Well, the nature will say, "Adds * to Atk, Subtracts * from SpD". Therefore, I interpret that as an addition to the main total of stars. Deck Knight said that if the stars go below 1 then you receive (-1) instead of (-), which means you take 1 more damage and if the stars go above 5 then you receive (+1) instead of (-). So basically as long as it stays within 1-5 stars, the nature ADDs a star to your stats.