There is no reason to believe that the natural world is divided up into separate pieces. As far as we can tell, everything and every non-thing in the universe is interconnected (why else would we call it the one-verse). It is, however, sometimes useful to use our imaginations to cut the universe into pieces. This is called analysis and humans do it all of the time. The universe seems to cleave at some points guiding us in our hacking it into pieces but there are always some sticking points whenever we cut the cake (e.g., separating human reproductive and urinary organ systems). Analysis is fine as long as we do it in the context of a wholistic perspective and remember that our hack jobs are always somewhat arbitrary and can lead us to lose track of the forest while attending to the trees.