De Levende Natuur nummer 3 van 2018 (English summary)

Unravelling key-processes for seagrass restoration in the Dutch Wadden Sea

L.L. Govers, T. van der Heide, Q.J. Smeele, J.H.T. Heusinkveld & A. van der Eijk

Seagrass beds are crucial habitats in a biodiverse and healthy Wadden Sea. However, subtidal seagrass beds have completely disappeared from the Dutch Wadden Sea and intertidal seagrass beds have become extremely scarce. Seagrass beds are considered as hotspots for foundation species and biodiversity, and a recent project faced the challenge to study the possibilities for eelgrass restoration in the Dutch Wadden Sea. This was done by combining large-scale restoration experiments with lab experiments. Through this unique dual approach of ‘doing & learning’ we revealed several bottlenecks for eelgrass restoration in the Wadden Sea. We developed innovative solutions to solve three of these bottlenecks: 1) natural seed loss in winter (>99%) was mitigated by controlled harvest and storage of seeds in winter. 2) We found a successful treatment for a newly discovered water mold (Phytophthora gemini) that previously killed >40% of all the seeds, and 3) We managed to increase restored plant densities in the field by 180x through innovations in seeding techniques. These positive developments unfortunately have not yet lead to self-sustaining seagrass populations, but certainly brought restoration of intertidal seagrass in the Wadden Sea a step closer. The knowledge and technology this project has yielded will be built upon in future seagrass project in the Dutch Delta (Grevelingen and Veerse Meer).

Tree nest of the red wood ant (Formica rufa)

A.A. Mabelis

In the forest near Amerongen (The Netherlands) it was found that red wood ants had been building  their nest at a height of 3.5  m in a dead beech tree, a rather exceptional nesting place for this species. At the end of 2016 the ants transported their nestmates to a new nest that was built 3 m from the foot of the tree (fig. 2). However, in spring they transported them back into the tree (fig. 3). About mid-March they transported also nesting material and at the end of March also prey, which indicates that at least one queen was inside the tree nest. The transport of large nesting material and heavy prey was not so easy as many workers fell down from the tree. About mid-May empty cocoons of males or females  were removed from the nest and in the beginning of June empty cocoons of worker ants. In September 2017 the ants transported again nestmates from the tree to the winternest on the ground. It is questionable what profit the ants may have of building their nest so high in a tree.

Parcel inundation for meadow bird chicks

T. Visser & Th.C.P. Melman

In the Netherlands, parcel inundation is evolving into a regular AES management tool (as a rather easy applicable measurement). Research has been performed on the effect of parcel inundation on the quality as foraging habitat for meadow bird chicks. A comparison has been made of 20 parcels with and 20 parcels without parcel inundation. This research covered insect abundance, vegetation structure and the abundance of meadow bird families. The assessed effects are on inundated parcels:

• insects were more abundant  (smaller as well as larger than four mm);
• the vegetation was more open and was more varied in stucture and height;
• chicks of the black-tailed godwit, common redshank and northern lapwing were more abundant. Based on these results it is concluded that parcel inundation enhances the foraging quality for meadow birds. 

Regeneration of bog vegetation in the Fochteloërveen

J. Wiegers

In 1980 a fire destroyed the aboveground vegetation in a part of the bog reserve. Following the fire two transects (50 x 5 meter) were plotted, one in a relatively wet situation and one in a relative dry situation. The distribution of the species in the research plots was mapped in 9 years over the period 1980 till 2016. At first the aim was to study the regrowth of the vegetation after the fire. From 1998 onwards dams and dikes were built in and around this part of the reserve in order to contain rain water in the area. This resulted over large surfaces in inundation of the vegetation. The reaction of the vegetation to this water management hence could be studied in the research plots. In the dry transect Calluna vulgaris showed a strong increase after the fire, followed by an increase of Erica tetralix. Both species disappeared following the increase of the water level but by 2016 Erica had returned. Eriophorum angustifolium seemed to react with reduced growth in periods of stable water levels, but increased growth in periods of changing water levels. In the wet transect there was only a minor return of Calluna following the fire. Here also Erica showed abundant growth until the water level rose to above the soil surface. The species did not completely disappear but became quite rare. Eriophorm angustifolium showed a comparable reaction to periods of water level stability and instability as in the dry transect. The appearance of Eriophorum vaginatum, Drosera rotundifolia, Sphagnum papillosum and S. magellanicum after 2012 indicate a positive change in  the direction of redevelopment of bog vegetation.

Population dynamics and management of a northern wheatear stronghold in the Dutch dunes

C.A.M. van Turnhout, F.A. Majoor & T. Zutt

The northern wheatear (Oenanthe oenanthe) has strongly declined as a breeding bird in the Netherlands since 1980 (fig. 1). In 2016, only 280-320 breeding pairs were left. During 2007-2017, we studied numbers, breeding success and survival of wheatears in a coastal dune population near Den Helder, NW-Netherlands, in relation to habitat management and nest protection. The number of breeding pairs fluctuated between 47 and 67, without a clear trend (fig. 3). However, breeding success decreased from 3.9 fledged young per pair in 2007-2011 to 2.6 in 2012-2017 (fig. 4). The proportion of successful nests decreased with 40%. Since 3.6 young per pair are needed for a stable population, the population turned from a source into a sink during the study period. Grass encroachment caused by decreased rabbit populations (fig. 2) and nest predation seem to be the main causes. Protection of nests in rabbit burrows using chicken wire appears a successful measure to prevent predation by foxes, but not by small mustelids. Furthermore, small-scale mowing and sod-cutting, in combination with sheep grazing during a short period in winter, appears a successful habitat management option. Trends in numbers and breeding success were more positive in parts of the study area where management was carried out, in comparison with parts without habitat management (fig. 5).

Prehistoric farmers and their role in the formation of sand drifts

N.W. Willemse

During the Holocene, Late Glacial aeolian deposits throughout the European sand belt were reactivated as sand drifts. The well-documented lack of synchronicity in the temporal and spatial distribution of Holocene dune activity clearly argues against any single major external force as the main trigger of this reactivation process, but rather points towards localised nuclei and human impact. In the Netherlands, most sand drifts date from the high- to post-medieval period. Their formation was probably connected to an intensification of land use in settlement peripheries (outfields), but archaeological evidence for this is scarce. In recent years, archaeological excavations have produced a growing body of evidence for the existence of much older prehistoric sand drifts, mainly situated along terraced Dutch river valleys. The archaeological context of these sand drifts strongly suggests that they are (largely) anthropogenic in origin and that they were originally situated in settlement infields. Possibly, natural soil depletion prior to the start of reclamation may have been an important trigger of intense aeolian sediment relocation at these sites. As they were ploughing fields and creating pasture for cattle, early farmers tore up already impoverished soils and over-exposed the until then fixated Late Glacial sandy landscape below, thereby sparking off intense sand drifting. This observed response suggests a form of geomorphic change that does not correspond to proportionally large external forcings but is instead characteristic of landforms in a state of incipient instability. These findings underpin the necessity for a thorough understanding of geomorphic controls and destabilization factors before taking sand drift reactivation measures. The findings also demonstrate the man-made background of many of the inland drift sand landscapes in the Netherlands, including the archaeological remains that are associated with the initiation of sand drifting at these locations.