De Levende Natuur nummer 3 van 2020 (English summary)

Impact of the invasive alien topmouth gudgeon and its associated parasite Sphaerothecum destruens on native fish Species 

F. Spikmans, P. Lemmers, H. op den Camp, G. van der Velde, R. Leuven & T. van Alen  

Alien fish are important invaders, with a high number of species introduced outside their native range worldwide. Pathogens associated with these invaders often remain undetected, while they potentially have detrimental effects on native fish species. The topmouth gudgeon (Pseudorasbora parva), a cyprinid fish, originates from Asia and is globally invasive. It is an asymptomatic carrier of Sphaerothecum destruens, an aquatic parasite known for causing mortality and spawning inhibition of native fish species. Pseudorasbora parva is a major threat to native fish communities and listed as an invasive alien species of European Union concern. This study aims to improve evidence-based knowledge on the impact of both P. parva and S. destruens on native fish species. We studied the fish assemblages and body condition of individuals in invaded and uninvaded water bodies in situ. Prevalence of the pathogen in native species was assessed. We explored the use of environmental DNA (eDNA) techniques to detect S. destruens. Fish samplings showed significantly negative correlations between the abundance of P. parva and the native sunbleak (Leucaspius delineatus), native ninespine stickleback (Pungitius pungitius) and three biodiversity indices (Simpson’s diversity index, Shannon-Wiener index and evenness). Contrastingly, the abundances of native three-spined stickleback (Gasterosteus aculeatus) and P. parva were positively related. In nearly all isolated water bodies with P. parva, this species is outnumbering native fish species. No effect of P. parva presence was found on body condition of native fish species. Sphaerothecum destruens was demonstrated in both P. parva and the native G. aculeatus. Gasterosteus aculeatus can aid further spread of S. destruens as an asymptomatic carrier. The presence of S. destruens coincides with that of P. parva. The ongoing invasion of both P. parva and S. destruens is predicted to pose a significant risk to native fish communities. Risks from introductions of invasive pathogens urgently require more attention. 

Developing herb-rich in the higher sandy regions through vegetation management 

E. Dorland, K. Eichhorn, T. van den Broek & M. Courbois  

In the higher sandy region of the Netherlands in particular, the vegetation management type ‘Herbs and fauna-rich grasslands’ [N12.02] is of high ecological importance. At present, however, despite years of mowing and removal of plant biomass, the majority of these grass-lands are dominated by a few grass species only and harbor a limited number of herb and insect species. The aim of this project was to investigate how these grasslands can be transformed into herb and fauna-rich grasslands. The field research took place at three locations distributed through the higher sandy region in the Netherlands. At each location, the effects of the treatments 1) mowing (control treatment), 2) temporary rye cultivation, and 3) one year fallow, all both with and without the introduction of seeds of target plant species, on soil chemistry, vegetation composition and insect fauna were investigated for a period of two years. The results showed that despite high soil phosphorus concentrations positive effects of both temporary treatments on plant species composition may occur. The success of these treatments appears to be greater in drier grasslands with relatively low biomass production, indicating the importance of continuation (or start) of mowing and removal of plant biomass, which is the regular management of this type of grassland. The treatments had no effect on soil chemical composition nor on insect species. Future monitoring will be necessary to investigate whether the positive effects of the treatments on vegetation composition will persist on the long term and if insect species will start to benefit from the increase in herb species. Summarizing in management advice: Realizing that long-term effects are not yet known, temporary arable cropping of grasslands (in particular in the form of laying fallow, and to a lesser extent as temporary rye cultivation) on relatively nutrient-poor, dry grasslands may be a suitable way to develop herb-rich grasslands. 

Developing herb-rich grasslands at topsoil with a high concentration of Phosphorus: What are the possibilities?  

K. Eichhorn, E. Brouwer, E. Dorland, R. Ketelaar & T. van den Broek  

Many grasslands on former agricultural land in nature reserves remain poor in herb species for a very long time. This is generally attributed to the heavy fertilisation in the past, resulting in a long lasting high concentration of phosphorus in the topsoil. However, our recent observations show that, under certain circumstances, herb-rich grasslands can develop on such phosphorus-rich soils. The observed examples seem to be the result of a lot of initial bare ground, in combination with a low production of biomass related to low levels of nitrogen and drought, and the availability of seed of the herbs. Especially grasslands developed from completely bare arable fields were rich in herb species and large attractive flowers. The implications for the development of natural herband fauna-rich grasslands are discussed. 

The flower-rich dike as a bee habitat 

C. Swinkels, C. Liebrand, N. van Rooijen, E. Visser & H. de Kroon 

 The Netherlands contain over 17,500 km of dikes, embankments of rivers, waterways and lakes, mostly covered by grasslands. Potentially, this network of grasslands can function as valuable bee habitat and as migration routes. Here we investigated the potential of dikes along the river Waal for wild bees, by comparing the bee fauna between locations with a flower-poor versus flower-rich vegetation. We find that flower-rich dikes harbour a much higher bee abundance and diversity. Nevertheless, the core of the bee community is similar at flower-rich and flower-poor localities, albeit impoverished at the latter. We thus expect that management to increase flower abundance at currently flower-poor locations will quickly lead to a restoration of a more abundant bee community. 

Smoddebos: development of vegetation related to (a)biotic conditions 

A. Kieskamp, H. Smeenge, M. Horsthuis & H. Koop  

The Smoddebos is an oak-hornbeam forest in the Natura 2000 site Landgoederen Oldenzaal. Since 1955, the development of flora and vegetation was followed along 47 permanent quadrates (PQ). In 2017, the vegetation on every PQ was described again and the soil and humus characteristics were examined in order to relate abiotic conditions to vegetation. Three forest zones were identified: Stellario-Carpinetum, Milio-Fagetum and Fago-Quercetum. Typically for the Stellario-Carpinetum zone in the Smoddebos is the stagnation of rainwater in winter on base-rich boulder clay. This results in an active soil life and therefore a ‘mull’ humus type. These are ideal conditions for species typical for oak-hornbeam forests like Primula elatior and Anemone nemorosa, which are strongly present in this zone. The Milio-Fagetum zone in the higher part of the forest is more prone to acidification since 1) the boulder clay dries out stronger and faster because there is less ground water influence; 2) there is no stagnant rain water that dissolves base elements from the clay and 3) oaks and beech (with acid litter) have a bigger share. It results in a ‘moder’ humus type. Comparison of vegetation surveys at 47 PQs in 1955, 1982, 1995 and 2017 and flora surveys in 1988 and 2016 show that indeed the Milio-Fagetum zone changed the last decades. Typical oak-hornbeam forest species retreat in the lower, more humid part of the Smoddebos, the Stellario-Carpinetum zone, which is a forest reserve since 1997 (no management since then). Less changes occurred in this part. There is a strong relation between humus type and presence of these typical species. Interviews with residents taught us that since the beginning of the 20th century, their families manually cut out some of the trees every winter for the purpose of the brickworks. These smallscale activities made scratches in the humus layer that resulted in ideal conditions for typical plant species to germinate. It is also plausible that the forest became darker during the last decades, to the disadvantage of the flora.