So a little bit of background. I am writing a fantasy novel (non-traditional setting), though the ecology is similar to our world so I'm simplifying to just discussing our world for ease of answering.
I've drawn some inspiration from Maoist China, particularly the Four Pests Campaign which practically wiped out the Eurasian tree sparrow and that led to drastic and devastating ecological effects.
Without going into the nuances of the story, I want to know other creatures (non-bird) which are important parts of a given ecosystem, that when pushed/wiped out could cause similar effects.
The biome in my story is more or less a temperate coniferous/mixed leaf forest along a mountainous region. But, feel free to provide examples of creatures that don't exist in this biome, as it might still provide some ideas.
It may also be related (either directly or indirectly) to the introduction of too many/an invasive top tier predator[s], but that's not necessitated.
To be clear, I'm asking ideas of real-world examples, but will probably use the information I receive to adapt it to a new fantasy creature.
This is my first post. Let me know if it needs clarification. Thanks!