There are a number of supplementary variables that can be derived from and complement essential variables for monitoring invasive alien species. These can be collected by countries in-situ or delivered by international initiatives and partnerships ex-situ (see Organisations & Resources).
|Cover, abundance or biomass and size or reproductive stage||Although this information could be collected in-situ, in some cases it could be obtained externally from existing information sources using remote sensing , or otherwise estimated using alien species occurrence data [2, 3].||These data are useful for measuring realized local impact and the extent of the problem, and for estimating costs of management actions.|
|Pathways of introduction and spread [4, 5, 6]||This information can be obtained from external resources but fine scale occupancy data can also be used to inform this variable when collected from sites associated with pathways, such as harbours or road verges.||These data can be used to prioritise sites at which occupancy should be monitored.|
|The biodiversity or ecosystem service value of the receiving environment (measured as, for example, threatened species richness)||These variables need to be measured in-situ unless already available.||These data can be used to prioritise sites at which occupancy should be monitored.|
|Characteristics of the receiving environment, such as habitat or climate [see also 7]||This information could be collected in-situ, but could also be obtained using remote sensing .||This information can be used to estimate future potential invasions and their associated impacts.|
Referenced material and links
 Barwell, L.J., Azaele, S., Kunin, W.E, and Isaac, N.J.B. (2014) Can coarse-grain patterns in insect atlas data predict local occupancy? Diversity and Distribution 20, 895-907. doi: 10.1111/ddi.12203.
 Hulme, P.E. et al., (2008) Grasping at the routes of biological invasions: a framework for integrating pathways into policy. Journal of Applied Ecology 45, 403-414. doi: 10.1111/j.1365-2664.2007.01442.x.