It depends on the species, but 1 common theme is that they are usually initially introduced by humans.
Aquatic non-native species can be introduced to estuaries (green crab, zebra mussel) by ballast water in big ships or on boat rotors and wader boots between streams and lakes (New Zealand mud snail). Terrestrial species are many times transported by direct trade, as with ornamentals (Japanese knotweed), agriculture (grasses), and cultural goods (Kudzu).
Once these species are released into the wild, they often have many new ways of getting around. Human transportation networks, such as roads and power right-of-ways, or hitching rides on wild animal fur, allow further penetration into natural areas. Even on the boots of the nature lover on a wilderness trail.
Perceiving the invasiveness of a nonnative species early is extremely important. Often, a nonnative plant or animal can remain out of public perception for years or even decades. Once the plant is recognized by the general public as being a problem, the species is often beyond feasible control and containment is the only option.
More State and Federal agencies and local governments are working to put in place early detection programs that foster public reporting of newly arrived and known invasive species. This reporting is largely based on volunteer support and can be done by anyone with an interest. Such reporting is crucial to preventing the establishment of new or potentially new invasive species.
In order to reduce the strain on the reporting network, reporting should be limited to State Listed invasive species whose presence in a local area (County) is either previously unknown or has a high chance of entering or is not abundant.
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.
