So, what makes some orders more or less sampled than others? Is it perhaps the number of species?
There isnât really a clear pattern here. Orders with less than 100 species have the highest average completeness at just over 20%, but orders with over 10,000 species have the second highest average completeness at 14.8%. Orders with between 100 and 1,000 species have the third highest average completeness at 14.15%, and orders with between 1,000 and 10,000 species have the lowest average completeness at only 13.47%. I guess orders with intermediate species counts have too many species to sample quickly and easily, but not enough species that they compose a massive part of the fauna which generate a lot of public interest? Perhaps.
Perhaps habitat might be the deciding factor?
I used the habitat of adults, since arthropods are primarily collected and identified by their adult stage. So this means stuff like dragonflies and mayflies are under terrestrial. Terrestrial orders do have a higher average completeness than aquatic orders. This makes sense, as terrestrial habitats are generally more accessible to humans than aquatic ones. Aquatic orders have an average completeness of 14.75% and terrestrial orders have an average completeness of 17.94%. Keep in mind this is not a perfect division since there are aquatic species in terrestrial orders and vice versa.
But there is one more factor to consider. Body size. People pay more attention to giant lobsters, tarantulas and butterflies than to tiny zooplankton, mites, and springtails.
And what do you know, itâs an extremely strong correlation! Large bodied orders have an average completeness of 20.94% and small bodied orders 7.3%. My assignment of orders as âlargeâ or âsmallâ was admittedly somewhat arbitrary, but generally if you need a microscope to see most species of a given order as anything more than a moving dot (or to see them at all), they are under the small bodied category. This includes most mite orders, all copepod, ostracod, and cladoceran orders, pseudoscorpions, palpigrades, pauropods, springtails, psocids, and several obscure crustacean orders. Most of everything else was lumped as âlarge bodiedâ. This is far from a perfect division, as âlarge bodiedâ taxa contain many very tiny species, and âsmall bodiedâ taxa also contain some fairly large species (compare fairy wasps vs giant velvet mites). If you could sort this out by family, the division would grow even sharper.
So to lay out all the possible differences, lets do a case study of two orders in particular, the Odonata and Thermosbaenacea. Odonata, the dragonflies and damselflies, is the most completely sampled arthropod order with more than 10 species, at 67.55%. Thermosbaenacea is a group of troglobitic (cave dwelling) crustaceans with no observations on INaturalist, and thus no observed species.
- Size. Odonates are generally pretty large and conspicuous insects, and thus easy to see. Thermosbaenaceans are sarcely more than a few mm long.
- Rarity. Odonates are reasonably common in many areas of the world, and at times form large swarms. Thermosbaenaceans on the other hand are quite rare.
- Accessibility. Odonates can be seen flying around in basically every kind of terrestrial habitat, and their young can be found in all manners of shallow freshwater bodies like ponds and creeks. Thermosbaenaceans live in caves and thermal springs, places where only people with specialized gear go into.
- Popularity. Everyone knows what a dragonfly is, and they are generally among the most well liked insects. Ask anybody if they know what a thermosbaenacean is and they will look at you like you came from outer space.
- Ease of identification. Odonates are generally among the easier insects to identify, and from what I hear many of them can be reliably identified from photos even which is atypical for arthropods. I donât know how easy it is to identify thermosbaenaceans to species level, but if its anything like other microarthropods its very hard and you need a microscope.
- Number of species. At around 6,000 species, odonata is certainly a a large order, but it is not incomprehensibly large. Several other taxa such as lepidoptera, coleoptera, and araneae share in common with odonates many of the advantages listed above, but their extremely high number of species makes sampling them to an even remotely close to complete level very difficult. Odonates have few enough species that all their other advantages can operate at full force. Thermosbaenacea actually has very few species, only 34, so in theory a truly dedicated team of people could put every species on INaturalist in a short amount of time. But in spite of the low species count, they have pretty much everything else working against them, so it keeps them from reaching any satisfactory level of sampling.
Allow me to get through one last proposition. Once INaturalist reaches 20% of all arthropods observed, we should make it a holiday!
