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A land bridge is an isthmus or some other land-based connection between two otherwise disconnected islands or continents. Land bridges are important ecologically because they allow the exchange of plants and/or animals that are otherwise separated, sometimes by millions of years of independent evolution. Land bridges are often transient, appearing and disappearing over geological time due to rising and falling sea levels.
The most famous land bridge in Earth's history is probably the Bering land bridge, which existed from present-day eastern Russia to Alaska about 20,000 years ago, during the last glacial period. During this time, sea levels were approximately 120 meters (394 ft) below today's levels, due to the huge quantities of ice locked up in continental glaciers. The Bering land bridge allowed humans to migrate from Asia to the Americas. After they crossed over, it is thought that they migrated down the coastline.
Though the Bering land bridge is the most famous, there were several other land bridges in the world at the same time. These include the large tract of low-lying tundra in what is today the south North Sea, which has been dubbed Doggerland, after the Dogger Bank, a large sand bank in the area today. Doggerland connected together England with continental Europe, and a separate land bridge connected England to Ireland. The earliest inhabitants of these islands made it across via land bridges.
There were extensive land bridges between islands in present-day Indonesia, connected them to the southeast Asian mainland. These made it possible for early humans to travel from Africa to islands like Borneo. In what is the first confirmed instance of humans traveling over a significant stretch of open ocean, early man built rafts and made it across the present-day Wallace Line, a deep sea channel in central Indonesia that separates the fauna of west Indonesia (which is more Asian) from east Indonesia (more Australian). From the east side of the Wallace Line, these people reached New Guinea and Australia, which were also connected by land bridges.
Further back in the past, about 2 million years ago, one of the world's most ecologically significant land bridges formed -- that between North and South America. Unlike the other land bridges discussed, this was formed by continental drift rather than lowered sea levels. As South America had previously been isolated for tens of millions of years, this exposed the continent to the flora and fauna of essentially the rest of the world (Europe, Africa, Asia, and North America were all connected). Generally speaking, the North American fauna dominated the South American fauna, though some South American animals did thrive in North America, for instance, the opossum.
Frequently Asked Questions
What exactly is a land bridge?
A land bridge, also known as an isthmus, is a narrow strip of land connecting two larger landmasses, allowing animals and humans to cross from one to the other. These geological formations can be created through tectonic activities, sea level changes, or glacial retreats. Historically, land bridges have played a crucial role in migration and species distribution, such as the Bering Land Bridge that once connected Asia and North America, facilitating the migration of people and animals between the continents.
How are land bridges formed?
Land bridges are formed through various geological processes. One common method is tectonic activity, where the Earth's plates collide or move apart, pushing up land from the seabed. Another way is through changes in sea levels, often associated with ice ages; when water is trapped in ice caps, sea levels drop, exposing land. Additionally, glacial retreat can carve out land paths between bodies of water. These processes can take thousands to millions of years to form a land bridge.
Can you give an example of a famous land bridge and its significance?
The Bering Land Bridge, also known as Beringia, is one of the most famous examples. It once connected Siberia and Alaska during the Pleistocene ice ages when sea levels were lower. This land bridge was significant for the migration of humans and animals, including the ancestors of Native Americans, around 20,000 years ago. According to the National Park Service, Beringia played a crucial role in shaping the biological diversity and cultural history of the Northern Hemisphere.
Are there any land bridges that exist today?
Yes, there are contemporary land bridges, such as the Isthmus of Panama, which connects North and South America. This land bridge has had a profound impact on the biodiversity of both continents by allowing the exchange of species in what is known as the Great American Biotic Interchange. The creation of the Isthmus of Panama is estimated to have occurred around 2.8 million years ago, drastically altering the ocean currents and climate patterns as well.
What is the ecological impact of a land bridge?
Land bridges have significant ecological impacts by enabling species migration, which can lead to genetic exchange and biodiversity. However, they can also introduce invasive species that outcompete native ones, leading to extinctions. The formation or disappearance of land bridges can alter habitats and climate patterns, influencing evolution and species adaptation. For instance, the Great American Biotic Interchange resulted in the mixing of North and South American faunas, profoundly affecting the ecosystems of both continents.