it’s because flowering plants are more often photographed. when i look out my windows, everything i see is a flowering plant. i know there are ferns (resurrection ferns, ornamental ferns), conifers (pines, cypresses, junipers), and cycads (sago palms) around somewhere, but they are relatively few and far between. i know there are mosses, but i’m unlikely to photograph them in part because they are so small and in part because i know they will be difficult to identify to species without microscopy. i know there are algae, but i’m unlikely to photograph them because of the same reasons as the mosses, and also because i have to find a natural body of water that contains them (unless it’s terrestrial algae).
this is fine, but i question a casual user’s ability to use these categories effectively. even though plants with inconspicuous flowers or plants which don’t often flower, like oaks, grasses, yuccas, asparagus ferns, etc. are all flowering plants, given the categories above, i bet a lot of casual users would choose one of the other above categories for them.
ferns and mosses categories might be useful for their respective enthusiasts, but that’s a very niche base. are there even dedicated conifer enthusiasts? really, most people interested in plants are interested in flowering plants or all plants, and so carving out flowering plants from plants to me doesn’t really offer most people any additional benefit.
(compare that to the animal side, insect folks are and bird folks and reptile folks are large and distinct bases.)
while i do believe that a lot of casual users, if asked “what kind of plant is this?”, are going to respond with something like tree, vine, grass, forb (plus fern, moss, algae), i think these form classifications are unreliable if you try to apply them in a mutually exclusive way, since some plants are both viny and tree-like, or grassy and forb-like.
so i am in no way endorsing that kind of classification. my point there was that your proposed classifications aren’t how most casual users would attempt to classify different plants. (compare that with what would happen if folks were asked “what kind of animal is this?”… i do believe most casual folks would respond with terms with correspond with iNaturalist’s iconic taxa, like bird, reptile, mammal, insect, etc.)
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one last note: if you’re not already aware of this, it is possible in the website to search for multiple taxa together by setting parameters in the URL directly. for example, to find mosses, liverworts, and hornworts together, you could use: https://www.inaturalist.org/observations?taxon_ids=64615,311249,56327