Been enjoying your musings on fanfic ecosystems and thought I'd just share some of my own small observations.
Fandom "migrations" are definitely a thing, where artists and writers move from fandom to fandom based on what's new and exciting like a herd searching for new growth to feed on. There are some fandoms I've been in where you can definitely see something like a population cycle, with a baseline of more permanent fans and more "seasonal" fans that wax and wane.
I'm not really into the MCU movies at all but it's interesting to hear it described as a post-apocylptic wreckage when I think it's doing okay for itself despite the mass extinction. But that's just me whining because smaller fandoms can be desolate and I'd LOVE for some of them to have as much fandom activity as MCU. I think of Harry Potter and Game of Thrones as being similar-ish ecosystems, in that they're big budget mainstream productions that gained enough critical mass to somewhat sustain themselves but nonetheless suffered mass die-off when their main series ended, and in response the IP holders attempt to prop up interest with new spin-offs.
Also, I think the type of content or "canon" has an effect on the type of fandom that grows and what kind of population cycles occur (if any). Some creators have a pretty contentious relationship with fans of their stuff while others I think, deliberately construct their stories in a way to encourage the flourishing of a fandom ecosystem. And a single one-and-done story is going to have a different ecosystem than an ongoing franchise.
I can't remember if you posted it on your blog at some point but there was a chart going around tumblr hypothesizing the sweet spot for a massive fandom to bloom. It has to be just good enough to be compelling, but not so good that all the emotional loose ends are tied up. There needs to be SPACE for that new growth. Related, I remember people observing that fluffy lighthearted source material tends to inspire angst, and dark stories end up with a disproportionate amount of fluff and fix-it fics.
I love all these thoughts. Fandom ecology!
There's a decent amount of activity in MCU fandom comparatively, but I would estimate it's collapsed by over 90% since the days of Avengers Endgame... well within the threshold to qualify for a mass extinction!
But there was so much content and so many fans, the primary productivity of the fandom ecosystem was incredibly high. think like a tropical ecosystem with high precipitation sustaining lots of plant growth and lots of energy to go around on all trophic levels...
Other fandom ecosystems might be like deep underground caves, or high alpine slopes, or deserts that receive moisture only through fog... processes are happening slower, nutrients and essentials of life are more limited
It's an interesting thought... what represents the primary energy source (in our world, sunlight) in this analogy? is it the media itself the fandom is based off of?
No...I think the sunlight must be the love and passion of the fans. The media itself might be the primary producers or maybe the rocks and water of the land...