Leo Zwarts

Seasonal variation in body weight of the bivalves Macoma balthica, Scrobicularia plana, Mya arenaria and Cerastoderman edule in the Dutch Wadden sea

Abstract The paper deals with the seasonal and annual variations in the weight of the soft parts of four bivalve species, Macoma balthica, Scrobicularia plana, Mya arenaria and Cerastoderma edule from tidal flats of the Dutch Wadden Sea. The paper reviews methodology and points to error sources. The variation in ash-free dry weight between individuals of the same size collected at the same time and place could be attributed to age, parasitic infestation, gametogenesis, burying depth and siphon size. The allometric relations between weight of soft parts and size are given in equations, averaged per month. The body weight of all four bivalve species peaked in May and June at a level approximately twice the lowest value, which occurred in November to March. The extent of this seasonal fluctuation varied, however, from year to year. The presence of gametes explained a part of the peak weight in summer. Winter losses of body weight were less at low temperatures, due to reduced energy expenditure when the animals are inactive. No large differences were found between the seasonal changes in body weight in the Wadden Sea and elsewhere in the temperate zone.

Year-to-year variability in the biomass of macrobenthic animals on tidal flats of the Wadden Sea: how predictable is this food source for birds?

Abstract The Wadden Sea is important as a stop-over and wintering area for several species of waders, foraging for shorter or longer times on its tidal flats. The size of the food stocks these birds encounter varies from place to place and from year to year. We studied characteristics of the variability in time of such prey stocks, using long-term data series of annual estimates of biomass of macrobenthic animals collected on tidal flats in various parts of the Dutch and German Wadden Sea. Year-to-year fluctuations were stronger in nearly all individual benhos species than in total macrozoobenthic biomass. The various species differed significantly in their year-to-year variability. Everywhere the tellinid bivalve Macoma balthica was relatively stable, whereas species such as the cockle Cerastoderma edule and the polychaetes Lanice conchilega, Nephtys hombergii and Anaitides mucosa fluctuated heavily and rapidly in all parts of the Wadden Sea where they were found. Within species, fluctuations in biomass of individual age or size classes were stronger than those in total biomass of the species. Several species showed minimal biomass values in the same years over vast areas. This synchronization of scarcity was caused particularly by similar responses to winter character, which was each year similar over the entire Wadden Sea. Such similar responses included low over-winter survival during severe winters and recruitment failure after exceptionally mild winters. Immediately after severe winters, such cold-sensitivee species as C. edule, L. conchilega and N. hombegii were scarce all over the Waden Sea, thus liimiting the possibilities for birds to switch to other parts of the Wadden Sea to find their preferred prey in sufficient quantitu. Simultaneous recruitment failure in several important bivalve species ( C. edule, Mytilus edulis , and Mya arenaria limited the possibility for specialized bivalve consumers to switch to alternative prey types in certain years.

Flocking and feeding in the fiddler crab (UCA tangeri): prey availability as risk-taking behaviour

For a full understanding of prey availability, it is necessary to study risk-taking behaviour of the prey. Fiddler crabs are ideally suited for such a study, as they have to leave their safe burrow to feed on the surface of the intertidal flats during low tide, thereby exposing themselves to avian predators. A study in an intertidal area along the coast of Mauritania showed that small crabs always stayed in the vicinity of their burrow, but large crabs wandered in large flocks (also referred to as droves) to feed on sea-grass beds downshore. Transplanting downshore feeding substrate to the burrowing zone of the small crabs proved that they too preferred to feed on it. Since small crabs can be preyed upon by more species of birds, this suggests that the decision not to leave the burrowing zone might be related to the risk of being fed upon by birds. We calculated predation risk from measurements on the density and feeding activity of the crabs, as well as the feeding density, the intake rate and the size selection of the avian predators. Per hour on the surface, crabs in a flock were more at risk than crabs feeding near their burrow. Thus, though flocking crabs may have benefited from ‘swamping the predator’ by emerging in maximum numbers during some tides only, this did not reduce their risk of predation below that of non-flocking crabs. Furthermore we found that irrespective of activity, large crabs suffered a higher mortality per tide from avian predators than small crabs. This suggests that large crabs could not sufficiently reduce their foraging time to compensate for the increased risk while foraging in a flock, even though they probably experienced better feeding conditions than small crabs staying near their burrow. The greater energy demands of large crabs were reflected in a greater surface area grazed. Thus, with increasing size a fiddler crab has to feed further away from its burrow and so may derive less protection from staying near to it. It seems that growing big does not reduce the risk of predation for fiddler crabs, as it does in many other species with indeterminate growth. As in such species, the most probable advantage of growing big is increased mating success. Ultimately, therefore, prey availability must be understood from the life-history decisions of the prey species.

The macrobenthos fraction accessible to waders may represent marginal prey

SummaryThe relationship between relative body condition (deviation from expected mean body weight) and burying depth was investigated in five macro-zoo benthic species living in a marine intertidal habitat. Body weight increased with depth when animals of the same size were compared. The increase amounted to 50% in the clamScrobicularia plana, ca. 40% in the wormNereis diversicolor, 25% in the clamMacoma balthica and 20% in the cockleCerastoderma edule and the clamMya arenaria. Only a part of the prey was within reach of some feeding wader species. Therefore prey value may be overestimated if one does not take into account the fact that shallow and accessible prey often have a relatively poor body condition.

Burying depth of the benthic bivalve Scrobicularia plana (da Costa) in relation to siphon-cropping

A field experiment tested the hypothesis that siphon-cropping influences the burying depth of the benthic tellinid Scrobicularia plana (da Costa). Cropping was simulated by removing part of the inhalant siphon and then burying the animals at the original depth. The depth was measured in situ each day afterwards with the aid of a nylon thread attached to the shell. The resulting increase of depth with siphon weight in the experimental animals resembles the relation between depth and siphon size in the natural population. Heavy cropping of the siphon reduced the body weight, whereas the loss of < 10 mg of the siphon had no discernible effect on the condition of the animals. When a Scrobicularia with a short siphon reduces its depth, it enlarges its feeding opportunity (radius around the burrow for deposit-feeding), but also increases the risk of being eaten by a predator. A short siphon alone does not force Scrobicularia to abandon the refuge provided by depth; only animals also in a poor body condition expose themselves to a larger predation risk. Siphon-cropping reduces the burying depth and makes the bivalves more accessible for wading birds like oystercatchers (Haematopus ostralegus L.). Burying depth is hence the outcome of conflicting demands; this implies that under each set of conditions an optimum can be defined.

Feeding radius, burying depth and siphon size of Macoma balthica and Scrobicularia plana

This paper investigates whether siphon weight limits the elongation of the siphon of deposit-feeding benthic bivalves under natural conditions. Were this to be so, it would imply that foraging and predator avoidance place conflicting demands on these animals, since an increase in the feeding radius on the surface would be associated with a decrease in the burying depth, and vice versa. The paper presents methods with which to measure siphon length and equations to transform siphon weight to siphon length in two benthic bivalves, Scrobicularia plana and Macoma balthica. Relatively heavy siphons are longer, but also thicker, than lightweight ones. We conclude that most individual bivalves stretch their siphons fully while feeding. However, bivalves with heavy siphons keep part of the siphon in reserve within the shell. Siphon cropping therefore results in an immediate reduction in siphon length, unless the siphon is heavy and a reserve is available. The feeding radius of M. balthica and S. plana is a linear function of shell size. S. plana use half of their siphon length to feed on the surface and the other half to bury themselves, but if the siphon weight is below average, the proportion extended over the surface decreases with siphon size. However, among the animals with a short siphon, those with a good body condition take no risks and live as deeply as possible. A comparison between species reveals that the weight of the extended inhalant siphon per cm amounts, on average, to 0.6% of the total body weight.

Does an optimally foraging oystercatcher obey the functional response

Summary(1)We describe an experimental test of the optimal diet model. An oystercatcher was offered bivalves Scrobicularia plana which were buried in the substrate at different depths. All prey were of equal length and thus of equal energy content. Since handling time increased with depth, deep-lying prey were less profitable.(2)From the known increase of handling and search time with depth we were able to predict for each prey density which depth classes should be ignored by the bird so as to maximize its intake rate.(3)The observed depth selection and intake rate were as predicted for the lower prey densities. At high prey densities the observed number of prey taken rose above the predicted intake rate. We suggest that the bird started to ignore closed bivalves with long handling times.(4)The observed relationship between prey density and intake rate could not be described by Hollings type-2 functional response, because the handling time and the rate of discovery were density-related. A multi-species functional response is necessary to describe the intake rate of an optimally feeding predator.

ENVIRONMENTAL-EFFECTS ON THE GROWTH-RATE OF INTERTIDAL INVERTEBRATES AND SOME IMPLICATIONS FOR FORAGING WADERS

The paper describes effects of intertidal height and sediment type on growth rate of the bivalves Cerastoderma edule, Macoma balthica, Mya arenaria, Mytilus edulis and Scrobicularia plana, and of the worms Arenicola marina, Nephtys hombergii and Nereis diversicolor in the eastern part of the Dutch Wadden Sea. In most species, exposure time was negatively correlated with length growth, although interfering effects of sediment type could not be ruled out. When controlled for the effects of exposure time, clay content of the sediment appeared to affect the growth of all species, but in different ways. The variation was related to the foraging methods of the invertebrates. Foraging waders may use the spatial variation in growth rate of the invertebrates to optimize the exploitation of individual cohorts.

The economic and ecological effects of water management choices in the upper Niger river: Development of decision support methods

One million people in the Inner Niger Delta make a living from arable farming, fisheries and livestock. Upstream dams (one built for electricity generation and one for irrigation) affect this downstream multifunctional use of water. Additionally, the Inner Niger Delta, which is one of the largest Ramsar sites in the world, is a hotspot of biodiversity and accommodates two of the largest known breeding colonies of large wading birds in Africa and in addition, is a vital part of the eco-regional network, supporting up to 3 to 4 million staging waterbirds, residents and migrants from all over Europe and western Asia. The hydrological and related ecological conditions in the Inner Delta largely determine the population size of these waterbird species. The major aim of the three-year study was to develop a decision-support system for river management in the Upper Niger, in which ecological and socio-economical impacts and benefits of dams and irrigation systems can be analysed in relation to different water management scenarios. The study involves various components: hydrology, arable farming, livestock, fisheries, ecology and socio-economics. An economic analysis has been conducted to determine the role of dams in the economy of the Inner Niger Delta and the Upper Niger region. By innovatively combining the above information on hydrology, ecology, fisheries, and agriculture, the study shows that building new dams is not an efficient way to increase economic growth and reduce poverty in the region. In fact, such efforts are counter-effective. Instead, development efforts should be aimed at improving the efficiency of the existing infrastructure, as well as of current economic activities in the Inner Niger Delta itself. This approach will also provide greater certainty for the essential eco-regional network functioning of the Inner Delta.

Moreau's Paradox Reversed, or Why Insectivorous Birds Reach High Densities in Savanna Trees

In West Africa, tree preferences of wintering migratory birds (and African residents) were quantified in order to assess the importance of wintering conditions on distribution, abundance and trends of insectivorous woodland birds. This study encompassed 2000 plots between 10–18°N and 0–17°W, visited in October-March 2007–2015, and covered 183 woody species and 59 bird species. Canopy surface (measured in a horizontal plane) and birds present were determined in 308,000 trees and shrubs. Absolute bird density amounted to 13 birds/ha canopy, on average, varying for the different woody species between 0 and 130 birds/ha canopy. Birds were highly selective in their tree choice, with no insectivorous birds at all in 65% of the woody species. Bird density was four times higher in acacias and other thorny species than in non-thorny trees, and seven times higher in trees with leaves having a low crude fibre content than in trees with high crude fibre foliage. Salvadora persica shrubs, but only when carrying berries, were even more attractive. Overall, densities of migratory woodland birds were highest in the (thorny) trees of the Sahelian vegetation zone. This counterintuitive finding, with highest numbers of wintering birds in the driest and most desiccated parts of West Africa (short of the Sahara), also known as Moreaus Paradox, can be explained by the foliage palatability hypothesis. The Sahelian vegetation zone has always been subject to heavy grazing from large herbivores, and as a consequence woody species have evolved mechanical defences (thorns) to withstand grazing of large herbivores, at the expense of chemical defence against arthropods. South of the Sahel, with a much lower grazing pressure, thorny trees (rich in arthropods) are replaced by (usually non-thorny) trees with less palatable foliage and a higher crude fibre content, and hence with less arthropod food for insectivorous birds.