The island soils derived largely from ultramafic rocks and have been a refuge for many native flora species that have adapted to their composition a long time ago; such flora can survive on acid soils with unfavourable compositions of nutrient elements. On New Caledonia examples of such soils commonly have an excess of magnesium, plus unusually high concentrations of phytotoxic compounds of heavy metals such as nickel. Not many invader species can compete successfully with plants adapted to such challenging soils.