With the possible exception of alpine and subalpine habitats, most areas of California contain significant expanses of exotic weeds. They deprive other plants of moisture and nutrients scarce in this environment. Their value to wildlife is low. They alter wind patterns that sculpt the dunes and they bind the dunes, preventing the natural disturbance required by some of the native species. Because coastal dunes support biological communities whose plant and animal inhabitants may exist nowhere else, their degradation represents a loss of biological diversity.
German ivy forms thick blankets which cut off light and air to plants which it covers.
Its aggressive root system outcompetes plants even much larger than itself. Tall plumes, bearing many light seeds rise several feet beyond the leaves, dispersing seed great distances in the wind. The plant is toxic to horses and stout spines render it inedible to sheep and cattle. Aside from its economic impacts, yellow star thistle increases roadside fire hazards and affects recreational values by invading campgrounds, lining trails, and reducing biodiversity. Seeds of all of them may be viable for decades, making reclamation of territory they occupy exceedingly difficult.