Not sure if this is the right place to put this, but it’s going to be a pretty big undertaking so here it goes.
So a paper came out a few years ago on how Opuntia humifusa in the US is actually a species complex. This is a big problem since on iNat, almost every prickly pear east of the Great Plains has been identified as O. humifusa (which is now only considered to be distributed from northern Virginia to the coast of New Hampshire, with a relict population in central Mississippi). Luckily enough, the majority of O. humifusa observations seem to be within this area, so we don’t have to change every single thing. However, there are still a ton of observations outside this area that need to be changed. I don’t know much about cacti, so here’s a key just based on distribution and a bit of my experience; if someone has better knowledge, feel free to add on!
- If the Opuntia is in the Mid-Atlantic (generally) or Northeast, it’s probably an O. humifusa.
- If the Opuntia is found along the Atlantic coast and the Gulf (where it goes much more inland) and it’s rather slender and relatively low to the ground, it’s probably O. drummondii. This appearance should set O. drummondii apart from all other Opuntia found in the same states.
- If the Opuntia is in NC, SC, GA, and (generally) AL and MS and (sometimes) FL and VA and it doesn’t look like O. drummondii, it’s probably O. mesacantha. However, it seems to be sympatric with the isolated O. humifusa population in central Mississippi, O. cespitosa in northern AL and MS, and with the southern extension of the main O. humifusa population in central VA. The biggest problem is in FL, where it’s completely sympatric with O. austrina. If anyone knows more about cacti, it would be nice if you can point out some differences!
- If the Opuntia is anywhere west of the Appalachians, especially the Midwest, it’s O. cespitosa. There do seem to be a few isolated populations in the mountains of VA and the Northeast (where O. humifusa is), but it shouldn’t be a prevalent issue. However, it seems to be sympatric with O. nemoralis in Missouri and Arkansas.
- If the Opuntia is west of the Mississippi River, in Louisiana, Missouri, Arkansas, and along the Texas gulf coast, it should be O. nemoralis.
The maps I based these on are found in the PDF on pages 17, 41, and 46