Countries differ widely in the level of existing information on invasive species, and also in the resources and capacity available to generate new data and to collate existing data. However, all countries can provide some information on the status of biological invasion. Over time this information can be built upon in a strategic and structured way to improve the quantity and coverage of information that feeds into National Observation and Monitoring for Biological Invasions [1-5].
Modular approach to the development of National Observation and Monitoring for Biological Invasions based on alien species occurrence, an essential variable for invasion monitoring that needs to be measured by countries (LTM: long term monitoring)
Determine baseline and develop a national inventory
- Countries without a national inventory can begin to develop one using existing data on alien species (existing data may include for example, well known groups of species such as plants, fish, mammals or other vertebrates).
- Develop complete lists by adding taxonomic groups. These lists can be further developed by adding taxonomic groups and improving on the completeness of existing lists.
- The national inventory should be updated every 5 years or less to reflect new incursions or effective eradication actions.
Alien species occurrence data collected in-situ is combined with species alien status information (which may also be sourced ex-situ) to create and regularly revise information for a national inventory or alien species.
Key questions to establish the baseline and to develop a national inventory
- What is the spatial resolution of available occurrence data  and how recent is it?
- Which taxonomic groups are included and excluded ?
- Is species alien status information up-to-date?
- Does occurrence data include true absences?
Prioritize species, sites and pathways .
- Countries will usually have some established priority sites for monitoring invasive species, such as sites that are susceptible to invasion (ports and harbours), sites that are sensitive to invasion (such as protected areas that have threatened species or habitats) and sites that have critical ecosystem functions (such as watersheds and riparian habitats).
- The network of priority sites can be developed simply by increasing the number of priority sites for monitoring as funding allows and updating site priorities as invasive species occurrence lists develop and, for example, as species expand their range into new sites, or as management actions successfully eradicate species from priority sites.
- Update priority species, site and pathway lists every 5 years or less to reflect current information on site sensitivity and species composition changes over time.
Alien species occurrence data (with attributed species alien status information) that is spatially referenced at an appropriate scale, can be combined with alien species impact information to inform site prioritisation. Species with the worst impacts present at important sites are prioritised .
Key questions to prioritise species, sites and pathways
- What is the impact of alien species?
- Are there priority sites or pathways that are missing [9, 10]?
- Are there management actions that can be taken for priority species, sites or pathways [11, 12]?
Increase the resolution of data
- Generate distribution data for priority species. This might initially be at a coarse scale, depending on existing data and the resources available to invest in new data collections.
- Increase the resolution of distribution data for all alien species across the country.
- Update data every 5 years or less as species distributions change.
Increase the survey capacity to include more priority sites and greater national coverage (alien species occurrence) of alien species (species alien status), starting with priority species with the worst impacts (alien species impacts; generated in the previous step and including surveys with true absences). Increase the number of species, range of taxa and the temporal resolution of data that can contribute to the National Observation and Monitoring System for Biological Invasions.
Key questions to increase the resolution of data
- What is the temporal resolution of data?
- Are there new priority sites or additional taxa that need to be included?
Establish a network of repeat monitoring sites
- Establish a network of long term monitoring (LTM) sites recording the occurrence of invasive alien species at a national scale
- Repeat monitoring every 5 years or less
Essential variables for invasion monitoring provide the building blocks for developing and informing the monitoring program as it progresses to this stage. At every stage of development and through time, essential variable data needs to be updated to reflect the dynamic nature of alien species occurrence (as species ranges and abundances change over time), species alien status and alien species impacts (as more or better information is gathered about invasive species and their impacts). This enables trends in invasive species to be determined and the evaluation of management actions . Monitoring of alien species could be incorporated into existing long-term ecological and biodiversity monitoring initiatives.
Referenced material and links McGeoch, M. A., Pyšek, P., Jeschke, J. M., Blackburn, T. M., Bacher, S., Capinha, C., Costello, M. J., Fernández, M., Gregory, R. D., Hobern, D., Hui, C., Jetz, W., Kumschick, S., Latombe, G., Pergl, J., Roy, H. E., Scalera, R., Squires, Z. E., Wilson, J. R. U., Winter, M. and Genovesi, P. (in review) A vision for global monitoring of biological invasions.
 McGeoch, M.A. and Squires, Z. (2015) An Essential Biodiversity Variable approach to monitoring biological invasions: Guide for Countries. GEO BON Technical Series 2, 13 pp. http://www.geobon.org/
 Pagad, S., Genovesi, P., Carnevali, L., Scalera, R., and Clout, M. (2015) IUCN SSC Invasive Species Specialist Group: invasive alien species information management supporting practitioners, policy makers and decision takers. Managing Biological Invasions 6, 127–135. doi: 10.3391/mbi.2015.6.2.01.
 Welch, B. A., Geissler, P. H., and Latham, P., (2014) Early detection of invasive plants—Principles and practices: U.S. Geological Survey Scientific Investigations Report 2012–5162, 193 p., doi: 10.3133/sir20125162.
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
 Berec, L., Kean, J. M., Epanchin-Niell, R., Liebhold, A. M. and Haight, R. G. (2015) Designing efficient surveys: spatial arrangement of sample points for detection of invasive species. Biological Invasions 17, 445-459. doi: 10.1007/s10530-014-0742-x
 Genovesi, P., Butchart, S.H.M., McGeoch, M.A. and Roy, D.B. (2013) Monitoring trends in biological invasion, its impact and policy responses In B. Collen, N. Pettorelli, S. Durant and J.E.M. Baillie (Eds.), Biodiversity Monitoring and Conservation: Bridging the Gaps between Global Commitment and Local Action. (pp. 138-158). Wiley-Blackwell, Cambridge. 10.1002/9781118490747.ch7.