iNat Isn't Slowing Down in Arizona

The iNaturalist website collects species observations from people all over the world.  It started in 2008 and grew slowly at first and then entered a period of rapid growth in 2017.  As a consequence, the number of species recorded on the website is constantly increasing, passing 300,000 in 2020.  The website is currently adding more than 50 million observations a year. This raises an interesting biodiversity question: how long can the number of species keep increasing?  Another way of stating the question: how many species are there?

Biodiversity scientists use species accumulation curves to estimate the total number of species in a given area.  As they investigate a new study site, they record new species and the date/time the species was observed.  For most sites, the number of new species increases rapidly as scientists describe common species; the number of new species slows as scientists search for more and more rare species.  Graphing the number of species over time should reveal a logarithmic curve.  Based on the equation for that curve, scientists can estimate the asymptote - the number of species the curve will eventually reach given enough time.  This allows scientists to estimate the total number even if they don't finish counting all of the species.

This slowing down does seem to be happening for total species count on iNat.  For example, the 2024 Year in Review showed 50 million observations over the year, and about 1,000 new species (not previously observed and posted to iNat) per month.  

From iNat 2024 Year in Review

In contrast, back in May 2019 more than 6,000 new species were added.  It appears that 2019-2020 was the peak for adding new species, and even as more new users have joined iNat, fewer and fewer new species are being observed.  

These charts show running totals, with new additions colored, so that the logarithmic curve is more visible in Newly Added Species:

From 2024 Year in Review

There were fewer observations and many fewer users in 2019-2020, than now, but the rate of newly added species was much greater.  This appears to indicate that it is getting harder and harder to find new species to add to iNat.  Observable species on iNat are those that can be distinguished with photographic evidence, usually limited to smartphone cameras.  So this estimate does not include microbial life, and probably excludes most microscopic life.  

Its possible that unobserved species are mostly in the middle of remote wilderness areas and that is why fewer and fewer are being observed.  But many of the new species are from the US and Europe - there's still lots to explore!

For example, in Arizona the species accumulation curve is still effectively linear, with about 700 new species each year.  No signs of slowing down here!

The same is true of smaller areas within AZ, for example the Prescott National Forest averages 186 new species observed each year.  

I considered whether the new species could be due to rare birds and insects showing up for the first time.  I also analyzed new plant taxa on Coconino National Forest.  Plants are well-studied and the forest has been extensively surveyed, so it seems unlikely that new species would be discovered yearly.  But, according to the iNat data, not only are new species being continuously discovered, there is no detectable slow down in the rate of discovery!

I'm not sure what conclusions to draw from this analysis.  The standard conclusion would be that we haven't sampled enough species yet to begin to see the rate of new species discoveries slowing down.  This implies that the total number of species is quite a bit greater than the number that have been recorded so far on iNat.  

Another interpretation could be that the actual number of species isn't constant.  In other words, there could be new plants showing up each year on the Coconino.  This could be due to new invasive species, shifting distributions of native species.  It could also be impacted by taxonomist naming conventions; the number of species in even well-explored areas could increase as botanists work to name and describe the huge floristic diversity of the world.

There is still a lot of biodiversity to explore, even in our backyards!