Aanpassingen van trekvogels aan een snel veranderende wereld

Oratie uitgesproken door Prof.dr. Jan A. van Gils op 21 september 2021 bij de aanvaarding van het ambt van bijzonder hoogleraar Global Change Ecology of Migrant Shorebirds aan Faculty of Science and Engineering Rijksuniversiteit GroningenVerkleining, vervorming, verplaatsing, vervrouwelijking en vermindering: wat klimaatverandering allemaal doet bij de Kanoet!Er is iets opmerkelijks gaande bij de kanoet, een gedrongen trekvogel die jaarlijks pendelt tussen pool en evenaar. Door de opwarming van de aarde komen er steeds minder kanoetenmannetjes. Op elke man die rondvliegt zijn er momenteel drie kanoetendames! Een probleem voor een monogame soort als de kanoet. En dat is niet het enige probleem voor deze vogel.De aarde warmt op en trekvogels zijn steeds vaker slachtoffer van deze ecologische ramp. Dit komt voor een groot deel doordat hun noordelijke broedgebieden sneller opwarmen dan hun zuidelijke overwinteringsgebieden. Reisschema’s gaan uit de pas lopen en het kroost komt vaak te laat uit het ei. Een extreem geval is de kanoet, welke broedt in het noordpoolgebied en, via onze eigen Waddenzee, afreist naar zijn winteroord in West-Afrika. Omdat zijn arctische broedgebied drie keer sneller opwarmt dan de rest van de Aarde fungeert de Kanoet als de ideale kanarie in de kolenmijn: effecten van klimaatverandering zijn het eerst zichtbaar bij deze vogel en vormen een voorbode voor wat andere diersoorten te wachten staat.En effecten zijn er! Ten eerste krimpt het kanoetenlijf. En wel met 20% over de afgelopen dertig jaar, een zeer waarschijnlijk gevolg van minder voedsel tijdens de kuikenfase. Hierop volgt een tweede effect: in West-Afrika zijn alleen die vogels wiens snavel het minst gekrompen is in staat de diep-ingegraven schelpdieren te vinden. Wat overblijft zijn gekrompen kanoeten met relatief lange snavels (zgn. ‘Pinokkio-vogels’). Ten derde: waar veel soorten noordelijker zijn gaan broeden, broedde de kanoet al op het ‘dak van de wereld’. De enige strohalm waar ze zich nog aan vast lijken te klampen is een verplaatsing naar de hoger gelegen delen van hun Siberische broedgebied. Echter, inmiddels zijn de toppen van de heuvels bereikt en is verdere verplaatsing onmogelijk. Tot slot neemt het aandeel mannen in de populatie snel af, momenteel tot een verhouding van één man op drie vrouwen. Een gevolg van hogere sterfte onder de mannen in West-Afrika, maar ook een afnemend aandeel mannetjes dat uit het ei komt. De kanoet is monogaam met alle gevolgen van dien.Het laatste effect laat zich dan ook gemakkelijk raden: vermindering. De kanoetenpopulatie is de afgelopen veertig jaar met 70% afgenomen. Het idee van de kanarie in de kolenmijn is te handelen wanneer Pietje het loodje legt. Het is dan ook vijf voor twaalf

Radio-telemetry observations of the first 650 km of the migration of Bar-tailed Godwits Limosa lapponica from the Wadden Sea to the Russian Arctic

In 1999 and 2000, 45 Bar-tailed Godwits Limosa lapponica were supplied with radio-transmitters during spring staging on the island Texel in the western Wadden Sea. With the use of Automatic Radio Tracking Stations (ARTS) on Texel and in south Sweden, and hand-held receivers on Texel, it was possible to follow the later part of the stopover period on Texel for 34 birds (76%) and the passage over south Sweden for 26 birds (58%). Thus, the method of automatic tracking of overflying migrating shorebirds works successfully where the migration corridor is narrow and predictable, as in the case with late spring shorebird migration from the Wadden Sea towards arctic Russia. The timing of departure from Texel and passage over south Sweden of radio-marked birds, with median dates of 30 May and 2 June respectively, were in agreement with published data on the spring migration of Siberian-breeding Bar-tailed Godwits L. l. taymyrensis. The individual variation in migration dates was larger than expected, with birds passing south Sweden between 25 May and 10 June, indicating that the time-window for departure might be broader than previously thought. There was no clear difference between males and females in timing of migration. The time difference between departure from Texel and passage over south Sweden (average 3.3 days) indicates that most Bar-tailed Godwits do not embark on the long flight towards Siberia directly from the western Wadden Sea, but are more likely to stop in the more easterly portion of the Wadden Sea before the final take-off. This pattern is similar to what has been found in other shorebirds and geese (e.g. Red Knots Calidris canutus and Dark-bellied Brent Geese Branta bernicla) migrating along the same route.

Holling's functional response model as a tool to link the food-finding mechanism of a probing shorebird with its spatial distribution

1. Knots Calidris canutus are high-arctic breeding shorebirds which spend the nonbreeding season in intertidal areas where they feed on small buried molluscs which are swallowed whole. We tested whether their intake rate can be adequately described by a functional response model (the disc equation of Holling) assuming prey detection by direct touch. 2. Knots were fed two bivalve species (Macoma balthica and Cerastoderma edule) of different size and at different density and in Macoma at variable depth on small areas of soft sediment. Five different individuals were used in feeding trials during which six prey were allowed detection and ingestion. 3. Knots perfectly obeyed the two basic assumptions of Holling's disc equation: neither the instantaneous area of discovery nor the handling time changed with variation in prey density. 4. However, two predictions for prey perception by direct touch (that the instantaneous area of discovery is a positive function of shell size and a negative one of prey depth) were rejected. The instantaneous area of discovery was usually also higher than predicted from the area touched by probing knots. 5. Knots may, therefore, use a 'remote sense' to locate buried hard-shelled prey. It was shown that knots do not use taste to discover bivalves at a distance. We argue that the detection involves a self-induced pressure mechanism rather than a passive sense for vibrations, since it is unlikely that static bivalve prey emit such signals just after experimental handling. 6. Whatever the exact prey detection mechanism, the recorded (high) value for the instantaneous area of discovery of deep-living bivalve prey is in accord with the observation that knots use large areas of intertidal flat in the western Dutch Wadden Sea where bivalve stocks are too low to support knots according to a previous functional response model invoking direct touch

Field measurements give biased estimates of functional response parameters, but help explain foraging distributions

Mechanistic insights and predictive understanding of the spatial distributions of foragers are typically derived by fitting either field measurements on intake rates and food abundance, or observations from controlled experiments, to functional response models. It has remained unclear, however, whether and why one approach should be favoured above the other, as direct comparative studies are rare. The field measurements required to parameterize either single or multi-species functional response models are relatively easy to obtain, except at sites with low food densities and at places with high food densities, as the former will be avoided and the second will be rare. Also, in foragers facing a digestive bottleneck, intake rates (calculated over total time) will be constant over a wide range of food densities. In addition, interference effects may depress intake rates further. All of this hinders the appropriate estimation of parameters such as the 'instantaneous area of discovery' and the handling time, using a type II functional response model also known as 'Holling's disc equation'. Here we compare field- and controlled experimental measurements of intake rate as a function of food abundance in female bar-tailed godwits Limosa lapponica feeding on lugworms Arenicola marina. We show that a fit of the type II functional response model to field measurements predicts lower intake rates (about 2·5 times), longer handling times (about 4 times) and lower 'instantaneous areas of discovery' (about 30-70 times), compared with measurements from controlled experimental conditions. In agreement with the assumptions of Holling's disc equation, under controlled experimental settings both the instantaneous area of discovery and the handling time remained constant with an increase in food density. The field data, however, would lead us to conclude that although handling time remains constant, the instantaneous area of discovery decreased with increasing prey densities. This will result into highly underestimated sensory capacities when using field data. Our results demonstrate that the elucidation of the fundamental mechanisms behind prey detection and prey processing capacities of a species necessitates measurements of functional response functions under the whole range of prey densities on solitary feeding individuals, which is only possible under controlled conditions. Field measurements yield 'consistency tests' of the distributional patterns in a specific ecological context.</p

Shellfish Dredging Pushes a Flexible Avian Top Predator out of a Marine Protected Area

There is a widespread concern about the direct and indirect effects of industrial fisheries; this concern is particularly pertinent for so-called “marine protected areas” (MPAs), which should be safeguarded by national and international law. The intertidal flats of the Dutch Wadden Sea are a State Nature Monument and are protected under the Ramsar convention and the European Union's Habitat and Birds Directives. Until 2004, the Dutch government granted permission for ~75% of the intertidal flats to be exploited by mechanical dredgers for edible cockles (Cerastoderma edule). Here we show that dredged areas belonged to the limited area of intertidal flats that were of sufficient quality for red knots (Calidris canutus islandica), a long-distance migrant molluscivore specialist, to feed. Dredging led to relatively lower settlement rates of cockles and also reduced their quality (ratio of flesh to shell). From 1998 to 2002, red knots increased gizzard mass to compensate for a gradual loss in shellfish quality, but this compensation was not sufficient and led to decreases in local survival. Therefore, the gradual destruction of the necessary intertidal resources explains both the loss of red knots from the Dutch Wadden Sea and the decline of the European wintering population. This study shows that MPAs that do not provide adequate protection from fishing may fail in their conservation objectives

統計力學の諸問題

Selective predation can lead to natural selection in prey populations and may alleviate competition among surviving individuals. The processes of selection and competition can have substantial effects on prey population dynamics, but are rarely studied simultaneously. Moreover, field studies of predator-induced short-term selection pressures on prey populations are scarce. Here we report measurements of density dependence in body composition in a bivalve prey (edible cockle, Cerastoderma edule) during bouts of intense predation by an avian predator (red knot, Calidris canutus). We measured densities, patchiness, morphology, and body composition (shell and flesh mass) of cockles in a quasi-experimental setting, i.e. before and after predation in three similar plots of 1 ha each, two of which experienced predation, and one of which remained unvisited in the course of the short study period and served as a reference. An individual's shell and flesh mass declined with cockle density (negative density dependence). Before predation, cockles were patchily distributed. After predation, during which densities were reduced by 78% (from 232 m-2 to 50 m-2), the patchiness was substantially reduced, i.e. the spatial distribution was homogenized. Red knots selected juvenile cockles with an average length of 6.9 mm (SD 1.0). Cockles surviving predation had heavier shells than before predation (an increase of 21.5 percentage points), but similar flesh masses. By contrast, in the reference plot shell mass did not differ statistically between initial and final sampling occasions, while flesh mass was larger (an increase of 13.2 percentage points). In this field-study, we show that red knots imposed a strong selection pressure on cockles to grow fast with thick shells and little flesh mass, with selection gradients among the highest reported in the literature

Estimation and Identifiability of Model Parameters in Human Nociceptive Processing Using Yes-No Detection Responses to Electrocutaneous Stimulation

Healthy or pathological states of nociceptive subsystems determine different stimulus-response relations measured from quantitative sensory testing. In turn, stimulus-responses measurements may be used to assess these states. In a recently developed computational model, six model parameters characterize activation of nerve endings and spinal neurons. However, both model nonlinearity and limited information in yes-no detection responses to electrocutaneous stimuli challenge to estimate model parameters. Here, we address the question whether and how one can overcome these difficulties for reliable parameter estimation. First, we fit the computational model to experimental stimulus-response pairs by maximizing the likelihood. To evaluate the balance between model fit and complexity, we evaluate the Bayesian Information Criterion. We find that the computational model is better than a conventional logistic model regarding the balance. Second, our theoretical analysis suggests to vary the pulse width among applied stimuli as a necessary condition to prevent structural non-identifiability. In addition, the numerically implemented profile likelihood approach reveals structural and practical non-identifiability. Our model-based approach with integration of psychophysical measurements can be useful for a reliable assessment of states of the nociceptive system