Alien species occurrence (made up of occupancy records) is the presence or absence of an alien species at a particular scale. This could be a list of species present (or absent) in a country or in a national park, or it could be point locality records for alien and invasive alien species resulting from field surveys. Harmonised occurrence data for aliens from several taxonomic groups across all countries would provide a sound global baseline – a significant step forward for quantifying the size, extent and nature of biological invasions . Repeated measurements of occurrence provide the data needed to track invasions and to evaluate the success of management and policy interventions at subnational, national and global scales .
Relationship between alien species occurrence and Essential Biodiversity Variables.
‘Species occurrence’ – including alien species occurrence – belongs to the class of Essential Biodiversity Variables called ‘Species populations’  and forms part of the ‘Species distributions’ EBV within this class. Observation data on alien species occurrence therefore contribute directly to the ‘Species distributions’ EBV.
The Essential Biodiversity Variable (EBV) ‘Species distribution’ includes species presence-absence records over sets of defined spatial units, and over a specified time, and aims to cover entire species ranges. The essential variable ‘alien species occurrence’ forms a natural subset of information provided by the species distribution EBV, and the value of this information will increase over time as the numbers of species and their taxonomic ranges, temporal detail and spatial detail become more comprehensive.
Necessary information associated with alien species occurrence
There are two pieces of information that must accompany an occupancy record. These are:
- The spatial unit of reference or georeference, such as the country, the land unit or the x-y co-ordinates of the locality record.
- The species identity.
A species may be known from only a small number of localities (geo-referenced, x y coordinates) while others may be more widespread. For example, there are 3568 occupancy records (individual point locality records) of brown trout (species identity: Salmo trutta) in Australia recorded in the Atlas of Living Australia (www.ala.org.au).
Alternatively, the country may be divided into geographic grid cells of, for example, a quarter degree. If a species is known to be present from 10 of these cells, then it has an occupancy of 10 at a quarter degree scale. This example shows how many records in the Atlas of Living Australia (www.ala.org.au) there are of brown trout (Salmo trutta) in Australia within each quarter degree grid cell (scaled colour bar).
- A national species inventory provides a list of the species that are present in a country. In this case only one locality is being considered (the country) and the maximum occupancy for any species is 1. Species not on the list are assumed to be absent from the country, unless the inventory is associated with a disclaimer that suggests that the list is incomplete.
- Occurrence may also be measured across a number of comparable sites or land-units of interest, such as ports or protected areas. For example, a species may be present in 5 of the national parks in a country, established at two of a country’s shipping ports, or present in 3 of a country’s 8 provinces or states [see table; 4, 5, 6].
Table: The number of aliens species in each national park in South Africa. These numbers are aggregates of presence – absence records (occupancy) at the scale of each park [4, 6].
Some properties of the variable occurrence (or occupancy)
- Occurrence is scale dependent. At a fine scale occurrence will be highest (for example, brown trout are found within 5 states in Australia) and at the coarsest scale it will be lowest (for example, at the country level brown trout occupancy is 1). This property is useful because occurrence scales in a predictable way.
- Occurrence information can be used to predict species abundance or to estimate species range size at finer scales .
- Not all species have discrete individuals that are readily distinguishable or readily countable. In this case occurrence can be measured as the presence of a group of individuals or of a habitat unit. For example, the number of nests, clumps or colonies of a species rather than the number of individuals.
- Occurrence data are most valuable when they include true absences. True absences can only be established when the objectives and corresponding methods of a particular survey, encompass the species or taxa of interest. It is therefore important to retain information about the survey aims and methods to establish true absences. This is often not the case, especially when sampling effort is low or when presence records are collated from pre-existing data originally collected for another purpose.
Referenced material and links
 McGeoch, M.A. et al.,(2010) Global indicators of biological invasion: species numbers, biodiversity impact and policy responses. Diversity & Distribution 16, 95–108. doi: 10.1111/j.1472-4642.2009.00633.x.