When, for artificial or natural reasons, the soil is almost absent the plants colonise the terrain. They establish in walls, rocks, collapsed rock areas, roads, scree or by a quarry. These are the pioneer plants or colonisers. They have little demand for nutrients and can germinate on open ground, then grow vigorous roots. Plants with impressive seed distribution methods also colonise here. These plants are tied to the passage of man and his livestock, for example at the sides of the roads. This group of plants includes thistles, buttercups, stonecrops, valerian, teasels, mallows, herb Robert, foxgloves and willowherbs. These plants prepare for other more demanding plants to arrive when the ground becomes deeper and more nutritious.