There is strong evidence from studies in Ireland and the UK, that species which breed in open habitats may experience greater population declines in more fragmented landscapes. A widely explored example of this, is of a higher predation pressure on birds which nest closer to the habitat edge.
In this context, the spread of trees can occur naturally, as the treeline moves further north, or can be imposed or accelerated when trees are planted in previously open environments. However, afforestation: the establishment of a forest or stand of trees in an area where there was no previous tree cover, can potentially influence mammalian predator communities, with impacts on prey species like ground-nesting birds. It also increases the fragmentation of these open habitats, which have traditionally provided homes for waders such as Dunlin, Lapwing and Curlew.
A recent study in Estonia investigated ‘edge effects’ in a system of wet grasslands fragmented by forests. Sixty years ago, the country’s coastal grasslands were full of breeding waders. Since then, farmland abandonment and afforestation, both as a result of commercial forestry and natural succession, have reduced the area of coastal grasslands by about 70%.
Nest survival of ten wader species on coastal grasslands was therefore conducted for three consecutive years. Nests were equipped with camera traps to record nest predation events and the predator’s association with the forest edge. The distance to nearest trees and forest and additionally, forest cover within a 1-km buffer around each nest, was also measured.
Throughout the study period, researchers recorded extremely low daily nest survival rates (0.903–0.922 for different species), with most nests lost to predation. Significantly, survival was lower closer to the forest edge and negatively affected by the proportion of forest within a 1-km buffer around each nest. Although Ravens were also recorded feeding on the nests, the red fox was the most commonly encountered predator operating close to forest edge. It was in these areas that eggs were most likely to be taken.
The study concludes, that future afforestation plans of open habitats need to acknowledge that the resulting fragmentation has a negative impact on nest survival of ground-breeding birds. In Ireland, it has been suggested that a patchy distribution of relatively new forestry plantations may be one of the factors contributing to the drastic decline of curlew numbers, which have declined by 96% in just 30 years. It has been proposed that some of these patches should be removed in an attempt to increase productivity.