De Levende Natuur nummer 3 van 2023 (English summary)

Effects of lower N-deposition in Dutch dunes

Atmospheric N deposition in Dutch dune grasslands decreased between 1992 and 2021 from 25 to 15 kg/ha/yr. We combined 11 separate studies to test whether this was associated with improvements in the vegetation. Although species richness was not affected, average aboveground biomass indeed decreased from 540 to 130 g/m2. However, sensitivity to high N deposition could be mitigated or enlarged by pH and soil organic matter content (SOM). Sensitivity would decrease with high pH and/or low SOM, but increase with low pH and high SOM, associated with changes in P-availability. High pH and low P availability also increased species richness, especially through plants that live together with arbuscular mycorrhiza. 

Closure Zuiderzee caused a rapid decline in biodiversity

The closure of the Zuiderzee, which was completed in 1932, transformed a 3500 km2 tidal-driven brackish water area into a freshwater lake (IJsselmeer). This caused a sharp and rapid decline in the original shellfish, snails, crabs, lobsters, shrimps, and mysids (from 42 species in 1931 to seven in 1935). IJsselmeer became populated by new species, but the overall result was more than a 40 % decrease in biodiversity (here: the number of species studied) within twenty years (1932-1952). Current climate change (sea level rise, but also the decrease in riverine outflows into the sea) challenges us again with the choice of whether to intervene on a large scale in Dutch coastal areas. The drastic changes after the closure of the Zuiderzee illustrate that we should care not to turn our backs to the sea, because such interventions also affect the (often highly valued) natural values of coastal areas. 

Climate change affects aquatic organisms in rivers

Riverine habitats in Dutch large rivers are exposed to climate change. We studied the effect of elevated water temperature on 175 river-characteristic aquatic species, by comparing literature-reported critical temperature- thresholds with expected temperature development in several types of backwaters and the main river. Results show that a considerable part of the species of fish, bivalves, mayflies as well as dragonflies are sensible to the higher temperatures that will become more common in the near future. This is especially valid for floodplain backwaters, where the temperature rises are higher than in the river itself. Real measured temperatures during hot summers (that are thought to be more common in the near future) in the floodplain backwaters rose to 32°C in the summer. This contrasts many characteristic riverine species that cannot endure temperatures above 24°C for long. There is a perspective for action though: mitigation measures can make the river ecosystem more resilient to rising water temperatures. 

Hard substrates Wadden Sea important for 17 benthic taxa

The Wadden Sea is an important soft-sediment dominated system. However, there are areas in this shallow sea where hard substrates are also present on the sediment surface. Boulder clay, peat, rocks, dead shell accumulations and wood are important habitats for certain benthic species. In this study, we show that in 35 % of the boxcore samples from the permanently submerged (subtidal) Dutch Wadden Sea, hard substrates were observed on the sediment surface. We identified 17 benthic taxa that were exclusively found in samples when a hard substrate was present on the sediment surface, but not in samples that only contained soft sediment. We also discuss the origin (Pleistocene, Holocene, contemporary and artificially introduced) of the most important hard substrates, and discuss the ecological values and possible consequences of the presence of hard substrates for nature management decisions in the Wadden Sea ecosystem. 

Little owls need good food conditions around their nests

Food is a key factor in the protection of little owls. Years of research in Southeast Achterhoek have shown that owls feed their nestlings mainly with insects and their larvae during the breeding period, but that mice – especially wood mouse and field common vole – have a major influence on important reproduction parameters such as clutch size, number of young and their body condition because of their biomass. Because the life of adult owls takes place in a relatively small area – a radius of 250 meters around the nest, it is important to create favourable conditions for mice and other important prey such as May beetles and earthworms.