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Resource subsidies exert critical influences on recipient habitats with relatively higher perimeter-to-area ratios, such as headwaters in watersheds. However, little is known about how those subsidies contribute to the energy sources in recipient habitats where the perimeter-to-area ratio is low, such as large stream channels. Here, we show that the diet of small Japanese eels (Anguilla japonica) Les apports de ressources exercent des influences cles dans les habitats recepteurs caracterises par des rapports perimetre:superficie relativement importants, comme les cours superieurs de bassins versants. Les connaissances sur la contribution de ces apports aux sources d'energie dans les habitats recepteurs ou le rapport perimetrersuperficie est faible, comme les chenaux de grands cours d'eau, sont toutefois limitees. Nous demontrons que le regime alimentaire de petites anguilles du Japon (Anguilla japonica) de moins de 500 mm de longueur totale vivant le long de rives naturelles de chenaux de grands cours d'eau est majoritairement constitue de petits lombrics terrestres (Metaphire spp.). Des analyses d'isotopes stables montrent que les lombrics constituent les proies animales les plus importantes dans le regime alimentaire des anguilles (45 %-47 %) et representent la plus grande proportion de leur contenu stomacal (7 %-93 %). Les anguilles ingerent des lombrics dans les deux jours suivant une pluie au printemps, a l'ete et a l'automne, et la consommation de lombrics augmente parallelement aux precipitations. Ces constatations indiquent que les apports episodiques de lombrics modules par la pluie pourraient biaiser provisoirement le regime alimentaire des anguilles vers cette ressource allochtone, ce qui pourrait expliquer l'importante contribution de cet apport pour les consommateurs vivant dans les chenaux de grands cours d'eau. En outre, differentes especes de lombrics pourraient produire de multiples episodes d'apport au fil des saisons et fournir aux predateurs un apport soutenu, rehaussant la contribution a long terme de l'apport au regime alimentaire des predateurs. [Traduit par la Redaction], Introduction Local habitats are linked tightly by reciprocal energy subsidies from contiguous habitats, and across-habitat transfers of both materials and organisms frequently have key effects on local consumers and their [...]

Theory on trophic interactions predicts that predators increase plant biomass by feeding on herbivores, an indirect interaction called a trophic cascade. Theory also predicts that predators feeding on predators, or intraguild predation, will weaken trophic cascades. Although past syntheses have confirmed cascading effects of terrestrial arthropod predators, we lack a comprehensive analysis for vertebrate insectivores--which by virtue of their body size anal feeding habits are often top predators in these systems--and of how intraguild predation mediates trophic cascade strength. We report here on a meta-analysis of 113 experiments documenting the effects of insectivorous birds, bats, or lizards on predaceous arthropods, herbivorous arthropods, and plants. Although vertebrate insectivores fed as intraguild predators, strongly reducing predaceous arthropods (38%), they nevertheless suppressed herbivores (39%), indirectly reduced plant damage (40%), and increased plant biomass (14%). Furthermore, effects of vertebrate insectivores on predatory and herbivorous arthropods were positively correlated. Effects were strongest on arthropods and plants in communities with abundant predaceous arthropods and strong intraguild predation, but weak in communities depauperate in arthropod predators and intraguild predation. The naturally occurring ratio of arthropod predators relative to herbivores varied tremendously among the studied communities, and the skew to predators increased with site primary productivity and in trees relative to shrubs. Although intraguild predation among arthropod predators has been shown to weaken herbivore suppression, we find this paradigm does not extend to vertebrate insectivores in these communities. Instead, vertebrate intraguild predation is associated with strengthened trophic cascades, and insectivores function as dominant predators in terrestrial plant-arthropod communities. doi/ 10.1073/pnas.1001934107 bottom-up and top-down control | intraguild predation | meta-analysis | trophic cascade | vertebrate predator exclusion

Trophic cascades are often a potent force in ecological communities, but abiotic and biotic heterogeneity can diffuse their influence. For example, inducible defenses in many species create variation in prey edibility, and size-structured interactions, such as cannibalism, can shift predator diets away from heterospecific prey. Although both factors diffuse cascade strength by adding heterogeneity to trophic interactions, the consequences of their interaction remain poorly understood. We show that inducible defenses in tadpole prey greatly intensify cannibalism in predatory larval salamanders. The likelihood of cannibalism was also strongly influenced by asymmetries in salamander size that appear to be most important in the presence of defended prey. Hence, variation in prey edibility and the size structure of the predator may synergistically affect predator--prey population dynamics by reducing prey mortality and increasing predator mortality via cannibalism. We also suggest that the indirect effects of prey defenses may shape the evolution of predator traits that determine diet breadth and how trophic dynamics unfold in natural systems. Key words: behavior; cannibalism; diet; food webs; Hynobius retardatus (Dunn); inducible defense; predation risk; Rana pirica (Matsui); salamander; trait-mediated indirect effects; trophic cascade.

Despite extensive empirical research and previous reviews, no clear patterns regarding the effects of habitat loss and fragmentation on predator--prey interactions have emerged. We suggest that this is because empirical researchers do not design their studies to test specific hypotheses arising from the theoretical literature. In fact, theoretical work is almost completely ignored by empirical researchers, perhaps because it may be inaccessible to them. The purpose of this paper is to review theoretical work on the effects of habitat loss and fragmentation on predator--prey interactions. We provide a summary of clear, testable theoretical predictions for empirical researchers. To test one or more of these predictions, an empiricist will need certain information on the predator and prey species of interest. This includes: (1) whether the predator is a specialist on one prey species or feeds on many kinds of prey (omnivore and generalist); (2) whether the predator is restricted to the same habitat type as the focal prey (specialist), can use a variety of habitats but has higher survival in the prey habitat (omnivore), or lives primarily outside of the focal prey's habitat (generalist); (3) whether prey-only patches have lower prey extinction rates than predator--prey patches: and (4) whether the prey emigrate at higher rates from predator-prey patches than from prey-only patches. Empiricists also need to be clear on whether they are testing a prediction about habitat loss or habitat fragmentation and need to conduct empirical studies at spatial scales appropriate for testing the theoretical prediction(s). We suggest that appropriate use of the theoretical predictions in future empirical research will resolve the apparent inconsistencies in the empirical literature on this topic. Key words: fragmentation; habitat loss; predator prey interaction; spatial scale; theoretical models.

A shift in outcomes of predator-prey interactions in plankton community may occur at sublethal dissolved oxygen concentrations that commonly occur in coastal waters. Laboratory experiments were conducted to investigate how a decline in dissolved oxygen concentration alters the predation rate on fish larvae by two estuarine predators. Behavior and consumption of larval fish by moon jellyfish Aurelia aurita (103.1 [+ or -] 12.4 mm in bell diameter) and by a juvenile piscivore, Spanish mackerel Scomberomorus niphonius (30.1 [+ or -] 2.1 mm in standard length: SL), were observed under four oxygen concentration treatments (1, 2 and 4 mg [1.sup.-1] and air-saturated: 5.8 mg [1.sup.-1]). Larvae of a coastal marine fish species, red sea bream Pagrus major (7.21 [+ or -] 0.52 mm SL), were used as prey for the experiment. Bell contraction rate of the jellyfish did not vary among the oxygen concentrations tested, indicating a tolerance to low oxygen concentration. Gill ventilation rate of the Spanish mackerel increased and swimming speed decreased as the oxygen concentration decreased, indicating that oxygen concentrations [greater than or equal to] 4 mg [1.sup.-1] are physiologically stressful for this species. The number of larvae consumed in 15 min. by jellyfish increased whereas those consumed by Spanish mackerel decreased with the decrease in oxygen concentration. Low oxygen concentrations that are commonly observed in coastal waters of Japan during summer have the potential to increase the relative importance of jellyfish as predator of fish larvae and to change the importance of alternative trophic pathways in estuarine ecosystems.

Large body size, carnivory, and endothermic costs lead to exceptionally high caloric demands in many mammalian predators. The potential impact on prey resources may be marked but is difficult to demonstrate because of the mobility, sparseness, and cryptic nature of these animals. In this study, we developed a method based on comparative bioenergetics and demographic modeling to evaluate predator effects and then used this approach to assess the potential impact of killer whales on sea otter and Steller sea lion populations in the Aleutian Islands. Daily caloric requirements of killer whales determined from allometric regressions for field metabolic rate show that an adult killer whale requires 51-59 kcal.[kg.sub.-1].[d.sub.-1] (2.5-2.9 W/kg). Caloric values of prey items determined by bomb calorimetry ranged from 41 630 kcal for an adult female sea otter to sequentially higher values for male otters, sea lion pups, and adult Steller sea lions. Integrating these results with demographic changes in marine mammal populations show that fewer than 40 killer whales could have caused the recent Steller sea lion decline in the Aleutian archipelago; a pod of five individuals could account for the decline in sea otters and the continued suppression of sea lions. The collapse of the historical prey base of killer whales due to human whaling may have contributed to a sequential dietary switch from high to low caloric value prey, thereby initiating these declines. This study demonstrates that a combined physiological-demographic approach increases our ability to critically evaluate the potential impact of a predator on community structure and enables us to define underlying mechanisms that drive or constrain top-down forcing in dynamic ecosystems. Key words: Aleutian Islands; caloric intake; community structure; energy comsumption; Enhydra lutris; Eumetopias jubatus; killer whale; metabolism; Orcinus orca; predators; sea otter; Steller sea lion.

The view of terrestrial arthropod communities as potentially exhibiting a mixture of three- and four-trophic-level dynamics creates a new challenge for researchers: the indeterminacy of predator function. Here we evaluate two predictions linking a predator's foraging behavior with its likely ecological role: (1) widely foraging predators may act as effective regulators of sedentary herbivore populations, and (2) sit-and-wait predators are unlikely to regulate sedentary herbivore populations but may act as top predators, consuming widely foraging intermediate predators and allowing herbivore populations to escape from control. We tested these predictions by manipulating the predator community associated with a sedentary herbivore, the spider mite Tetranychus cinnabarinus, feeding on papaya, Carica papayae. The predators included a widely foraging specialist consumer of spider mites, the ladybird beetle Stethorus siphonulus, and a sit-and-wait generalist predator, the tangle-web spider Nesticodes rufipes. The experiments provided support for both predictions. The widely foraging predator Stethorus when tested alone was capable of effective suppression of spider mite populations. In contrast, the sit-and-wait predator, Nesticodes, never suppressed spider mites. Instead, Nesticodes consistently increased the population growth rates of spider mites. This effect was most likely due to Nesticodes suppressing populations of the intermediate predator Stethorus. Our results underscore the presence of both three- and four-trophic-level dynamics within this arthropod community and begin to address the challenging problem of the indeterminacy of predator function. Key words: biological control; food webs; foraging behavior; generalist predator; higher order predation; indirect effects; intraguild predation; trophic cascades.

The ecological and evolutionary dynamics of fruit color polymorphisms remain poorly known because patterns and agents of selection have rarely been identified. Here, we examine Acacia ligulata, a shrub of the Australian arid zone characterized by a red/yellow/orange aril color polymorphism. Seed production patterns over four populations and three years suggested that spatially variable selection may be acting to maintain the polymorphism: red and yellow aril color morphs each had the highest seed production in alternate sites. Seed production differences between morphs were a function of both intrinsic plant characters (fruit production) and predispersal seed predation, which affects the number of viable seeds matured per ovule. Fruit production differences are hypothesized to result from a genotype-by-environment interaction, perhaps related to plant vigor. In contrast, morph differences in the numbers of viable seeds per ovule are produced via differential seed predation by heteropteran insects, as demonstrated by exclusion experiments. Because these predators feed when aril color is not visible, differential predation is evidently a response to pleiotropic effects of fruit color alleles. We suggest that such pleiotropic effects may be a common feature of fruit color polymorphisms, and that the most obvious selective agents (that is, seed dispersers) may not always be the most important. Keywords: acacia; Acacia ligulata; Australian arid zone; Coleotichus; fruit color polymorphism; insect seed predators; Melanterius; pleiotropic effects; pre-dispersal seed predation; seed production patterns; spatially variable selection; Trichilogaster.

Tropical rain forests, some of the most diverse natural communities, have been characterized either as inherently stable or as extremely fragile. Theoretically, complex food webs allow for alternative pathways and overlapping functions within and among trophic levels, such that cascading effects are buffered and perturbation is resisted. On the other hand, strong interdependence among species and trophic levels in these same systems can make them vulnerable to perturbation. To assess the ability of a tropical rain forest community to resist perturbation or to amplify perturbation through trophic cascades, we measured changes in overall vegetative cover and species richness of understory plants in response to the experimental addition of a top predator beetle. In August 2000, 30 randomly located patches of naturally occurring Piper cenocladum shrubs were found at sites with variable soil quality and light availability, and top predator addition treatments were randomly assigned to half the patches. Using an 8-[m.sup.2] subplot within each P. cenocladum patch, we estimated (nondestructively) vegetative cover of understory plants for nine mouths and measured herbivore damage, plant longevity and plant species richness for ~ 1.5 years. For all understory species in our subplots, vegetative cover and total numbers of leaves declined significantly more in predator addition patches than in control patches. Understory cover also declined significantly more on poor soils than on rich soils. Mortality of small plants was high overall, but longevity was significantly greater in control patches than in patches with top predators. Herbivore damage was lowest, plant establishment highest, and the accumulation of new plant species highest in control plots on rich soils. The cumulative effect of top predators interacted with plant resources, such that plant species richness was suppressed when beetles were present in understory communities on rich soils. Patterns of herbivore damage suggest that trophic cascades from apex predators through ant predators to herbivores altered the structure of understory vegetation. The presence of indirect effects supports the characterization of rain forests as vulnerable ecosystems whose integrity may be threatened by the slow erosion of complex tight interactions among species. Keywords: ant-plant; biodiversity; bottom-up forces; consumer vs. resource control; Costa Rica; Piper; Pheidole; soil nutrients; trophic cascade; tropical rain forest.

It is well established that plants provide alternative foods to predators of herbivorous arthropods. This provision may facilitate protection against herbivory. However, plants often cannot prevent other organisms from utilizing these foods as well. There are many examples of herbivorous arthropods that can feed on plant-provided foods such as extrafloral nectar and pollen. The question therefore arises whether individual plants still gain protection when not only the predators, but also the herbivores, can feed on these foods. We investigated this question using a mathematical model and experiments that assessed the impact of supplementary pollen on the dynamics of predatory mites (Iphiseius degenerans (Berlese)) and herbivorous thrips (Frankliniella occidentalis (Pergande)), two arthropods capable of using pollen for reproduction. Replicated greenhouse experiments showed that addition of pollen every two weeks to one young mature leaf of a male-sterile cucumber plant increased predator population growth and greatly reduced herbivore numbers. A stage-structured predator-prey-pollen model with experimentally established parameters gave reasonably accurate predictions of population trends observed in the greenhouse experiments with and without pollen. Model analysis yielded three important results. First, herbivore (prey) equilibria always settled to lower values in the presence of pollen. Second, mean herbivore numbers during the transient phase following predator release were not always lower under pollen supply, depending on the initial numbers of predators and prey. Third, limiting the plant area covered with pollen led to a decrease in mean herbivore numbers, provided that the predators aggregated in (and thereby 'monopolized') pollen patches. The latter result may explain why plants provide alternative foods at specific sites. Key words: apparent competition; Frankliniella occidentalis; indirect plant defense; intraguild predation; Iphiseius degenerans; omnivory; plant-predator mutualism; pollen; predator-prey interaction; spatial distribution; supplementary food; tri-trophic interactions.

Jefrey A. Crooks (2) Abstract: Musculista senhousia (Benson in Cantor, 1842) is a soft sediment- dwelling mussel that has spread anthropogenically from its native Asia to North America, Australasia, and [...]

Research indicates that population densities for predatory animals tend to be lower than those of non-predators, irrespective of body size. Data were obtained from studies of macroinvertebrate colonies in detrital microcosms.

Many animals live in temporary or permanent groups, either as gregarious or social species, to reduce predation risk. The solitary midge, Aphidoletes aphidimyza, preys specifically on aphids and spends its entire larval development within a prey colony where it is susceptible to intraguild predation. We hypothesized that midge larvae profit from a dilution effect produced by aphids which enhances their chances of survival. We examined the defensive behaviors of aphids in response to foraging midges, and investigated the effect of aphid density on the predation risk of A. aphidimyza by the lacewing Chrysoperla rufilabris. We found that a foraging midge displays furtive hunting behavior which triggers little defensive reaction by aphids, and does not stimulate significant disturbance of the gregarious prey. Within the aphid colony the midge benefits from a dilution effect which reduces the incidence of predation by lacewing larvae. However, the effectiveness of such a mechanism is determined by the level of disturbance caused by foraging intraguild predators. Large lacewing larvae tend to dislodge aphids from their feeding sites, thereby eliminating the dilution effect. We conclude that prey not only provide food to midge larvae, but also protection against natural enemies. Key words: aphid; Aphidoletes aphidimyza; Chrysoperla rufilabris; defensive mechanism; dilution effect; ectosymbiosis; furtive behavior; intraguild predation; predatory midge.

This article discusses the complex nature of the Maasai's coexistence with predators. Topics include wildlife conservation, predator persecution, and biological diversity.

A comparison of chlorophyll/nutrient dynamics at two survey sites near South Georgia, one of the Falkland Islands, is discussed. The potential role of planktonic nitrogen as recycled by warm-blooded land-based predators has been studied by examination of small-scale distributions of phytoplankton biomass as chlorophyll and nutrients along with physical oceanography in two mesoscale surveys near the island. The island and smaller islands nearby have large populations of macaroni penguins, other breeding birds, and fur seals.

Effects of predators and nutrients on stream food chains are discussed as related to trophic interaction in open systems. The combined effects of predators and nutrients on community organization have been tested in an open system in Sycamore Creek, a central Arizona perennial desert stream. It was not possible to conclusively determine mechanism for the increase in algal and herbivore biomass seen as a response to nutrient addition. REsults suggest adjustment of consumer dispersal rates in response to predators and resources can influence biomass at multiple trophic levels

The survival of the coral-reef fish Acanthochromis polyacanthus is negatively related to predator abundance at the larger spatial scale and unrelated at that smaller spatial scale. On the large scale, there is higher mortality in locations having high number of predators. The spatial differences in rank abundance of surviving juveniles is due to mortality. Initial juvenile abundance is the cause of temporal differences in rank abundance. This indicates that predatory fish effects are large in space than time.

Neonatal, previously unfed garter snakes (Thamnophis sirtalis) exposed to aqueous chemical cues from earthworms tongue flicked at higher rates during the 5 min after withdrawal of the stimulus than after aqueous control trials. If snakes struck at but did not touch, the worm stimulus, they showed a much greater increase over control trials in postexposure tongue flicking than when they did not strike, and the increased rate persisted longer. These results demonstrate strike-induced chemosensory searching (SICS) by ingestively naive snakes. It is highly probable that other squamate reptiles show a similar well-integrated suite of sequential foraging behaviors at birth or hatching. Differences among species in SICS may be related to details of sequential prey capture tactics in reconstruction of broad phylogenetic patterns.

Use of prey, and topographic and habitat features by mountain lions (Felis con-color), bobcats (Lynx rufus), and coyotes (Canis latrans) in central Idaho was investigated to determine how syntopic carnivores coexist where resource use may overlap. There were significant differences in use of elevation, forest types, terrain, overstory, density, and exposure by these predators during summer. Despite morphological and behavioral differences permitting these predators to partition resources, resource use overlapped during winter snow confined prey and predators to lower elevations. Overlap in their diets was significant during winter resulting in mountain lions killing bobcats and coyotes while defending or usurping food caches.

There are two sides to animal species conservation. One side is the biology: learning everything we can about an animal such as what it eats, how it cares for its [...]

The ecological consequences of predator behavioural response to global climatic variation are reported, based on 40 years of data on wolf predation from Isle Royale, USA. Wolves hunted in larger packs, tripling the number of moose killed per day, in response to a rise in winter snow linked to the North Atlantic Oscillation. Moose abundance declined and growth of understory fir increased.

The assumption that fledging success would result in reproductive success is investigated. A study of the predation of northern shrikes by little owls was carried out by banding the shrikes and following their growth in relation to the presence of little owls within the territory. Remains of fledgling shrikes were noted in the owls' nests. Overall fledging success of the shrikes was 79%, but survival rate was a mere 27%, suggesting that reproductive success is controlled by other factors such as predation.

IT HAS BEEN SUGGESTED THAT THE high primary production of the Southern Ocean [1-4] could be further increased by the addition of iron [5] to take advantage of the generally [...]

A snow-tracking stat is used to characterize the dynamics of small mutelids collected from different parts of Finland. The results have indicated the dependence of stoat and weasel dynamics on their main prey and a decreasing temporal trend in the abundances of both species as was reported for voles.

The recovery of total abundance, diversity and community composition of forest soil mesofauna, which included both predators and detritivores/fungivores, after a six-year climatic disturbance was studied to examine the resilience of soil animal communities to large-scale disturbances. Results reveal that there is a tendency for more mobile groups to recover faster than the slow-moving oribatids, suggesting the importance of dispersal ability for the resilience of soil communities.

Red foxes, raccoons, end striped skunks' use of levees and the edges associated with them within a wetland environment are studied. It is found that the use of road and levees within the refuge increases the possible travel distance and penetration of predators into wetlands environments.

A series of field experiments were conducted to test the population response of two ecologically-divergent wolf spider species (Hogna helluo and Pardosa milvina) to correlates of land fragmentation. It was found that these two synoptic wolf species have distinctly different responses to landscape fragmentation.

Detection rates for mammalian predators were 11-fold higher for riparian corridor areas than in vineyards, and more predator species were present in wide corridors than in denuded or narrow ones. Additional comparisons between riparian and vineyard study areas are included.

A study that will determine the extent to which the hypotheses about nest predators and fragmentation are invoked and compared and a meta-analysis of tested predator effects to evaluate whether predator responses to fragmentation depend on spatial scale, landscape type, geographic region, or predator taxa is presented. Conservation efforts for declining avian species need to be customized according to the nest-predator species primarily responsible for local nest mortality and the nature of the landscape mosaic.

The four complexities, which can modify the effects of competition on the population dynamics of carnivores, namely habitat fragmentation, counterintuitive effects of prey density, predator-prey-size ratios and habitat type is presented using data from the African wild dogs. Competitions effects can vary dramatically even for a single pair of competitors and it is concluded that realistic assessments of extinction risk or the quality of reintroduction sites might have to be highly tailored or case-specific.

We conducted a two-year predator-exclusion study to assess the magnitude and timing of larval predation in non-outbreaking populations of a geometrid moth, Epirrita autumnata. Laboratory-produced newly hatched larvae were placed on the experimental trees which were assigned to five treatments within two larval densities: (1) all predators, including parasitoids, excluded by mesh bag, (2) birds excluded by cage, (3) ants excluded by glue ring, (4) birds and ants excluded, and (5) control without any predator exclusion. Thereafter, larvae were censused every 3-4 d throughout the five-instar larval period. Mortality of E. autumnata larvae in these populations was high and mostly due to natural enemies. In control trees, only ~10% of larvae survived, while survival was ~90% in mesh bags preventing all natural enemies. Bird exclusion significantly improved larval survival, as survival was almost three times higher in trees with cages than in those without cages. On the other hand, ant exclusion did not have any overall effects on larval survival, mostly because ants were only detected in about half of the trees without glue rings. Larvae survived longer in high-density trees from which ants were excluded, but the effect was masked by high mortality, unrelated to ant exclusion, in the late larval season. The results suggest that the effect of ant predation on survival of E. autumnata larvae may be spatially restricted and not important at a larger scale. The same result applies for crab spiders, as they caused high mortality in ~20% of the study trees. Our results emphasize the importance of considering the spatial scale as well as assessing the impact of multiple predators in order to detect predators affecting survival at the population level. Exclusion of all predation had a significantly stronger effect on larval survival than exclusion of birds alone. Further, mortality was highest during the late larval period, when parasitoids emerge. Thus, a large proportion of larval mortality was most likely due to parasitism. Our results suggest that predation by passerine birds and parasitism may contribute to maintenance of low E. autumnata densities by strong suppression of the number of larvae entering the pupal stage. Key words: ants; birds; crab spiders; Epirrita autumnata; field experiment; insect herbivore; larval predation; parasitism; predator exclusion; stability; suppression; survival analysis.

Detection of density dependence in animal populations is a primary goal of population ecology, and the processes causing density dependence play a major role in population regulation. Predation can strongly regulate populations by populational and behavioral responses of predators to their prey. Here I evaluate the existence and strength of density-dependent mortality in local populations of a reef fish, the kelp perch (Brachyistius frenatus), caused by its predator, the kelp bass (Paralabrax clathratus). Specifically, I examine both the functional response and a potential aggregative response by kelp bass as mechanisms underlying predator-induced density-dependent mortality. I calculated the per capita mortality of kelp perch as a result of the functional response of its predator by subjecting a range of densities of kelp perch to a low, constant number of kelp bass and different amounts of giant kelp (Macrocystis pyrifera) as habitat structure and a potential prey refuge. The potential for an aggregative response by kelp bass was determined by exposing densities of kelp perch to natural levels of kelp bass in the field. Per capita mortality of kelp perch in the laboratory was inversely density dependent to density independent with increasing habitat structure. By contrast, per capita mortality in the field was strongly density dependent, with evidence for an aggregative response by kelp bass. Furthermore, emigration and other nonpredatory losses of kelp perch from field plots were negligible, indicating that the product of the functional and aggregative responses by kelp bass induced density-dependent mortality in kelp perch. My results indicated that increasing habitat structural complexity at low densities of kelp perch, coupled with a strong aggregative response by kelp bass, was responsible for the observed pattern of density-dependent mortality. In addition, the distribution and relative abundances of kelp perch and kelp bass at larger spatial scales (entire reefs) were consistent with predation as an important process structuring local populations of kelp perch. The contrasting patterns of short-term per capita mortality found here underscore the need to evaluate both the functional and aggregative responses of predators to their prey. Ultimately, the pattern of mortality expressed will be determined by the relative strengths of these behavioral responses and the degree to which habitat structure provides an effective prey refuge. Key words: aggregative response; Brachyistius frenatus; density-dependent mortality; functional response; habitat structure; kelp (Macrocystis pyrifera); Paralabrax clathratus; population regulation; predation; prey refuges; reef fish.

BECKY M. PIERCE [1,2] VERNON C. BLEICH [1,2] R. TERRY BOWYER [1] Abstract. Mountain lions (Puma concolor) are thought to regulate their populations via social behavior. The proposed mechanism is [...]