The first six did not create any other races at first. They were content to live within a world where only witches roamed, but curiosity and need eventually changed that. One by one each of the original six created their own race. How each of these races came to be were unique to the one creating them. For nature, it was needed that convinced him to create the fairies. He needed a race that understood its ebb and flows just as he did. He needed a race to be connected to it just as he was. So, they can cause plants to grow and die, and the seasons to change. Most of all he needed someone to defend his dominion against those that wish to destroy it.
Nature actually only created one fairy named Irial, but that was all that was needed. The lone fae wandered the forest for a while, but it wasn't long before he stumbled upon two groups of people fighting over which one of them should have the right to destroy the forest. This was not something that Irial would stand for, and he leaps into action. He commanded the forest to render their enemies still. It didn't take these two groups long to find themselves entangled within the plant life of the forest, and any effort to break free only tighten their bonds. A fact that pleased Irial to no end. He watched them struggle for a while before making his presence known.
Not knowing he was the one that ordered the forest to attack them, they pleaded with him to help them get down, and when they came to realize his role in it all they pleaded with him to let them go. They vowed to leave the forest and never return. These words went upon deaf ears. Irial wasn't interested in empty promises nor did he believe they should receive such a light punishment for their crimes. Their war might not have completely destroyed the forest, but it had damaged it greatly. They needed to pay.
Irial slit the men's throats, and from their blood other fairies sprung into being. There was another way fae gained life that way, and it was done by Irial planting his seeds inside the woman's womb. He willed those seeds to grow fast--much faster then their bodies could handle--and when they gave birth a few hours later each one of them died. The babies grew into an adult within the next hour. Irial realized that his work wasn't done. There were still those within the forest that wished it harm.
Wasting no time, Irial and the other fae spread out into the forest, hunting and killing those with ill will towards it. However, in order for those outside the forest to understand what fate awaits them if they enter the fairies dominion with ill intent a few were allowed to live. Some of them returned with armies---believing with bigger forces they could crush the fairies. Irial was more than happy to prove them wrong, and again when they foolish thought challenging him outside the trees would ensure their victory. The earth and its various life from--both plants and animals--obey his commands.