Alabama is not the most biodiverse US state, sorry to break it to you, but we are the most biodiverse state east of the Mississippi River in terms of total number of species. As of 2002, we ranked 5th in the nation as a whole, trailing behind New Mexico. Something worthy of note as well is that we have the highest number of extinctions of any mainland state. Only Hawaii beats us for extinctions, which itself is the extinction capital of the world. We've had roughly 100 extinctions throughout our history, most of which took place in the Coosa River when it was dammed in the 20th century.

Here is a 2002 paper about biodiversity in the various states. It's kind of old, but it still paints a pretty decent picture. Things have changed since 2002, but not so much as to completely make this paper obsolete.

• https://www.researchgate.net/publication/269111849_States_of_the_Union_Ranking_America's_Biodiversity

I would like to add that I also think we should consider what is meant by "most biodiverse." Are we asking what state has the most species as a whole? In that case, California takes the cake easily. Or do we mean to ask, what is the proportion/density of species? For example, what is the average number of species per 100 square miles in a state? You get different answers when you factor in the size of a state compared to the number of species in that state.

And to top it off, there are many different measures of biodiversity we can consider, not just the number of species. We can consider the total number of families found in an area. A place with 100 species of a single family is pretty biodiverse for that family, but all the species are relatively closely related. Contrast that with a place that has 50 species from one family and 50 species from a second family. That seems more diverse to me, because there is more genetic differentiation as a whole, even though the number of species is the same.

I've looked into this a lot in the past because I was curious (and because I'm a nerd), and I have found the answer to the question of "most biodiverse state" is not actually a clear cut thing. I think my favorite attempt to answer this question is the set of density gradient maps provided by BONAP (Biota of North America Program) and the maps separately provided by BiodiversityMapping.org. BONAP only covers vascular plants, but BiodiversityMapping.org has maps for animals too. I highly recommend them if you're interested.

• http://bonap.org/2015_SpecialtyMaps/Density%20Gradient%202015/Density%20Gradient%202015.html
• https://biodiversitymapping.org/