Our model also incorporates the effects of search rate on foragers' probability to detect prey. The model predicts that foragers encountering cryptic prey should devote all attention to a single prey type. When encountering conspicuous prey, foragers should divide attention among the different prey types. For prey types that differ in conspicuousness but are equal in energy content, handling time, and density, foragers should give more attention to the more conspicuous prey if the conspicuousness values of all prey types are relatively small. However, when all prey are more conspicuous, foragers should devote more attention to the less conspicuous prey type. We suggest that our model may serve to explain and predict some of the foraging decisions of animals searching for cryptic or conspicuous prey, and especially studies on 'search image' formation.

We studied piping plovers (Charadrius melodus) on Assateague Island (Md., Va.) in 1986-87 to estimate population size and to identify factors affecting productivity. Fledging rates (0.19-1.11 chicks/pair) appeared to be lower than the level necessary to maintain a stable population. Fifty-four percent of the nests were unsuccessful. Predators accounted for most (91%) of the known causes of nest losses. Only 1 nest (2.2% of losses with known cause) was lost due to direct human destruction, and we found no evidence that suggested recreational disturbance was a factor affecting productivity. Mean chick fledging success was 69% for broods foraging at bay flats or tidal pools and 19% for broods foraging on ocean beach (P < 0.05).