So this is a cool thing to notice as you go about your life: there are no unexploited corners in a balanced ecosystem (reference: Sylvia Earle); take note of what in your locale consumes what. If there is energy to consume, something has evolved to consume that. Ants are amazing at making use of small caloric particles that other organisms will ignore because the bits are too small.
Plastic is an interesting case. Plastic is made of long chains of lipids (an energy storage molecule), polymerized into somewhat durable longer molecules. When some critter learns to crack those molecules, things could get very interesting for humans if we're still around.