Submodule 2: Where Invasive Plants Are Found

Invasive Plants Lack Natural Controls

Plants evolve along with other organisms in their native environment. Most invasive rangeland plants evolved in Europe and Asia, with a particularly heavy concentration of weeds originating in the Mediterranean area. These plants were not considered “invasive” in their native habitats because along with them evolved some natural controls. These controls include insect predators, plant pathogens such as viruses, bacteria, and fungi, other plants, and herbivores. When plants are allowed to grow in new habitats, factors that once naturally controlled native plant population sizes may not be present. This can be a major factor contributing to the “invasiveness” of non-native plants. On a positive note, natural predators of these newly invasive plants are often investigated as possible biological control methods.

Nodding plumeless thistle (photo courtesy invasivespecies.gov)