That's not really true. In nature there are no filters...everything is a cycle.
Plants will naturally take in ammonia and nitrite. If there were no plants in there- that would be a different story.
All depends on what you consider a filter. Most, if not all bodies of water are feed by either streams or rivers, which are in a sense filters. When moving water passes over and through rocks, gravel, etc. it is in a sence being "filtered". Mother Nature is the best biological filter you could ever hope to use.