Shorebird Predation Research Articles

Shorebird predation on benthic invertebrates after shrimp-pond harvesting: Implications for semi-intensive aquaculture management

How to improve habitat quality for wildlife is of particular importance in areas dedicated to food production, due to increasing pressures of global demands associated to human population growth. Semi-intensive aquaculture ponds can provide a potentially important foraging habitat for migratory shorebirds throughout the non-breeding season. Therefore, understanding the availability of benthic invertebrates in aquaculture ponds will help to identify proper management decisions for shorebird conservation. We used an exclosure experiment during the first three days after shrimp harvesting at semi-intensive aquaculture ponds in north-west Mexico to assess shorebird predation on benthic invertebrates. We found that shorebird predation did not deplete total benthic invertebrate density (particularly polychaete worms), but significantly affected the prey size distributions and biomass in the ponds during a short-time window of just three days. Shorebirds removed 0.6 g ash-free dry weight m−2, equivalent to 43% of the initial biomass and showed high selectivity for polychaetes larger than >40 mm as prey, potentially explaining the absence of large polychaetes at the end of the experiment. This depletion was the likely cause of the daily decrease observed in overall density of foraging shorebirds at recently harvested ponds. These results can serve to identify management actions that allow an extended use of semi-intensive aquaculture ponds as foraging sites for migratory shorebirds during the non-breeding season, with potential applications to develop standards for a friendlier aquaculture management.

Egg Exhumation and Transport on a Foreshore Under Wave and Swash Processes

Several aquatic species use beach foreshores to lay their eggs. Understanding the dynamics of egg entrainment versus egg retention in foreshore sediment is important to delivering eggs to foraging species, and to predicting fecundity and population responses of beach nesting species to harvest and habitat management. Exhumation of these eggs is primarily by bioturbation and wave action. This study isolates effects of wave and swash processes on exhumation, entrainment, and transport of horseshoe crab eggs by conducting a 1-day field study designed to avoid effects of bioturbation by spawning and migratory shorebird predation. Dyed sand and horseshoe crab (Limulus polyphemus) eggs were injected into the foreshore of an estuarine beach in Delaware Bay, USA and sampled during 110 swash events using streamer traps. Significant wave heights ranged from 0.30 to 0.47 m. The quantity of eggs exhumed from the mid-foreshore by waves and swash was 3.8 times greater than from the upper foreshore where activation was by swash alone. Swash flows were skewed offshore, but egg transport was predominantly onshore from bore collapse. Egg release was greatest in early rising tide, making this the most productive time for shorebird foraging. Release of eggs from mid-foreshore may be dominated by wave action during high energy days with little spawning or by bioturbation by horseshoe crabs during intensive spawning. Activation depths in the swash were low, indicating that eggs buried on the upper foreshore can reach later stages of development after spawning ceases.

The interactive role of predation, competition and habitat conditions in structuring an intertidal bivalve population

Habitat characteristics, predation and competition are known to interactively drive population dynamics. Highly complex habitats, for example, may reduce predation and competition, allowing more individuals living together in a certain area. However, the strength and direction of such interactions can differ strongly and are context dependent. Furthermore, as habitat characteristics are rapidly changing due to anthropogenic impacts, it becomes increasingly important to understand such interactions. Here, we studied the interactive effects of predation and competition on common cockle (Cerastoderma edule) recruitment, growth and survival under different habitat characteristics in the Wadden Sea, one of the world's largest intertidal ecosystems. In a predator-exclosure experiment, we manipulated cockle densities (100 vs. 1000 individuals m-2) and shorebird predation at two sites differing in habitat characteristics, namely at the wake of a blue mussel bed (Mytilus edulis) and at an adjacent sandy site. We found that recruitment was higher in the mussel-modified habitat, most likely due to reduction of hydrodynamic stress. Although bird predation strongly reduced recruit density, the combined effects still yielded more recruitment at the vicinity of the mussel bed compared to the sandy area. Furthermore, we found that high cockle densities combined with high densities of other potential prey (i.e. mussels) at the mussel-modified site, mitigated predation effects for adult cockles. Apart from these positive effects on adults, mussel-modified habitat reduced cockle growth, most likely by reducing hydrodynamics in the wake of the mussel bed and by increasing inter-specific competition for food. Our study experimentally underpins the importance of habitat characteristics, competition and predation in interactively structuring intertidal communities.

Impact of shorebird predation on intertidal macroinvertebrates in a key North African Atlantic wintering site: an experimental approach

Shorebirds, as migratory aquatic birds and top predators in intertidal ecosystems, can be affected by global environmental changes and escalations in local impacts on coastal lagoons and estuarine trophic networks. Many shorebirds winter in North African Atlantic coastal sites, most likely because these locations provide constant and reliable food supplies with less energy costs in comparison with the wintering sites of northern Europe. Although more information is available for other important southern coastal sites (e.g. Saharan Atlantic coastal desert and Guinean mangroves coast), very little information is available for the North African Atlantic coast. Here, we focus on the impact of shorebird predation on benthic macroinvertebrates in a major wintering site in this area—Sidi Moussa coastal lagoon, Morocco—using an exclosure experiment. For most of the macroinvertebrate species there was no significant effect of the exclusion of shorebird predation. Overall, our results do not show evidence that predation by shorebirds influenced the overall standing biomass of the benthic community. This may indicate that the benthic productivity is high enough to provide constant and reliable food supplies for non-breeding shorebirds.

Behavioral Response of Corophium volutator to Shorebird Predation in the Upper Bay of Fundy, Canada

Predator avoidance is an important component of predator-prey relationships and can affect prey availability for foraging animals. Each summer, the burrow-dwelling amphipod Corophium volutator is heavily preyed upon by Semipalmated Sandpipers (Calidris pusilla) on mudflats in the upper Bay of Fundy, Canada. We conducted three complementary studies to determine if adult C. volutator exhibit predator avoidance behavior in the presence of sandpipers. In a field experiment, we monitored vertical distribution of C. volutator adults in bird exclosures and adjacent control plots before sandpipers arrived and during their stopover. We also made polymer resin casts of C. volutator burrows in the field throughout the summer. Finally, we simulated shorebird pecking in a lab experiment and observed C. volutator behavior in their burrows. C. volutator adults were generally distributed deeper in the sediment later in the summer (after sandpipers arrived). In August, this response was detectably stronger in areas exposed to bird predation than in bird exclosures. During peak predator abundance, many C. volutator adults were beyond the reach of feeding sandpipers (>1.5 cm deep). However, burrow depth did not change significantly throughout the summer. Detailed behavioral observations indicated that C. volutator spent more time at the bottom of their burrow when exposed to a simulated predator compared to controls. This observed redistribution suggests that C. volutator adults move deeper into their burrows as an anti-predator response to the presence of sandpipers. This work has implications for predators that feed on burrow-dwelling invertebrates in soft-sediment ecosystems, as density may not accurately estimate prey availability.

Effects of shorebird predation and snail abundance on an intertidal mudflat community

Top-down effects of predation are well documented in a variety of ecological communities, including marine soft-sediment systems. It has been proposed that intertidal mudflats in the upper Bay of Fundy, Canada, which host a large population of foraging shorebirds each summer, may exhibit this community dynamic. Biofilm (consisting mainly of diatoms) forms the base of the mudflat community food web, which is dominated by the amphipod Corophium volutator. To assess the potential for a trophic cascade, we conducted a manipulative field experiment examining individual and combined effects of the shorebird Calidris pusilla, a primary predator of C. volutator, and the eastern mudsnail (Nassarius obsoletus), an intraguild predator, on community structure (including macrofauna and large meiofauna retained by a 250-μm screen). Snails exhibited density-dependent top-down effects, primarily from strong negative interactions with juvenile and adult C. volutator, likely due to interference, consumption and emigration. Medium and high densities of snails reduced chlorophyll a concentration (a measure of diatom abundance), likely through consumption and disturbance of the sediment. When present at higher densities, snails also increased variability in community structure. Shorebirds were less influential in determining community structure. They reduced C. volutator biomass through consumption, but there was no resulting effect on primary production. Top-down effects of snails and birds were cumulative on C. volutator, but did not generate a trophic cascade. We suggest that a combination of omnivory and intraguild predation by shorebirds and snails, coupled with relatively low grazing pressure by C. volutator, prevented transmission of top-down effects.

Shorebird predation of horseshoe crab eggs in Delaware Bay: species contrasts and availability constraints

1. Functional responses -- the relationship between resource intake rate and resource abundance -- are widely used in explaining predator-prey interactions yet many studies indicate that resource availability is crucial in dictating intake rates. 2. For time-stressed migrant birds refuelling at passage sites, correct decisions concerning patch use are crucial as they determine fattening rates and an individual's future survival and reproduction. Measuring availability alongside abundance is essential if spatial and temporal patterns of foraging are to be explained. 3. A suite of shorebird species stage in Delaware Bay where they consume horseshoe crab Limulus polyphemus eggs. Several factors including spawning activity and weather give rise to marked spatial and temporal variation in the abundance and availability of eggs. We undertook field experiments to determine and contrast the intake rates of shorebird species pecking for surface and probing for buried eggs. 4. Whether eggs were presented on the sand surface or buried, we demonstrate strong aggregative responses and rapid depletion (up to 80%). Depletion was greater at deeper depths when more eggs were present. No consistent give-up densities were found. Type II functional responses were found for surface eggs and buried eggs, with peck success twice as high in the former. Maximum intake rates of surface eggs were up to 83% higher than those of buried eggs. 5. Caution is needed when applying functional responses predicted on the basis of morphology. Our expectation of a positive relationship between body size and intake rate was not fully supported. The smallest species, semipalmated sandpiper, had the lowest intake rate but the largest species, red knot, achieved only the same intake rate as the mid-sized dunlin. 6. These functional responses indicate that probing is rarely more profitable than pecking. Currently, few beaches provide egg densities sufficient for efficient probing. Areas where eggs are deposited on the sand surface are critical for successful foraging and ongoing migration. This may be especially true for red knot, which have higher energetic demands owing to their larger body size yet appear to have depressed intake rates because they consume smaller prey than their body size should permit.

MIGRANT SHOREBIRD PREDATION ON BENTHIC INVERTEBRATES ALONG THE ILLINOIS RIVER, ILLINOIS

Abstract We evaluated the effect of shorebird predation on invertebrates at a wetland complex along the Illinois River, west-central Illinois, during spring migration. Using a new exclosure experiment design adapted to the shifting nature of foraging microhabitat of interior wetlands, we found that shorebird predation did not significantly deplete total invertebrate density or total biomass in open (no exclosure) versus exclosure treatments. Chironomids and oligochaetes were the most common invertebrates occurring in substrate samples. The density of oligochaetes was lower in open treatments, though the degree of difference varied both spatially and temporally. Shorebird density was positively correlated with the amount of invertebrate biomass removed from the substrate during the late-May sampling period. Our results suggest that shorebirds use an opportunistic foraging strategy and consume the most abundant invertebrate prey. The dynamic hydrology at our study site likely played a role in preventing inv...

Shorebird predation affects density, biomass, and size distribution of benthic chironomids in salt pans: an exclosure experiment

AbstractChironomus salinarius larvae are the dominant benthic organisms in the salt pans of the Odiel marshes, southwest Spain, an area internationally important for migratory shorebirds. Predator exclosures were used to examine the effects of shorebirds on chironomid larvae for periods of 32 to 46 d during spring migration. During the 1st experimental period, shorebird numbers were relatively low, peaking at 4 feeding birds/ha. During the 2nd period, shorebird numbers were higher, peaking at 32 feeding birds/ha. The changes in larval density and biomass from the beginning to the end of each period were significantly greater in exclosures than in controls. Predation decreased larval density by 35% and 32% during periods 1 and 2, respectively. Predation decreased larval biomass by 37% and 49% during periods 1 and 2, respectively. Predation also changed prey size distributions, reducing the proportion of larger larvae. These effects did not vary significantly between periods, suggesting that increased preda...

Surface activity of Corophium volutator: A role for parasites?

In soft-bottom intertidal habitats, the normally infaunal amphipod Corophium volutator is often found active on the sediment surface during low tide, exposed to desiccation and shorebird predation. Here we examine whether such risky behaviour is related to parasite infections. Surface-active and buried C. volutator were collected during a low tide period in the Danish Wadden Sea, and the infection patterns of the two groups were described in relation to sex and size. Surface-active males and females were more heavily infested by microphallid trematodes (four species) than buried specimens of the same sex and size class. Although the density of surfaced amphipods decreased as a function of exposure time, the mean parasite load of those that remained on the surface increased. A narrow size-specific parasite intensity threshold above which the amphipods were always surface active did not exist: heavily infected individuals were also found buried in the substrate. Although likely to be beneficial to the parasites, this suggests that the behavioural alteration is a side-effect of the infections rather than a consequence of direct parasitic manipulation. Besides the presumed mortality associated with the parasite-related surface activity in a range of size-classes, the intensity-size frequency distribution indicated that larger and hence heavily infected hosts are removed from the population. Together it demonstrates that microphallid trematodes impact the population dynamics of C. volutator.

Variable sex-specific mortality due to shorebird predation on a fiddler crab

Male fiddler crabs compete for access to mates mainly by displaying an enlarged claw. One cost of this large claw is presumed to be higher exposure to predators. The fiddler crab Uca uruguayensis is used as a food source by several Neotropical migratory shorebirds. We investigated whether crab density and male claw-waving behaviour increase predation by shorebirds. The sex ratio of crabs on the surface was strongly male biased. Male crabs showed a daily succession of feeding–waving–feeding activities and their dry mass at the site decreased with a high density of crabs, suggesting stronger male–male competition than at sites with a low density of crabs. Sex-specific predation varied according to shorebird species. The Ruddy Turnstone (Arenaria interpres) preyed intensively on male crabs when they were courting and at sites with a high density of crabs, the Black-bellied Plover (Pluvialis squatarola) and the Lesser Golden Plover (Pluvialis dominica) preyed on crabs of both sexes, and the Whimbrel (Numenius phaeopus) foraged intensively on females. When estimating the net effect of all predators, we found that the four shorebird species together were able to consume between 0.1 and 0.2% of the crab population, and consumption did not differ between the sexes of crabs. These mortality rates indicate a low source of mortality, likely unable to produce deviations in the sex ratio of the population. Thus, our results indicate that a trait thought to be sexually selected does not necessarily lead to a disproportionate increase in mortality due to predation.

Testing the effects of nutrient additions on mudflat macroinfaunal assemblages in the presence and absence of shorebird predators

An issue that is of increasing concern worldwide relates to the possibility that areas subject to conservation agreements, owing to the resident and migratory bird populations they support, will be adversely affected by attempts to reduce organic inputs into nearshore environments. In the present study, we investigated the effects of nutrient additions and shorebird exclusion on the infaunal assemblages of an intertidal mudflat. Where responses to nutrient additions were observed they only occurred in the high-dose treatments, indicating that a high nutrient loading is required before infaunal responses are initiated at this site. There was no evidence to support the idea that nutrient additions would only stimulate macroinfaunal assemblages where shorebird foraging was reduced. Instead, nutrient additions were detectable separately from any effects of shorebird predation and, contrary to expectations, appeared to be of more importance than shorebird predation in this environment. There was also no evidence to suggest that shorebird predation has a strong interaction with the infaunal prey assemblage and, although further studies will be needed to support this statement, it is possible that moderate changes in nutrient status will not impact on the shorebird populations at this site.

Invertebrate response to snow goose herbivory on moist-soil vegetation

Foraging activity by snow geese (Chen caerulescens) often creates large areas devoid of vegetation (“eat-outs”) in moist-soil impoundments and coastal wetlands. Open-water habitats that result from eat-outs may be valuable foraging areas for other wetland-dependent birds (i.e., waterfowl and shorebirds). However, few studies have examined the effects of goose-induced habitat changes on invertebrates, an important food source for both waterfowl and shorebirds. We quantified changes in abundance and composition of benthic invertebrates in response to snow goose herbivory in moist-soil impoundments at Prime Hook National Wildlife Refuge, Delaware, USA. We found invertebrate taxon richness and diversity and abundance of Chironomidae, Coleoptera, and Total Invertebrates to be higher in goose-excluded sites than in adjacent eat-outs. These effects were most pronounced during January, February, and early April. We also measured invertebrate abundance in shorebird exclosures in eat-outs but found few detectable effects of shorebird predation on invertebrates. Our study demonstrated that abundant snow geese may negatively influence availability of invertebrates for other nonbreeding waterbirds, suggesting that management actions to reduce local goose populations or deter feeding in impoundments may be warranted.

Intense episodic predation by shorebirds may influence life history strategy of an intertidal amphipod

Life history theory suggests that selective predation on older or larger individuals in prey populations or a disturbance such as intense and episodic predation should lead to smaller size at maturity, and a tendency toward semelparity. Many populations of the intertidal amphipod, Corophium volutator, in the western Atlantic are subjected to an intense period of size‐selective predation for about one month in summer, during the southward migration of shorebirds. We compared size at maturity and fecundity of populations of C. volutator from mudflats that are intensively used by shorebirds with populations that are visited by very few birds. We found that mature females were of similar size in May, but those from bird mudflats produced more offspring during the first reproductive episode. In July, females of the summer generation began to reproduce at a smaller size on bird mudflats, and as a consequence, produced fewer offspring that grew more slowly. The results of this correlative study suggest that shorebird predation has shaped C. volutator life history in two ways. First, females on bird mudflats concentrate their reproductive effort into a larger early brood, probably because later broods would come to maturity during the period of intense predation. Second, in summer, amphipods begin to reproduce at a smaller size so as to produce a brood before the arrival of the birds in mid‐July. Predators are not present at the time that these responses occur, and thus they are probably not a plastic response to perceived predation risk. Thus the patterns we observed agree with the predictions of recent theories: individuals faced with a predictable disturbance alter their life cycles so as to minimize the effect of that disturbance.

Predicting shorebird mortality and population size under different regimes of shellfishery management

Summary Human interests often conflict with those of wildlife. In the coastal zone humans often exploit shellfish populations that would otherwise provide food for populations of shorebirds (Charadrii). There has been considerable debate on the consequences of shellfishing for the survival of shorebirds, and conversely the effects of shorebird predation on the shellfish stocks remaining for human exploitation. Until now, it has been difficult to determine the impact of current shellfishery practices on birds or to investigate how possible alternative policies would affect their survival and numbers. One long‐running contentious issue has been how to manage mussel Mytilus edulis and cockle Cerastoderma edule shellfisheries in a way that has least effect on a co‐dependent shorebird, the oystercatcher Haematopus ostralegus, which also consumes these shellfish. This study used a behaviour‐based model to explore the effects that the present‐day management regimes of a mussel (Exe estuary, UK) and a cockle (Burry inlet, UK) fishery have on the survival and numbers of overwintering oystercatchers. It also explored how alternative regimes might affect the birds. The model includes depletion and disturbance as two possibly detrimental effects of shellfishing and some of the longer‐term effects on shellfish stocks. Importantly, model birds respond to shellfishing in the same ways as real birds. They increase the time spent feeding at low tide and feed in fields and upshore areas at other times. When shellfishing removes the larger prey, birds eat more smaller prey. The results suggest that, currently, neither shellfishery causes oystercatcher mortality to be higher than it would otherwise be in the absence of shellfishing; at present intensities, shellfishing does not significantly affect the birds. However, they also show that changes in management practices, such as increasing fishing effort, reducing the minimum size of shellfish collected or increasing the daily quota, can greatly affect oystercatcher mortality and population size, and that the detrimental effect of shellfishing can be greatly increased by periods of cold weather or when prey are unusually scarce. By providing quantitative predictions of bird survival and numbers of a range of alternative shellfishery management regimes, the model can guide management policy in these and other estuaries.

Elevated predation risk changes mating behaviour and courtship in a fiddler crab

The fiddler crab, Uca beebei, lives in individually defended burrows, in mixed-sex colonies on intertidal mud flats. Avian predation is common, especially of crabs unable to escape into burrows. Mating pairs form in two ways. Females either mate on the surface at their burrow entrance ('surface mating') or leave their own burrow and sequentially enter and leave ('sample') courting males' burrows, before staying in one to mate underground ('burrow mating'). We tested whether perceived predation risk affects the relative frequency of these mating modes. We first observed mating under natural levels of predation during one biweekly, semi-lunar cycle. We then experimentally increased the perceived predation risk by attracting grackles (Quiscalus mexicanus) to each half of the study site in two successive biweekly cycles. In each experimental cycle, crabs were significantly less likely to mate on the side with more birds. Moreover, on the side with elevated predation risk, the number of females leaving burrows to sample was greatly reduced relative to the number of females that surface-mated. Males waved less and built fewer mud pillars, which attract females, when birds were present. We discuss several plausible proximate explanations for these results and the effect of changes in predation regime on sexual selection.

Shorebird predation may explain the origin of Holocene beds of stout razor clams in life position

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 167:301-306 (1998) - doi:10.3354/meps167301 Shorebird predation may explain the origin of Holocene beds of stout razor clams in life position Oscar Iribarne*, Juan Valero, Mariano M. Martinez, Luis Lucifora, Silvina Bachmann Departamento de Biología (FCEyN), Universidad Nacional de Mar del Plata, CC 573 Correo Central (7600), Mar del Plata, Argentina *E-mail: osiriba@mdp.edu.ar Several hypotheses have been proposed to explain the origin of death assemblages of stout razor clams Tagelus plebeius buried in life position found in Holocene sediments in coastal Argentina and Uruguay: (1) a rapid decrease of sea level, (2) obstruction of the mouth of a coastal lagoon and episodic deposition of fluvial sediments in the mouth of a creek, and (3) a 'taphonomic feedback' process. Hypothesis 1 has been discarded, Hypothesis 2 only has a local explanation, while based on current information Hypothesis 3 is unlikely. A bird predation hypothesis is proposed here on the basis of a study of an extant population. The density of an extant population at the Mar Chiquita coastal lagoon, Argentina (37°46'S, 57°27'W), is higher in the lower intertidal (up to 200 clams m-2) and decreases towards the upper intertidal, and clam size is correlated with burial depth. However, live individuals there coexist with large densities (up to 1500 clams m-2) of dead clams in life position, with densities peaking at the middle-upper intertidal. These shells showed no size-depth of burial correlation. Stout razor clams in fossil deposits (1500 yr BP) also showed large densities of shells in life position (528 clams m-2, SD = 16, n = 20) with no clam size-burial depth relationship. Predation by the American oystercatcher Haematopus palliatus is an important cause of mortality year-round, taking 28 clam h-1 during 3:43 h d-1. Half of the successful attacks left shells in life position buried in the sediment. A computer simulation based on this information showed that predation by 1 oystercatcher produces 9.2 life-position shells m-2 yr-1 (SD = 3.7 life-position shells m-2 yr-1, n = 10000). Thus, 89.5 yr (95% CL = 50-422 yr) are required to generate a mean shell density similar to the one found on our study site (824 clams m-2). Thus, predation by oystercatchers generates a pattern similar to fossil assemblages and is the most likely explanation for the Holocene shell assemblages of the stout razor clams. Intertidal · Predation · Paleoecology · Stout razor clams · Oystercatchers Full text in pdf format PreviousExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 167. Publication date: June 18, 1998 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1998 Inter-Research.

Genetic isolation of populations of the gammaridean amphipod, Corophium volutator, in the Bay of Fundy, Canada

Selection experiments suggest that evolutionary modifications in amphipod demography can respond to local environmental changes and that local races of amphipods may be common. We tested this hypothesis in mudflat populations of Corophium volutator in the Bay of Fundy, Canada. Due to the unique topography of the Bay of Fundy, distinctive environmental conditions are prevalent in different branches of the Bay, while the impact of shorebird predation has also been shown to vary between populations. Several hypotheses have been suggested to explain ecological evidence which indicates that Corophium volutator (Pallas), a common amphipod crustacean, exhibits extensive life history variation in Bay of Fundy populations. We used RAPD‐PCR techniques to examine populations of C. volutator in an investigation of genetic isolation in marine environments. Our data suggest that variation in selection pressures have played a significant role in the genetic divergence of populations of C. volutator in the Bay of Fundy.

Shorebird–prey interactions in South Carolina coastal soft sediments

We investigated the simultaneous influence of benthic prey (primarily insect larvae, oligochaetes, and polychaetes) on spring-migrating shorebirds (Charadriiformes) and shorebirds on prey in brackish managed wetlands of South Carolina. We proposed that positive correlations between shorebird and prey densities and between prey reduction and shorebird density might result in a negative feedback loop. The loop would be characterized by shorebird redistribution over time and equalization of shorebird and prey densities (and biomass) across the site. In support of the feedback loop (i) there were positive correlations between shorebird and prey densities (and biomass) early in migration; (ii) shorebird predation was responsible for declines of 49% in prey density and 50% in prey biomass in a shorebird exclosure experiment, with shorebird density positively correlated with prey decline in open plots; (iii) there was equalization of prey biomass across the site, and relative prey species abundance appeared to become more equal. Contrary to predictions, shorebirds did not redistribute after prey reduction, and patchiness of shorebird and prey densities persisted across the site. This failure was attributed to high prey density, even at the season's end, and high water depths. There were no confounding indirect trophic-level effects.

The life history of the amphipod, Corophium volutator: the effects of temperature and shorebird predation

The life cycle of the amphipod, Corophium volutator (Pallas), is described from an intertidal mudflat in Maine, near the southern limit of the species' range in the western Atlantic Ocean. This population has two generations per year, with one generation born in May to mid–June and growing to reproduce and give rise to a second generation in August. The individuals born in August form the over-wintering generation. Comparison to sites in the Bay of Fundy, where different populations of C. volutator display one and two generations per year, rejects the hypothesis that episodic predation by shorebirds selects for the life history with two generations per year. Water temperatures during the summer may control the number of generations per year for C. volutator in the western Atlantic.