COMPARISON OF THE PERFORMANCE OF SPECIES RESPONSE MODELS IN SEVERAL LANDSCAPE UNITS IN THE PROVINCE OF NOORD-HOLLAND, THE NETHERLANDS

In The Netherlands, empirical statistical response models are used in the Dutch nature conservation policy to predict vegetation response to management options. The development of empirical statistical models i s time consuming and costly, due to the need for databases that extens ively describe the biotic and environmental variation in the research area. For this reason it is important to optimize the sampling procedu re and thus reduce costs. Our research aimed at finding out to what ex tent species response models will improve when an environmental gradie nt is sampled outside the range optimal for a species, which may be es pecially relevant where species presence is related to geographical la ndscape units. We tested for 46 species and two landscape units in our research area, a dune and a polder area in the province of Noord-Holl and, if it would be worthwhile to extend sampling to the other landsca pe unit. Herefore we compared the predictive values of response models based on either a dune or polder subset with those of response models based on a combined dune/polder set. In general the predictive values of models based on the combined set were not significantly higher tha n those of models based on the subsets. Thus, where time and financial resources are limited, we recommend focusing sampling attention espec ially on the regions of interest. Extrapolation of subset response mod els to the other landscape unit was also tested. Considering the extre me poor predictions, we advise against the extrapolation of response m odels to regions with different environmental conditions. (C) 1998 Els evier Science B.V. All rights reserved.