I am a postdoctoral researcher studying how detritivorous earthworms could be used in a more nature friendly farming system. During my PhD project (2012–2017), I studied the interactions between earthworms, meadow birds and dairy farming in Friesland (The Netherlands). During my PhD I focused on how management of grasslands used for dairy production influences different earthworm species and how this could influence the foraging ecology of waders using dairy farmland to feed on earthworms (e.g. Black-tailed Godwits, Lapwings, Ruffs, Golden Plovers etc.).

Ruffs and Golden Plovers feeding on earthworms. Photo: Jeroen Onrust

The main result I found was that there are two main groups of earthworms, species that surface (detritivorous species) and species that don’t surface (geophagous species). Especially the surfacing species are important because by coming to the surface they are then also available for visual hunting birds such as Lapwings. A probing Black-tailed Godwit could feed on both groups. However, intensive land use is detrimental for surfacing species. This will not only affect visual hunting predators, but also the whole food web as this group of earthworms play a crucial role in soil formation and nutrient cycling.

To measure earthworm availability for visual hunting earthworm predators, I developed a cart that can be pushed slowly with the legs along a transect and then all the earthworms could be counted form close distance without creating too much disturbance (see picture). It is night work as earthworms only surface then.

Jeroen counting earthworms by pushing himself slowly along a transect using a self-developed cart. Photo: Lars Soerink

Furthermore, I studied nocturnal movements of Golden Plovers wintering in dairy farmland to investigate whether they use earthworm-rich patches for foraging. For two years, we applied TOA transmitters on 50 birds and measured number of surfacing earthworms in the patches an individual used at night and compared it with nearby unused patches. By performing indoor feeding experiments, we revealed how Ruff catch earthworms. Also during my study biology at the University of Groningen I worked with waders (Oystercatchers and Redshanks at the saltmarsh and Red Knots in Banc d’Arguin).

Profile photo: Lars Soerink