Insect and plant diseases were determined to be the prime causes of problems in forests. Extinction is a natural process. The U.S. and the rest of the world have virtually infinite capacity to deal with garbage. There's ample space for safe landfills.
Incinerators can handle the rest.
The entire population of America lives on just over three percent of the nation's land mass. Most of the population lives within fifty miles of either the East or West coast. Elsewhere, while the earth's population is currently projected to be six billion, there is ample room for humanity. People tend to crowd into urban centers, seeking employment, creating a multitude of health and social problems. As more nations begin to enjoy the prosperity and living standards of the US and European nations, their populations will decrease.