I follow an interesting pro-Bayesian blog called "Overcoming Bias," which just recently had a post about reductionism. The post refers to the alleged fact that "there is only the most basic level - the elementary particles and fundamental forces." Now, I'm a great fan of reductionism (it has been some months since my dissertation was completed and accepted, but I haven't changed my mind about the central views I defend there yet). But the idea of a fundamental level is not necessary (or, I think, desirable) for a reductionist view. All of the benefits of reductionism come from accepting that there is only one world. The point of reductionism is to establish the unity of science, as some past advocates of reductionism have put it. But unity only requires that everything be linked to everything else, it doesn't require that there's some privileged foundation that all the connections flow through.
Insisting on a fundamental level saddles reductionism with unprovable metaphysics (as I ask in the comments thread, how do we know there is a basic level?) and also encourages misleading implications. The word "fundamental" sometimes means "most important," and those who make a big deal about the bottom level not infrequently seem to be misled into thinking that the bottom level (whatever they think it is) is fundamental in this sense as well, and not merely in the sense of just being the bottom.