Kolonisatietrends bij watersalamanders modelmatig berekend

Calculating colonisation trends of newts by modelling. By digging new ponds in the woods and meadows of a rural landscape in the Dutch province of Limburg, the number of potential breeding sites for amphibians increased from 47 to 131 in the years 1987- 2009. During this period, to see if newts were present, the old and new sites were visited irregularly and more or less randomly. Each year, 25% (range 4 – 54%) of the sites were visited and three species detected: Triturus cristatus, Mesotriton alpestris and Lissotriton vulgaris. Extrapolation of the visited sites to all sites leads to a strongly fluctuating occupancy pattern for all species. Once a species is seen at a site, the occupancy at all sites shows that T. cristatus and M. alpestris increase only slowly and slightly, which is in contrast with L. vulgaris. The data were analysed using “site occupancy”-models and Akaike’s Information Criterion using the software programme PRESENCE. The models calculate site occupancy, colonisation, extinction and detection probability. No clear occupancy trend could be proved for T. cristatus. The best model of M. alpestris indicates a slight increase in overall occupancy. L. vulgaris appears to be a very good and fast coloniser of new ponds. Modelling site occupancy using randomly collected data in a long-term series seems to be a suitable method for detecting trends.