21. However, this is a natural system and temperatures will vary. 22. Past development hugely altered the natural systems of the region. 23. Culture was now just another natural system that could be explained in mathematical terms. 24. Alaska has strict laws governing the protection of natural systems such as salmon streams. 25. Dynegy got the Northern Natural system out of a broken merger agreement with Enron. 26. These changes could affect the availability of freshwater for natural systems and human uses. 27. For natural systems , those data are very difficult to collect. 28. The losers will be the natural systems and human environment of the Pacific Northwest. 29. Complex natural systems arise from millions upon millions of organisms living, dying, and evolving. 30. Some experts say natural systems are intrinsic to psychological well-being.