Some of the terms associated with the given clues:
1. Mexico
2. Great Lakes
3. Rio Grande
4. Cascade Range
5. Neartic
6. Isthmus of Panama
7. Guatemala
8. Lake Erie
9. narrow land curve connecting North and South America (Isthmus of Panama)
10. endemic species in Sierra Gorda
11. Central American country parallel to Cocos ocean plate (Guatemala)
12. nearly 20% of world's fresh surface water (Great Lakes)
13. natural border between U.S. and Mexico (Rio Grande)
14. ecoregion of North America and Canada (Neartic)
15. location of Mount St. Helens (Cascade Range)
16. one of 17 recognized mega-diverse countries (Mexico)
17. natural border between U.S. and Canada (Great Lakes)
18. endangered species in Sierra Gorda (endemic species)