The interplay between foraging mode, habitat structure, and predator presence in antlions

Antlion larvae are sand-dwelling insect predators, which ambush small arthropod prey while buried in the sand. In some species, the larvae construct conical pits and are considered as sit-and-wait predators which seldom relocate while in other species, they ambush prey without a pit but change their ambush site much more frequently (i.e., sit-and-pursue predators). The ability of antlion larvae to evade some of their predators which hunt them on the sand surface is strongly constrained by the degree of sand stabilization or by sand depth. We studied the effect of predator presence, predator type (active predatory beetle vs. sit-and-pursue wolf spider), and sand depth (shallow vs. deep sand) on the behavioral response of the pit building Myrmeleon hyalinus larvae and the sit-and-pursue Lopezus fedtschenkoi larvae. Predator presence had a negative effect on both antlion species activity. The sit-and-wait M. hyalinus larvae showed reduced pit-building activity, whereas the sit-and-pursue L. fedtschenkoi larvae decreased relocation activity. The proportion of relocating M. hyalinus was negatively affected by sand depth, whereas L. fedtschenkoi was negatively affected also by the predator type. Specifically, the proportion of individual L. fedtschenkoi that relocated in deeper sand was lower when facing the active predator rather than the sit-and-pursue predator. The proportion of M. hyalinus which constructed pits decreased in the presence of a predator, but this pattern was stronger when exposed to the active predator. We suggest that these differences between the two antlion species are strongly linked to their distinct foraging modes and to the foraging mode of their predators. Reut Loria and Inon Scharf contributed equally to the paper.     

Habitat structure helps guide the emergence of colony-level personality in social spiders

Studying the environmental factors that guide the emergence of collective behaviors is instrumental to understanding the ecology and evolution of animal societies. Although recent work has provided insights into the demographic factors that influence inter-colony variation in collective behavior (i.e., colony-level personality or collective personality), relatively few studies have investigated how the physical environment (e.g., habitat structure) affects colony-level personality. Here, we study the emergence of collective personality in prey capture behavior in the social spider, Stegodyphus dumicola. We measured collective prey capture behavior four times over 36 days in a classic repeated measures design. We used four different artificial habitat (web support) structures in three different treatments: habitat structure was either (1) fixed and undisturbed, (2) disturbed with a complete removal of webbing between each measurement, or (3) disturbed with changes of habitat structure between each measurement. Our results revealed that repeatability in colony-level personality was retained as long as habitat structure was not altered. However, the repeatability of colony-level personality declined precipitously when groups were forced to build their webs on novel habitat structures. Furthermore, habitat structure affected collective capture behavior, that is, latency to attack and the number of attackers differed among colonies on different habitat structures. Collectively, our data demonstrate that habitat structure is instrumental in shaping both the mean and repeatability of the collective behavior of colonies and may influence overall foraging success.     

How temperature and habitat quality affect parasitoid lifetime reproductive success—A simulation study

Adult parasitoid females lay their eggs in or on host insects. Most species are incapable of de novo lipogenesis as adults, and lipids accumulated during the larval stage are allocated either to egg production or to adult survival. Lipid consumption increases with distance covered by the parasitoids and thus with the distance between available hosts within a habitat. Temperature should affect parasitoid fitness because it changes the constraint imposed by a limited reserve of lipids and because it influences behaviour. Climate change involves both an increase in average temperature and an increased frequency of extreme weather such as heat waves. We investigated how the predicted increase of temperature will affect parasitoid fitness and how this depends on habitat parameters (spatial distribution of hosts and lipid cost of habitat exploitation). We studied optimal behaviour and calculated fitness at different temperatures and in different habitats using a stochastic dynamic programming model for pro-ovigenic parasitoids (which mature all their eggs before becoming adult). We show that an increase in temperature decreases fitness of parasitoids adapted to lower temperatures. This decrease in fitness depends on habitat quality. In field conditions (assuming small costs of intra-patch foraging), the loss of fitness should be larger in habitats with high inter-patch distance and in habitats with a more aggregated distribution of hosts. The foraging behaviour of parasitoids is also affected; at higher temperature we show that intra-patch foraging becomes less efficient, and patch residence times are longer.     

Do brain parasites alter host personality? — Experimental study in minnows

Despite that the existence of animal personalities is widely recognized, no consensus has been reached on the relative importance of different ecological factors behind their expression. Recently, it has been suggested that parasites may have a crucial role in shaping animal personalities, but only a very few studies have experimentally tested the idea. We infected Eurasian minnows (Phoxinus phoxinus) with the brain-encysted trematode parasite, Diplostomum phoxini, and studied whether infection could modify the personality of their hosts. Our results show that D. phoxini infection did not affect the mean levels of boldness, activity or exploration, but infected minnows showed higher repeatability in boldness and activity, and reduced repeatability in exploration. We also found that D. phoxini may be able to break the associations (behavioral syndromes) between behavioral traits, but that this effect may be dependent on parasite intensity. Furthermore, the effect of D. phoxini infection on personality of the hosts was found to be nonlinearly dependent on infection intensity. Taken together, our results suggest that D. phoxini parasites may shape the personality of their hosts, but that behavioral consequences of ecologically relevant infection levels may be rather subtle and easily remain undetected if only the mean trait expressions are compared.     

Male sperm whale (Physeter macrocephalus) acoustics in a high-latitude habitat: implications for echolocation and communication

Sperm whales (Physeter macrocephalus) are deep-diving predators foraging in meso- and bathypelagic ecosystems off the continental shelves. To investigate the ecophysiological and communicative function of various click types from male sperm whales in a high-latitude habitat, we deployed a large-aperture array of calibrated hydrophones off northern Norway (N69, E15). Data show that sperm whales in this habitat produce three click types: usual clicks, creak clicks and, occasionally, slow clicks. Usual clicks and creak clicks exhibit short duration, profound directionality and a frequency content suited for echolocation on meso- and bathypelagic fish and squids. The acoustic properties and low repetition rate of usual clicks are suited for long-range echolocation, whereas creak clicks have properties equivalent to signals in buzzes, the terminating pulse trains known from echolocating bats during prey capture. From these source parameters and the high acoustic activity during foraging dives, it is concluded that echolocation is an important sensory cue in prey location. Sound pressure levels of creak clicks and usual clicks measured off the acoustic axis suggest that sperm whales may be subjected to eavesdropping by conspecifics, thereby conveying information about food aggregations at estimated ranges of 6 km for creak clicks and 16 km for usual clicks. Slow clicks exhibit low directionality, low-frequency emphasis and a reduced repetition rate, suggesting that this click type is more suited for communication than for echolocation. Slow clicks can be detected by submerged conspecifics at ranges up to 60 km. Thus, sperm whales producing slow clicks may represent an odontocete species that utilizes long-range sound communication.     

The influence of subsurface thermal structure on the diving behavior of northern fur seals (<Emphasis Type="Italic">Callorhinus ursinus</Emphasis>) during the breeding season

In the heterogeneous marine environment, predators can increase foraging success by targeting physical oceanographic features, which often aggregate prey. For northern fur seals (Callorhinus ursinus), two prevalent oceanographic features characterize foraging areas during summer in the Bering Sea: a stable thermocline and a subsurface “cold pool”. The objective of this study was to examine the influence of these features on foraging behavior by equipping fur seals from St. Paul Island (Alaska, USA) with time-depth recorders that also measured water temperature. Foraging bout variables (e.g., mean dive depth and percent time diving in a bout) were compared with respect to subsurface thermal characteristics (thermocline presence and strength and cold pool presence). Over 74% of bouts occurred in association with strong thermoclines (temperature change > 5°C). Few differences were found for dive behavior in relation to the presence of a thermocline and the cold pool, but for epipelagic bouts, a strong thermocline resulted in increased bottom times, number of dive wiggles, and percent time diving when compared to moderate thermoclines. There was also a positive relationship between mean dive depth and thermocline depth. The combination of increasing foraging effort in areas with strong thermoclines and diving to depths closely related to the thermocline indicates this feature is important foraging habitat for northern fur seals and may act to concentrate prey and increase foraging success. By recognizing the environmental features northern fur seals use to find prey, managers will be better equipped to identify and protect foraging habitat that is important to northern fur seals, and possibly other marine predators in the Bering Sea.     

The pit-trapping foraging strategy of the ant lion,Myrmeleon immaculatus DeGeer (Neuroptera: Myrmeleontidae)

Summary Larvae of Myrmeleon immaculatus in large pits captured both large and samll prey, while larvae in small pits captured only the small prey. Larvae in small pits did not respond to large ants, although they always responded by sand-flinging to small ants. Larvae in medium-sized pits often captured large ants only after prolonged and vigorous sand-flipping. Larvae in large pits usually captured large ants with relatively little sand-flipping. Pit enlargement and pit relocation in the laboratory were not significantly correlated with reduction of rations in the first 3 weeks after a pit was built. However, after a month without food, larvae on the average moved once every 10 days, built successively smaller pits, and moved longer distances before building a new pit. In the field pits were dug primarily in response to microclimatological factors and possibly edge-effects. The presence or absence of suitable prey at a site, per se, had no effect on whether or not a larva would dig a pit there. We conclude that these sit-and-wait predators have a relatively large repertoire of behavior that enhances their foraging success, and we contrast it with previously made optimal foraging models relating to pit locations, pit relocations, pit size and ant lion responses.     

Nomadic behavior of army ants in a desert-grassland habitat

Summary Nomadic behavior of the army ant Neivamyrmex nigrescens was studied in a desert-grassland habitat. Six colonies were followed through eight nomadic phases (94 nomadic days) while direction and distance of emigrations, growth of larvae, number of adults and larvae per colony were determined.In all colonies, the nomadic phase began when newly eclosed adults and small larvae were present, and ended when the larvae were fully grown. Average emigration distance was positively related to number of larvae in the colony. These findings support Schneirla's theory that brood stimulation is a proximate cause of the nomadic phase.Failures to emigrate were equally likely at all points in the nomadic phase, and there was no systematic increase in emigration distance as the phase progressed. These findings do not support Schneirla's version of brood-stimulative theory.Number of adults per colony was positively related to the directionality of the nomadic phase; however, both the direction and distance of emigrations varied unpredictably from one nomadic day to the next, in marked contrast to predictions from optimal foraging theory.Schneirla's theory is useful in predicting phase differences in colony behavior, but it does not account for characteristics such as frequency, direction, or distance of emigrations within the nomadic phase. These aspects of nomadic behavior are more closely related to characteristics of the habitat such as prey density and availability of nest sites. Army ant nomadism in this habitat may depart from the optimum because of high prey density, small colony size, or lack of nesting sites.     

Pattern-oriented modeling of bird foraging and pest control in coffee farms

We develop a model of how land use and habitat diversity affect migratory bird populations and their ability to suppress an insect pest on Jamaican coffee farms. Bird foraging—choosing which habitat patch and prey to use as prey abundance changes over space and time—is the key process driving this system. Following the “pattern-oriented” modeling strategy, we identified nine observed patterns that characterize the real system's dynamics. The model was designed so that these patterns could potentially emerge from it. The resulting model is individual-based, has fine spatial and temporal resolutions, represents very simply the supply of the pest insect and other arthropod food in six habitat types, and includes foraging habitat selection as the only adaptive behavior of birds. Although there is an extensive heritage of bird foraging theory in ecology, most of it addresses only the individual level and is too simple for our context. We used pattern-oriented modeling to develop and test foraging theory for this across-scale problem: rules for individual bird foraging that cause the model to reproduce a variety of patterns observed at the system level. Four alternative foraging theories were contrasted by how well they caused the model to reproduce the nine characteristic patterns. Four of these patterns were clearly reproduced with the “null” theory that birds select habitat randomly. A version of classical theory in which birds stay in a patch until food is depleted to some threshold caused the model to reproduce five patterns; this theory caused lower, not higher, use of habitat experiencing an outbreak of prey insects. Assuming that birds select the nearby patch providing highest intake rate caused the model to reproduce all but one pattern, whereas assuming birds select the highest-intake patch over a large radius produced an unrealistic distribution of movement distances. The pattern reproduced under none of the theories, a negative relation between bird density and distance to trees, appears to result from a process not in the model: birds return to trees at night to roost. We conclude that a foraging model for small insectivorous birds in diverse habitat should assume birds can sense higher food supply but over short, not long, distances.     

Colony nutritional status modulates worker responses to foraging recruitment pheromone in the bumblebee Bombus terrestris

Foraging activity in social insects should be regulated by colony nutritional status and food availability, such that both the emission of, and response to, recruitment signals depend on current conditions. Using fully automatic radio-frequency identification (RFID) technology to follow the foraging activity of tagged bumblebees (Bombus terrestris) during 16,000 foraging bouts, we tested whether the cue provided by stored food (the number of full honeypots) could modulate the response of workers to the recruitment pheromone signal. Artificial foraging pheromones were applied to colonies with varied levels of food reserves. The response to recruitment pheromones was stronger in colonies with low food, resulting in more workers becoming active and more foraging bouts being performed. In addition to previous reports showing that in colonies with low food successful foragers perform more excited runs during which they release recruitment pheromone and inactive workers are more prone to leave the nest following nectar influx, our results indicate that evolution has shaped a third pathway that modulates bumblebee foraging activity, thus preventing needless energy expenditure and exposure to risk when food stores are already high. This new feedback loop is intriguing since it involves context-dependent response to a signal. It highlights the integration of information from both forager-released pheromones (signal) and nutritional status (cue) that occurs within individual workers before making the decision to start foraging. Our results support the emerging view that responses to pheromones may be less hardwired than commonly acknowledged. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Interactive effects of rabbit grazing and environmental factors on plant species-richness on dunes of Norderney

On the German island Norderney rabbit grazing effects on vegetation, soil and microclimate were investigated with exclosures in different dune vegetation. Differences in species composition between grazed and ungrazed plots increased with grazing activity. A decline in growth height resulted in higher light availability and temperature on the grazed sites. One of the most visible effects was the decline in flowering plants. Most grazing effects varied between habitat types. Grey dunes were distinctly affected by rabbit grazing. In relation to the habitat type, life forms were differently affected: in closed vegetation, rabbit activities further open patches, resulting in more therophytes. In contrast, in open vegetation rabbit activities were too strong and following sand dynamics too high for many therophytes. In general, grazing diversified habitats, slightly supported the total species-richness, and enhanced the heterogeneity of species composition. Highest species-richness in total and of herbs was reached at intermediate levels of grazing pressure. The study is in line that highest species-diversity will be found at intermediate levels of disturbance and environmental stress and that grazing effects vary with the habitat type.     

Meta-analysis of foraging and predation risk trade-offs in terrestrial systems

Although there is ample evidence for the generality of foraging and predation trade-offs in aquatic systems, its application to terrestrial systems is less comprehensive. In this review, meta-analysis was used to analyze experiments on giving-up-densities in terrestrial systems to evaluate the overall magnitude of predation risk on foraging behavior and experimental conditions mediating its effect. Results indicate a large and significant decrease in foraging effort as a consequence of increased predation risk. Whether experiments were conducted under natural or artificial conditions produced no change in the overall effect predation had on foraging. Odor and live predators as a correlate of predation risk had weaker and nonsignificant effects compared to habitat characteristics. The meta-analysis suggests that the effect of predation risk on foraging behavior in terrestrial systems is strongly dependent on the type of predation risk being utilized.     

Epiphyte Biodiversity in the Coffee Agricultural Matrix: Canopy Stratification and Distance from Forest Fragments

Abstract: Quality of the agricultural matrix profoundly affects biodiversity and dispersal in agricultural areas. Vegetatively complex coffee agroecosystems maintain species richness at larger distances from the forest. Epiphytes colonize canopy trees and provide resources for birds and insects and thus effects of agricultural production on epiphytes may affect other species. We compared diversity, composition, and vertical stratification of epiphytes in a forest fragment and in two coffee farms differing in management intensity in southern Mexico. We also examined spatial distribution of epiphytes with respect to the forest fragment to examine quality of the two agricultural matrix types for epiphyte conservation. We sampled vascular epiphytes in a forest fragment, a shade polyculture farm, and a shade monoculture farm at 100 m, 200 m, and 400 m from the forest. Epiphyte and orchid richness was greater in the forest than in the monoculture but richness was similar in the forest and polyculture farm. Epiphyte species composition differed with habitat type, but not with distance from the forest. In the forest, epiphytes were distributed throughout tree canopies, but in the farms, epiphytes were primarily found on trunks and larger branches. Epiphyte richness and species similarity to forest species declined with distance from the forest fragment in the monoculture, but richness and similarity to forest species did not decline with distance from forest in the polyculture. This suggests polyculture coffee has greater conservation value. In contrast, monoculture coffee is likely a sink habitat for epiphytes dispersing from forests into coffee. Coffee farms differ from forests in terms of the habitat they provide and species composition, thus protecting forest fragments is essential for epiphyte conservation. Nonetheless, in agricultural landscapes, vegetatively complex coffee farms may contribute to conservation of epiphytes more than other agricultural land uses.     

Foraging habitat and echolocation behaviour of Schneider's leafnosed bat, Hipposideros speoris, in a vegetation mosaic in Sri Lanka

The Hipposideridae and Rhinolophidae are closely related families of bats that have similar echolocation (long-duration pure-tone signal, high duty cycle) and auditory systems (Doppler-shift compensation, auditory fovea). Rhinolophid bats are known to forage in highly cluttered areas where they capture fluttering insects, whereas the foraging habitat of hipposiderid bats is not well understood. Compared to rhinolophids, hipposiderid calls are shorter in duration, have lower duty cycles, and they exhibit only partial Doppler-shift compensation. These differences suggest that the foraging habitat of the two families may also differ. We tested this hypothesis by studying foraging and echolocation of Hipposideros speoris at a site with a range of vegetation types. Bats foraged only while in flight and used all available closed and edge habitats, including areas adjacent to open space. Levels of clutter were high in forest and moderate in other foraging areas. Prey capture (n=42) occurred in edge vegetation where it bordered open space. Echolocation signals of H. speoris lacked an initial upward frequency-modulated sweep and were of moderate duration (5.1-8.7 ms). Sequences had high duty cycles (23-41%) and very high pulse repetition rates (22.8-60.6 Hz). Variation in signal parameters during search phase flight across foraging habitats was low. H. speoris showed a greater flexibility in its use of foraging habitat than is known for any rhinolophid species. Our study confirmed that there are differences in habitat use between hipposiderid and rhinolophid bats and we suggest that this divergence is a consequence of differences in their echolocation and auditory systems.     

Antipredator strategies of house finches: are urban habitats safe spots from predators even when humans are around?

Urbanization decreases species diversity, but it increases the abundance of certain species with high tolerance to human activities. The safe-habitat hypothesis explains this pattern through a decrease in the abundance of native predators, which reduces predation risk in urban habitats. However, this hypothesis does not consider the potential negative effects of human-associated disturbance (e.g., pedestrians, dogs, cats). Our goal was to assess the degree of perceived predation risk in house finches (Carpodacus mexicanus) through field studies and semi-natural experiments in areas with different levels of urbanization using multiple indicators of risk (flock size, flight initiation distance, vigilance, and foraging behavior). Field studies showed that house finches in more urbanized habitats had a greater tendency to flock with an increase in population density and flushed at larger distances than in less urbanized habitats. In the semi-natural experiment, we found that individuals spent a greater proportion of time in the refuge patch and increased the instantaneous pecking rate in the more urbanized habitat with pedestrians probably to compensate for the lower amount of foraging time. Vigilance parameters were influenced in different ways depending on habitat type and distance to flock mates. Our results suggest that house finches may perceive highly urbanized habitats as more dangerous, despite the lower number of native predators. This could be due to the presence of human activities, which could increase risk or modify the ability to detect predators. House finches seem to adapt to the urban environment through different behavioral strategies that minimize risk.     

Nesting Success of a Neotropical Migrant in a Multiple-Use, Forested Landscape

Abstract: We studied the nesting success of an individually marked population of Kentucky Warblers ( Oporornis formosus ), a species that nests in disturbed and undisturbed forests, in a heterogeneous, managed forest site in the Shawnee National Forest in southern Illinois from 1992 to 1995. We examined the effects of forest stand type (clearcuts of various ages, tree plantations, and older forest) and distance from habitat edges on rates of nest predation and brood parasitism by Brown-headed Cowbirds (   Molothrus ater ). Brood parasitism levels gradually decreased from 60% to 3% ( nests) over a distance of 2 km from an agricultural edge proximal to a known cowbird foraging site (a pig feedlot), but they did not vary with distance from any other kinds of edges or with forest stand type. Rates of nest predation ( nests) did not vary with distance from any edges, but they were significantly lower in older forest than within even-aged clearcuts, a tree plantation, and in successional vegetation adjacent to a residential facility. These results suggest that, even in fragmented landscapes with high overall levels of parasitism and nest predation, management practices within and immediately adjacent to forest tracts can affect the nesting success of some species, but not necessarily as a simple function of distance from edge. For the Kentucky Warbler, our results suggest that a management strategy that avoids even-age silviculture and leaves core stands of older forest far from cowbird feeding areas can increase nesting success to levels similar to those measured in more forested landscapes.     

Linking movement, diving, and habitat to foraging success in a large marine predator

Establishing where and when predators forage is essential to understanding trophic interactions, yet foraging behavior remains poorly understood in large marine carnivores. We investigated the factors leading to foraging success in gray seals (Halichoerus grypus) in the Northwest Atlantic in the first study to use simultaneous deployments of satellite transmitters, time depth recorders, and stomach-temperature loggers on a free-ranging marine mammal. Thirty-two seals were each fitted with the three types of instrumentation; however, complete records from all three instruments were obtained from only 13 individuals, underscoring the difficulty of such a multi-instrument approach. Our goal was to determine the characteristics of diving, habitat, and movement that predict feeding. We linked diving behavior to foraging success at two temporal scales: trips (days) and bouts (hours) to test models of optimal diving, which indicate that feeding can be predicted by time spent at the bottom of a dive. Using an information-theoretic approach, a Generalized Linear Mixed Model with trip duration and accumulated bottom time per day best explained the number of feeding events per trip, whereas the best predictor of the number of feeding events per bout was accumulated bottom time. We then tested whether characteristics of movement were predictive of feeding. Significant predictors of the number of feeding events per trip were angular variance (i.e., path tortuosity) and distance traveled per day. Finally, we integrated measures of diving, movement, and habitat at four temporal scales to determine overall predictors of feeding. At the 3-h scale, mean bottom time and distance traveled were the most important predictors of feeding frequency, whereas at the 6-h and 24-h time scales, distance traveled alone was most important. Bathymetry was the most significant predictor of feeding at the 12-h interval, with feeding more likely to occur at deeper depths. Our findings indicate that several factors predict feeding in gray seals, but predictor variables differ across temporal scales such that environmental variation becomes important at some scales and not others. Overall, our results illustrate the value of simultaneously recording and integrating multiple types of information to better understand the circumstances leading to foraging success.     

Parent-offspring aggression in moorhens

Summary Colonial orb-weaving spiders from Mexico were studied to test predictions of risksensitive foraging theory: 1. group foraging increases prey capture/individual, and reduces prey variance; 2. spiders should be expected to exhibit risk-averse behavior (forage in groups) when the average level of prey exceeds individual needs, and exhibit risk-prone behavior (forage solitarily) when prey are searce. Laboratory and field studies show that group foraging increases capture efficiency and reduces variability in prey captured per spider. In desert/mesquite grassland habitat, where prey availability is low, M. atascadero forage solitarily in most cases. In tropical rainforest/agriculture sites, M. increassata forage in large colonies of thousands of webs. In intermediate habitats, M. spinipes forages solitarily or in groups, depending on prey availability. Over a range of sites with varying levels of prey, M. spinipes shifts from a risk-prone to a risk-averse group foraging strategy as prey increases.Group foraging behavior observed in colonial Metepeira fits the predictions of risk-sensitive foraging models. These findings explain why spiders tend to group webs together only in areas of superabundant prey. The role of risk-sensitivity in the evolution of coloniality in spiders is discussed.     

Differences in relative abundance and size structure of the sea stars Pisaster ochraceus and Evasterias troschelii among habitat types in Puget Sound, Washington, USA

We surveyed patterns in the relative abundance and size structure of the sea stars Pisaster ochraceus and Evasterias troschelii in five habitat types of varying structural complexity and prey availability (sand/cobble, boulder, and rocky intertidal; pilings; and floating docks) in Puget Sound and the San Juan Islands, Washington. For both species, small sea stars were most abundant in the most structurally complex habitat type (boulder), where they occurred almost exclusively under boulders during low tide. Larger individuals became more abundant as structural complexity decreased, occurring more frequently in open habitat types (rocky shores, pilings, and docks) known to have greater abundances of prey resources. Gull foraging observations and experiments demonstrated that exposed small sea stars of both species were highly vulnerable to predation, suggesting that small sea stars require structural complexity (crevice microhabitat) as a predation refuge. Large sea stars, once attaining a size refuge from predation, appear to migrate to more exposed habitat types with more abundant food resources. These results suggest parallel ontogenetic habitat shifts in two co-occurring consumer species related to a shared predation risk at early life stages and demonstrate how the relative importance of top-down and bottom-up processes may differ with ontogeny.     

Fish feeding on mobile benthic invertebrates: influence of spatial variability in habitat associations

Patterns of habitat association and foraging were examined for a group of tropical goatfishes (family Mullidae) that feed on mobile benthic invertebrates at Lizard Island (Great Barrier Reef). All goatfish possess barbels that disturb the substratum during feeding. Foraging methods were examined for the six most common species and used in conjunction with data on habitat associations to estimate the distribution and potential impact on the benthic invertebrate assemblage of foraging-related disturbance. Particular species exhibited broad habitat associations which differed little over two surveys (January 1989, January 1990). All species showed different preferences for the substrata they foraged. Preferences for substrata exhibited by the most common reef-associated species, Parupeneus multifasciatus, differed among locations separated by 1 km, between sites 150 m apart, and between depths (shallow and deep). Habitat preferences changed with ontogeny. Based on their habitat associations and foraging preferences, species were divided into habitat generalists and specialists. Specialists associated primarily with soft sediments. Habitat generalists, such as P. multifasciatus and P. cyclostomus, are likely to have an impact on their mobile invertebrate prey that is localised, diffuse and transitory, making any experimental analysis difficult and expensive. Habitat specialists form a guild of fishes with complementary feeding modes that efficiently exploit soft sediments and are more amenable to experimental manipulation. Experiments designed to detect the impact of foraging by these fishes must be repeated at different locations and times and must account for depth differences in foraging pressure.