you can filter for endemic=true status (meaning a plant is found only in a particular place), but this method would rely on the quality of the taxon establishment means setups. (it’s possible establishment means are not set up for a particular place, that they’re defined incorrectly, or that endemic status is set up for a giant place like a continent even when the organism is found only in a tiny part of the place.)
here’s an example of Texas observations of taxa which have endemic=true: https://www.inaturalist.org/observations?place_id=18&endemic=true.
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however, if you’re searching within a particular place or area and want to get a ratio of obs within the place vs obs outside the place by taxon, this is the way i would approach it: https://forum.inaturalist.org/t/how-to-find-endemic-species/41139/7.