ecology 452 environment 155 environment protection 53 permanently sustainable development 35 environmental pollution 23 ecosystems 20 city planning 16 more ... recycling of wastes 15 agriculture 15 landscape 14 geography 13 environmental management 13 microbiology 13 wastes 13 water courses 12 development 11 savings 11 energy 10 forestry 10 landscape protection 10 environmental policy 10 biodiversity 9 landscape forming 9 teaching aids 9 production 9 architecture 8 construction 8 forests 8 unconventional energy sources 8 plants 8 less ...