APA style for reference list

In the Publication Manual of the American Psychological Association (APA) 6th edition, it has been featured, especially, the Digital Object Identifiers (DOIs) about reference list.

The online environment has become popular to research recently, but the Uniform Resource Locators (URLs) are changeable. Therefore, the DOIs have being assigned to substitute for URLs.
                    NOTE: The URLs are still vital matters in the reference list.

Where can we found DOI?

• the first page on the electronic journals
• the database landing page
• in the webpages which are hidden behind buttons

What is the exact format of DOI?

          e.g.) Wittemyer, G., Elsen, P., Bean, W. T., Burton, A. C. O., & Brashares, J. S. (2008).  

                             Accelerated human population growth at protected area edges. Science, 321
                             (5885), 123-126. doi:10.1126/science.1158900