Harmonising approaches in the calculation of the range 1st draft Definition of the range

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ETC/BD 2010/10

Harmonising approaches in the calculation of the RANGE

1st draft

Definition of the range

The range of the habitat types and species is one of the parameters required in order to assess the conservation status. In order to evaluate the status of the range we need to look at two principal characteristics of the range, first at the size of the range in relation to the size of the favourable range and second at the range trend.

In the ‘Explanatory notes and guidelines’ from the previous reporting the range was defined as ‘the outer limits of the overall area in which a habitat or species is found at present. It can be considered as an envelope within which areas actually occupied occur.’

Furthermore the ‘range’ as a parameter must allow for the assessment of change from one period to another, so it can not be regarded as too large an envelope around the distribution.

The range should represent a parameter suitable for assessing the spatial aspects of the conservation status. However for both, habitat types and species, the spatial component is also included in other parameters, namely ‘area’ for habitat types and ‘area of habitat’ for species. The ‘range’ should be able to describe and detect changes in the extent of the distribution and, ideally, to some extent in its internal structure (e.g. fragmentation).

In future Article 17 reports the range will be a technical parameter allowing for assessing the extent and the changes in the habitat type or species distribution. The range will be calculated based on the map of the actual distribution using a standardised algorithm. The standardised process is needed to ensure repeatability of the range calculation in different reporting rounds.

The standardised process will basically consist of 2 principal steps:

Gap closure using a predefined set of rules specifying when two distribution points/grids will be joined together to form a single range polygon, and where an actual gap in the range will be left.

Fitting to environmental parameters The polygons created by filing in the gaps will be then fitted to the environmental parameters to avoid for example the range of a terrestrial species covering marine areas.

ETC/BD will develop a Range Tool to facilitate an estimation of the range. However Member States can still use their system to calculate ranges. The main requirements are repeatability of the estimation and sensitivity to the spatial changes of the distribution.

Calculation of range

Discontinuities in the range

Most of the basic principles for the range estimation, including the size of gaps which will represent a discontinuity in the range, were established so far during the 2000-2006 reporting and will be still valid in future. Range should exclude the discontinuities that are natural i.e. caused by ecological factors. What is considered as a natural discontinuity is largely dependant on ecological characteristic of the habitat/species and a character of the surrounding landscape. A discontinuity of at least 40 – 50 km is suggested to be considered as a gap in the range. This value may be modified on the basis of an expert judgement, for example dependent on dispersal and migration potential of a species.

The choice of gap distance has some consequences for the evaluation of the conservation status of range. A range calculated with larger gap distances (40-50km) is more sensitive to the processes on the margins of the distribution and large scale changes within the outer limit of the distribution. On the other hand range calculated with smaller gap distances (20km) better reflect small scale changes (see figure 1 of Leucorrhinia caudalis).

Figure 1 Image shows a difference between the range calculated with 20km and 50km gap distances. In the case that one population occupying two grids on the map is lost (dark brown grids) the range calculated with 50km gap distance will decrease by more than 15% of its original area (orange grids). While if the gap distance of 20 km was used the decline in the range area will be around 3%. With 12 years reporting period the same situation would lead to different conclusions; ‘unfavourable bad’ for the range with 50km gap and ‘unfavourable inadequate’ for the range with 20km gap.

The gap distance should reflect ecological characteristic of the habitat types and species. This means that for mobile species the range will be calculated using larger gaps and conversely smaller gaps will be used for sedentary species. The exact knowledge on the dispersal capacity of many species is still lacking and in addition the possible dispersal distance is highly influenced by the quality of the surrounding landscape matrix. Proposed gap distances are therefore rather broad and reflect major ecological differences between broad species groups. The recommended gap distances by the species group are outlined in table 1.

Table 1 Gap distance for major species groups

Species group

Gap distance

Lower plants


Higher plants












Marine mammals and reptiles


For relatively localised habitat types the gap distance of 40km is recommended, which is equal to the recommended gap distance for plant species which represent main structural components of the majority of the habitat types. However for wide spread habitats which are structurally similar to the surrounding landscape matrix the gap distance should be increased to 50km.

For very rare and localised species and habitat types, occurring in particular environmental conditions (e.g.1130 Estuaries, 8340 Glaciers, Alpine habitats and species) the range should be equal to the distribution.

The updated reporting format suggests that the distribution data will be provided as presence/absence in a 10 x10 km grid (ETRS LAEA 5210 10km grid). However this method is not appropriate for highly mobile or migratory species. For these species distribution is mostly mapped on home-range basis, which is then converted into 10x10km grid system. The range in this case will represent a spatial generalisation of the space that is used regularly by the population(s). If distribution is represented as relatively broad polygons the range Tool may not be the most appropriate method for determining ranges. Here the expert judgement would be the most suitable.

Defined gap distances will be then implemented in the calculation procedure. Technically the range will be calculated by filling in unoccupied grids between grids of distribution. A gap distance should be understood as the distance between two distribution grids, that will not be joined together to form a single polygon, component of range.


The range calculated by automated filling of gaps will be fitted to the environmental and biogeographical constraints. The following types of unsuitable areas will be excluded from the calculated range.

  • marine areas from the range of terrestrial species

  • terrestrial areas from the range of marine species

  • areas in the biogeographical region where habitat type/species does not occur

  • areas more than 20km from coastline for coastal habitats

  • if data exist areas that do not overlap with limnic environment for freshwater habitats and species

Although the distinction between suitable and unsuitable areas is very coarse the purpose of fitting is to solve only most important contradictions resulting from automated calculation. The process of fitting should be simple and applicable across all Member States.

Technically the grids that occur only in the unsuitable areas will be excluded from range. Grids will not be cut by the limits of the area with unsuitable conditions, or limits of biogeographical region.

Description of the Range tool

The range tool generates a standardised grid based range using the rules discussed in this document. The tool uses two inputs to calculate the range. The first input is the distribution, which can be any spatial object (point, polygon,or grid). The second input is the reference grid system. Both inputs need to be in the same projection. The tool is based on calculating the distances between the centroids of grid cells and then constructing a series of polyline and polygons to connect other centroids of grid cells based on the ‘gap distance’ specified. All cells that intersect these polyline and polygons, as well as all distribution cells, are used to create the range. A separate set of technical guidelines regarding the range tool will be developed in the future.

Some particular issues.

Occasional occurrences, outlying occurrences

The Article 17 range is drawn as an external envelope around the habitat type/species distribution which excludes principal discontinuities. Size and shape of the range is then to a large extent determined by the occurrences on the outer limits of the distribution. The species are often occasionally recorded apart from their usual area of distribution, and these occasional records should not influence the shape and size of the range. The map of distribution than includes only regular occurrences of the habitat/species.

On the other hand, particularly on the boundaries of it natural geographical range, habitats/species occurs in limited numbers in atypical conditions. These exclaves are included in the distribution of the habitat/species if they represent regular or stable occurrence.


Many species including some listed in the Annexes if the Directive have a metapopulation structure, which is characterised by repeated extinctions and re-colonisations. Although as was previously stated the range is a spatial generalisation of the actual habitat/species distribution, in this case the range should represent the space which is used by metapopulation(s). Those localities with repeatedly recorded absence of the species, which can be based on the biology of a species interpreted as locally extinct, are included in the distribution map, as it takes part of the space used by metapopulation.

Incomplete distribution data.

Some of the gaps in the distribution, as well in the range maps, are due to gaps in the data. After automated calculation of range it is possible to correct the gaps resulting from incompleteness of data. The resulting range map will then be a combination of the automated procedure completed by expert judgement

Another option for common and widespread habitats and species would be to increase a gap distance.

How the calculated area of range will be used.

The map of range created by the agreed upon algorithm (with a use of the Range Tool) will be used directly or indirectly to fill in information requested by the reporting form.

Area of range

Automatically calculated range maps are a starting point for evaluation of the conservation status of range. The first information on range needed to fill in the reporting form is a range area. This is derived directly from the range map as its GIS calculated area.

Range maps are created for entire Member State, but the range parameters are reported separately for each biogeographical region. The range in the biogeographical region is represented by all grids which occur or partly occur within the region. So the grids on the boundary of the biogeographical regions are counted twice, once in each region.

Range trend

Ideally range trend is estimated by comparison of range maps created in different periods. The short-term trend period, including the period for evaluating the range trend, was set to 12 years. It is obvious that in the next reporting the range trend will be estimated based upon the expert judgement. If the changes in the distribution are known over these12 years it is possible to recreate a distribution map from before 12 years, calculate the surface of the range and compare it to the surface of the range at the end of the 2007-2012 reporting period.

For many countries the range for 2000-2006 was estimated by different method than it will be in the next reporting period. To make full use of the data harvested in the 2000-2006 reporting it is suggested, if data allow for that, to recalculate ranges based on the 2000-2006 distribution and use it in the trend estimation.

Favourable reference range.

The favourable reference range will be estimated with respect to the calculated range.

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