Migration is typified as the movement of a large number of animals into a common location. It is also a very prolific occurrence in many different animal orders, and serves a number of different adaptive purposes There are also different reasons for migration,ranging from environmental pressures to mate selection.
Many animal species migrate simply because of a need to stay in warmer weather during the cold seasons. Animal habitation is usually dictated by the availability of food in their immediate surroundings, and in many locales food becomes scarce during the winter months. This in turn forces animals to move into new areas where there is sufficient food available to support a population.
Conversely, some species are forced to migrate in order to avoid the hottest weather which often means access to water becomes limited at certain points of the year. In many regions drought and increased competition for limited resources means that animals capable of moving over long distances are benefited by moving to cooler climates during the summer months. Those who cannot move great distances are often pressured and face reduced populations and are more vulnerable to unseasonable weather.
For example if a drought or dry season lasts much longer than usual, the species who are forced to remain in the same location will often face large drops in population. Whereas migratory species will remain unaffected and upon returning will benefit from the reduced number of predators and competitor species.
Some species also migrate in the short term to take advantage of seasonal blooms in certain food sources, despite the fact that there is adequate food where they are already. During plankton blooms for example, many different species will move from their normal feeding grounds to follow the bloom for the brief period that it lasts. Because plankton are the basis of many food chains in the sea, prey and predators alike are attracted to the event, and will often feed around the clock while there is a high abundance of food available.
Mating is one of the main reasons for migration, and can benefit species in a number of ways. Often within an adult population of animals there are numerous predators and often competition for space, food and resources such as nesting sites. By migrating, species often assure that their offspring are born away from many of the predators which might otherwise prey on them. For example many bird species will migrate to isolated islands and cliff faces where the usual predators found in the lowlands cannot gain access to them.
The restricted access to young and developing individuals in a particular species helps to keep the population at a stable level rather than rising and falling. The limited space available at most mating grounds also ensures that the population doesn't get too large, because there is often not enough room for a great increase in the population to occur in a space limited location.
This in turn enables the young to have a better chance of survival and keeps the species from becoming threatened by environmental factors in their usual habitat. A surge in certain predator populations during a certain period for example is then negated by the fact that the offspring are born elsewhere, and are much less at risk from sudden shifts in population. Examples of sudden population shifts include over protection of predator species on game reservations, or the sudden availability of new prey species.
Another benefit of migratory mating is that there are a greater number of mates to choose form for each individual. In smaller, relatively isolated populations gene pools may become stagnant and mutations can occur. If smaller pockets of overall populations migrate to a common location for breeding purposes however, the gene pool is constantly diluted and mixed, stopping mutations and inbreeding from happening as much.
Some species also need to move to a new area to migrate because their life cycles dictate it. Many insects and amphibians hatch in water and begin their lives requiring a water source. Tadpoles are good examples of animals which require water early in their life cycle but later grow and change so that as adults they no longer need an immediate water source at all times. This necessity means that adults are forced to return to a water source to breed and lay their eggs.