Hey I just read your post about jumping spiders and it was very informative! I have been thinking about getting a pet invertebrate, but I want to do so in an ethical manner for both the ecosystem and the bug. Maybe this is way too much overthinking, but is picking up a wild caught bug and putting it in an enclosed space harmful to the critter? I think I would make a very nice terrarium, it seems like less of a commitment and more in tune with my local ecosystem, but still, it would be less free space than they’d get in the wild. That is also my concern with purchasing wild caught insects/invertebrates from sellers (like you mention in the post) and I’m not sure if purchasing captive bred is better. I worry about disrupting ecosystems by demand for pets, too. Bit lost!
Hey! Wild catching bugs can be a bit of a debate, I mainly keep US native scorpions which are not commonly bred in captivity. I'm trying to actively change that by starting captive breeding projects myself, but this requires buying wild caught bugs initially to have captive bred generations later. For the most part, inverts breed relatively quickly, and from an environmental impact standpoint catching a few bugs from your backyard is not going to noticeably impact the environment at all. If you're buying wild caught bugs though, there's a few things to keep in mind. There are some bugs that are very rarely bred in captivity and in high demand, these bugs are the most impacted by wild catching. Blue death feigning beetles are a huge issue, though they can be captive bred it's very difficult to do, so essentially every blue death feigning beetle for sale is wild caught. Because the demand is so high, this can lead to hundreds being caught at a time.
It can be difficult to talk about impact, because individual species like this are not commonly tracked. I definitely believe that Phidippus regius, blue death feigning beetles, sonoran desert millipedes, and desert hairy scorpions are negatively impacted by over collecting, but as of right now there are no population tracking or studies that prove it one way or the other. All these animals have demands that far outweigh captive breeding though. If you aren't planning on breeding yourself I would definitely recommend sticking to captive bred. That being said, some animals like isopods are mostly not native to the united states and breed readily in captivity, so there's basically no negative impact from collecting them from outside.
As far as from a welfare standpoint, depends on what you provide for care and what animal you're talking about. For most common pet bugs, isopods, millipedes, ect, though a terrarium is obviously less space than the wild, these animals aren't really seeking out large amounts of space. Isopods and millipedes prefer to be in moist environments filled with decaying leaves and wood, which is why they're commonly found under logs or in leaf piles. As long as they have those needs provided for, they aren't traveling very far or patrolling large tracks of land. Obviously more active animals are going to need more space, jumping spiders can be very space intensive and need fairly big enclosures for their size because they're so active, but this is still easy to provide in captivity. For other types of spiders, such as widows, cellar spiders, steatoda, web builders, they will rarely if ever leave their webs at all. Providing a suitable space to web provides them as much space as they would patrol in the wild, similar things can be said about animals that spend most of their time in a burrow or crevice. I don't believe in the logic that animals should only be given a burrow, or any "the enclosure is the burrow" type reasoning, an animal should still be provided with enough space to explore if they want to, but again this is still easy to provide in captivity. If you are properly taking care of the animals you have theres nothing immoral or unethical about keeping an animal in captivity. Hope this answered your question!
Edit for clarity: Even if you dont intend on breeding being conservative with your collecting isn't going to hurt the local bug population, especially since most things you're likely to encounter easily are going to be nonnative anyway (but always get a positive ID!). I would absolutely only buy CB for animals that are in high demand and/or breed slowly because those are going to be the most impacted by over collecting. Also there are some bugs I don't think are suitable for captivity at all, namely solifugae. They have not been kept reliably in captivity long term, there seems to be something we're missing with their care, so unless you are a very dedicated solifugae keeper that is going to intentionally study them to figure out how to keep them successfully I don't think you should have a solifugae at all.