Great American Interchange - The Great American Biotic Interchange

The Great American Biotic Interchange

The formation of the Isthmus of Panama led to the last and most conspicuous wave, the great interchange, around 3 Ma ago. This included the immigration of North American ungulates (including camelids, tapirs, deer and horses), proboscids (gomphotheres), carnivorans (including felids like cougars and saber-toothed cats, canids, mustelids, procyonids and bears) and a number of types of rodents into South America. The larger members of the reverse migration, besides ground sloths and terror birds, were glyptodontids, pampatheres, capybaras and the notoungulate Mixotoxodon (the only South American ungulate known to have invaded Central America).

In general, the initial net migration was symmetrical. Later on, however, the Neotropic species proved far less successful than the Nearctic. This misfortune happened both ways. Northwardly migrating animals often were not able to compete for resources as well as the North American species already occupying the same ecological niches; those that succeeded in becoming established were not able to diversify much. Southwardly migrating Nearctic species established themselves in larger numbers and diversified considerably more, and are thought to have caused the extinction of a large proportion of the South American fauna. (There were no extinctions in North America obviously attributable to South American immigrants.) Although terror birds were initially able to occupy part of North America, their success was temporary; this lineage disappeared about two million years ago. The other large warm-blooded Neotropic predators fared no better. Native South American ungulates also did poorly, with only several of the largest forms, Macrauchenia and a few toxodontids, withstanding the northern onslaught. (Among the notoungulates, the mesotheriids and hegetotheriids did manage to hold on until the Pleistocene.) On the other hand, South America's small marsupials survived in large numbers, while the primitive-looking xenarthrans proved to be surprisingly competitive and became the most successful invaders of North America. The African immigrants, the caviomorph rodents and platyrrhine monkeys, generally held their own during the interchange, although the largest rodents (e.g. the dinomyids) disappeared. With the exception of the North American porcupine and several extinct porcupines and capybaras, however, they did not migrate past Central America.

The initial wave of southwardly migrating Nearctic carnivorans rapidly occupied the South American predatory niches, displacing phorusrhacids and sparassodonts, as well as eliminating Chapalmalania. The paucity of early competition and plentiful prey seems to have allowed short-faced bears to rapidly evolve into the largest known bear or terrestrial mammalian carnivore species; Arctotherium angustidens is estimated to have weighed around 1600 kg. Later species of Arctotherium exhibited a trend towards smaller size and a more omnivorous diet, probably due to increasing competition from later-arriving or evolving carnivores. In contrast, Smilodon showed a trend toward increasing body size that culminated in the appearance of S. populator, at up to nearly 500 kg the most massive felid known.

Due in large part to the success of the xenarthrans, one area of South American ecospace the Nearctic invaders were unable to dominate was the niches for megaherbivores. Before 12,000 years ago, South America was home to about 25 species of herbivores weighing more than 1000 kg, consisting of Neotropic ground sloths, glyptodontids and toxodontids, as well as gomphotheres and camelids of Nearctic origin. Native South American forms made up about 75% of these species. However, none of these megaherbivores have survived.

The presence of armadillos, opossums and porcupines in North America today is explained by the Great American Interchange. Opossums and porcupines were among most successful northward migrants, reaching as far as Canada and Alaska, respectively. Most major groups of xenarthrans were present in North America up until the end-Pleistocene Quaternary extinction event (as a result of at least eight successful invasions of temperate North America, and at least six more invasions of Central America only). Among the megafauna, ground sloths were notably successful emigrants; Megalonyx spread as far north as the Yukon and Alaska, and might well have eventually reached Eurasia if the extinction event had not intervened.

Generally speaking, however, the dispersal and subsequent explosive adaptive radiation of sigmodontine rodents throughout South America (leading to over 80 currently recognized genera) was vastly more successful (both spatially and by number of species) than any northward migration of South American mammals. Other examples of North American mammal groups that diversified conspicuously in South America include canids and cervids, both of which currently have 4 genera in North America, 2 or 3 in Central America, and 6 in South America. Although Canis currently ranges only as far south as Panama, South America still has more extant canid genera than any other continent.

The effect of formation of the isthmus on the marine biota of the area was the inverse of its effect on terrestrial organisms, a development that has been termed the "Great American Schism". The connections between the east Pacific Ocean and the Caribbean were severed, setting now-separated populations on divergent evolutionary paths. Caribbean species also had to adapt to an environment of lower productivity after the inflow of nutrient-rich water of deep Pacific origin was blocked.