Great American Interchange

The isthmus of Panama connects the North American and South American continents.

It became a complete land bridge for the first time about 3,000,000 years ago, in the Middle Pliocene.

At the same time, the Andes Mountains were undergoing their final uplift, which doubled their height from ~ 2000 to ~ 4000 meters.

This led to dramatic climatic changes in South America at exactly the time that significant faunal exchanges were occurring between the two continents.

Before this exchange, which has come to be known as the Great American Interchange, each continent had 26 families of land mammals.

About 16 families from each dispersed to the other continent during the interchange.

Of the North American mammals, 26 genera dispersed southward, mainly in the Late Pliocene/Early Pleistocene (about 2.5 million years ago).

At the same time, 12 genera of South American mammals dispersed northward.

The fauna that was exchanged between NA and SA was one that was adapted to a savanna type community. This suggests that this must have been the environment of the isthmus when the exchange took place.

Although the percentages of dispersing mammals was similar for the two continents, the North American mammals fared better in South America than their South American counterparts did in North America.

In South America, 50% of the living land mammal genera are descended from North American immigrants.

The corresponding figure for South American descendants among North American mammals is only 29.1%.

Great American Interchange migrants. Those with a cross by their name became extinct in their new continent.

What was the reason for the greater success of the immigrants from North America?

One reason was undoubtedly climate. Tropical climate zones, which are warm year-round and moist at least part of the year (blue zones Af, Am and Aw) cover nearly all of Central America and much of South America, but very little of North America.

So, animals that were clearly tropical-adapted had a much greater area in which to be successful.

It has also been suggested that the North American mammals may have been shaped by millions of years of competition between mammal groups in the Northern Hemisphere, while South American mammals had been protected in a relatively isolated environment.

It appears that size may have played a role. Larger species were more likely to go extinct, and South American species were larger than North American ones.

Other notable factors:

All 13 species of endemic South American ungulates went extinct, probably because they were unable to cope with North American carnivores or with competition from North American ungulates.

Tapir terrestris

Tapirus indicus

An interesting result.

Tapirs and llamas originally dispersed from North America. They later became extinct here, but survived in South America.

As a result, both groups show a disjunct distribution in which their surviving relatives are found in Asia.