Daan van der Elsken is an ecologist and works for his own ecological consultancy firm: Elsken Ecologie. He advises and conducts research, particularly regarding the Nature Conservation Act. This law requires initiators of spatial interventions (building, demolition, post-insulation) to rule out negative effects on protected species. In built-up areas these include house sparrows, swifts and bats. They must not be killed, injured or their habitats destroyed. He conducts research into where these animals are located and thus prevents them from being negatively affected.