Maggi, both species are native and quite widespread here, specially M.speciosum. This species species is an erosion indicator, so actually it will grow well in very poor "soil". Sheep eat it sparingly and it gives meat a foul taste so paddocks with it will tend to be closed off to grazing at least if the sheep are being raised for meat. T.polyphylum grows in exact same place plus in very sandy conditions. In otherwords where sand has accumulated possibly blown off the eroded soil. I let T.polyphyllum grow naturally here...I'm not particularly attracted to the other species, but grows wild across my fence. Now its the beginning of winter and we have pouring rain. They don't seem to mind that at all!. Tropaeolum is ephemeral. If the substrate were poor soil plus sand added I wouldn't worry in trying to grow it in humid lowland conditions. I hope this helps. Perhaps you can refer your e-mail sender to me if it were needed to expand this info.
Arturo
