Non-native plants are usually introduced directly or indirectly by humans and often thrive in the wild, despite not belonging naturally to the habitat it lives on. Most of these species do not pose a threat to the ecosystem but some can spread rapidly and end up outgrowing native flora. These plants are called invasive because they lack natural control mechanisms (such as herbivores), spread rapidly (by seed or vegetatively) or end up killing native species (such as allelopathy or competition for resources).