Direct and indirect effects of the most widely implemented Dutch agri-environment schemes on breeding waders.
In the Netherlands, agri-environment schemes are an important tool for halting the ongoing decline of meadow birds and, in particular, waders breeding on wet meadows. The effectiveness of the main scheme, postponed mowing, is heavily debated because it does not result in higher breeding densities. Recently, agri-environmental collectives have become involved in the co-ordination of scheme applications and additional measures have been introduced. One of them is per-clutch payment: farmers are paid per wader clutch without being restricted in their farming practices. We evaluated the effectiveness of the combination of the two measures, postponed mowing and per-clutch payment, by determining the number of birds and territories on 12.5-ha plots where both measures (on average 1.6 ha postponed mowing and 10.9 ha per-clutch payment) were being implemented. Conventionally managed grasslands served as controls. Additionally, on a field with postponed mowing and a paired control field, we measured a number of environmental factors that might influence wader distribution. On plots operating a combination of postponed mowing and per-clutch payment, more territories of all bird species were found and more redshanks Tringa totanus were observed. The same pattern occurred on fields with per-clutch payment alone. On fields with postponed mowing alone, we found more territories of the most abundant wader species but on conventional fields we observed more lapwings Vanellus vanellus. The positive effects of postponed mowing on wader territories were probably caused by small differences in soil moisture and groundwater level between the two field types, as inclusion of these factors in a general linear model rendered all scheme effects insignificant. Postponed mowing affected the form and amount of fertilizer applied to the fields as well as available nitrogen, but none of the other environmental factors that were measured. Additional analyses identified groundwater depth, penetration resistance and prey density (earthworms, Lumbricidae, and leatherjackets, Tipulidae larvae) as the main factors determining wader density. Synthesis and applications. Our results show that conservation measures consisting of postponed mowing and per-clutch payment implemented by agri-environment collectives do not support a higher abundance of waders but do support marginally higher breeding densities of waders compared to conventional farms. These results are probably due to differences in soil moisture and groundwater depth. The effectiveness of agri-environment schemes directed towards conservation of waders might be enhanced by including raised groundwater levels into scheme prescriptions.