5. Oil spills are increasingly common. This is threatening marine life. (so)
-> Oil spills are increasingly common, so they pose a threat to marine life.
6. Because the old forests have been heavily logged, global warming is becoming
worse. (therefore)
-> Therefore, global warming is becoming worse due to the heavy logging of old forests.
7. In order to feed and accommodate the growing population, humans take lands
which are wild animals' natural habitat. (since)
-> Since humans need to feed and accommodate the growing population, they take lands that serve as the natural habitat for wild animals.