Koa'ki meiru Guardian Effect

Does the effect work on effects like Dandylion, Treeborn Frog, Plaguespreader etc? If so, or not, why?

It can negate any monster effect that starts a chain, so Dandylion, Treeborn and Plaguespreader can all be negated by it. The only thing that can't be negated, is summoning conditions like Cyber Dragon, Chaos Sorcerer (though you can negate the effect that removes a monster) and so on.

Thanks, i think that answers my question.