Blog split into topics?

I’d like to have one blog on one site with multiple pages where each page has posts about a different topic. The overall subject will be environmental issues, so using that as an example:

• Opening page contains general posts
• Page 2 contains posts about trees
• Page 3 contains posts about animals
• etc.

How do I do this? Do I need to set up separate blogs (databases?) for each of those pages? Can I somehow set up filters or categories for each page so one blog (database?) can be used for all pages?

Thanks for any pointers or links.
EdB