Sonoran Desert Tortoise - Follow Up

In my previous post "Sonoran Desert Tortoise Population Status", I argued that a decline in the ratio of tortoise observations to total iNaturalist observations indicated a possible decline in the actual population of tortoises.  

However, that conclusion rests on the assumption that opportunistic "citizen science" observers have not changed their preferences in photographing other animals.  This could happen if people became less interested in tortoises, or if they became more interested in other taxa.  

Part 1: Trends in Other Taxa

I analyzed various other well-represented taxa in the iNat data to look for possible trends in observer preferences.  Specifically, I looked at all Research Grade (RG) iNat observations of amphibians, reptiles, insects, birds, and plants in the study area: 

Data table with total RG amphibian, reptile, insect, bird, and plant observations in the study area each year.

These are large taxa made up of many species and I expected that the ratio of observations would remain relatively consistent. 

Count of observations by taxa 2013-2024

Observations of all taxa have increased over the last 10 years, but some appear to have increased more than others. I divided the count of each taxa's observations by total observations to investigate proportional changes in the ratio of various taxa:

Ratio of various taxa to total observations 2013-2024

The ratio of various taxa has changed over time.  Specifically, the ratio of plant observations to total observations doubled in 2017.  I don't think this could be due to an actual increase in the number of plants in AZ.  

Instead, I think this must be an indication of a changed bias toward plants, perhaps due to specific iNat users who focus on plants, or a general trend toward more plant-focused observers on iNat.  Plants, because they don't run away, are arguably the easiest organisms to photograph; it could be that as iNat has grown there are now more casual users biased toward photographing easier to observe organisms.  

Regardless of why iNat users are observing proportionally more plants, this appears to be a source of bias that should be corrected in my analysis.    

To correct for the large increase in plant observations, I compared various taxa to total non-plant observations: 

Ratio of various taxa to total non-plant ("total2") observations 2013-2024

This shows a fairly consistent observation ratios for the major animal taxa over the last 10 years.  While there is some year to year variability, there are no long term trends, except reptiles.  Reptiles in general are photographed 71% as often in 2024 compared to 2013. Most of this drop occurred from 2015-2017, with no major changes since then.  It is not clear why reptiles as a group declined in representation.  There are other reptile species of conservation concern besides tortoises and it is possible that reptiles actually are declining, or there could be other sources of observer bias in the data, similar to the trend in plant observations above.

Except for reptiles, all taxa ratios stayed within approximately +/- 15%:

Table: percent change in ratios of various taxa since 2013 and 2017.

Part 2: Another Look at the Tortoise Trend

The analysis in Part 1 led to a refinement in the total observation count used to create ratios, and helped set a baseline for expected change in ratios over the last 10 years.  I used this information to reassess the observed decline in ratio of tortoise observations.  

Tortoise observations were compared to various taxa and to total non-plant observations:

Percent change in the ratio of tortoises to various taxa and to total non-plant ("total2") observations 2013-2024

The ratio of tortoise observations has decreased since 2013 for all 4 animal taxa investigated, and for total non-plant observations.  The decline is remarkably consistent for amphibians, birds, and total non-plant observations ("total2"), and fairly consistent for insects.  The ratio of tortoise to reptile observations tends to fluctuate over time, while maintaining the same overall negative trend.

While most animal taxa ratios show less than 15% change from baseline (Part 1), tortoises are observed 66-84% (mean 72%) in 2024 compared to 2017, and 35-57% (mean 43%) in 2024 compared to 2013:

Table: percent change of tortoise observations to various taxa and to total non-plant observations.

Conclusions

Tortoises show large declines compared to various representative animal taxa and compared to all non-plant observations.  This is the same result I found in my original blog post when I compared tortoises to all observations.  

The result was not affected by removing a potential source of user bias, showing that the original result is robust to some observational biases.  Of course, there will always be more sources of bias that could be analyzed and corrected for.  However, the fact that correcting for one source of bias didn't change the result makes me somewhat more confident that this result is directionally correct.

Conversely, the fact that I did find a large source of observer bias makes me wonder whether there are other large biases in iNat observation trends.  Without analyzing all sources of bias (why did reptiles change in 2017?) these results must remain clouded by potential uncertainties.  

The result was also not affected by which taxa I compared tortoises with, showing that the original result is robust to choice of comparison.  Tortoises appear to be declining, whether they are compared to all observations, all animals, amphibians, reptiles, insects, or birds.  While it is possible that one or more of these taxa are affected by observational bias, the fact that they all point in the same direction makes me more confident that this result is directionally correct and reflects an actual downward population trend.