Species are introduced either intentionally or unintentionally. For unintentional introductions, species-based prioritization is not always feasible because which species will arrive is difficult to predict, and the biology and life history of the species that do arrive are sometimes poorly known. 
The focus on species must therefore be balanced with a focus on pathways of introduction and spread, with the purpose of preventing the propagules that they carry from arriving and spreading. 
Prioritization of pathways uses information on the full suite of vectors and routes by which alien propagules are introduced, and the propagule loads of such pathways. 
A framework that categorizes pathways is needed to compare data within regions and across countries, and also to facilitate regulatory approaches that have to deal with the many potential taxon–pathway combinations.
A detailed categorization of pathways has been developed by the IUCN Invasive Species Specialist Group, and endorsed by the CBD [2, 3].
This hierarchical system of categorization encompasses three broad mechanisms: importation of a commodity, arrival of a transport vector, and spread from a neighbouring region. These are further subdivided into six principal pathway categories: intentional release, escape from containment, transport as a contaminant, transport as a stowaway, spread through corridors, and spread through unaided natural dispersal .
Parties to the CBD have called for the use of this pathway framework for the purpose of assessing and prioritizing the risk posed by pathways , which will facilitate the reporting envisaged in Aichi Target 9. 
Referenced material and links
McGeoch, M.A., Genovesi, P., Bellingham, P.J., Costello, M.J., McGrannachan, C. & Sheppard, A. Early Online 2015. Prioritizing species, pathways, and sites to achieve conservation targets for biological invasion. Biological Invasions. Doi: 10.1007/s10530-015-1013-1
 UNEP (2014) Pathways of introduction of invasive species, their prioritization and management. UNEP/CBD/SBSTTA/18/9/Add.1, Subsidiary Body on Scientific, technical and Technological Advice, Eighteenth Meeting, Montreal. (www.cbd.int/doc/meetings/sbstta/sbstta-18/official/sbstta-18-09-add1-en.pdf, Decision XII/17 CBD COP12) Essl F, Bacher S, Blackburn TM et al (2015) Crossing frontiers in tackling pathways of biological invasions. Bioscience 65: 769-782