Wildlife migration is the seasonal movement of animals from one habitat to and from another in search of food, for better climatic conditions or for reproductive purposes. It occurs in a wide range of species, including some insects, crustaceans, fish, amphibians, reptiles and mammals.
Wildlife migration is distinguished from wildlife emigration, which is the long-term movement of animals and plants to a new location as a result of permanent changes in the condition of their habitat, such as a warming trend or the loss of a food source.
Migrating animals might travel by land, water or air to reach their destination, and the distances can range from a few tens of meters to more than 10,000 kilometers. The migrations are often in a north-south direction, but can also instead involve a change of altitude or other environmental conditions. Individuals in some species will migrate many times over their lifetime and some may do it only once, while some species may pass through multiple generations over a single round trip.
The earth's largest and possibly most spectacular wildlife migration is the enormous, hundreds of kilometers in length, clockwise loop that more than two million wildebeest, zebras and other species of large mammals make across the enormous Tanzanian Serengeti and Kenyan Mara ecosystem in East Africa.
Another amazing example is that of monarch butterflies, which every autumn escape the freezing winter weather of southern Canada and the eastern U.S by flying all the way to central Mexico, where they huddle together in dense clusters in trees. The return trip is made over multiple generations, stopping on the way to lay eggs on milkweed plants. The emerging striped caterpillars voraciously eat the milkweed and then, after transforming into butterflies, continue the northward journey.
Although animals most commonly migrate together in herds or flocks, some migrate alone. An example is the aptly named solitary sandpiper, a North American shorebird that lives mostly alone along the the shaded edges of streams, ponds and ditches.
Much remains unknown about wildlife migration, including how the animals know when and where to go, especially for individuals who have not made the journey previously. Among the several possible navigation aids they might use are the earth's magnetic field, the position of the sun, and smells.
Human activity has been increasingly interfering with long-established animal migration patterns in numerous ways. In addition to human-caused climate change, examples include the damming of rivers used by salmon and other fish, the draining of wetlands used for stopovers by migrating birds, the cutting down of trees used used as the winter home for monarch butterflies, and the blocking of land migration routes with fences, roads and urban sprawl.
This interference can have a cascading effect on ecosystems by not only threatening species directly and leading to their population declines, loss of genetic diversity and even extinction in some cases, but also harming additional species that are dependent on these species.