Great American Interchange - Reasons For Success or Failure

Reasons For Success or Failure

The eventual triumph of the Nearctic migrants was ultimately based on geography, which played into the hands of the northern invaders in two crucial respects. The first was a matter of climate. Obviously, any species that reached Panama from either direction had to be able to tolerate moist tropical conditions. Those migrating southward would then be able to occupy much of South America without encountering climates that were markedly different. However, northward migrants would have encountered drier and/or cooler conditions by the time they reached the vicinity of the Trans-Mexican Volcanic Belt. The challenge this climatic asymmetry (see map on right) presented was particularly acute for Neotropic species specialized for tropical rainforest environments, who had little prospect of penetrating beyond Central America (Central America currently has about 40 mammal species of Neotropical origin, compared to 3 for temperate North America).

The second and more important advantage geography gave to the northerners is related to the land area available for their ancestors to evolve in. During the Cenozoic, North America was periodically connected to Eurasia via Beringia, allowing multiple migrations back and forth to unite the faunas of the two continents. Eurasia was connected in turn to Africa, which contributed further to the species that made their way to North America. South America, on the other hand, was connected to Antarctica and Australia, two much smaller continents, only in the earliest part of the Cenozoic, and this land connection does not seem to have carried much traffic (apparently no mammals other than marsupials and perhaps a few monotremes ever migrated by this route). Effectively, this means that northern hemisphere species arose over a land area roughly six times larger than was available to South American species. This calculation may not be entirely fair, in that migrations between continents would have been more difficult and less frequent than migrations within South America. Nevertheless, it is clear that North American species were products of a larger and more competitive arena, where evolution would have proceeded more rapidly. They tended to be more efficient and brainier, generally able to outrun and outwit their South American counterparts. These advantages can be clearly seen in the cases of ungulates and their predators, where South American forms were replaced wholesale by the invaders.

Against this backdrop, the ability of South America's xenarthrans to compete effectively against the northerners represents a special case. The explanation for the xenarthrans' success lies in part in their idiosyncratic approach to defending against predation, based on possession of body armor and/or formidable claws. The xenarthrans did not need to be fleet-footed or quick-witted to survive. Such a strategy may have been forced on them by their low metabolic rate (the lowest among the therians). Their low metabolic rate may in turn have been advantageous in allowing them to subsist on less abundant and/or less nutritious food sources. Unfortunately, the defensive adaptations of the large xenarthrans would have been useless against humans armed with spears and other projectiles.