Experiments on epibenthic predation in the Wadden Sea

Field experiments were designed to evaluate the role of predators in the Wadden Sea, small predators like shore crabs, shrimps and gobies, and large ones like flatfish and birds. Exclosures, maintained in aCorophium volutator bed, an eelgrass bed, in a sandy and a muddy flat, protected the infauna from such epibenthic predation. The resulting changes in the macrofauna were recorded and compared with an unaffected control area. In sandy and muddy flats of the lower intertidal zone, cages (mesh size5 mm) altered abundance and composition of the infauna almost entirely. Nearly all species achieved higher population densities than in the control area, and in addition, the number of species increased as well. In the the scarcely populated mud flat a dense suspension-feeder assemblage emerged, associated with numerous tube-building polychaetes. The sand flat, normally dominated by deposit feeders, also became occupied by a dense suspension-feeder assemblage, mainly cockles. In contrast, narrowly meshed cages had only little effect in the beds of eelgrass and ofCorophium volutator. Both are positioned in the upper intertidal zone. Although a number of species still responded with significant increases in abundance, many remained indifferent or even tended to be less abundant within cages. Cages provided with a 20-mm mesh nylon net, excluding only birds, flatfish and the biggest crabs, increased significantly the survival of large-sized infaunal members. This was only apparent in the upper intertidal zone. It was concluded, that small sized epibenthic predators are the major determinants of the dynamic species abundance pattern of the lower intertidal flats. In the eelgrass bed, the meshwork of rootlets constitutes an important spatial refuge from these predators. The complex habitat structure causes a diversified faunal assemblage. TheCorophium bed in the uppermost intertidal zone is less accessable to predators like crabs, shrimp and gobies. The monotonous appearance of this faunal assemblage is assumed to be the outcome of competitive exclusion and of occasional harsh physical conditions.     

Moderate predation on meiofauna by the macrobenthos of the Wadden Sea

The role of meiofauna in the trophic web of marine ecosystems is a controversial topic. During an experimental study on tidal flats in the eastern part of the North Sea, potential predators on meiofauna were kept in enclosures. A gobiid fish, shrimp, crabs, an amphipod, four polychaete worms and a nemertine were tested for their ability to decimate nematodes, turbellarians, ostracods and benthic copepods. Only few macrobenthic species preyed heavily on permanent meiofauna: juveniles of the shore crabCarcinus maenas, the hermit crabPagurus bernhardus with the colonial hydrozoanHydractinia echinata on its shells, and the rag wormNereis diversicolor. When benthic infauna was protected from these predators with exclosures, juvenile macrofauna responded with a marked increase in number, while the permanent meiofauna remained unaltered except for a limited increase in nematodes. It is concluded that in the Wadden Sea abundance of permanent meiofauna (Nematoda, Turbellaria, Ostracoda, Copepoda) is only locally or temporarily regulated by macrobenthic predators.I acknowledge a grant from the Deutsche Forschungsgemeinschaft (DFG).     

Consumption of benthic fauna by carnivorous birds in the Wadden Sea

Consumption by carnivorous birds was estimated for the Sylt-Rømø tidal inlet in the northern part of the Wadden Sea, as well as the subarea Königshafen, a small, tidal bay. The bird community of the Sylt-Rømø Wadden Sea was dominated by Dunlin (35% of all birds counted), Eider (9%), Oystercatcher (8%), Knot (8%), and Shelduck (7%). The community in the Königshafen was dominated by Eider (20%), Knot (17%), Bar-tailed Godwit (17%), Dunlin (13%), and Oystercatcher (8%). Annual consumption was estimated at 3.4 g AFDW · m^?2 · year^?1 for the entire Sylt-Rømø Wadden Sea and 19.2 g AFDW · m^?2 · year^?1 for the Königshafen. Restricting the calculations to the intertidal area resulted in a consumption of 8.7 g AFDW · m^?2 · year^?1 for the Sylt-Rømø Wadden Sea and 17.6 g AFDW · m^?2 · year^?1 for the Königshafen. In the two areas, consumption was dominated by the Eider with 37% and 60% of the total consumption, respectively. In comparison to the western parts of the Wadden Sea the seasonal pattern of consumption as well as species composition differed, most probably as an effect of different climatic conditions, whereas annual consumption on intertidal flats seems to be in the same order of magnitude. On average, 15–25% of the mean annual macrozoobenthic biomass seems to be taken by carnivorous birds in the Wadden Sea, which is in the same order of magnitude as in other northern temperate estuarine areas.     

Effects of herbivorous birds on intertidal seagrass beds in the northern Wadden Sea

During autumn migration (September to December), brent geese (Branta b. bernicla) and wigeon (Anas penelope) feed on the seagrass Zostera noltii in the nearshore, upper tidal zone leeward of the island of Sylt (eastern North Sea). To graze on leaves and shoots above the sediment and on rhizomes and roots below, these birds reworked the entire upper 1 cm layer of sediment eight times within this 3-month period. In addition, brent geese excavated pits 3–10 cm deep by trampling in order to feed on below-ground phytomass. About 12% of the seagrass beds became pitted to an average depth of 4.5 cm. Using net exclosures, it was estimated that birds removed 34 g dry weight m^–2 of above-ground and 28 g of below-ground phytomass. This corresponds to 45% of the phytomass in September. Of the overall loss of phytomass from September to December, 63% was caused by birds. Roughly half of the leaves fell off anyway until December and the other half were taken by the birds. Below the ground, phytomass remained almost constant where birds were excluded, while with birds phytomass of rhizomes and roots was halved. In spite of this strong effect, in the next vegetation period the blade density was lower at former exclosure sites compared to the ambient seagrass bed. The underlying process seems to be a self-inhibition of dense overwintering seagrass by mud accretion. Assuming our experimental results can be scaled up to the entire seagrass bed, we hypothesize that in the sheltered upper intertidal zone, seasonal erosion caused by herbivorous geese and ducks is necessary for the persistence of Z. noltii. Received: 7 January 1999 / Received in revised form: 23 August 1999 / Accepted: 25 August 1999     

On the population development of the introduced razor clamEnsis americanus near the island of Sylt (North Sea)

The American razor clamEnsis americanus (=E. directus) was introduced into the eastern North Sea in the late 1970s. By larval and poslarval drifting the species rapidly extended its distribution, now ranging from the English Channel to the Kattegat. Near the island of Sylt in the eastern North Sea it has been recorded since 1979. Recruitment was rather irregular, with about six strong year-classes within two decades. Growth seems comparable with populations in its native range (Atlantic North America). Although present in the lower intertidal zone, maximum densities occurred in shallow subtidal sand with a biomass similar to that of dense beds of native cockles and mussels in the adjacent intertidal zone.Ensis americanus established in otherwise sparsely faunated sand (channels exposed to strong currents) as well as in dense infaunal assemblages (lower intertidal and subtidally). There were no significant interactions with resident species. In dense beds of razor clams, however, fine sediment particles accumulated which may have altered abundances of polychaetes. In spite of high annual variability,E. americanus has become a prominent component of the macrobenthos in shallow subtidal sands of the North Sea.     

The introduced clam Ensis americanus in the Wadden Sea: field experiment on impact of bird predation and tidal level on survival and growth

In the Danish Wadden Sea the intertidal distribution of the introduced bivalve Ensis americanus (syn. E. directus) is restricted to a narrow zone around the mean low water level. To test the possible impact of birds and submersion time on dynamics and distribution of the clams, adult specimens of E. americanus collected near the low water line were transplanted to two intertidal sites and established in open and net-covered experimental plots for 9 weeks (autumn 2001). The lowest survival of clams was registered at the low-shore-site (LSS) in plots open to bird predators, suggesting that birds such as Common Eider (Somateria mollissima) or Oystercatcher (Haematopus ostralegus) may control the abundance of E. americanus at the lower tidal levels. For clams showing increment in shell length during the study period, the shell growth rates were highest at the LSS and lowest in the open plots at the high-shore-site (HSS). Differences in immersion time and thus food supply may explain this pattern. Body mass index (BMI) of the clams showed basically the same pattern as the survivorship: lowest BMI in open plots at the LSS and highest in the covered plots at this site. Clams from the HSS were intermediate in their BMI. Disturbance by birds in the open plots at the LSS may explain the low BMI. In conclusion birds may be an important factor controlling abundance of E. americanus in the lower intertidal zone.     

Grazing on green algae by the periwinkleLittorina littorea in the Wadden Sea

On sedimentary tidal flats in the Wadden Sea near the Island of Sylt, the periwinkleLittorina littorea occurred preferentially on clusters and beds of mussels and on shell beds (100 to 350 m⁻²), achieved moderate densities on green algal patches or mats (20 to 50 m⁻²), and remained rare on bare sediments (<5 m⁻²). Green algae covering>10% of sediment surface appeared in summer on approximately one third of the tidal zone, mainly in the upper and sheltered parts and almost never on mussel and shell beds. In feeding experiments,L. littorea ingested more of the dominant alge,Enteromorpha, than ofUlva, irrespective of whether or not algae were fresh or decaying. The tough thalli ofChaetomorpha were hardly consumed. Snails feeding onEnteromorpha produced fecal pellets from which new growth ofEnteromorpha started. In the absence of periwinkles,Enteromorpha developed on mussels and the attached fucoids. Experimentally increased snail densities on sediments prevented green algal development, but the snails were unable to graze down established algal mats. It is concluded that natural densities ofL. littorea hardly affect the ephemeral mass development of green algae on sediments. However, where the snails occur at high densities, i.e. on mussel beds, green algal development may be prevented.     

The impact of predation by birds on bat populations in the British Isles

The role played by predation of birds in the mortality of British bats is assessed. A review of dietary studies and anecdotal accounts revealed eleven species of bird which occasionally feed on bats–Herring Gull Larus argentatus , Black-headed Gull Larus ridibundus , Rook Corvus frugilegus , Carrion Crow Corvus corone , Little Owl Athene noctua , Short-eared Owl Asio flammeus , Kestrel Falco tinnunculus , Hobby Falco subbuteo , Merlin Falco columbarius , Peregrine Falco peregrinus and Sparrowhawk Accipiter nisus. A further three species feed more frequently on bats–Barn Owl Tyto alba , Tawny Owl Strix aluco and Long-eared Owl Asio otus. Rates of predation were very low accounting for only 00034^o_o of items taken by small hawks and falcons ( n items = 29 519) but 0035^o, of prey taken by owls ( n items = 99 479). By multiplying together the average annual prey capture rates of the predators, assessed from their energetic food requirements and direct observations of prey intake rates, the British populations of the predators and the contribution to the diet made by bats, the annual number of bats which die each year as a result of predation was estimated. The total losses of bats to predation might amount to about 201 400 bats/annum. The most significant predators are Tawny Owl ( c. 168 850 bats/annum), Barn Owl (c. 8800 bats/ annum), Long-eared Owl (c. 10 200 bats/annum) and Kestrels (c. 8400 bats/annum). This predation by avian predators would account for about 11 % of the annual mortality of British bats. An assessment of the biases involved in this calculation suggests it is probably a minimum estimate. Despite the apparent low representation of bats in the diets of predatory birds, the effects of this predation on bat behaviour and population dynamics cannot be ignored.     

Identification and evaluation of the Atlantic razor clam (Ensis directus) for biologically inspired subsea burrowing systems

In this article, we identify and analyze a subsea organism to serve as a model for biologically inspired burrowing technology to be used in applications such as anchoring, installation of cables, and recovery of oil. After inspecting myriad forms of life that live on or within ocean substrates, the Atlantic razor clam, Ensis directis, stood out as an attractive basis for new burrowing technology because of its low-energy requirements associated with digging (0.21?J/cm), its speed and depth of burrrowing (～1?cm/s and 70?cm, respectively), and its size and simplicity relative to man-made machines. As anchoring is a prime application for the technology resulting from this work, the performance of an Ensis directus-based anchoring system was compared to existing technologies. In anchoring force per embedment energy, the E. directus-based anchor beats existing technology by at least an order of magnitude. In anchoring force per weight of device, the biologically inspired system weighs less than half that of current anchors. The article concludes with a review of E. directus's digging strategy, which involves motions of its valves to locally fluidize the substrate to reduce burrowing drag and energy, and the successful adaptation of E. directus's burrowing mechanisms into an engineering system: the RoboClam burrowing robot, which, like the animal, uses localized fluidization to achieve digging energy that scales linearly with depth, rather than depth squared, for moving through static soil.     

Seed predation by birds and small mammals in semiarid Chile

We studied spatial and temporal patterns in foraging activity among diurnal birds and nocturnal mammals at a semiarid site in northern Chile using artificial foraging trays. Small mammals foraged more extensively under shrubs than in open microhabitats, but birds showed no such selection. Moreover, avian foraging was more extensive than that by small mammals in all seasons and both microhabitats. Avian foraging was highly seasonal, as many birds at our site migrate to the Andean prepuna or to Patagonia during the austral summer. Birds have tended to be overshadowed by small mammals and ants in studies of granivory, but this study suggests that their importance may be underestimated in some systems.     

A mathematical model of cell growth and alkane degradation in Wadden Sea sediment suspensions

In a series of 16 experiments, hexadecane in different concentrations was mixed with natural Wadden Sea sediments, and the degradation of the alkane was observed under controlled conditions (bioreactor). The aim of the experiments was to quantify the impact of the dissolved oxygen tension and the initial alkane concentration on degradation time and degradation completeness. A mathematical model was constructed which is able to reproduce reasonably well the observed-time series under all conditions: aerobic and nearly anaerobic, diluted and alkane-saturated. The model contains hypotheses on the chain of reactions; it attempts to bridge existing gaps in the understanding of the degradation process.     

Non-lethal effects of predation in birds

Predators can affect individual fitness and population and community processes through lethal effects (direct consumption or ‘density’ effects), where prey is consumed, or through non‐lethal effects (trait‐mediated effects or interactions), where behavioural compensation to predation risk occurs, such as animals avoiding areas of high predation risk. Studies of invertebrates, fish and amphibians have shown that non‐lethal effects may be larger than lethal effects in determining the behaviour, condition, density and distribution of animals over a range of trophic levels. Although non‐lethal effects have been well described in the behavioural ecology of birds (and also mammals) within the context of anti‐predation behaviour, their role relative to lethal effects is probably underestimated. Birds show many behavioural and physiological changes to reduce direct mortality from predation and these are likely to have negative effects on other aspects of their fitness and population dynamics, as well as affecting the ecology of their own prey and their predators. As a consequence, the effects of predation in birds are best measured by trade‐offs between maximizing instantaneous survival in the presence of predators and acquiring or maintaining resources for long‐term survival or reproduction. Because avoiding predation imposes foraging costs, and foraging behaviour is relatively easy to measure in birds, the foraging–predation risk trade‐off is probably an effective framework for understanding the importance of non‐lethal effects, and so the population and community effects of predation risk in birds and other animals. Using a trade‐off approach allows us to predict better how changes in predator density will impact on population and community dynamics, and how animals perceive and respond to predation risk, when non‐lethal effects decouple the relationship between predator density and direct mortality rate. The trade‐off approach also allows us to identify where predation risk is structuring communities because of avoidance of predators, even when this results in no observable direct mortality rate.     

Begging and the risk of predation in nestling birds

Theoretical models of the evolution of begging in nestling passerinesassume that begging is costly, either energetically or in termsof predation. However, few empirical measures of these costsexist. We examined whether nestling begging calls could attractpredators to nests by comparing predation rates at artificialnests with and without playbacks of tree swallow begging calls.Nests were baited with quail eggs and placed in pairs on theground or in modified nest-boxes. Nests with playbacks of beggingcalls were depredated before control nests significantly moreoften in both the ground and nest-box trials, suggesting thatpredators may use begging calls to locate nests. These resultssuggest that the risk of nest predation may be increased becauseof calling by nestlings and provide further support for theassumption that conspicuous begging is costly in terms of predation     

Nest predation and the species diversity of birds

In a recent series of papers, Thomas E. Martin has suggested that selective, density-dependent predation on nests may promote ecological diversification of birds and constrain the number of coexisting species in a community. In an experimental study, Martin has demonstrated density-dependent predation on artificial nests constructed and placed so as to mimic natural ones. Comparisons between habitats indicate that availability of suitable nest sites may influence community diversity more strongly than does diversity of foraging niches. Although his case is circumstantial at this point, Martin argues compellingly for considering predation more seriously in studies of community ecology. Indeed, many patterns attributed to competition for food resources may arise, instead, from the partitioning of predator-escape space.     

Archaeological evidence of former occurrence and changes in fishes, amphibians, birds, mammals and molluscs in the Wadden Sea area

Animal remains are well preserved in archaeological sites, especially the terp sites, of the Wadden Sea area of Denmark, Germany and The Netherlands. Here, we provide an overview on the wild mammals, birds, fishes, amphibians and molluscs found in coastal sites dating from 2700 to 2600 B.C. and 700 B.C. to A.D. 1600. Coastal people used a variety of animal species for food and other purposes. Hunting, fowling, fishing and agriculture did not have much influence on wild stocks in the period from the late Bronze Age/early Iron Age until the late Middle Ages. However, large changes to the landscape were made in the late Middle Ages by diking and damming. As a result, some species such as the northern vole (Microtus oeconomus) and the natterjack toad (Bufo calamita) disappeared from the area except for some dune districts on the islands, and others became rare, such as the grey seal (Halichoerus grypus) and the lagoon cockle (Cerastoderma lamarcki). New habitats arose for birds of dry meadows and fields, like lapwing (Vanellus vanellus) and black-tailed godwit (Limosa limosa). Sturgeon (Acipenser sturio) disappeared from the Wadden Sea within a few decades since A.D. 1890 due to the destruction of spawning grounds by damming and high exploitation pressure. Our findings are important for the ecological history of the region.     

Frequency-dependent predation by birds at edges and interiors of woodland

Structural variations between edge and interior areas within forest fragments may bring about differences in food availability that may influence the selective behaviour of predators and prey population dynamics. The purpose of this paper was to assess patterns of artificial prey selection by wild birds (House Sparrow Passer domesticus and Rock Dove Columba livid) between edge and interior areas of woodland, taking into account differences in prey frequency (10% of one food type and 90% of the other) and density (30 baits/m^-2 and 50 baits/m-²). Experiments were conducted at 24 plots in 3 forest fragments in the city of Madrid, Spain. Selectivity did not vary among parks or between densities. However, selectivity did vary with the frequency and location of baits, showing an anti-apostatic trend (baits were preferred at low rather than at high frequencies) that was more pronounced at interiors than at edges. Two possible factors that may account for stronger anti-apostatic selection at edges are the higher densities of predators and pedestrians found there. However, there are many other possible explanations, and no specific conclusion can be supported with the current data. The results of this study also point out that site heterogeneity should be taken into account in the experimental design of future studies on frequency-dependent food selection by wild birds, particularly in fragmented landscapes.