Natural pathways (i.e., those not aided by humans) include wind, currents (including marine debris), and other forms of natural dispersal that can bring species to a new habitat.
Man-made (or human-mediated) pathways are those which are created or enhanced by human activity. These are characteristically of two types:
- Intentional, which is the result of a deliberate movement of a species by humans outside of its natural range. Examples include the introduction of biological control organisms or the movement of species for the horticultural or pet trade. Intentional introductions as a whole should not be labeled as either good or bad. A specific intentional pathway can only be judged by the positive or negative impact of the specific organisms that are moving along that means.
- Unintentional, which is the inadvertent movement of species as a byproduct of some other human activity. Examples of unintentional pathways are ballast water discharge (e.g. red-tide organisms), pests and diseases in imported plants, firewood, and other agricultural products (e.g. fire ants), the movement of recreational watercraft (e.g. zebra mussels), and the international movement of people (e.g. pathogens). In these and countless other unintentional pathways, the movement of non-native species is an indirect byproduct of human activities.
For our purposes, the term "vector" is viewed as a biological pathway for a disease or parasite (i.e. an organism that transmits pathogens to various hosts) and is not completely synonymous with the much broader definition of a pathway. The Asian citrus psyllid is an example of a vector of the serious citrus greening disease or Huanglongbing.
The best way to fight invasive species to prevent them from occurring! Learn more about How They Spread to help stop them from USDA's Animal and Plant Health Inspection Service Hungry Pests outreach campaign.
Examples of ways invasive species spread include:
- Agricultural materials
- Aquaculture farming
- Ballast water and shipping
- Classroom or science lab (escapes or introductions)
- Fishing gear
- Food trade and its packing material
- Fouled hulls of commercial and recreational vessels
- Internet sales
- Outdoor gear (hitchhikers on boots, clothes, and camping equipment)
- Passenger baggage
- Pets (unwanted or escaped)
- Plants and plant parts (escaped or disposals)
- Recreational boats
- Recreational vehicles (RVs)