Don looked at some bushland infested with lantana (Lantana camara). He pointed out that the weeds did well in this area because it was very moist and rich in nutrients.
Simply removing the lantana would be a complete waste of time, because birds would deposit more lantana seeds in their droppings. The seeds would germinate and thrive in the perfect growing conditions, soon outcompeting any native vegetation in the vicinity.
Keeping such an area weed free would involve a huge amount of time and effort. It would make more sense and make a bigger difference to the environment if the nutrient-rich runoff was traced and stopped at the source.