[Edit: curse you, Levnik. Stole it out from under me while I was typing away]

Lets see!
I dont recall any relevant notes from any champion profiles, comments, or JoJ articles, but we do have this line from the learning center art-- Nevermind.

So. Minions are indeed just Things. Relatively uncomplicated homunculii assembled from the extreme potency of a nexus simply to facilitate that sense of War that would otherwise be lacking. I say this because isnt the thought process strange? The summoners decided to settle political affairs via arena combat, but not Just arena combat. You remark about clearing wildlife, but what about everything else? This isnt Just champions in a gladiatorial Colosseum, fighting it out. The Summoners decided to take the chaotic business of War and simulate it. They decided it was fine to take these unique landscapes full of trees and hills and make them part of the process, just as they would be were opposing armies to do battle. They decided to replicate soldiers without needless loss of life. They even added a sort of arms race; either side fighting for the superior magical arsenal.

I do not know how the wildlife fits in. Perhaps once upon a time a tribe of lizard men Did live in those forests until an army crashed upon them, destroying their humble lizardy lives to take the Elder's mysterious blessing whilst the enemy General slayed the great Dragon, inspiring his men and wearing its hide into battle. In memory of these actions, we now simulate that as well. Then again, not every part of gameplay has a 1:1 translation to lore. Perhaps the way we, as players, see the field of justice isnt how they really are in the story? Maybe the fields are as large and natural as they appear in the season one trailer, and packs of monstrous wolves really do live in those woods.