After his ministerial period, he became chairman of the Vereniging Natuurmonumenten.
In 2010 the organization had 768,000 members and it is Headquartered in 's-Graveland.
The park is owned and managed collectively by Vereniging Natuurmonumenten (Natural Monuments Association), Staatsbosbeheer (State Forestry), NV PWN Waterleidingbedrijf Noord-Holland (Water Supply Company North Holland), IVN, Hoogheemraadschap van Rijnland (Water Board Rijnland), and the surrounding municipalities and private owners.