Complete the crossword, then click on "Check" to check your answer. If you are stuck, you can click on "Hint" to get a free letter. Click on a number in the grid to see the clue or clues for that number.

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Across: 1. An organism that breaks down dead tissues into simple chemical components, thereby returning nutrients to the physical environment. 2. (____ cycle) A type of nutrient cycle in which the nutrient does not enter the atmosphere easily. 3. A process in which species in a community are replaced over time. For a given location, the order in which species are replaced over time is fairly predictable. 7. (food ____) A summary of the movement of energy through a community. A food ____ is formed by connecting all of the food chains in the community to one another. 8. (____ cycle) A type of nutrient cycle in which the nutrient enters the air easily. 10. (____ succession) Ecological succession that occurs as communities recover from disturbance, as when a forest grows back when a field ceases to be used for agriculture. 12. An association of populations of different species that live in the same area. 14. (two words, no space) Rainfall with a low pH. 15. A community of organisms, together with the physical environment in which the organisms live. Global patterns of air and water circulation link all the world's organisms into one giant ____, the biosphere. 16. (food ____) A single sequence of feeding relationships describing who eats whom in a community. 17. An organism that uses energy from an external source such as the sun to produce its own food without having to eat other organisms or their remains. 18. (____ community) A community, typical of a given climate and soil type, whose species are not replaced by other species. A ____ community is the end point of succession for a particular location.

Down: 1. An event, such as a fire or windstorm, that kills or damages some organisms in a community, thereby creating an opportunity for other organisms to become established. 4. (____ species) A species that, relative to its own abundance, has a large effect on the presence and abundance of other species in a community. 5. (____ consumer) An organism that eats a producer. 6. An organism that obtains its energy by eating other organisms or their remains. Includes herbivores, carnivores, and decomposers. 9. (____ species) A species that does not naturally live in an area but has been brought there either accidentally or on purpose by humans. Also known as nonnative species. 11. In an ecosystem context, an essential element required by a producer. Such essential elements include carbon, hydrogen, oxygen, nitrogen, phosphorus, and many others. 13. (____ level) A level or step in a food chain. ____ levels begin with producers and end with predators such as lions that eat other organisms but are not fed upon by other predators.