This is for a novel I'm writing. The basic premise is that massive networks of mycelium running under the soil combine to form a decentralised intelligence that has been managing life and conditions on the planet for hundreds of millions of years. Think Gaia theory, but way weirder. Humans occasionally making contact with the mycelial intelligence explains much religious and supernatural experience in my world.
The big conflict/mystery of my novel is that the mycellium have decided humanity have outlived their usefulness and engineer a population cull. But what I need to explore further is: how if at all might what in essence amounts to entire ecosystems acting with self awareness impact humanity?
Edited to add: Thank you for all the responses. Its given me a lot of food for thought.
My thinking is the mycelia is "magically" intelligent as a result of its current biology. We already know that mycelia will link up, share resources and communicate, so its not too much of a stretch.
MY thinking is that the mycelia doesn't have much ability beyond what actual mycelia has. They mostly manage the ecosystem by nurturing certain plants, or feeding them particular chemicals in order to achieve specific changes.
In terms of knowledge and psychology, i'd see them as having extensive ecological knowledge, but a very poor understanding of anything that happens at human timescale - not none, but little. We're talking immensely long-lived intelligence. They're quite happy to invest a few decades into a plan, since thats a trivial length of time for them.
As for methods, I see them going for something subtle and targetted. They don't need to wipe out humanity completely - a 90% population drop would be more than sufficient. And I imagine their preferred method would be to have us do it to ourselves somehow, preferably in a way that would keep collateral damage to other species or wider ecosystems to a minimum. My thinking is drugs that undermine critical thinking, and promote the kind of psychology that would breed violent ideologies or something like that.