Large-scale drainage and breeding success in boreal forest grouse.
The breeding success of Finnish grouse has been in decline for decades. While it has been shown that fragmentation and modern forestry practices such as clear-cutting affect the viability of grouse populations, little is known about effects of large-scale drainage. The drainage network in Finland has increased dramatically during the past decades. By 1988, 6 million ha of bog ecosystems had been drained. This is likely to have had a profound direct and indirect effects on grouse productivity. Because ditches persist in time, large-scale drainage may therefore have strong potential for affecting the long-term breeding success of three forest grouse species in Finland. Using a mixed-model approach on nation-wide data, we estimate the effects of drainage on breeding success of three grouse species for the period from 1966 to 1988. The possible additive effect of precipitation is investigated using regional precipitation data. For each species, we compare a set of hypotheses and select the best model(s) according to the Akaike information criterion. For all species, the model which included the main effects of both ditch density and precipitation outperformed the models with just one of the main effects as well as the interaction model. The effects are restricted to southern and central Finland, where drainage is most intensive. The effects of drainage are likely to be both direct and indirect, but the mechanisms are less clear. Both bottom-up, such as decreased availability of insect food, and top-down effects, such as increased predation risk, may account for indirect effects. Direct effects are likely to involve drowning of chicks. Synthesis and applications. Bogs are classified as key biotopes by the European Union, and Finland is obliged to maintain their favourable conservation status and to protect typical bog communities. While this goal may be achieved for state-owned land, restoration of drainage systems on private land continues. Considering the persistence of ditches, new legislation are therefore needed to favour the recovery of former bogs and mires on private land as well as to protect remaining ones.