Migratory species are species that move from one habitat to another during different times of the year, as they cannot live in the same environment all year round due to seasonal limitations in factors such as food, sunlight, and temperature. The movement between habitats, which can exceed thousands of miles/kilometers in length for some migratory birds and mammals such as whales, is referred to as migration.
A migration route can involve resting and nourishment stops along the way, and also requires the availability of habitats before and after each migration. Thus, migrating species are at particular risk of changes in environment or land use.
Efforts to protect migratory species include government, private, and individual initiatives within the concerned countries, organizations such as the Sierra Club and Ducks Unlimited, and international agreements such as the Convention on Migratory Species (the Bonn Convention). As of January 2008, 104 nations have signed the Bonn Convention.
Historical Background and Scientific Foundations
Migrating birds were recorded about 3,000 years ago by the ancient philosophers Homer and Aristotle. Also, the Old Testament of the Bible has been interpreted as describing the migration of several bird species.
Migratory species can be capable of very long journeys. For example, the Arctic tern route between the Arctic and Antarctic is over 13,670 mi (22,000 km) long. The Bar-tailed Godwit migratory route, for example, is over 6,800 mi (11,000 km) in length, between Alaska and New Zealand, a journey that is made nonstop.
Other species that migrate include bats, whales, seals, turtles, and insects. An example of the latter is the Monarch butterfly, which migrates from southern Canada to winter in central Mexico.
Migratory species exploit the resources that are present in different areas of the globe at different times of the year. Because they cannot live in the same area all year—due to seasonal limitations in temperature, food availability, weather, or another factors—they have evolved to be capable of movement from one region to another.
Stopping en route to rest and feed is common among migratory species. The fact that they migrate and the need for space and resources such as food and water en route can make migratory species at risk from changing environments. For example, the drying up of a wetland or its development can take away a rest stop that is vital for the success of the migration. Similarly, any threat to the environments occupied at either end of the migratory route threatens species survival.
An example is the Monarch butterfly, whose returning populations from Mexico have been dwindling since the mid-1990s. Conservation scientists who have studied their decline feel that land use changes in central Mexico, specifically logging, have adversely altered the over-wintering grounds of the butterfly.
Climate change can affect migratory species. For example, the increased warming of more northern latitudes has shifted the territory of some migrating bird species farther north, adding length to their migratory journey. Migration itself has been affected for species whose route takes them over deserts. Regions such as the Sahel region of Africa have become drier, decreasing the opportunity to stop and refuel with food and water.
Impacts and Issues
Migratory species can be an indicator of environmental change, particularly of changes due to human activities, since their lives involve movement between different environments. These changes can be direct such as logging, or can be indirect. The warming of the atmosphere due to the production of greenhouse gases by various human-related activities is an important indirect change. As of 2008, only the Ramsar Convention on Wetlands—which was signed in 1971 and which, as of 2008, has 158 parties and includes 161 million hectares of wet-land—considers the influence of climate change on migratory species.
Climate changes that could threaten migratory species include a sea level rise, which could bury Caribbean and Mediterranean beaches used as breeding, nesting, or rearing sites by some turtle and seal species. Also the warming of shallow coastal waters would adversely affect whales, dolphins, and manatees, which require cold and shallow water as breeding sites.
WORDS TO KNOW
DUCKS UNLIMITED: An international non-profit organization founded in Canada in the 1940s to preserve and protect wetlands.
HABITAT: The natural location of an organism or a population.
LIGHT POLLUTION: Also known as photopollution and luminous pollution, refers to the presence of excessive amounts of light in the atmosphere.
SIERRA CLUB: Environmental organization founded in 1892 by American naturalist John Muir (1838–1914), whose mandate includes the protection and, when needed, restoration, of natural environments.
Climate change could also affect migratory species by altering the geographical distribution of other species that are food sources. Because migration can be over national boundaries, international cooperation is necessary to help protect migratory species. The Convention on Migratory Species, which was struck in Bonn, Germany, in 1979 under the United Nations Environment Programme (UNEP), and which is concerned with the global conservation of migratory wildlife and habitats, involves 104 nations. Although North America and coastal waters are part of the migratory routes for a variety of bird and mammal migratory species, the United States, Canada, and Mexico have not signed the convention.
Chiras, Daniel D., John P. Reganold, and Oliver S. Owen. Natural Resource Conservation: Management for a Sustainable Future. New York: Prentice-Hall, 2004.
Monarch Watch. http://www.monarchwatch.org/ (accessed April 21, 2008).