In three years the largest limestone quarry in the Netherlands, operated by the ENCI cement plant near Maastricht, will be closed. Large area of Mount St. Peter is already being transformed into an area for nature, biodiversity, ecosystem services and other public benefits. Can we expect red-backed shrikes to appear in the area, as an indicator of a functioning ecosystem?
Five months ago I wrote about the interesting research project carried out on the spot and at control sites in Limurg by the Biosphere Science Foundation. The researchers wanted to examine closely some elements of the ecosystem in order to evaluate the chances of successful colonisation of the area by red-backed shrikes Lanius collurio.
We asked Dr. Arnold van den Burg to present his ongoing research, funded by BirdLife- HeidelbergCement and the province of Limburg.
Is the insect community present in the area sufficiently diverse and abundant to provide enough food for the shrikes? In other words, what’s on the menu?
First of all it was important to study the food preferences of the shrikes and to compare the insect communities among sites already occupied by the red-backed shrike in South Limburg. As the areas around the quarry offer seemingly suitable habitat but no shrikes, they were also sampled for their insect community. The two sets of data were analysed for species diversity, abundance and similarities using common statistical methods.
Arnold, explained further the field methods: Red-backed shrike nests were monitored with the help of Bushnell wildlife camera traps to minimise disturbance. The drawbacks of this approach are that it is impossible to determine the exact feeding frequency. The cameras were often not fast enough to capture the exact chick feeding moment which resulted in many photos of chicks that have just been fed. Often it was impossible to determine exactly the food item. And finally we could only see these problems at the end of the field season when the young fledged and we could look into the camera. On the other hand, good data was collected from four nests and we plan to expand this method next year, now that we know how to set them up better, what to do and what not. In addition to filming the nests we sampled the grasslands for insects by sweep netting. The samples were frozen for analysis after the field season. The insects were identified down to family. Both methods were necessary to allow us to compare insect availability and prey choice.
The second question is: given the prey choice and prey availability for the shrikes, what are the land (nature) management interventions that favour or hinder the development of suitable insect community?
The relative composition of the grassland insect communities at the shrike territories in south Limburg differ clearly from the grassland sites around ENCI (on the graph shrike territories appear at the top center and upper right corner). This divergence of insect groups is explained mainly by presence of Sepsidae, Anthomyidae and Cercopidae which are directly or indirectly associated with grazing and (cow) dung. The Chloropidae are an ecologically diverse group, with representatives feeding on other insects, plants, and organic waste. The Miridae are present at all sites, but in much higher numbers in the shrike breeding area, indicating higher plant productivity. Even the most-productive ENCI-site visible on the graph falls in the lower-left corner (so most dissimilar to the shrike sites). The insect groups responsible for this difference are all flower visitors (Oedemeridae, Stratiomyidae) and species of ungrazed higher vegetation (Araneae, spiders). The Orthoptera are also well-represented at this site.
So, what are your preliminary conclusions?
Overall our data indicate that grassland productivity at the ENCI area is too low to support red-backed shrikes. As management is primarily focussed on promoting species-rich chalk grasslands, where low productivity is desired, we cannot expect that insect populations will be rich enough to support red-backed shrike colonisation. If return of the shrike will be one of the goals of the restoration project here we would recommend to diversify the management of the area to allow higher grassland productivity and cattle grazing at least in some places. In the same time, we are not suggesting to fertilize the chalk grasslands! The desired effect can be achieved by promoting a mosaic of habitats: at some parts of the site that are naturally enriched, like bottom of hills and along streams, higher productivity can be promoted. We have put such sites on maps. It will also be important to create nesting and hunting opportunities there. We will continue our line of research in 2015 where (besides expanding the dataset) we will focus on Dutch species-rich chalk grassland sites that are specifically managed as such and also have Red-backed shrikes (although these may be hard to find).