Vergeten adderhabitat en het behoud van adderpopulaties

In de meeste literatuur en door veldwerkers wordt de adder (Vipera berus) beschouwd als een typische bewoner van (vochtige) heide. Logisch dus dat studies én het beheer van adderpopulaties zich vrijwel uitsluitend richten op heideterreinen. Maar dat is een te beperkt uitgangspunt: je kan ook adders vinden buiten de heide. Het gaat daarbij niet om toevallige waarnemingen van verdwaalde dieren, neen, adders brengen wel degelijk een belangrijk deel van hun activiteitscyclus en levensloop door in niet-heidevegetaties. Welke zijn die “vergeten” adderhabitats en wat is hun belang voor het behoud van adderpopulaties? The adder (Vipera berus) is generally considered as a typical species of (wet) heathland. Hence, studies and management of adder populations focus almost exclusively on heathland areas. However, that view has turned out to be a too limited one. Already the first ecological studies of adder populations showed that individuals undertake seasonal migrations between so-called winter and summer habitats. Remarkably, this behaviour has received very little attention in the Dutch literature. Our observations on a large population at the Groot Schietveld in northern Belgium confirm that adders spend large amounts of time in their summer sites. These sites are located near, but outside of the heathlands. The adders use a wide variety of habitat types: edges of hayfields, abandoned meadows and arable fields, scrub, rugged fields with Soft Rush (Juncus effusus) or Reed Canary-grass (Phalaris arundinacea), bramble patches (Rubus fruticosus), roadside verges with rough vegetation, and the edges of deciduous or coniferous woodland. An essential characteristic of the various summer habitats is that there is a greater food supply than in the nutrient-poor heathlands. Accordingly, the proportion of adders with food in their stomachs was often more than double in the summer habitats than in the winter habitats. Because adders forage mainly in the summer habitats, building up fat reserves, these habitats are essential for the survival, reproduction and persistence of the populations. Moreover, year-to-year variation in adder abundance at the heathland sites, where they are commonly monitored, may reflect fluctuations in food availability in the summer habitats. Projects of adder conservation often overlook the summer habitats, and the migration corridors towards them, even though they are of fundamental importance for the projects’ success.