[@ERode] Oooh interesting points!

Originally, there were more than seven types of dragons controlled by their Listeners, but during the course of the Great Wars many of them died out or went into hiding. Each faction in the Great War had an assortment of dragons, but their leaders determined which race was dominant among their ranks. There are still an assortment of dragons that live in each country, but they're extremely domestic and not in touch with their abilities anymore (including that which makes them able to have Listeners; they're more like pets). The environment of each country also plays a role in which dragon race makes it their home. 

Dwarf dragons are an interesting case. Dwarf dragons have never been part of the other factions in the Great War. They were always a fiercely kept secret by the Woodwyn people. As you may recall, Woodwyn is pretty much kept apart from others, and the location of their villages and dragons is kept secret. This is how they've protected the Dwarves.

Humans [i]can[/i] kill dragons, but it's extremely difficult (depending on the dragon). For physically powerful ones, a special military force is required. They might even need the help of other dragons. For less physically powerful ones, the problem is finding them and keeping them in one place long enough to strike a deadly blow.