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!

"Trying to define the undefinable": are taxonomists too focused on species?

A recent article in the New Yorker includes a good overview of why identifying species can be problematic:  

"You Name It: Carl Linnaeus and the effort to label all of life" by Kathryn Schulz, August 21, 2023 

Extended quote from article:

"What Linnaeus sought to do was organize nature according to its fundamental, intrinsic divisions--to carve it at the joints, in Plato's famous formulation.  But what he actually did, for the most part, was impose artificial categories on the natural world for the convenience of scientists.

This is not a retroactive assessment; Linnaeus himself knew full well the limitations of his classification method.  To achieve a system completely in accordance with nature was, he wrote, "the first and last wish of botanists."  But the more closely you looked at her bounty the more difficult that prospect became--so, in the meantime, "artificial systems are absolutely necessary."

In philosophy, this tension between intrinsic and imposed categories takes the form of a debate between nominalism and realism.  Realists believe that nature is full of real and discrete categories, from 'amphibian" to "zinc," and that the job of the scientist is to discern them accurately.  Nominalists believe that nature lacks clearly defined categories, and that we simply impose those distinctions upon it--creating, as it were, the illusion of joints where none really exist.  

This is not just the position of post-truth relativists.  "I look at the term 'species' as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other": that is Charles Darwin, in the second  chapter of "On the Origin of Species."  That book, of course, trumpeted to the world a very large problem with the entire notion of a species.  According to evolutionary theory, species are constantly changing--emerging, diverging, going extinct.

The very concept of a species is in radical flux, too, with more than twenty competing definitions in circulation.  Choosing a definition is not just a matter of what goes in the dictionary under "species"; which one you use will determine how you divide up nature, such that a group of creatures that would be regarded as a species by one standard might not merit the label by another.  

All this confusion comes, as Darwin wrote, "from trying to define the undefinable."  Yet committed realists continue to promulgate more and more definitions, in the belief that one of them will map perfectly onto some intrinsic and stable feature of nature.  Darwin called that idea "laughable," a word that captures the impossibility but not the gravity of arbitrarily imposing categories on living beings.

Do endemic taxa correlate?

One of the main assumptions of many biodiversity assessments and conservation efforts is that biodiversity correlates across taxa. In other words, an ecosystem with high plant diversity might be expected to also harbor high lichen diversity, high arthropod diversity, and a great many birds, bees, and bloomin' confusion. 

If this assumption is true, than scientists could study one taxa, say lichen, and use the results as a surrogate for studying all of the other possible taxa. But a new study by Dr. Che-Castaldo questions this "surrogacy" assumption:

"Testing Surrogacy Assumptions: Can Threatened and Endangered Plants Be 
Grouped by Biological Similarity and Abundances?"

Abstract:

"There is renewed interest in implementing surrogate species approaches in 
conservation planning due to the large number of species in need of 
management but limited resources and data. One type of surrogate approach 
involves selection of one or a few species to represent a larger group of 
species requiring similar management actions, so that protection and 
persistence of the selected species would result in conservation of the 
group of species. However, among the criticisms of surrogate approaches is 
the need to test underlying assumptions, which remain rarely examined. In 
this study, we tested one of the fundamental assumptions underlying use of 
surrogate species in recovery planning: that there exist groups of 
threatened and endangered species that are sufficiently similar to warrant 
similar management or recovery criteria. Using a comprehensive database of 
all plant species listed under the U.S. Endangered Species Act and 
tree-based random forest analysis, we found no evidence of species groups 
based on a set of distributional and biological traits or by abundances 
and patterns of decline. Our results suggested that application of 
surrogate approaches for endangered species recovery would be unjustified. 
Thus, conservation planning focused on individual species and their 
patterns of decline will likely be required to recover listed species."

Similar conclusions have been reached by a other studies. For example, Erhlich et. al. 2002 found that in subalpine meadows in Colorado, indicator taxa show no skill in predicting diversity of other taxa, even among phylogenetically related species (in this case, butterflies and moths).