Background: current state of knowledge < Definition of migratory Species > Knowledge about migratory species

2.1 — Definition of migratory species

First of all, a clear definition of migration is needed. Some scientists use a rather broad definition, such as "Migration: the act of moving from one spatial unit to another" (Baker 1978, p. 23). In contrast, ‘migratory species’ as defined by the CMS means

"the entire population or any geographically separate part of the population of any species or lower taxon of wild animals, a significant proportion of whose members cyclically and predictably cross one or more national jurisdictional boundaries" (CMS 1979, Article 1)

An important component of the definition is its emphasis on the return component of migrations ("cyclically and predictably"). This is close to the biological concept of ‘true migration’ (Dingle 1996). The latter has been defined as a seasonal movement that implies some element of a return to some initial starting point – the traveller needs a ‘return ticket’ (see McKeown 1984, p. 3, for a discussion of different definitions). This definition was adopted by GROMS. As a biological concept it is independent of national boundaries, and therefore includes species migrating within large range states. However, for reasons of practicability, small-scale migrants such as amphibians were not included in GROMS, and the minimum migration distance was set at 100 km.

The original CMS proposals for species to be included in Appendix I classify migration with respect to international boundaries as follows (BELF 1979, explanatory notes):

  1. Migration wholly outside national jurisdictional boundaries,
  2. Migration between areas outside national jurisdiction and those under the jurisdiction of one or more states,
  3. Migration between areas under the jurisdiction of two or more states,
  4. Migration involving movements across borders by members of populations living in contiguous areas on either side of one or more national jurisdictional boundaries (border taxa).

Category 4 (border taxa) contains several species confining their migratory movements to short distances, but between different range states. Examples are the mountain gorilla (Gorilla gorilla beringei), occurring between Rwanda, Burundi, and Democratic Republic of Congo (Figure A2.14), or the South American sea otters (Lontra spp.) with home ranges around 30 km within the border region between Chile and Argentina. CMS Appendix II contains several non-migratory European bats, which are protected by the EUROBATS agreement (Figure A2.2). These species are labelled as ‘technical migrants’ within the GROMS database.

In addition, the scientific evidence for an exact assessment of migration behaviour is often insufficient. Migratory behaviour might be unknown, or current studies reveal that species are nomadising, emigrating, or exhibit range extensions after breeding (see Table 2.1). In particular the regular return component of ‘true migrants’ is often difficult to assess, and classifications are not always clear and heavily disputed. As with all living systems, such categories have to be treated with a certain elasticity – too rigid definitions would result in what Williamson (1999) criticised as ‘bureaucratic ecology’. Last not least, species evolve, and it has been shown recently that genetic programs controlling migratory behaviour can change within a few generations (Berthold 1998).

Fish migrations have their own classification and terminology. Many species cross the important habitat barrier between fresh and sea water, which requires major physiological changes during migration. The classification of the principal migration strategies identified by ichthyologists is based on the direction of this important transition (cf. McDowall 1988):

Migrations between fresh and sea water are summarised as diadromous. They have been subdivided by Myers (1949) into the following categories:

Examples are discussed in chapter 4.3.4.

The register contains all species listed by CMS, together with all species identified as true migrants according to the GROMS definition. Potential migrants were added to the database, and their migration behaviour was classified according to the categories listed in Table 2.1.

All diadromous fishes are classified as migrants in the GROMS, because the change of medium (sea to fresh water) is comparable to a major distance. Species which turned out to be non-migratory were marked up and are not printed in Annex 3. However, these species were not deleted from the database, because often they play an important role as competitors within species communities.

Table 2.1 shows the differences between the definition of migration by GROMS and by CMS. CMS does not include migrants within large range states, as they do not cross national boundaries. For example, the endangered black-fronted tern (Chlidonias albostriatus) migrates more than 100 km within New Zealand, but it does not cross political boundaries. On the other hand, technical migrants listed by CMS, such as the whiskered bat (Myotis mystacinus), probably move within the 100 km limit, but cross European borders, where they are protected by the EUROBATS agreement (see Table 2.3).

Tab. 2.1: Classification of migratory behaviour as used in GROMS.

For CMS, a migratory species has to cross political boundaries, while GROMS focuses on "true migrants" covering more than 100 km, or crossing from sea to freshwater. A species with intracontinental migration is not necessarily a CMS migrant, as migration might occur within one range state. Therefore, the respective category is put in brackets (+).

Tab. 2.1: Einteilung des Wanderverhaltens im Weltregister wandernder Tierarten.

Gemäß der Bonner Konvention muß eine wandernde Art politische Grenzen überqueren, während das Weltregister nur "echte Wanderarten" berücksichtigt, die mehr als 100 km zurücklegen. Eine intrakontinental wandernde Art erfüllt nicht notwendigerweise die Definition der CMS, da die gesamte Wanderung innerhalb eines Staates stattfinden kann. Die entsprechende Kategorie steht deshalb in Klammern (+).

Category Explanation CMS-migrant GROMS-migrant
(> 100 km)
Major category
Non-migratory Non-migratory
GROMS migrant Migratory according to GROMS definition (+) +
Technical migrant Movements across borders by members of populations living in contiguous areas on either side of one or more national boundaries (border taxa) + (+)
Partial Minor part of population migratory (+) (+)
Possibly migratory Some references indicate possible migration    
Data deficient Possible migrant for theoretical reasons, but no data available    
Subdivisions of GROMS migrants
Intracontinental Within continents (+) +
Intercontinental Between continents + +
Nomadising Following resources, often without predictable temporal patterns. (+)
Emigration Mass migrations after population explosions
Range extension E.g. post-breeding dispersal of birds or bats (+) (+)
Marine organisms (except fish)
Interoceanic Movement between different oceans or transoceanic + +
Intraoceanic Within oceans or continental shelves + (+)
Fish migrations
Anadromous Return from ocean to fresh water for reproduction, subdivision of diadromous (+) +
Catadromous Migration into the sea for reproduction, subdivision of diadromous (+) +
Amphidromous Migration between sea and fresh water for feeding, subdivision of diadromous (+) +
Potamodromous Well-defined migration within streams and rivers (+) (+)
Limnodromous Within lakes (+) (+)
Oceanodromous Within sea water (+) (+)

There might be a considerable number of additional border taxa which cross political boundaries during small-scale or vertical migration. Johnson & MacLean (1994) address short-distance, but regular migration of birds from high to low altitude with season in Natal, where steep altitudinal gradients span huge changes of climate. Such migrations might involve crossing of boundaries. For example, the species-rich regions between Ecuador and Columbia would probably reveal a considerable number of technical migrants qualifying for CMS-listing. However, the identification of these short-distance migrants is much more difficult and far beyond the present scope of GROMS. In addition, it might be asked if a considerable increase of CMS-appendices by technical migrants can be handled efficiently. Finally, it must be reminded that political boundaries changed considerably during the last decades. In the future, the GROMS listing will allow a rapid identification of migrants which suddenly do qualify for CMS listing in case of political subdivision of large range states.

Background: current state of knowledge < Definition of migratory Species > Knowledge about migratory species

This document should be quoted as part of the publication "Riede, K. (2001): The Global Register of Migratory Species ­ Database, GIS Maps and Threat Analysis. Münster (Landwirtschaftsverlag), 400 pp." + CD

 by Klaus Riede