The word migration has its roots in the Latin language from the word “migrationem” which refers to a change in living area by either people or animals. Ecologists define migration as the movement of a large number of organisms of a particular species from one region to another. For many animals, migration is a common occurrence, and it occurs for some reasons such as to search for food, water, and mates. Other animals, particularly those preyed upon, migrate in search of safety from their predators. Ecologists classify migration according to the type of organism moving from one place to another with some of the most common types being bird and fish migration.
Some mammal species migrate in large numbers with one of the most well-known instances being the wildebeest migration from the Masai Mara in Kenya to the Serengeti in Tanzania. Rainfall patterns in the area significantly influence the movement of wildebeest in search of grazing land. Estimates indicate that close to 2,000,000 wildebeest participate in the migration across the Kenya-Tanzania border. Apart from wildebeest, other animals that make the journey include gazelles and zebras. Research by Y.A. Darman, A.A. Danilkin and A.N. Minayev indicates that some Siberian Roe Deer migrate seasonally. The populations that migrate move from their wintering areas to areas experiencing spring to breed. They stay in the breeding range rearing their fawns until autumn when they move back to the wintering range. The Mongolian gazelle has also been observed to migrate particularly during the spring and autumn. In North America, mammals that migrate include bison, pronghorns, and wapiti. The migration of some species such as the scimitar-horned oryx, the springbok, and the blesbok has stopped due to human influence.
Migration is a common phenomenon among numerous bird species, and they usually fly from the northern regions to the south. Several journeys that birds make are not considered migrations since they do not fit all the requirements. Birds typically move from the areas where they breed and raise their young to their wintering grounds. Bird migration was well-recorded during ancient times by well-known authors such as Aristotle and Homer who chronicled the movement of swallows, storks, and turtle doves. Johannes Leche was one of the modern pioneers in studying bird migration. Leche's work mainly involved recording the dates when birds arrived in Finland during spring. Ecologists have identified about 1,800 bird species that engage in long-distance migration. Ornithologists have identified several patterns within the migratory habits of birds with one of the most important being the tendency to fly from north to south while following a path known as a flyway. Some of the well-known birds that follow north-south flyways include swallows, northern wheatears, and the Amur falcon. Several bird species also migrate to higher or lower altitudes to escape harsh environmental conditions. Some of the birds observed to practice altitudinal migration include the white-throated dipper and the wallcreeper.
Reverse migration is a phenomenon observed in many bird species where they travel in a route in the opposite direction of the traditional path followed by their species. Scientists have determined that birds learn their migratory routes through two methods; following their instincts or following their parents. Birds that follow their instincts are usually more susceptible to reverse migration. Some of the bird species that have recorded cases of reverse migration include the Pallas's Warbler and the yellow-throated Vireos. Most of the birds that follow the wrong route usually die, but some survive. In some instances, the birds that survive go back to the same region over successive seasons while others reorient themselves and follow the traditional migratory paths.
A large number of fish species move from one region to another with some moving thousands of miles. Like other animals, fish typically migrate to reproduce or search for food. Fish migration may be anadromous, where mature fish living in the salt water move to fresh water bodies to reproduce, or catadromous, where mature fish residing in fresh water bodies move to salt water bodies such as the sea to breed. Salmon are some of the most well-known anadromous fish that migrate from the sea to the fresh water rivers. On the other hand, eels are the best example of catadromous fish that migrate that migrate from fresh water rivers to the seas. Other fish species, particularly those that forage, migrate over vast distances and scientists have linked their migration to ocean currents and the search for food. Several scientists have suggested that since the fish cannot recognize their young, their movement ensures that they do not engage in cannibalism. Due to the importance of fish to the global economy, the UN coined the term highly migratory species to describe fish that move within the Exclusive Economic Zones of separate countries. Some of the fish in this category include tuna, sailfish, and yellowfins.
Due to a large number of insect species, scientists have a difficult time coming up with an exact definition of insect migration. John Kennedy, one of the premier etymologists, came up with one of the most widely used definitions based on the insects' behavior. Locusts, butterflies, beetles, and moths are some of the insects that migrate most often. The term "Lepidoptera migration" was coined to describe the migratory behavior of butterflies and moths which occurs in every continent apart from Antarctica. One of the best-studied cases of Lepidoptera migration is that of the Monarch butterfly which move to Mexico from Canada. Another type of insect migration is the Odonata, the long-distance movement of the dragonfly.
Diel Vertical Migration
Diel vertical migration is considered the most significant migration in the world because of the large biomass of the organisms that participate in the migration. Diel vertical migration usually occurs when organisms move to higher levels of a water body during the night and during the day they move to the lower levels. The primary cause of the migration is a difference in light intensities during the day and night. Animals that practice diel vertical migration include squids, copepods, and trout.
Effects of Human Activities on Animal Migration
Human activity has a direct impact on animal migration with some of the most affected species being white storks and salmons. Dams built across rivers sometimes prevent salmons from reaching their breeding grounds which significantly reduces the number of salmon in the wild. Fish ladders were developed to allow the fish to overcome barriers such as dams. Dumpsites filled with junk foods have been known to divert birds such as white storks from their traditional migration routes.