Final answer:
An invasive plant species is a non-native plant that disrupts the ecosystems it invades by out-competing native species. The purple loosestrife is a prominent example, notably altering wetlands in North America.
Step-by-step explanation:
An invasive plant species is defined as a non-native organism that, upon introduction to an area outside its native range, begins to alter the community and disrupt the ecosystem balance. These species often spread aggressively, out-competing local flora and posing significant ecological and economic threats.
One prominent example of an invasive plant species in the United States is purple loosestrife (Lythrum salicaria). Originally hailing from Europe, this plant has overwhelmed wetlands across North America, displacing native vegetation and reducing biodiversity.