11.   One positive sign is that a fledgling effort to restore natural ecosystems is beginning to pick up steam in several parts of the country.

12.   Park managers say that the critics expect a natural ecosystem to look the same as one managed to produce maximum grass for cattle.

13.   Since the warming would be unusually rapid, many natural ecosystems might be unable to adjust, and whole forest types could disappear.

14.   Some environmentalists believe such projects disturb the natural ecosystem.

15.   Some of these possible effects, according to the IPCC, are impacts on human health, natural ecosystems and food supply.

16.   The rays can cause skin cancer, cataracts and can damage immune systems and natural ecosystems.

17.   The three new studies ranged from experiments in ecosystems created by researchers to observations of natural ecosystems, from studies of grasslands to boreal forests.

18.   They increase property values, attract visitors who patronize local retailers and provide outdoors classrooms for lessons on natural ecosystems and national history.

19.   Through agriculture and development, people have cut natural ecosystems into disconnected fragments.

20.   To many scientists, that is a major argument for conserving what is left of natural ecosystems.