Environmental protection Environmental policy 80 Conservation of natural resources 49 Environmental aspects 40 Pollution 34 Congresses 33 Periodicals 26 Human ecology 25 Ecology 24 Power resources 18 Citizen participation 16 Economic aspects 12 Environmental engineering 12 Natural resources 12 Bibliography 11 Nature conservation 10 Planning 10 International cooperation 9 Management 9 Directories 8 Research 8 Economic development 7 Environmental law 7 Land use 7 Case studies 6 Conservation and restoration 6 Environmental impact analysis 6 Environmental sciences 6 Mathematical models 6 National parks and reserves 6