Here’s the info iNat gives about charts when you click on the question mark next to the chart:
About Charts
Seasonality
This chart shows the number of observations of this taxon grouped by month. Keep in mind that these are numbers of observations, so they are influenced both by when the organism can be observed and when people bother to observe them. So a bird might seem to be very active in May, but that could also be due to more people birding in May who tend to ignore that species in later months. Similarly, if you see more dragonflies in June than in January, that’s probably because we have more people observing in the northern hemisphere than in the southern hemisphere and not because dragonflies are more active in June, so check the map when considering these charts. It’s always a good idea to be skeptical of these charts when there are low numbers of observations and/or large discrepancies between the number of “Verifiable” and “Research Grade” observations.
History
This chart shows the number of observations of this taxon by month for the last ten years. Again, it is biased by the number of people observing, but it will show you unusual spikes in observations, and if it seems flat or decreasing despite an increasing number of observers, that might suggest a change in abundance.
Relative Observations
Showing frequency as a relative proportion of all observations helps smooth out the effect of the overall growth of the site. For example, if the site is growing as we get more observations with every passing year, we get more observations of any individual taxon, which doesn’t tell you anything about whether there are more of that taxon around to observe, just that there are more people observing it. Showing the relative proportion means that if there are 100 observations total but 20 observations of this taxon, the proportion is 0.2 (20 / 100). If people observe 2000 observations the next year and 400 observations of this taxon, the proportion is still 0.2 (400 / 2000). This causes some aberrations when there are very few observers in an area, or for taxa that are very infrequently observed, but that’s true of total counts as well.
You can switch between ‘relative observations’ and ‘actual counts’ in the settings next to the chart, but that doesn’t seem to make as much of a difference as I would have expected.
The thing that I always find fun in ‘seasonality’ is trees - I’m pretty sure they exist all year, but if you just take the stats at face value, most of them disappear in winter!
I like the idea of taking out the artificial spike of CNC, but it doesn’t eliminate the issue; people are likely to iNat more in spring because we can finally get out of the house, and there’s probably an increase in new users who continue to iNat just after the CNC and gradually peter out.
Info is skewed in terms of what is observed, too. My city has more observations of deer than squirrels, and more observations of Great Blue Herons than House Sparrows, but I doubt those stats reflect actual populations.
I think the main thing with using iNat data, just like all data, is to be aware of how it’s collected, never take it at face value, and don’t let it lead you to make conclusions that it doesn’t truly support.