Hi out there. I recently started becoming interested in the life of the octopus and find them fascinating. Something I am still wondering about is if they are a significant part of the food chain. Are they vital for our ecosystem?
Welcome to Tonmo! The more you learn about animals, the more you'll find that pretty much every animal is vital in it's own niche of the ecosystem. Octopus and squid fill one of the larger predatory roles in the ocean, and many species are important for the food intake of other animals, including humans!
I'm sure you will find out a lot about cephs here on Tonmo, it is a fabulous site.
The role of squids in the ecosystem is pretty well documented, since they're both prey and predators for commercially important fish, as well as a commercial fishery themselves, and a major food for the much-studied cetaceans. I haven't seen much about octos' place in the ecosystem, though...
The book Cephalopods: Ecology and Fisheries by Boyle and Roadhouse talks about the ecology in some detail, but really focuses on squids, I just flipped through and didn't see much on octopuses. The FAO guide may have some info, as well... there are two versions, the old-and-complete and new-but-unfinished: