Trivia: The creature that hunts down the Doctor before being shot down by the Sniper girl is actually the original design of the Predator that was scrapped during Predator 1 due to issues with the costume and the jungle. The original Predator was supposed to be very slender with backwards knees and long arms, but because of the jungle setting, it was scrapped because the director was unable to make the costume work properly.
Trivia: The famous line from the original Predator "You're one ugly motherf**ker" is included in Predators, but most people have missed it unless they speak Russian. Nikolai says the line before blowing himself up.
Trivia: At one point, the actor who played Nikolai whacked himself in the face during a scene and started bleeding. He ignored it, and apparently the blood added to the effect of the scene.
Trivia: Topher Grace was the only cast member (apart from the Predators of course) who did at least some of his own stunts, such as the scene where the group falls down a waterfall.
Trivia: Hanzo has only three lines of dialogue in the entire movie.
Trivia: When the Yakuza guy faces with the Predator, takes his shirt off and cuts himself across the chest, it's almost exactly the same as in the original Predator movie when the Indian guy faces down the Predator, takes his shirt off and cuts himself across the chest. And on both occasions, they die, although Yakuza guy manages to kill the Predator too.
Chosen answer: Indeed, it's an interesting question, and not one to which any answer is given. In the absence of any other information, your speculation that the predators have taken various plant species from other worlds to study and just make their hunting environment more interesting is as good an explanation as any other.
Tailkinker ★