During the western expansion, what animals, plants did the settlers encounter on the California trail?
Look at the map in this article:
http://en.wikipedia.org/wiki/California_Trail
The California trail covers half of our country! So read through several of these articles and list a few. There's no way you'll be able to list them all, but you should have many.
http://www.google.com/search?q=animals+confronted+on+california+trail&oq=animals+confronted+on+california+trail&aqs=chrome..69i57.7189j0j4&sourceid=chrome&espv=210&es_sm=119&ie=UTF-8