Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
Basis for division of periods are the same as Fig 10. Content in gray squares indicate the main human activities of different stages. Content in green squares show ecological phase of different stages.
