How does species counterpoint increase in difficulty across its five types?

Species counterpoint is a traditional pedagogical method for teaching counterpoint by requiring students to write an added voice against a preexisting melody, known as a cantus firmus. The difficulty increases across the five types by introducing progressively complex rhythmic and melodic requirements:

• First Species: The simplest form, where the added voice moves in note values identical to the cantus firmus (note-against-note).1
• Second Species: The added voice introduces two notes for every single note of the cantus firmus, increasing rhythmic activity.2
• Third and Fourth Species: These types further increase complexity by introducing more intricate rhythmic patterns and specific techniques for handling dissonance and melodic flow.34
• Fifth Species: Also known as florid counterpoint, this is the most advanced stage. It integrates the resources and techniques developed in the previous four species, allowing for a more fluid and varied melodic line.54

By moving through these stages, students learn to balance the "vertical" requirements of harmony with the "horizontal" independence of melodic lines.6