Ecology 394 Human ecology 304 Congresses 115 Plant ecology 96 Environmental aspects 74 Forest ecology 73 Environmental protection 52 Pollution 46 Conservation of natural resources 42 Marine ecology 42 Nature 41 Periodicals 41 Environmental policy 40 Effect of human beings on 37 Botany 32 Agricultural ecology 30 Animal ecology 29 Research 27 Bibliography 25 Forests and forestry 23 History 23 Study and teaching 23 City planning 21 Coastal ecology 21 Economic development 21 Mathematical models 21 Natural history 21 Natural resources 20 Microbial ecology 19 Urban ecology 19