The higher biodiversity in an ecosystem means that there is a greater variety of genes and species in that ecosystem. A great variety of genes and species means that the ecosystem is better able to carry out natural processes in the face of external stress. Thus, the ecosystem is more sustainable.
The ability of an animal to maintain a stable internal environment is called homeostasis.
9.organ system, 10. organ level I guess.
Because science is always changing and every time you cam come up with a new outcome.