I feel I have to clarify that the issue of introduced vs native organisms has very little to do with the one raised by Pablo.
Often plants are also cultivated in the same area where they are native and alien species are already marked .
Concerning cultivated plants, sometimes users answer that they did not notice the box to be checked. More often they just go on posting unflagged cultivated plants as it was the norm here. The pop-up too could intended as a way to make users think if the organism they are goig to post is wild or not. This could also allow other users to understand if there is someone here who has an imprecise idea of what wild and cultivated/captive mean.