Pitrus Juncus effusus en Sprinkhaanzangers Locustella naevia: de discrepantie tussen theorie en praktijk

In an area of 45 km5 in western Drenthe, distribution and abundance of soft rush. Juncus effusus exploded in the course of the 1990s. Although already widespread in the early 1990s, it was still an isolated plant species which nowhere occurred in dense carpets over large areas. This changed radically from the mid-1990s onwards. Especially on former farmlands, Juncus effusus has become the dominant plant species; a similar trend is observed in swampy areas around fens. In 2001, dense mats of soft rush were recorded at 27 sites in the above-mentioned region, varying in size between 0.1 and 43 ha (mean 5.04,±7.98 ha), whereas such sites had been completely lacking in 1990. In the early 1990s, Grasshopper Warblers were scarce breeding birds in this part of Drenthe, with a few solitary territories near large fens (>5 ha). The species showed a steep increase in the latter part of the 1990s and early 2000s (Table 1), in parallel with the booming spread of soft rush on land recently withdrawn from farming practices. Within 2-3 years, almost all these agriculturally improved but botanically poor fields were covered by the pioneer soft rush. The present breeding population of Grasshopper Warblers is confined to 13 of the 27 sites with extensive growth of Juncus effusus. Soft rush sites that were occupied by Grasshopper Warblers were on average larger than unoccupied sites, i.e. respectively 6.79 ha±l 0.51 ha (range 0.5-43.0 ha) and 3.16±2.55 ha (range 0.1-7.5 ha). However, apart from size several other factors may have been important in determining habitat choice, such as the presence of a scattering of trees or shrubs and distance from the forest edge (Appendix 1). In 1999-2001,29 out of 35 singing birds were checked for being paired or not, and whether or not nesting attempts were made. In five cases, neither a mate nor behaviour associated with nesting was noted. All remaining birds appeared to have nests, in three cases confirmed by the location of an active nest (Appendix 1). Nesting behaviour was recorded from the second half of May through early August (Table 2), indicating that two broods are regularly produced (in an estimated 50% of the pairs). These and other data implicate that large carpets of the emergent macrophyte Juncus effusus, despite being dreaded by conservationists and biologists alike as indicative of disturbed and acidified areas, can have great ecological value for birds ( Luscinia svecica, Saxicola torquata, Locustella naevia, Sylvia communis, Emberiza schoeniclus), amphibians (Rana temporaria, Ratio arvalis) and reptiles ( Vipera berus, Natrix natrix, Lacerta vivipara, Anguis fragilis). This unexpected outcome deserves a more tolerant and wait-and-see attitude from conservationists, instead of trying to eradicate Juncus effusus at all cost, and provides biologists food for thought regarding theories on dispersal and metapopulations.