Page 69 - A bird’s-eye view of recreation - Rogier Pouwels

Relating local impacts to regional conservation targets
2002). Variables were selected with forward and backward stepwise variable selection with the R-function ‘step’ (Venables and Ripley, 2002). Given the limited amount of count data as compared to the available co-variables the number of interactions was held limited in order to prevent over-fitting. The habitat model thus accounts for differences in the number of breeding birds resulting from differences in habitat variables, traffic disturbance, year of the census, mapping method and area and is treated as a base model. For each species the best base model was selected by means of stepwise model selection based on the lowest AIC (Venables and Ripley, 2002). To this base model the four variables that represent the disturbance effects (the output of the MASOOR-SCAN model) were added separately. For each species the best model was selected by the lowest AIC (Burnham and Anderson, 2002).
4.3.3 Assessing local impacts with regards to regional conservation targets
The statistical models were used to make a prediction for the total population size
within the Veluwe area. For this aim we made predictions on a 1 × 1 km grid. We
determined all the relevant explanatory variables in these 1 × 1 km squares and, 4 using the coefficients of the best statistical models, made predictions for the expected
breeding bird numbers in each cell. We did this for two scenarios: one with the current
recreation pressure and one without recreation. The difference between the sums of
the two estimates gives an estimate of the impact of disturbance by visitors on the
population size. The difference between the population size and population targets
for the Veluwe was used to determine whether the impact of recreation disturbance
would result in a population size falling below the conservation target.
4.4 Results
4.4.1 Visitor densities
The area of the Veluwe accommodates over 200 parking areas. This includes almost 50 small car parks that were not on the GIS-maps, but were encountered during the field trip. A large proportion of the parking areas with a large capacity are located near sand dunes and heathlands. We estimated that over 8.5 Million visitors hike and walk in the Veluwe each year. The models predicted large differences in recreational use in the area. The southeast part of the Veluwe near Arnhem in particular is intensively used with up to 200,000 visitor groups per year per ha, while some central parts are almost free of visitors (Appendix 6).
67