A species is considered non-native when it occurs outside of its historical natural range — or to put it more simply — when a species is brought to a new place where it never existed before. While a species will naturally spread across an area, there are physical barriers that restrict their movement; these could be mountains or oceans, but also a change in biome or climate. Bumblebees, for example, are susceptible to overheating, and this prevented them from crossing the equator to reach areas such as sub-saharan Africa, Australia and New Zealand. However, with the aide of man, they now call some of these places home.

A species becomes invasive when its introduction causes harm to an ecosystem, usually by outcompeting other species for resources, but also by directly killing or injuring a particular species. Having evolved in another part of the world, they are far removed from their natural predators, competitors and diseases, all of which kept their numbers in check.

In their new environment, they have an advantage over everything else around them. For a plant, this means few to no animals munching on their leaves, flowers, roots, stems, or seed; and so, they grow stronger, faster, and reproduce more.

Without human intervention, invasive species become dominant, resulting in a loss of biodiversity. Fewer native plants means fewer insects that rely on them, and the loss of insects soon trickles up the food chain.