Hi @dlevitis, I’m actually going to be working on some analysis related to this question in the coming weeks. I’m trying to put together a short talk for the Ecological Society of America meeting this summer on the topic. There are a lot of great answers here about various methods for dealing with rarefaction curves, Chao estimator etc.
In general, I don’t think the unstructured nature of iNat data presents a big problem. The alternative, structured surveys, e.g. bird point counts or vegetation transects, will greatly underestimate the richness of a larger park or region. So if you want to estimate what’s in your park, it might be better to have observers search non-randomly (as in iNat) for rare species to get a more complete accounting of biodiversity in the region.
This does become a problem if you want to compare parks or compare time periods. Then you need a way to account for any differences in the number of individuals sampled, the area and the sampling effort. The most helpful paper I’ve read on this is by Nick Gotelli (also his book is linked to in @ahospers comments). This link should give you access, let me know if it doesn’t work: https://onlinelibrary.wiley.com/share/SQYPFNNEKK4PRERWXTKS?target=10.1046/j.1461-0248.2001.00230.x
You can find another interesting paper comparing species area curves to species time curves here: https://onlinelibrary.wiley.com/share/XYYHKHUU9AZQYEU3XUIC?target=10.1046/j.1461-0248.2003.00497.x
Great to see this discussion and all the good ideas, I will follow up on this if I come up with anything interesting.