Petra de Goeij

Facilitation on an intertidal mudflat : the effect of siphon nipping by flatfish on burying depth of the bivalve Macoma balthica

During deposit feeding on benthic micro-algae, the siphon of buried tellinid bivalves like Macoma balthica is vulnerable to nipping by plaice Pleuronectes platessa and other flatfish. We experimentally tested the hypothesis that siphon nippers facilitate predation on the entire bivalve (by shorebirds, for example) by inducing a decrease in burying depth. Three experiments in May and in September, during which likely siphon nippers (juvenile plaice P. platessa) were allowed to feed on M. balthica siphons, demonstrated that the bivalves indeed lost siphon mass and came closer to the surface. However, the strength of the burying response and the speed of recovery of siphons after nipping varied greatly between experiments, partly as a consequence of body condition-related differences in initial burying depth. Our experimental study confirms that siphon nippers can enhance the availability of bivalve prey to probing predators, and will thus facilitate avian predation on intertidal flats.

Deep-burying reduces growth in intertidal bivalves: field and mesocosm experiments with Macoma balthica

Abstract On intertidal flats in north-west Europe, the bivalve Macoma balthica buries deep in the sediment in early winter, but ascends to the surface in late winter. Close to surface Macoma runs the risk of being eaten by epibenthic predators. We hypothesise that living shallow leads to more rapid growth in spring by allowing for higher food-intake rates. In three experiments, of which two took place in the field, we placed Macoma at fixed burying depths. We measured body mass and survival after 10–16 weeks. At the end of all three experiments, body mass of shallow-buried animals was significantly higher than that of the surviving deep-buried animals. Survival was lowest in the animals buried deepest; deceased animals may have died of starvation and/or perhaps suffocation. In the mesocosm experiment there was a negative relationship between burying depth and feeding (grazing) radius. Our experimental results are consistent with the idea that Macoma comes to the surface to feed efficiently in order to grow fast.

Reversed optimality and predictive ecology: burrowing depth forecasts population change in a bivalve.

Optimality reasoning from behavioural ecology can be used as a tool to infer how animals perceive their environment. Using optimality principles in a ‘reversed manner’ may enable ecologists to predict changes in population size before such changes actually happen. Here we show that a behavioural anti-predation trait (burrowing depth) of the marine bivalve Macoma balthica can be used as an indicator of the change in population size over the year to come. The per capita population growth rate between years t and t+1 correlated strongly with the proportion of individuals living in the dangerous top 4 cm layer of the sediment in year t: the more individuals in the top layer, the steeper the population decline. This is consistent with the prediction based on optimal foraging theory that animals with poor prospects should accept greater risks of predation. This study is among the first to document fitness forecasting in animals.

A double test of the parasite manipulation hypothesis in a burrowing bivalve

Abstract. The parasite manipulation hypothesis predicts that parasites should be selected to manipulate host behaviour to facilitate transmission to the next host. The bivalve Macoma balthica burrows less deep when parasitized by the trematode Parvatrema affinis. Shallow burrowing increases the likelihood of ingestion by birds, their final hosts, and therefore this has been interpreted as manipulation by the parasite. When unparasitized, M. balthica displays seasonal changes in burrowing depth, becoming less accessible to predators in winter. If shallow burrowing of parasitized individuals is due to direct manipulation by the parasite, the availability of parasitized individuals should be high throughout the year, or at least especially in the season when most birds are present and potential transmission rates are highest. We compared burrowing depths of parasitized and unparasitized individuals in a single population during seven consecutive years. Parasitized individuals showed reduced burrowing depths but, in contrast to the prediction, the effect of parasites on availability to predators was smallest, not largest, in the season with the highest bird numbers. The parasite P. affinis competes for energy with the host, and M. balthica with low energy stores are known to reduce depth of burrowing. When we included size-corrected somatic ash-free dry mass (as an estimate of the energy stores) in our statistical analysis, the effect of infection on burrowing depth disappeared. Thus the effect of infection on burrowing depth is likely to be an unavoidable, indirect effect of the channelling of energy towards the parasite, causing the starving individual to take greater risks in the acquisition of food. Since both the seasonal pattern and the magnitude of increased availability of parasitized individuals are inadequate, the increased exposure of parasitized M. balthica to the final host does not seem to represent an example of adaptive host manipulation by the parasite.

A modern landscape ecology of Black-tailed Godwits : Habitat selection in southwest Friesland, The Netherlands

For a long time, agricultural areas had considerable ornithological value, an ecological richness which in The Netherlands was epitomised by the term ‘meadow birds’. However, over the last half century, agricultural intensification has negatively affected the quality of meadow bird habitats. Here we provide a quantitative characterization of agricultural habitats and their use by Blacktailed Godwits Limosa I. limosa in the south-western part of the province of Friesland, The Netherlands, in 2009, to provide a yardstick to evaluate further change. We used groundwater level, vegetation typology, relief in the landscape, the occurrence of foot drains, land use and soil characteristics such as textures and peat to describe the landscape that comprised 43 polders covering 8480 ha. We used a Principal Component Analysis to summarize landscape characteristics and find that much of the habitat variation is explained by a combination of herb richness of the vegetation, the presence or absence of foot drains and groundwater level. The modern agricultural landscape of southwest Friesland consists of 80% of uniform, intensively managed landscape with herb-poor meadows and low groundwater levels, the remaining 20% being taken by remnants of the former herb-rich meadows. The whole study area was searched weekly and Black-tailed Godwits were mapped between arrival and egg-laying. The positive relationship between godwit density and the first PC axis indicates that Black-tailed Godwits preferred herb-rich polders with high groundwater levels and the presence of foot drains. Soil texture was poorly correlated with godwit breeding densities for intensively managed (herb-poor) parcels, but for herb-rich meadows, soils of sandy clay loam and sandy clay harboured the highest densities of godwits. To protect Black-tailed Godwits, areas should have a herb-rich vegetation, contain foot drains and high groundwater tables should be re-established.

Experimental effects of immersion time and water temperature on body condition, burying depth and timing of spawning of the tellinid bivalve Macoma balthica

Abstract. The burying depth of many bivalve molluscs on intertidal mudflats varies throughout the year and differs between places. Many factors are known to influence burying depth on a seasonal or spatial scale, with temperature and tidal regime probably being very important. Burying depth, body condition and gonadal development of Macoma balthica were followed throughout winter and spring in an experiment in which water temperature and immersion time were manipulated. Unexpectedly, relative water temperature, in contrast to the prediction, did not generally affect body condition or burying depth. This was probably a consequence of the exceptionally overall low water temperatures during the experimental winter. Differences in temperature did, however, result in different timing of spawning: M. balthica spawned earlier at higher spring temperatures. Longer immersion times led to higher body condition only late in spring, but led to deeper burying throughout almost the whole period. There was no effect of immersion time on the timing of spawning. We conclude that a longer immersion time leads to deeper burying, independent of body condition. We also conclude that burying behaviour of M. balthica is not determined by the moment of spawning.

The effect of immersion time on burying depth of the bivalve Macoma balthica (Tellinidae)

Abstract As a characteristic buried tellinid bivalve, Macoma balthica has a long inhalent siphon that enables it to feed in two different ways: deposit and suspension feeding. To deposit feed efficiently on benthic microalgae, Macoma has to live close to the sediment surface, where it can graze an extensive surface area, but is within reach of many predators. Individuals that are more safely buried at a greater depth can only suspension feed, or deposit feed from a small surface area. We expected local differences in burying depth on intertidal mudflats to be caused by differences in immersion time (i.e. time available for feeding, particularly suspension feeding), since immersion time has been shown experimentally to affect body condition positively, and since body condition and burying depth in Macoma are postively related in the field. To test this we experimentally manipulated immersion time, and followed changes in burying depth and body condition. In the experiments, longer immersion time went consistently with greater burying depth of Macoma and higher body condition. On a transect in the western Wadden Sea, the deepest Macoma were indeed found at the intertidal level with the longest immersion time, but these were at that time not the animals with the highest body condition. Within each locality, however, body condition was positively correlated with burying depth. The experimental data and the within-locality data support the hypothesis that longer immersion time may influence burying depth through body condition. However, the fact that between-locality differences in burying depth seemed to be consistently related to immersion time, but not to body condition, indicates that body condition alone does not explain place-to-place variation in burying depth.

Colony-Breeding Eurasian Spoonbills in the Netherlands: Local Limits to Population Growth with Expansion into New Areas

It has been suggested that in most colony-breeding birds, food availability in the feeding areas surrounding the colonies limits, and thereby regulates, population size. However, population size is also determined by adult survival, which will additionally be influenced by circumstances outside the breeding season. Most Eurasian Spoonbills Platalea leucorodia leucorodia in The Netherlands breed on the Wadden Sea barrier islands. After 30 years of exponential growth, the breeding population in the Dutch Wadden Sea area is now levelling off towards a maximum of nearly 2000 nests. For these Spoonbills, density-dependent effects on survival by the different age-classes and in the different seasons have already been demonstrated. However, the mechanisms underlying the densitydependent survival of juveniles before and after fledging remain unclear. To examine whether these density-dependent effects reflect limitations at the colony level, we compared colony growth, chick condition and reproductive success among the Wadden Sea colonies. Population growth rates from 1988 to 2015 varied widely between the 10 existing colonies, and so did the statistically predicted maximum colony sizes. Chick condition, measured for 781 chicks in six different colonies between 2011 and 2015, was lower in stable colonies than in growing colonies, although not for the very late chicks, and reproductive success tended to be lower as well. Over the longer period of 1991 to 2011, reproductive success showed a strong negative relationship with colony size. We propose that the levelling off of colony sizes in the Wadden Sea is caused by local food limitations, and suggest further research in this direction. The continuing growth of the Dutch population is now being fuelled by exponentially increasing numbers of Spoonbills breeding in the Delta area.

Unexpected dietary preferences of Eurasian Spoonbills in the Dutch Wadden Sea: spoonbills mainly feed on small fish not shrimp

AbstractAfter an historical absence, over the last decades Eurasian Spoonbills Platalea leucorodia leucorodia have returned to breed on the barrier islands of the Wadden Sea. The area offers an abundance of predator-free nesting habitat, low degrees of disturbance, and an extensive intertidal feeding area with increasing stocks of brown shrimp Crangon crangon, the assumed main prey of P. leucorodia leucorodia. Nevertheless, newly established and expanding colonies of spoonbills have surprisingly quickly reached plateau levels. Here we tested the often stated assertion that spoonbills mainly rely on brown shrimp as food, by quantifying the diet of chicks on the basis of regurgitates and by analysis of blood isotopes using stable isotope Bayesian mixing models. Both methods showed that, rather than brown shrimp being the staple food of spoonbill chicks, small flatfish (especially plaice Pleuronectes platessa) and gobies (Pomatoschistus spp.) were their main prey. Unlike shrimp, small flatfish have been reported to be rather scarce in the Wadden Sea in recent years, which may explain the rapid saturation of colony size due to food-related density-dependent recruitment declines of growing colonies. By way of their diet and colony growth characteristics, spoonbills may thus indicate the availability of small fish in the Wadden Sea. We predict that the recovery to former densities of young flatfish and other juvenile/small fish in the Wadden Sea will be tracked by changing diets (more fish) and an increase in the size of Eurasian Spoonbill colonies across the Wadden Sea.ZusammenfassungUnerwartete Nahrungsvorlieben bei LöfflernPlatalea leucorodia leucorodiaim niederländischen Wattenmeer: Statt Garnelen fressen Löffler hauptsächlich kleine Fische Nach längerer Abwesenheit in der Vergangenheit ist der Löffler Platalea leucorodia leucorodia während der letzten Jahrzehnte als Brutvogel auf die vorgelagerten Inseln des Wattenmeeres zurückgekehrt. Dieser Lebensraum bietet reichlich prädatorfreies Nisthabitat, ein geringes Maß an Störungen sowie räumlich scheinbar grenzenlose Nahrungsflächen in der Gezeitenzone mit zunehmenden Beständen an Nordseegarnelen Crangon crangon, welche als die Hauptbeute der Löffler gelten. Dennoch haben die neugegründeten und sich ausdehnenden Kolonien überraschend schnell ihre Sättigungsgrenze erreicht. Hier überprüften wir die oft geäußerte Behauptung, dass Löffler hauptsächlich auf Nordseegarnelen als Nahrung angewiesen sind, indem wir die Kükennahrung anhand von Speiballen und durch Analysen der Isotope im Blut mittels Bayes’scher gemischter Modelle stabiler Isotope quantifizierten. Bei beiden Methoden zeigte sich, dass nicht, wie erwartet, vorwiegend Nordseegarnelen an die Löfflerküken verfüttert wurden, sondern dass kleine Plattfische (besonders Scholle Pleuronectes platessa) und Grundeln (Pomatoschistus spp.) die Hauptbeute darstellten. Anders als die Garnelen sind kleine Plattfische im Wattenmeer in den letzten Jahren selten geworden, was aufgrund nahrungsbedingter und dichteabhängiger Abnahmen der Rekrutierungsraten wachsender Kolonien das schnelle Erreichen der Sättigungsgrenze bezüglich der Koloniegröße erklären könnte. Aufgrund ihrer Ernährungsweise und ihrer Koloniewachstumseigenschaften könnten Löffler so die Verfügbarkeit kleiner Fische im Wattenmeer anzeigen. Wir sagen voraus, dass eine Bestandserholung junger Plattfische und anderer juveniler beziehungsweise kleiner Fische zu früheren Dichten im Wattenmeer an einer veränderten Nahrungszusammensetzung (mehr Fische) und einer wachsenden Koloniegröße der Löffler im gesamten Wattenmeer erkennbar sein wird.