Changes in range provide important information about status and trends of species or habitats. Such changes result from the loss of populations at the edge of their distribution or from thriving populations expanding into new areas. Natural regression and anthropogenic destruction as well as natural or human-assisted expansion of habitats may not only affect the range of the habitat but also of species depending on it. Thus, range change usually implies a considerable trend in the abundance of a species or habitat at least at the margins of their geographic distribution. Here we first explain how the range of a habitat or species can be determined. For reasons of standardization, we strongly recommend to follow the approach developed by the Scientific Working Group of the Habitats Committee, DG Environment. Second, we provide guidelines how trends in range can be analysed.
Definition of range. Range is the area over which a species or habitat is usually found; it can be shown on a map or given as a surface area. Several types of range can be described, such as present day, historical, potential, etc. (Peterken 1996). Here range is taken to be the outer limits of the overall area in which a habitat or species is found at the time period, for which range is determined. It can be considered as an envelope within which areas actually occupied occur as in many cases not all the range will actually be occupied by the species or habitat. This is similar to the IUCN definition of range as 'the area contained within the shortest continuous imaginary boundary that can be drawn to encompass all its known or inferred sites of present occurrence (excluding cases of vagrancy)'. The population distribution will be a subset of the range and is defined as the area actually occupied.
The natural range includes also areas that are not permanently used: for example for migratory species 'range' means all the areas of land or water that a migratory species inhabits, stays in temporarily, crosses, or overflies at any time on its normal migration.
Range may be discontinuous. Obviously, a habitat or species restricted to high mountains will not be found on the low ground between two mountainous areas. However, other species and habitats may also have a discontinuous range because of various other biogeographic or climatic reasons; e.g. the alpine newt (Triturus alpestris).
Determining range. Range is estimated by use of a grid map at a suitable scale as the area of the minimum convex polygon encompassing the distribution area of a species or habitat. For reasons of consistency and standardizations, we suggest to follow the methods recommended by the Scientific Working Group of the Habitats Committee, DG Environment, in the guidelines of reporting under Article 17: A polygon is drawn around the occupied cells of a 10 km x 10 km grid. Small gaps in the distribution are considered as part of the range but larger gaps are considered as breaks in the range. There should be at least 4 or 5 non-occupied grids (about 40-50 km distance) to justify a break in the range. This value may need to be modified on the basis of the dispersal and migration potential of a species; no standards for different dispersal potential have been established as yet. Significant areas ecologically not suitable should be excluded from the habitat/species range. The approach is shown using the example of habitat '2110: embryonic shifting dunes' in Germany in the following figure.
Discontinuous range of the alpine newt (Triturus alpestris). Source: GASC et al. (1997): SEH Atlas. With courtesy of Natural History Museum Paris
The Scientific Working Group of the Habitats Committee, DG Environment, recommends to use the same method to calculate the range of habitats and species having a linear occurrence (e.g. riverine habitats, fish species). While this is a valid suggestion, the length of the river sections, along which a habitat or species occurs, may be a more precise measure of range for such habitats and species.
The range of habitat '2110 Embryonic shifting dunes' of the Habitats Directive in Germany (from Axel Ssymank & Eckhard Schröder, BfN)
Statistical analysis. If range is determined as outlined above, then range data are of the form "size of area occupied", "number of grid cells occupied", or "length of a landscape element occupied". The statistical analysis for each of these three data types can use the same approach. The range data are regressed against time with standard linear regression models. This approach assumes that the complete range edge is surveyed on each occasion and that the probability of detecting the species or habitat within any grid cell is one, if it is present in that grid cell. This may be the case for most habitats and for conspicuous species but may be violated for others. In that case the trend estimate may still hold, if detection probability remained constant through time, which requires among others that survey effort remained constant. Generally, survey efforts for documenting the distribution of species and habitats have increased with time. This makes it more difficult to detect a negative trend against the increased probability of registering the presence of a habitat or species in a grid cell. Thus, a statistically detected negative trend of range likely reflects indeed a real and large trend and should receive appropriate management and policy response. Notwithstanding, detection probability should be estimated if there are doubts that a species or habitat is detected with certainty within a grid cell. A further problem is that range changes require the foundation of new populations or the extinction of existing ones or massive regression or expansion of their habitats. As these processes usually occur at the scale of decades rather than years, monitoring schemes must run for a very long period of time.
- Gasc, J.-P.; Cabela, A.; Crnobrnja-Isailovic, J.; Dolmen, D.; Grossenbacher, K.; Haffner, P.; Lescure, J.; Martens, H.; Martínez Rica, J.P.; Maurin, H.; Oliveira, M.E.; Sofianidou, T.S.; Veith, M. & Zuiderwijk, A. (eds.) (1997): Atlas of Amphibians and Reptiles in Europe. - Societas Europaea Herpetologica & Muséum National d'Histoire Naturelle, Paris, 469 Pp.
- Peterken, G. (1996). Natural Woodland - Ecology & Conservation in Northern Temperate Regions, Cambridge University Press, Cambridge.
EuMon core team; May 2013