Predation by gulls on crabs in rocky intertidal and shallow subtidal zones of the Gulf of Maine

Most organisms in intertidal areas are marine in origin; many have distributions that extend into the subtidal zone. Terrestrial predators such as mammals and birds may exploit these animals during low tide and can have considerable effects on intertidal food webs. Several studies have shown that avian predators are capable of reducing densities of sessile and slow-moving intertidal invertebrates but very few studies have considered avian predation on mobile invertebrate predators such as crabs. In this study, we investigated predation by Great Black-backed Gulls (Larus marinus Linnaeus) on three species of crabs (Cancer borealis Stimpson, Cancer irroratus Say, and Carcinus maenas Linnaeus). The study was at Appledore Island, ME (a gull breeding island) and 8 other sites throughout the Gulf of Maine, including breeding islands and mainland sites. On Appledore Island, intertidal and subtidal zones provided over one-third of prey remains found at gull nests, and crabs were a substantial proportion (∼ 30% to 40%) of the total remains. Similarly, collections of prey remains from intertidal areas indicated that crabs were by far the most common marine prey. C. borealis was eaten far more often and C. irroratus and C. maenas less often than expected at each site. Comparing numbers of carapaces to densities of crabs in low intertidal and shallow subtidal zones at each site, we estimated that gulls remove between 15% and 64% of C. borealis during diurnal low tides. The proportion of C. borealis eaten by gulls was independent of proximity to a gull colony. Approximately 97% of the outer coast of Maine is within 20 km of a breeding island. Thus, a lot of gull predation on crabs may occur throughout the Gulf of Maine during summer months. Crabs are important predators of other invertebrates; if predation by gulls reduces the number of crabs in intertidal and shallow subtidal areas, gulls may have important indirect effects on intertidal food webs.     

Why are intertidal snails rare in the subtidal? Predation, growth and the vertical distribution of Littorina littorea (L.) in the Gulf of Maine

The North Atlantic gastropod Littorina littorea exhibits a characteristic “intertidal” distribution: the snail is abundant in the littoral zone but scarce in the shallow subtidal and the relatively few subtidal individuals are larger (in shell size) on average than those in the intertidal zone. For highly mobile species like L. littorea, this vertical distribution is primarily determined by directional movement. Biotic and abiotic factors vary across tidal heights, and natural selection for movement to shore levels where fitness is maximized provides the ultimate (evolutionary) explanation for vertical distribution patterns. In this study, we asked whether variation in growth rate and/or predation pressure among tidal heights provide an ultimate explanation for vertical gradients in L. littorea size and abundance. We used a cage experiment to compare juvenile growth rate among tidal heights and a series of field and laboratory experiments to examine variation in predation pressure among tidal heights and snail size classes. Juvenile growth rates were highest in the low intertidal zone, declining at both higher and lower levels. Predation risk for tethered L. littorea increased with both decreasing tidal height and decreasing body size (shell height). Almost all tethered prey were consumed by shell- breaking predators and a census revealed that the two most abundant such predators were the crabs Carcinus maenas and Cancer borealis. Laboratory feeding experiments were used to compare size-dependent prey vulnerability and prey-size preferences for these two key predators. We found that L. littorea vulnerability decreased with increasing snail size and increased with increasing size of both predator species. However, whereas C. borealis were capable of consuming even the largest L. littorea, most Carcinus were unable to feed on individuals larger than 10 mm in shell height. Additionally, C. borealis preferred larger sizes of L. littorea than did Carcinus. Thus, Carcinus, which co-occurs with L. littorea in the intertidal, is a much less effective predator than C. borealis, which is found primarily in the subtidal. We conclude that predation on L. littorea by C. borealis and other subtidal consumers has resulted in the scarcity of this ecologically important grazer in the subtidal. This effect has been produced both through direct predation and by imposing strong selection for movement of L. littorea to higher tidal zones.     

How climate change might influence the starvation–predation risk trade-off response

Climate change within the UK will affect winter starvation risk because higher temperatures reduce energy budgets and are likely to increase the quality of the foraging environment. Mass regulation in birds is a consequence of the starvation–predation risk trade-off: decreasing starvation risk because of climate change should decrease mass, but this will be countered by the effects of predation risk, because high predation risk has a negative effect on mass when foraging conditions are poor and a positive effect on mass when foraging conditions are good. We tested whether mass regulation in great tits (Parus major) across the UK was related to temporal changes in starvation risk (winter temperature 1995–2005) and spatial changes in predation risk (sparrowhawk Accipiter nisus abundance). As predicted, great tits carried less mass during later, warmer, winters, demonstrating that starvation risk overall has decreased. Also, the effects of predation risk interacted with the effects of temperature (as an index of foraging conditions), so that in colder winters higher sparrowhawk abundance led to lower mass, whereas in warmer, later, winters higher sparrowhawk abundance led to higher mass. Mass regulation in a small bird species may therefore provide an index of how environmental change is affecting the foraging environment.     

Subtidal-intertidal trophic links: American lobsters [Homarus americanus (Milne-Edwards)] forage in the intertidal zone on nocturnal high tides

Homarus americanus (Milne-Edwards), the American lobster, is a predator in New England subtidal communities, feeding on ecologically important grazers (sea urchins), mesopredators (crabs), and basal species (mussels). In this study, we provide the first report of adult American lobsters foraging in rocky intertidal habitats during nocturnal high tides. Censuses by SCUBA divers in the low intertidal (Chondrus crispus Stackhouse) zone showed mean densities of 2.2 lobsters/20 m² on nocturnal high tides, with contrasting low densities of 0.18/20 m² during diurnal high tides. Nocturnal high-tide intertidal densities were 62% of those reported in a previous study of lobsters in nearby subtidal rocky areas (Novak, 2004). The average carapace length of lobsters in the intertidal at night was > 50 mm. These lobsters were actively foraging in the intertidal with collected individuals having a mean stomach fullness of 67%. Prey found in the stomach contents primarily consisted of crabs, mussels and snails. Field experiments showed that lobsters rarely fed on medium to large size individuals of the common intertidal snail, Littorina littorea (L.). In contrast, experiments with local crab species demonstrated that lobsters actively and readily prey on Cancer irroratus (Say) and Carcinus maenas (L.), but were significantly less likely to consume Cancer borealis (Stimpson). The abundance of Carcinus maenas and blue mussels (Mytilus edulis L.) in the intertidal zone may explain the upshore movement of lobsters. Since nocturnal migration of Homarus americanus into the intertidal zone has not been documented before, our understanding of the dynamics of New England intertidal communities needs to be expanded to include this predator.     

A dynamic-bioenergetics model to assess depth selection and reproductive growth by lake trout (Salvelinus namaycush)

We coupled dynamic optimization and bioenergetics models to assess the assumption that lake trout (Salvelinus namaycush) depth distribution is structured by temperature, food availability, and predation risk to maximize reproductive mass by autumn spawning. Because the model uses empirical daily thermal-depth profiles recorded in a small boreal shield lake (lake 373 at the Experimental Lakes Area, northwestern Ontario) during 2 years of contrasting thermal stratification patterns, we also assessed how climate-mediated changes in lakes may affect the vertical distribution, growth, and fitness of lake trout, a cold-water top predator. The depths of acoustic-tagged lake trout were recorded concurrently with thermal-depth profiles and were compared to model output, enabling an assessment of model performance in relation to the observed fish behavior and contrasting thermal conditions. The depths and temperatures occupied by simulated fish most closely resembled those of the tagged fish when risk of predation was included in the model, indicating the model may incorporate the most important underlying mechanisms that determine lake trout depth. Annual differences suggest less use of shallow (warm), productive habitats, resulting in markedly less reproductive mass, during the year with the warm stratification pattern. Mass for reproduction may be lower in warmer conditions because of reduced reproductive investment, yet survival may be inadvertently higher because risky surface waters may be avoided more often in warmer, shallower, and metabolically costly conditions. At a minimum our study suggests that lake trout reproductive mass and fitness may be expected to change under the anticipated longer and warmer stratification patterns.     

Foraging costs and accessibility as determinants of giving-up densities in a swan-pondweed system

We measured the patch use behaviour of Bewick's swans (Cygnus columbianus bewickii) feeding on below ground tubers of fennel pondweed (Potamogeton pectinatus). We compared the swans’ attack rates, foraging costs and giving‐up densities (GUDs) in natural and experimental food patches that differed in water depth. Unlike most studies that attribute habitat‐specific differences in GUDs to predation risk, food quality or foraging substrate, we quantified the relative importance of energetic costs and accessibility. Accessibility is defined as the extent to which the animal's morphology restricts its harvest of all food items within a food patch. Patch use behaviours were measured at shallow (ca 0.4 m) and deep (ca 0.6 m) water depths on sandy sediments. In a laboratory foraging experiment, when harvesting food patches, the swan's attack rate (m³ s^?1) did not differ between depths. In deep water the energetic costs of surfacing, feeding and trampling were 1.13 to 1.21 times higher than in shallow water with a tendency to spend relatively more time trampling, the most expensive activity. Taking time allocation as measured in the field into account, foraging in deep water was 1.26 times as expensive as in shallow water. In the lake the GUD in shallow water was on average 12.9 g m^?2. If differences in energetic costs were the only factor determining differences in GUDs, then the deep water GUD should be 14.2 g m^?2. Instead, the mean GUD in deep water was 20.2 g m^?2, and therefore energetic costs explain just 18% of the difference in GUDs. At deep sites, 24% of tuber biomass was estimated to be out of reach, and we calculated a maximum accessible foraging depth of 0.86 m. This is close to the published 0.84 m based on body measurements. A laboratory experiment with food offered at a depth of 0.89 m confirmed that it was just out of reach. The agreement between calculated and observed maximum accessible foraging depths suggests that accessibility largely explains the remaining difference in GUDs with depth, and it confirms the existence of partial prey refuges in this system.     

Diel variation in predator abundance, predation risk and prey distribution in shallow-water estuarine habitats

Predation by visual predators is often affected by light conditions and may therefore exhibit strong diel variation. The dominant predators on grass shrimp, Palaemonetes pugio, are finfish predators that are thought to locate their prey by visual cues. We examined the response of grass shrimp to diel variation in predation risk in the nearshore shallow waters of the Chesapeake Bay. We used diel shoreline seines to assess the relative abundance of predators. We assessed the relative risk of predation with shrimp tethered at refuge (30 cm) and nonrefuge (60 cm) depths. To measure grass shrimp response to predation risk, we used dipnets to monitor habitat use. Four predominantly visual predators dominated the shoreline seine catches, Fundulus heteroclitus, Micropogonias undulatus, Morone americana and Morone saxatilis. Total predator abundance had a diel component, with dramatic nighttime decreases in total abundance, whereas guild composition and relative abundance remained unchanged. Relative predation risk for tethered shrimp exhibited significant time by habitat interaction. During the day, depth negatively affected survivorship of tethered shrimp while at night overall survivorship increased and there was no effect of depth. Shrimp habitats use reflected diel predation risks. Abundances in the near shore were highest during the day with decreased abundances at night. Together, the seine and tethering data highlight the importance for a refuge (e.g., shallow water) from predation during the daytime and a relaxation of predation pressure at night.     

An environmentally induced tidal periodicity of microgrowth increment formation in subtidal populations of the clam Ruditapes philippinarum

The periodicity of increment formation in the shell of the Manila clam Ruditapes philippinarum was investigated in the subtidal zone of the Auray River estuary (South Brittany, France). Calcein markings were performed at different periods between May and October 2007 using in situ benthic chambers tented by scuba divers. This study shows that shell microgrowth increments were well-defined and deposited with a tidal periodicity in the subtidal zone, providing the calendar base for high-resolution ecological studies and environmental reconstruction from these R. philippinarum shells. Endogenous rhythmicity in shell microgrowth increment formation and oxygen consumption was previously documented in this species from intertidal flats. Our study suggests that, in the subtidal zone, Manila clams' rhythmic activity may be controlled by such an endogenous process, synchronized by tidal cues. As in other bivalves, R. philippinarum is an osmoconformer euryhaline bivalve. The tidal rhythmicity of shell microgrowth increments in subtidal specimens of this species could be explained by a behavioral adaptation of valve closure at low tide to protect the clam from low salinities and/or to synchronize with food availability. Finally, large inter-individual variability in tidally associated growth rates and asynchronous growth breaks were observed, and could be due to genetic variability between individuals, asynchronous partial spawning events or predation.     

Native species vulnerability to introduced predators: testing an inducible defense and a refuge from predation

To manage the impacts of biological invasions, it is important to determine the mechanisms responsible for the effects invasive species have on native populations. When predation by an invader is the mechanism causing declines in a native population, protecting the native species will involve elucidating the factors that affect native vulnerability. To examine those factors, this study measured how a native species responded to an introduced predator, and whether the native response could result in a refuge from predation. Predation by the green crab, Carcinus maenas, has contributed to the decline in numbers of native soft-shell clams, Mya arenaria, and efforts to eradicate crabs have proven futile. We tested how crab foraging affected clam burrowing, and how depth in the sediment affected clam survival. Clams responded to crab foraging by burrowing deeper in the sediment. Clams at shallow depths were more vulnerable to predation by crabs. Results suggest soft-shell clam burrowing is an inducible defense in response to green crab predation because burrowing deeper results in a potential refuge from predation by crabs. For restoring the native clam populations, tents could exclude crabs and protect clams, but when tents must be removed, exposing the clams to cues from foraging crabs should induce the clams to burrow deeper and decrease vulnerability. In general, by exposing potential native prey to cues from introduced predators, we can test how the natives respond, identify whether the response results in a potential refuge, and evaluate the risks to native species survival in invaded communities.     

Axes of fear for stream fish: water depth and distance to cover

To better understand habitat-specific predation risk for stream fish, we used an approach that assumes animals trade off food for safety and accurately assess risk such that predation risk can be measured as a foraging cost: animals demand greater harvest rates to occupy riskier locations. We measured the foraging cost of predation risk for juvenile salmonids within enclosures in a natural stream at locations that varied in water depth and distance to cover. Measurements relied on a food delivery apparatus and direct observations that allowed estimation of “giving-up” harvest rates – food delivery rates at which animals left the feeding apparatus. Juvenile steelhead about 120 mm fork length exhibited sharp increases in giving-up harvest rate with decreasing water depth and refused to use the feeding device even when offered extreme food delivery rates in water ≤20 cm deep. Giving-up harvest rates were less affected by the distance to cover. Assuming the gradients we observed in giving-up harvest rates reflect predation risk, the results of this study can be applied to spatially explicit models of stream fish populations that incorporate risk into both habitat selection and mortality due to predation.     

A new late Katian (Late Ordovician) trilobite association from Zhenxiong,northeastern Yunnan,Southwest China and its palaeoecological implications

A new late Katian (Late Ordovician) trilobite association is documented from the Daduhe Formation in Zhenxiong, northeastern Yunnan, including 17 species of 15 genera, among which four species (Malongullia sinensis, M. zhenxiongensis, Taklamakania paucisegmentatus and Amphitryon constrictus) are new. Lithologic and faunal evidence enables the recognition of a new association with medium diversity, named the Taklamakania Association. It is a typical representative of the Raphiophorid Community that lived in a deep subtidal environment close to the anoxic basin. A total of five trilobite ecological associations were found to occur in South China during the late Katian, in relation to the environmental gradients ranging from shallow subtidal zone, deep subtidal zone, slope to dysoxic shaly basin.     

Growth-mortality tradeoffs and 'personality traits' in animals

Consistent individual differences in boldness, reactivity, aggressiveness, and other 'personality traits' in animals are stable within individuals but vary across individuals, for reasons which are currently obscure. Here, I suggest that consistent individual differences in growth rates encourage consistent individual differences in behavior patterns that contribute to growth-mortality tradeoffs. This hypothesis predicts that behavior patterns that increase both growth and mortality rates (e.g. foraging under predation risk, aggressive defense of feeding territories) will be positively correlated with one another across individuals, that selection for high growth rates will increase mean levels of potentially risky behavior across populations, and that within populations, faster-growing individuals will take more risks in foraging contexts than slower-growing individuals. Tentative empirical support for these predictions suggests that a growth-mortality perspective may help explain some of the consistent individual differences in behavioral traits that have been reported in fish, amphibians, reptiles, and other animals with indeterminate growth.     

FACTORS CONTRIBUTING TO SPATIAL HETEROGENEITY IN THE ABUNDANCE OF THE COMMON PERIWINKLE LITTORINA LITTOREA (L.)

Although Littorina littorea (L.) exhibits a relatively consistentpattern of vertical distribution throughout the North Atlantic,ranging from the mid-intertidal to the shallow subtidal zone,its horizontal distribution and abundance are highly variable.In this study, we first described the snail's horizontal distributionpatterns on Appledore Island, ME, USA and then asked whetherwave exposure, rugosity, predator density (e.g. Carcinus maenasand Cancer borealis), the percentage of the substrate coveredby Ascophyllum nodosum, Chondrus crispus, Polysiphonia spp.,and ephemeral green algae, or the percentage of uncovered substrate(bare rock) were correlated with L. littorea abundance in theintertidal zone (0.6 to 0.0 m relative to Mean Lower LowWater [MLLW]) and the shallow subtidal zone (–1.5 to –3.0 mMLLW) at nine study sites. Intertidal densities of L. littoreawere highly variable across sites, ranging from 0 to >600 m^–2.In this zone, L. littorea density showed a significant positivecorrelation with rugosity and percentage of bare rock. Densitieswere very low in the subtidal zone (range: 0–19 m^–2)and varied little among sites. Exploratory multiple regressionanalysis suggested that L. littorea density was positively correlatedwith the density of C. maenas in the shallow subtidal zone.Additionally, snails in the subtidal zone had thicker shellsthan snails of the same size in the intertidal zone. Our resultssuggest that structural elements of the habitat, such as rugosityand percentage of uncovered substrate, are among the most importantpredictors of L. littorea abundance on moderately protected,rocky intertidal shores. (Received 9 February 2005; accepted 10 August 2005)     

Relative predation risk and risk of desiccation co-determine oviposition preferences in Cope’s gray treefrog, <Emphasis Type="Italic">Hyla chrysoscelis</Emphasis>

Habitat permanence and threat of predation are primary drivers of community assembly and composition in lentic freshwater systems. Pond-breeding amphibians select oviposition sites to maximize fitness and minimize risks of predation and desiccation of their offspring, typically facing a trade-off between the two as predation risk often increases as desiccation risk decreases. To experimentally determine if Hyla chrysoscelis partition oviposition along gradients of relative desiccation risk and predation risk, we tested oviposition site preference in a natural population of treefrogs colonizing experimental ponds that varied in water depth and contained predatory larvae of two Ambystoma salamander species. Hyla chrysoscelis selected habitats with both lower predation risk, avoiding A. talpoideum over A. maculatum, and lower desiccation risk, selecting ponds with three times greater depth. We demonstrate that adult oviposition site choices simultaneously minimize relative predation risk and desiccation risk and that closely related salamander species produce functionally different responses among colonizing animals.     

Assessing the risk effects of native predators on the exotic American bullfrog (Lithobates catesbeianus) and their indirect consequences to ecosystem function

Understanding the factors and mechanisms that affect the impacts of invasive species in invaded environments has been widely debated among researchers. However, few studies about invasive species have explored the effects of predation risks by native predators on exotic prey. Herein, the traditional invasive predator-native prey framework was reversed. We tested if tadpoles, of the worldwide invasive American Bullfrog Lithobates catesbeianus, were affected by the predation risk imposed by native predators. We used two different species of belostomatid predators and tested whether and how predation-induced phenotypic plasticity on L. catesbeianus reverberated in morphological, physiological, and ecosystem-level processes. Individuals of L. catesbeianus modified their morphological (tail muscle width), behavioral (activity and foraging), and physiological (growth and growth efficiency) traits in the presence of predation risk. Based on the observed morphological changes, our results suggest that prey may recognize predator-specific cues. In addition, we observed that L. catesbeianus' responses to predation risk can affect ecosystem-level properties, by inducing trophic cascades and reducing animal-mediated nutrient recycling rates. In summary, our study supports that exotic prey species who are subjected to native predators may display anti-predator responses, with implications for their development, as well as possible ecosystem-level effects.     

Type and nature of cues used by Nucella lapillus to evaluate predation risk

The ability of prey to detect and adequately respond to predation risk influences immediate survival and overall fitness. Chemical cues are commonly used by prey to evaluate risk, and the purpose of this study was to elicit the nature of cues used by prey hunted by generalist predators. Nucella lapillus are common, predatory, intertidal snails that evaluate predatory risk using chemical cues. Using Nucella and a suite of its potential predators as a model system, we explored how (1) predator type, (2) predator diet, and (3) injured conspecifics and heterospecifics influence Nucella behavior. Using laboratory flumes, we determined that Nucella responded only to the invasive green crab (Carcinus maenas), the predator it most frequently encounters. Nucella did not respond to rock crabs (Cancer irroratus) or Jonah crabs (Cancer borealis), which are sympatric predators but do not frequently encounter Nucella because these crabs are primarily subtidal. Predator diet did not affect Nucella responses to risk, although starved predator response was not significantly different from controls. Since green crabs are generalist predators, diet cues do not reflect predation risk, and thus altering behavior as a function of predator diet would not likely benefit Nucella. Nucella did, however, react to injured conspecifics, a strategy that may allow them to recognize threats when predators are difficult to detect. Nucella did not react to injured heterospecifics including mussels (Mytilus edulis) and herbivorous snails Littorina littorea, suggesting that they are responding to chemical cues unique to their species. The nature of cues used by Nucella allows them to minimize costs associated with predator avoidance.     

Mussel beds in deeper water provide an unusual situation for competitive interactions between the seastars Leptasterias polaris and Asterias vulgaris

The common seastars Leptasterias polaris and Asterias vulgaris show competitive interactions in shallow subtidal communities in the northern Gulf of St. Lawrence, particularly during summer when aggregations of the two seastars forage on mussel beds at 1-2 m in depth. We examined interactions between the two seastars in a different situation, in a mussel bed at 6 m in depth (a rare situation in this region). In the deeper mussel bed, seastars were three times more abundant than in the shallower beds, and the mussels were larger. The deeper bed disappeared rapidly due to the intense predation. Although decreased prey abundance should have favored interference interactions, we did not detect either partitioning of mussels by size or avoidance of A. vulgaris by L. polaris as previously reported when mussels are in short supply in shallower water. The lack of an avoidance behavior by L. polaris, together with the higher proportion of L. polaris than A. vulgaris that were feeding, suggests that in this situation, the dominance of A. vulgaris (observed in shallower water) is attenuated, or that L. polaris may dominate.     

Potential energy expenditure by litter-roosting bats associated with temperature under leaf litter during winter

In temperate portions of North America, some bats that remain active during winter undergo short periods of hibernation below leaf litter on the forest floor during episodes of below-freezing weather. These winter roosts may provide above-freezing conditions, but the thermal conditions under leaf litter are unclear. Further, little is known of the relationship between temperatures under litter and potential energy expenditure by bats. Therefore, I characterized thermal conditions below leaf litter, compared temperatures encountered under different litter depths, and evaluated the quality of these sites as hibernacula based on potential energy use by eastern red bats (Lasiurus borealis) during winter in forests of the Ouachita Mountains, Arkansas, USA. Over an averaged 24-h period, there was no significant difference in temperature among different depths of leaf litter, but temperatures under litter remained significantly warmer than air temperatures, especially during nighttime and under snow cover. Temperatures below leaf litter were significantly warmer on south-facing slopes than north-facing slopes, but predicted metabolic rates did not differ among aspects. Predicted metabolic rates of eastern red bats were lowest under the deepest leaf litter measured (8 cm) and highest under ambient air conditions. Depending on depth of leaf litter cover, predicted energy savings based on O₂ consumption from roosting under litter were 1.9 to 3.1 times greater than remaining in ambient air during periods of freezing weather and around 5.6 times greater when roosting under leaf litter with snow cover. A model for predicted total energy consumption (estimated as the total oxygen consumption during a 24-h period) by eastern red bats indicated that when roosting below leaf litter, energy consumption would be reduced with greater ground temperatures, greater leaf litter moisture, and when located on south-facing slopes. Predicted metabolic rates and total energy consumption may provide more insight on the quality of roost sites for wintering bats than temperature of roost sites alone.     

Testing the mass-dependent predation hypothesis: in European blackbirds poor foragers have higher overwinter body reserves

As foraging becomes more unpredictable animals should increase their body reserves to reduce the risk of starvation. However, any increases in reserves may increase the risk of predation because extra mass probably compromises escape ability. Because of differences in foraging ability not all individuals will be affected in the same way by changes in foraging conditions. Relatively poor foragers will have more unpredictable foraging success for any given availability of food and therefore should carry larger body reserves. The mass-dependent predation hypothesis then predicts a negative correlation between levels of body reserves and foraging ability, although this may be modified by state-dependent compensation. I measured foraging rates and body masses of wintering European blackbirds, Turdus merula. Individuals with the lowest foraging rates had the largest gain in mass for the winter and had relatively high mass overall, independently of age and sex. That foraging rate determined mass rather than the reverse was demonstrated because foraging rate was independent of daily and seasonal mass change. Foraging rate within the experimental system was also independent of predation risk (as measured by distance from protective cover) and so the relation between mass and foraging rate was unlikely to have been confounded by any changes in vigilance to compensate for increased mass-dependent predation risk. The results suggest that blackbirds with high relative foraging rates have lower body reserves during the winter. Therefore there is probably a direct link between overwinter condition and fitness at least in blackbirds. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Effect of an invasive crab upon a marine fishery: green crab, Carcinus maenas, predation upon a venerid clam, Katelysia scalarina, in Tasmania (Australia)

Nonindigenous species are increasingly recognized as altering marine and estuarine communities, causing significant changes in abundance and distribution of native species. Such effects are of particular concern to coastal fisheries. We experimentally determined the effect of the nonindigenous European green crab, Carcinus maenas, upon the stepped venerid clam, Katelysia scalarina, the basis for a fledgling clam fishery in Tasmania, Australia. First, we observed a trend of decreased juvenile (<13-mm shell length or SL) abundance of K. scalarina at sites with C. maenas relative to those without this invasive predator. Additionally, relative predation intensity on these juveniles was significantly higher in invaded areas. To better understand the dynamics of predation by this invader, we conducted a number of manipulative experiments. In cage experiments testing per capita predation rates, we found that: (1) of the various sizes of C. maenas, large C. maenas were the most significant predators; (2) the smallest size class of K. scalarina tested (6-12-mm SL) was preferred by C. maenas; (3) C. maenas had much higher predation rates than any native predator tested; and (4) while the native shore crab, Paragrapsus gaimardii, was found to have a constant predation rate over an eightfold range of densities of juvenile K. scalarina (16-128 individuals·m⁻²), C. maenas significantly increased its per capita predation with increasing prey density. Notably, in open field plots at a site where C. maenas was abundant, predation was constant over the range of tested prey densities. We predict, therefore, that the invasion of C. maenas will have significant negative consequences for the Tasmanian K. scalarina fishery.