1.   And after they let grassy areas build up for a year or two, they are setting fires to restore the natural ecosystem.

2.   As in the rice paddies, in natural ecosystems, when there is a greater diversity of plants, it is simply more difficult for the disease to spread.

3.   All of this underscores a pervasive uncertainty that has developed about how natural ecosystems actually behave in a changing climate.

4.   But a number of scientists question the assumption that alien species are never acceptable in a natural ecosystem.

5.   But some scientists question the assumption that alien species are never acceptable in a natural ecosystem.

6.   By relating the story of Los Angeles, Davis draws larger lessons about the relationship between urban growth and natural ecosystems.

7.   Climatic zones would shift away from the Equator, upsetting natural ecosystems.

8.   In complex natural ecosystems, however, researchers say such straightforward identification of crucial species is impossible.

9.   It is especially difficult to measure the value of natural ecosystems.

10.   Natural ecosystems, being untended, would be even more vulnerable than cropland.