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Across 1. species that have rapidly decreasing numbers of individuals 6. type of urban air pollution resulting from a combination of chemical pollutants, particulate matter, and sulfur dioxide 8. types of wastes that are not easily broken down and can exist in the enrionment for many years 16. factors that limit population density 19. keeping an organism or an area from harm or destruction by the establishment of parks, wildlife habitats, and other refuges 20. proportions of any given population that are either in their pre-reproductive, reproductive, or post-reproductive years 22. rain or snow more acidic than unpolluted rainwater 23. fresh water from rain and surface streams that accumulates in underground reservoirs 24. coal, oil, and natural gas formed from the buried remains of organisms 25. occurs when the last members of a species die 26. contamination of air, water, or land by wastes produced in such excess that they cannot be recycled by natural processes

Down 2. most often weather-related occurrences that affect populations regardless of their density 3. species with numbers of individuals so low that it is in danger of extinction 4. study of populations that includes birth rates and death rates, fertility rates, age structure, and geographic distribution 5. resources replaced or recycled by natural processes 7. protective layer at the top of the stratosphere; it absorbs most of the sun's harmful radiation 9. resources available in limited amounts that cannot be replaced and cannot be recycled quickly by natural means 10. explosive population growth in which the number of reproducing individuls increases by an ever-increasing rate 11. a natural phenomenon by which carbon dioxide and other atmospheric gases prevent heat from escaping into space 12. maximum, stable population size an environment can support over time 13. renewable or nonrenewable parts of the natural environment that are used by humans 14. planned management of wildlife habitats and other natural areas to prevent exploitation or destruction 15. movement of individuals into a population 17. solid particles of soot contained in smoke released by burning fossil fuels 18. solid wastes that can be broken down by natural processes 21. the movement of individuals out of a population