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Please help me define the boundaries of community creation
I would ask, how do you want people to use Mander? Do you want it to be the singular home for your members and host to all of their varied and disparate interests (bringing in subscribers from a thousand other instances who are not remotely interested in nature or science), OR a destination repository of nature and science communities (topic-centered)?
I wouldn't worry about people needing to hop between instances, unless you'd prefer to eventually become a generalized server like beehaw. If you look at the issues they are running into right now, even though their communities are admin-created, catch-all is a model I wouldn't want to touch with a ten foot pole as an admin. Some of the instances they've had to block are just... shocking.
My own use case is I had already joined a handful of different instances, and set them each up to give myself some semblance of what multireddits used to do for me.
This is my brain-food account. From here I subscribe to communities about nature (my heart), science, and more broadly, those focused on curiosity, learning, research, and with more academically-oriented (or at least thoughtful) discussion. So my own extension of your theme has some pretty soft edges.
In terms of how to draw your boundaries, I honestly thought what you have in the sidebar should have been enough, it's short and clear and I wouldn't have thought to add anything... but then I saw communities for games/deals, a horror fiction author fan group, and a re-creation of the 'boring dystopia' doomscroll subreddit lol
Rather than getting into the absolute headache of putting hard boundaries around communities, another idea might be to just add a question/item to your registration process, reiterating that the focus of Mander is nature and science in general, and asking registrants to explicitly agree to consider this before creating a new community in this space?
This would leave the theme open to interpretation, while still respecting it. Like, the science of cooking is a super cool topic I would never have considered. Someone else mentioned a beekeeping community. Those are awesome and absolutely belong here. I think keeping the boundaries soft is a good idea, and I think "nature and science" should be perfectly fine, but a confirmation filter of some kind (either in registration or in the create community form, or both) could be helpful.