Populatieonderzoek boomkikker IJzerenbosch 1987-2016

Long-term monitoring of a population European Tree Frogs, 1987- 2016. An example of a successful natural colonization after management measures. For a long time the biggest population of Tree Frogs (Hyla arborea) in the Dutch Province Limburg occurred in a nature reserve located in the approximate centre of the Province. Several areas nearby had potential but where not (permanently) occupied by the species. One of the most promising of these areas is called “IJzerenbosch”, which is situated approximately four kilometres to the South from the core population. This area is mostly comprised of deciduous woods and pastures but lacked suitable breeding waters. From 1987 on, new ponds were created as potential breeding sites. Currently the total number of ponds is 32 with a maximum potential water area of 7000 m2. This area depends on pond and groundwater level maintenance. After the year 2000 another measure was the development of 800 metres of new terrestrial habitat, such as sun exposed shrubs and brambles. To monitor the development of the population and the colonization of the area researchers surveyed the area on the number of calling males. In the first years the population was relatively small but in recent years 150-200 males are present in the area. This case study is compared to two other studies from the Netherlands and Switzerland. These studies focused on the development of annual population growth rates by means of a model that takes in account calling males and environmental factors such as climate and groundwater levels. As in previously mentioned studies the population in the IJzerenbosh is regulated by density dependent- and climatic factors. However, the most important factor for explaining the variance is the total amount of sun exposed water area that is available two years before the chorus counts. The mean day temperature in the breeding season is also important. The impact of new terrestrial habitat in the last decade could not be modelled, because of its slow and irregular growing. Since the population is still increasing this might be an important extrinsic factor for its positive development. De verspreiding van de boomkikker (Hyla arborea) was in Limburg gedurende een langere periode beperkt tot Midden- Limburg, met als belangrijkste leefgebied de Doort. Door het uitvoeren van inrichtingsmaatregelen in enkele nabij- gelegen natuur- en cultuurgebieden heeft deze zeldzame kikkersoort zich echter sterk kunnen uitbreiden (Vergoossen & van Buggenum, 2009). Een van deze gebieden is het IJzerenbosch, waar de ontwikkeling van de populatie vanaf de vroegste kolonisatie nauwlettend is gevolgd. Dit heeft geresulteerd in een ononderbroken meetreeks van 30 jaar.