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Across 2. Act of one organism feeding on another organism. 4. Physical area in which an organism lives. 9. Symbiotic relationship in which both organisms benefit. 11. Close mutualistic, parasitic, or commensal association between two or more species that live together. 13. Each step in a food chain or food web. 15. Simplified model that shows a single path for energy flow through an ecosystem. 20. Heterotroph that preys on other heterotrophs. 23. Exchange of matter through the biosphere involving living organisms, chemical processes, and geological processes. 25. Process in which nitrogen gas is captured and converted into a form plants can use. 27. Symbiotic relationship in which one organism benefits at the expense of another organism. 28. Heterotroph that consumes both plants and animals. 29. Chemical substance that living organisms obtain from the environment to carry out life processes and sustain life. 30. Organism that cannot make its own food and gets its nutrients and energy requirements by feeding on other organisms.

Down 1. Symbiotic relationship in which one organism benefits and the other organism is neither helped nor harmed. 3. Total mass of living matter at each trophic level. 5. Heterotroph that eats only plants. 6. Process in which fixed nitrogen compounds are converted back into nitrogen gas and returned to the atmosphere. 7. Group of organisms of the same species that occupy the same geographic place at the same time. 8. Relatively thin layer of Earth and its atmosphere that supports life. 10. Any living factor in an organism's environment. 12. All the interacting populations of different species that live in the same geographic location at the same time. 14. Role, or position, of an organism in its environment. 16. Any nonliving factor in an organism's environment, such as soil, water temperature, and light availablity. 17. Anything that takes up space and has mass. 18. Large group of ecosystems that share the same climate and have similar types of communities. 19. Model that shows many interconnected food chains and pathways in which energy and matter flow through an ecosystem. 21. Biological community and all the nonliving factors that affect it. 22. Heterotroph that decomposes organic material and returns the nutrients to soil, air, and water, making the nutrients available to other organisms. 24. Organism that captures energy from sunlight or inorganic substances to produce its own food; provides the foundation of the food supply for other organisms; also called a producer. 26. Scientific study of all the interrelationships between organisms and their environment.