| Abyssal (4000 - 6000m) [Bathymetric zone] |
| Acores (=Azores; Portugal) [Geographic region] |
| Afghanistan [Geographic region] |
| Africa [Geographic region] |
| Alabama [Geographic region] |
| Alaska (United States) [Geographic region] |
| Alaskan North Slope Coastal Tundra [Global 200 ecoregion] |
| Albania [Geographic region] |
| Alberta (Canada) [Geographic region] |
| Albertine Rift Montane Forests [Global 200 ecoregion] |
| Albian (112 - 99.6 Mya) [Geologic time] |
| Algal-feeder (algae) [Nutritional category] |
| Algeria [Geographic region] |
| Alsace (France) [Geographic region] |
| Altai-Sayan Montane Forests [Global 200 ecoregion] |
| Alto Paraguay (Paraguay) [Geographic region] |
| Ambush predation [Feeding strategy] |
| American Samoa (United States) [Geographic region] |
| Americas [Geographic region] |
| Ancash (Peru) [Geographic region] |
| Andaman and Nicobar Islands (India) [Geographic region] |
| Angola [Geographic region] |
| Anisian (245 - 241 Mya) [Geologic time] |
| Annamite Range Moist Forests [Global 200 ecoregion] |
| Antarctic Region [Geographic region] |
| Antarctica [Geographic region] |
| Antioquia (Colombia) [Geographic region] |
| Antioxidant [Functional class] |
| Appalachian and Mixed Mesophytic Forests [Global 200 ecoregion] |
| Aquatic (water) [Lifestyle] |
| Arabian Highland Woodlands and Shrublands [Global 200 ecoregion] |
| AraucanÃa (Chile) [Geographic region] |
| Arboreal (trees) [Lifestyle] |
| Archipel des Marquises [Geographic region] |
| Archipel des Tuamotu [Geographic region] |
| Arctic Region [Geographic region] |
| Argentina [Geographic region] |
| Arizona [Geographic region] |
| Arkansas [Geographic region] |
| Arthropod-feeder (arthropods) [Nutritional category] |
| Ascension (administrative area) [Geographic region] |
| Asia [Geographic region] |
| Astrakhan' (Russia) [Geographic region] |
| Atacama-Sechura Deserts [Global 200 ecoregion] |
| Atlantic Dry Forests [Global 200 ecoregion] |
| Atlantic Forests [Global 200 ecoregion] |
| Atlantic Ocean [Geographic region] |
| Australasia [Geographic region] |
| Australia [Geographic region] |
| Australian Capital Territory (Australia) [Geographic region] |
| Australian Region [Geographic region] |
| Austria [Geographic region] |
| Autotrophic (CO2) [Nutritional category] |
| Azerbaijan [Geographic region] |