I am hoping to find a Phd. I have found one which is really interesting yet I am wondering if your post doc would need to be in the same area? The project has themes that are interchangeable between species yet I am a little unsure whether a phd in vertebrate field would impact on future invertebrate research. I am also unsure whether not to apply and try to seek voluntary work and wait until more phd's are advertised. Has anyone been in the same state of confusion over what to do? Any advice welcome.

If the field is somehow relevant, then you can do a PhD and then follow a post-doc in a different field. E.g., my PhD is in law and human rights but I am discussing a post-doc in journalism.

Yes you can. My husband's PhD was in theoretical computer science. He got a post-doc in space technology research, and now works in that as a research fellow.