you could pull ecology from both the Bio book (majors level) and Concepts (non-majors)
These are part of the OpenStax platform so it should be easy to build in with what you find above
This is actually for K-12 but the HS content is very good. I use this in my GE classes and the students really like them. In addition to the textbook there are activities, videos, etc.
This focuses more on society rather than science, but there may be some interesting things here.
The following are free but not Open (meaning you have to use their site)
I don’t know much about this site, but it seems to have some good content. I can find out more if you like what’s here
I’m not sure if this is adaptable or not (it might be)
This is written by students; but it might have some interesting ideas
these are mostly highly specialized but there are some great reads here! They all have free PDFs online (but they have to be used as is).
these may be too complicated for students; but fun summer reads for you :)