An invasive species is often described as an organism that is introduced into an area from which it did not naturally (without human help) evolve or originate (non-native), and that exhibits aggressive and competitive behavior that results in serious biological, economic, and ecological damage, often to intact native ecosystems.
It is important to note that some native species can sometimes exhibit localized invasive behavior due to changes in the local environment, such as soil disturbances, but this typically does not have the same ecological impact as a true nonnative invasive species. To come full circle, weeds are a moniker given to native or nonnative species that are deemed a nuisance.
Truly invasive species often have 1 or more advantages over native species, such as a lack of effective predators or pathogens, or an ability to utilize resources more effectively, or not being used by other local species. In some cases, invasive species can alter the local ecosystem through biochemical release (allelopathy) that can inhibit other species from growing.
The result of an invasive species is often complete dominance (monocultures) of a community. With invasive plants, for instance, this dominance can alter and disrupt animal and insect food webs, structural or functional integrity of the local ecosystem, and the original balance of hydrological (water), nutrient, and abiotic (rock and soil) cycles.