Migration (biology)

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Migration is a concept to describe the movements of organisms above a certain (species and scale dependent) range. In the case of animal species, a distinction is usually made between more or less everyday movement patterns that occur within an action space or territorytake place, and which primarily serve to seek out and obtain food or the search for mating partners, and movements beyond that, which lead the animal far outside of its range of action, migrations are only the movements that lead beyond the range of action. These movements are not purely random, as would be the case if organisms would drift through a severe storm, but are usually predetermined by morphological and behavioral adaptations and are meaningful and necessary in the life cycle and population structure of the respective species. Depending on their characteristics and biological meaning, these movement processes are classified differently:

  • Migrations in the narrower sense are directed migrations, usually leading to a destination and back to the original habitat, which are carried out by entire populations, usually synchronously (see also: animal migrations ). The same individuals can migrate back and forth (e.g. bird migration , ungulate migrations in the African savannah , migration of whales between tropical and arctic seas) or their offspring (mostly insects, e.g. aphid species that change host ).
  • Propagation or dispersion processes (English: dispersal) are directed migrations out of the traditional habitat, mostly with the aim of settling in new habitats (colonization). Every biological species must at least to some extent be capable of such dispersal processes, otherwise it would die out immediately with any change in the living conditions in the original habitat . Spreading can vary depending on the species, from movements in the centimeter range to movements across continents. Many species have special life phases or stages that are specifically designed to allow them to spread.

Both the delimitation of these processes from one another and from the “normal” movement patterns in the action area is blurred. For example, there are animal species that have no permanent defined action space at all; these are called “nomadic” species. Migrations in the second sense ( dispersion ) affect all types of organisms, e.g. B. also plants, fungi and microorganisms. Depending on the type, both types of migration can always (obligatory) or only on special occasions and conditions (e.g. food shortages) (optional) occur in a certain phase of life.

When considering migrations, two levels of observation exist side by side, both of which have a long tradition, but are rarely linked to one another:

  • Level of the individual. Individual adaptations of the respective organism to migration processes are examined here, e.g. B. Behavioral adjustments.
  • Population level. The main focus here is on the impact of migration processes on the ecology and distribution of populations and species. A typical approach is e.g. B. the theory of metapopulations .

The level of genes is equally important for both levels, but has so far only been researched in part. If spread is to be adaptive , it must have a genetic basis. In research, this is considered on a descriptive level (through analysis of heredity , e.g. behavior) and causally (through direct analysis of behavior-determining genes). The processes have only just begun to be solved.

Types and forms of migration

There are numerous approaches to typify migrations alongside one another. These have different advantages depending on the level and question. The approaches to processing birds, mammals, insects and marine organisms are also traditionally somewhat different.

According to the spatial movement pattern, e.g. B. the following cases:

  • Outward and backward migrations. This is the classic case that has been shaped primarily by the study of bird migration . When birds migrate, the same individuals tend to move back and forth between summer and winter habitats. Other well-studied cases include: B. bats or whales. The East Pacific gray whale population migrates between the Bering Sea and the Bay of California every year, with individual whales traveling more than 18,000 kilometers a year. For each of these groups, the outward and return routes can be different, creating a loop. Especially with insects, but also z. B. In the case of migratory fish species such as salmon and eels , it is not the individuals themselves, but rather their offspring who begin the return migration. Daily migrations in the water column up and down, which can be several hundred meters, are typical for numerous representatives of marine plankton .
  • directed migrations. Numerous species have an obligatory migration phase in their life cycle. In many animal groups this is a larval stage. In the case of the flying insects (Pterygota), the young adult animals (Vollkerfe, Imagines) usually carry out so-called dispersion flights . The expansion phase is firmly integrated into the life cycle. Usually the maturation of the gonads is delayed and only takes place after the migration phase; this has been systematized as the "oogenesis flight syndrome". Numerous insect species coexist with flightless (long-winged or macroptere ) and flightless (short-winged or wingless, brachyptere or aptere) individuals. This dimorphism very often has a genetic basis: As a rule, in holometabolic insects (with complete metamorphosis) a single allele is responsible, in which the long-winged form is recessive and the short -winged form is dominant. More rarely there is a real spread morph induced by environmental influences, as in the case of the migratory locusts . Species that migrate back and forth usually also carry out directed migrations, often at a different stage of life. There is also a gender difference in bats: males migrate back and forth more frequently, females more frequently. (In the terminology of vertebrate researchers, the term migration is not always applied to these directed movements.)

Most migrations happen regularly; B. annually (annually) or seasonal (seasonal). For most of the regularly migratory species, special timers have developed which determine the migration behavior, even if the underlying reason is ecological (e.g. lack of food). This is because it is risky to start the hike only when the deterioration occurs. The extent and duration of the migration are also often hereditary. Such species do not stop migrating if they encounter a habitat with favorable conditions on their route. Irregular and non-seasonal migrations are also called irruptions . These are usually triggered directly by the deterioration in living conditions. Migrations of aquatic organisms between freshwater and saltwater habitats are called diadromous migrations. In addition to migratory fish, they also occur in invertebrates (e.g. the woolly crab ). Migrations with the medium, i.e. through air or water currents, are often referred to as drift .

Evolution of migration behavior

Migration is associated with considerable effort for the organisms involved. Not only the energy consumption for movement is important. Even the possession of locomotion organs such as wings and the necessary muscles is associated with energy expenditure. In insect species with short- and long-winged individuals (wing dimorphism), the short-winged ones can usually lay more eggs or finish their development earlier. If migration is an adaptive feature promoted by evolution, this effort must be offset by corresponding gains. For an individual organism, there is an optimization problem : If it does not migrate, it is in a habitat with environmental conditions that were at least good enough for it to successfully complete its own development. On the other hand, conditions can (accidentally or predictably) worsen. Habitats newly populated by migration serve as a risk buffer against accidental extinction due to disasters. In addition, competition from conspecifics is likely to be less or even absent in a newly colonized habitat. It is immediately evident that species that live in habitats that only exist for a short time (ephemeral) or in habitats with strongly fluctuating conditions have to invest more in migration than species in stable habitats with predictable conditions. (As an alternative to migration, however, resistant permanent stages can also serve: dormancy .) Since it can already be too late when the conditions actually deteriorate, the evolution of timers (such as the photoperiod) or endogenous rhythms may be promoted. Some definitions of migration only allow changes of location based on such factors to apply. If migrating individuals succeed in founding new populations, it is to be expected that in this new population the proportion of mobile individuals who are willing to migrate will initially be very high (since all of them descend from such individuals). If this habitat is stable and long-lived (in relation to the development period of the species under consideration), the wingless individuals have an advantage here because of their higher reproduction rate. So your share should increase rapidly. If the colonization rate of new habitats is too low, the ability to migrate can possibly be completely lost - with a long-term greatly increased risk of extinction (investigated e.g. in ground beetles and weevils ).

swell

  • Hugh Dingle (1996): Migration: The Biology of Life on the Move . New York: Oxford University Press.
  • Hugh Dingle & Alistair Drake: What is migration? . In: BioScience . 57, No. 2, 2007, pp. 113-121. doi : 10.1641 / B570206 .
  • Derek A. Roff & Daphne J. Fairbairn (2007): The Evolution and Genetics of Migration in Insects . BioScience Vol. 57, No. 2: 155-164.

Individual evidence

  1. ^ William Henry Burt (1943): Territoriality and Home Range Concepts as Applied to Mammals. Journal of Mammalogy Vol. 24, No. 3: 346-352.
  2. ↑ Melissa S. Bowlin, Isabelle-Anne Bisson, Judy Shamoun-Baranes, Jonathan D. Reichard, Nir Sapir, Peter P. Marra, Thomas H. Kunz, David S. Wilcove, Anders Hedenstrom, Christopher G. Guglielmo, Susanne Akesson, Marilyn Ramenofsky, Martin Wikelski (2010): Grand Challenges in Migration Biology. Integrative and Comparative Biology, Vol. 50, No. 3: 261-279. doi : 10.1093 / icb / icq013
  3. ^ Theodore H. Fleming & Peggy Eby: Ecology of bat migration. In: Thomas H. Kunz & M. Brock Fenton (editors): Bat Ecology. University of Chicago Press 2006.
  4. ↑ Cecil George Johnson (1969): Migration and dispersal of insects by flight. London (Methuen).
  5. ^ TRE Southwood (1962): Migration of terrestrial arthropods in relation to habitat. Biological Reviews 37: 171-211. doi : 10.1111 / j.1469-185X.1962.tb01609.x
  6. ^ PJ den Boer: Dispersal power and survival: carabids in a cultivated countryside. Miscellaneous papers (Landbouwhogeschool Wageningen) 14 (1977). 190pp.
  7. ^ PJ den Boer (1990): Density limits and survival of local populations in 64 carabid species with different powers of dispersal. Journal of Evolutionary Biology 3 (1/2): 19–48 (Basel)
  8. ↑ VW Stein (1977): The relationship between biotope age and occurrence of short-wingedness in populations of dimorphic weevil species (Col., Curculionidae). Journal of Applied Entomology 83: 37-39. doi : 10.1111 / j.1439-0418.1977.tb02372.x