Abstract
The Great American Interchange lasted for about two million years between North America and South America. The objective of the essay is to look at the symmetrical results of the great American interchange in the beginning and how the interchange became asymmetrical after time progressed. The unbalanced nature of the interchange offers a model to explain the asymmetrical results of the land-mammal interchange between the two Americas. Both Geological and biological data are looked into the get insights the past and present distributions of fauna.
Introduction
The Great American Interchange is a crucial paleo zoogeographic event that occurred between North America and South America. The Interchange took place about three million years ago during the Piacenzian age (Marshall, 1988). The sudden exchange of mammals between the two isolated continents is one of the most celebrated events in the faunal history (Marshall, 1988). There was a noticeable increase in a number of genera for the South American fauna during the peak times of the Great American Interchange(Marshall, 1988).The land and freshwater fauna migrated to South America via Central America as the volcanic activity of Isthmus of Panama bridged the continents that were separated before (Woodburne, 2010). The Biotic Interchange, the sea level changes and change in the vegetation of the Central and northern South America too were part of the big change (Woodburne, 2010) along with the land mammal exchanges. The diverse non-tropical ecologies promoted the Pleistocene glacial intervals (Woodburne, 2010).
The apparently symmetrical interchange of land mammals between the two continents takes place after the appearance of the Panamanian land bridge (Larry et al., 1982). There was a significant rise in the number of species of land mammals in South America that rose after the interchange began(Marshall, 1988). There was a comparable decline in the number of families in North America during the same period (Marshall, 1988). . The imbalance in the later period is attributed (Larry et al., 1982) to the secondary immigrants that evolved from initial immigrants.
The exchange led to the rapid merging of faunas that had evolved in complete isolation from each other for millions of years (Marshall, 1988). However, because of incomplete fossil record, the faunal mixing in the groups is not understood properly (Marshall, 1988). When studying the molecular dating (Weir et al., 2009) of the unique faunas of the two continents, the results support the part played by the land bridge in the merger of the tropical faunas of North and South America. However, interestingly, the traffic direction for the birds was primarily south to north (Weir et al., 2009), and the Great American Interchange changed the fauna of both the continents significantly.
Initially, there was equal reciprocal mingling between both continents, but the impact faded away and became imbalanced (Woodburne, 2010) with time. Today, more than half of the South America mammalian fauna stem from northern immigrants when measured at the generic level (Woodburne, 2010) . In addition, extinctions in the immigrant families were more severe in North America (Lessa, Valkenburgh& Fariña, 1997) as compared to the South America.
The appearance of Panamanian bridge Before looking into the famous exchange of fauna between the two condiments, it is essential to understand how the Isthmus of Panama (Woodburne, 2010) developed. There are different factors and collisional events that work behind the emergence of tropical islands and peninsulas (Woodburne, 2010). The beginning of the Panamanian land bridge began at the Pliocene when the Panamanian arc (Woodburne, 2010) as almost completed. The narrow strip of land links North and South America and lies between the Pacific Ocean and Caribbean Sea (Woodburne, 2010). The bridge became complete about 3,000,000 years ago, in the Middle Pliocene (Woodburne, 2010). Today, it comprises of Panama and the Panama Canal and like many other isthmuses, its location carries an enormous strategic value. As the bridge between two massive land masses, it is no surprise to find the Panamanian biosphere filled with overlapping fauna and flora from both continents (Woodburne, 2010). The tropical climate encourages a myriad of species from the south. The results of creation of the land bridge were far-reaching and (Woodburne, 2010) led to the Great American Interchange. During the same period, the Andes Mountains in South America (Woodburne, 2010) were experiencing a final uplift, and their height had almost doubled.
According to the scientists, the creation of the Isthmus of Panama is one of the most significant geologic happenings to take place in the last 60 million (Woodburne, 2010) years on Earth. Even though it is a tiny strip of land, it led to a huge impact on Earth’s climate, (Woodburne, 2010), the environment and the fauna. The movement of water between the two oceans was cut short, and a new current pattern known as Gulf Stream developed (Woodburne, 2010). The Atlantic, now that it was no longer circulating with the Pacific grew saltier. Thus, the Isthmus of Panama indirectly and directly influenced oceans, weather and the fauna (Woodburne, 2010). As the volcanic Isthmus of Panama rose up from the sea floor (Woodburne, 2010), it bridged the two separated continents, thus creating the land bridge—the Panamanian Isthmus and resulted in the migration of land and freshwater fauna (Woodburne, 2010).
Before the exchange During the Age of Dinosaurs, the bird and mammal families had already (Marshall, 1988). originated. The island continent of South America remained well separated from the other lands for well over 85 million years (Marshall, 1988). The birds and mammals in both continents were undergoing diversification until the formation of (Marshall, 1988). the Panama Land Bridge. Before the Great American Interchange took place, there were about 26 families of land mammals (Marshall, 1988) in each continent.
The total surface area of North America is much greater than South America and the North continent enjoyed a (Larry et al., 1982) greater generic diversity. Why the explosive diversification became unique and asymmetrical is not easy to understand (Larry et al., 1982). There were more species that migrated southward than northward. The resultant diversity in flora and fauna depend on the size of the area to attain equilibrium (Larry et al., 1982). Thus, continents with a bigger area, greater diversity and lower species turnover will require a longer time span to achieve an equilibrium as compared to the oceanic islands (Larry et al., 1982). The land mammal faunas in North and South America seemed to have attained an equilibrium (Larry et al., 1982) diversity before the great exchange took place. Despite continuous extinction and origination, the equilibria in both continents were dynamic, and the diversity remained steady (Larry et al., 1982). The higher generic diversity North America was consistent (Larry et al., 1982) with its total area. By the time of the dinosaurs became extinct, South America was an island, and its mammals continued to evolve in isolation (Larry et al., 1982). North America, on the other hand, remained in sporadic contact with the rest of the world for most of the (Woodburne, 2010) Cenozoic.
The emergence of the Panamanian bridge brought an end to the simple phase of equilibria (Webb, 1991) in both the continents and ushered in the great exchange of fauna. Some of the genera and species did face barriers when migrating because of the tropical areas of North and South America (Webb, 1991). It is seen that families with temperate species did not take part in the migration. Only those families that were distributed in the tropical or subtropical areas participated in the grand exchange (Larry et al., 1982).
South America was an isolated island and its mammals evolved in a (Lessa, Valkenburgh& Fariña, 1997) biosphere of their own. Thus, most of the species were endemic and are not seen in any other part of the world. The Great American interchange brought the species living in South America face to face with other potential predators and competitors (Lessa, Valkenburgh& Fariña, 1997) for the first time. South American fauna entered into the Northern continent and were the first-time immigrants (Marshall, 1988) in their history. The contribution of a completed land bridge
The completed land bridge contributed mainly to the faunal and floral mixing (Woodburne, 2010) between North and South America. When analyzing interchange events, one finds several families of passerine birds inhabiting both temperate and tropical regions of the New World. However, as good fossil records are not possible in case of birds, avian interchange (Weir et al., 2009) between the two continents is still not understood clearly. Today, molecular phylogenetic data (Weir et al., 2009) is used to compare rates of avian interchange before and after Isthmus completion. The formation of the land bridge did initiate an abrupt mixing (Webb, 1991) of the faunas of North and South America. The tropical rainforests (Webb, 1991) in Central America are primarily comprised of floral makeup of South American elements, thus suggesting an invasion of the North America by the rainforest flora and avifauna of South America post-land-bridge. Great American Biotic Interchange (GABI) researches (Woodburne, 2010) on the distribution of arid‐adapted species found in South and North American xeric regions. The analyses on the historical biogeography of the bee genus Diadasia (Wilson, Carril & Sipes, 2014) suggests the divergence between North and South American occurred long before the formation of the Isthmus of Panama.
The Great Interchange Begins The elements of the isthmian land bridge were physically present even before the great exchange of species between the north and south (Woodburne, 2010). The climatic stimulus too has a role to play as the primary (Woodburne, 2010) dispersal instigator. Research shows that the main overland dispersal of the mammalian taxa did not occur until favorable climatic conditions (Marshall, 1988) developed to support the temperate-adapted taxa. Study on the Fossil mammals suggest a north-to-south movement primarily and in contrast, the molecular evidence from birds (Weir et al., 2009) indicates a south-to-north transfer. The mammal record is derived from latitude fossils and tropical forests fossils (Weir et al., 2009), that help to explain the direction of traffic between the continents. The time of interchange in birds and mammals agree closely (Weir et al., 2009), and a limited number of interchange has been found in late Miocene and this was before land bridge completion.
Some families exhibit a good example of greatest numbers of interchange events include antbirds, wood creepers, tanagers, etc. These families are found in high diversity (Marshall, 1988) in tropical regions of North and South America and also present habitat specialization and dispersal abilities. Raccoons, camels, proboscideans, peccaries and other species (Larry et al., 1982) that were families of North American origin moved south. The interchange among the tropical birds families and mammals (Webb, 1991) accelerated shortly after land bridge completion and continues even today.
Climate was an important factor as any species that reached Panama needed to tolerate moist (Woodburne, 2010) tropical conditions. The species' marketing from North to South America did not have to face markedly different climates, however, northward migrants had to encounter drier and colder conditions as they moved northward (Woodburne, 2010). The climatic asymmetry was acute for the species used to the tropical rainforest environments of the south. Thus, climate (Woodburne, 2010) to played a role in the unequal interchange but was not the only deciding factor.
South American Fauna
South American mammals currently can be segregated into three major sections (Marshall, 1988). based on the sequence of tectonic events. The old groups have descended directly from the mammal lineages in the continent and had no close relatives anywhere else in the world(Marshall, 1988).. These are marsupials and the placental mammals such as anteaters, armadillos and sloths (Marshall, 1988). The early immigrants make the next group and are known to have colonized the island. These too have no close relatives to the current fauna of North America and are made of squirrel monkeys, marmosets, howlers, which are very different from those found in Africa and Asia. Capybaras, pacas, porcupines, guinea pigs, and others are the caviomorph rodents (Marshall, 1988). The last group comprises of the diverse modern forms that are the product of the Great Exchange (Marshall, 1988). They have close relatives in North America and include pigs, squirrels, bears, rabbits, deer, cats and dogs (Marshall, 1988).
There were distinct strata of land mammals and were the first to fill the adaptive land zones in South America in early Cenozoic. Pliometanastes and Megalonychidae sloth families (Marshall, 1988) make their first appearance in local fauna near New Mexico, central Florida and central California, and soon become firmly established. It is about 2.5 mya (Webb, 1991) that one finds the first record of South American animals walking across the new bridge in the late Blancan age. Several species of land mammals and ground birds (Weir et al., 2009) appeared during the same period. There were armadillos, sloths, capybara, phororhacoid, etc .ground bird among these immigrants. Titanis bird (Weir et al., 2009) is perhaps the most interesting of these early dispersants and these flightless, carnivorous ground birds were the only large South American carnivore to move to North America. The records of the next significant contingent of South America have only been established recently and comprised of ground sloth and rhino-like Mixotoxodori, (Marshall, 1988) who reached the southern part of the United States. However, most of these species (Marshall, 1988) did not make their way towards the north.
North American fauna
The first North American land mammals that walked across to South America date back to 2.8 to 2.5 mya (Marshall, 1988).. There are records of a skunk, a horse and a peccary that appear in the northwestern Argentina. It is about 2 mya that 16 genera from 9 families (Marshall, 1988) have been found that include dogs, cats, saber-tooths, bears, deer, tapirs, horse, rabbits and squirrels. Field mice (Marshall, 1988) from North America first appear in South America.
Today, there are many species in South America that are restricted to savanna habitats, and one finds a similar situation is found in Central and North America (Larry et al., 1982). The recurrent glacial events during the time of the interchange worked as a kind of filter and determined as to what kind of animals (Marshall, 1988) could move and when.
The asymmetrical interchange
During the late Cenozoic, it is found that the familial diversities in both continents remain similar and remains at 35 for ‘south and 34 for the North continent (Marshall, 1988). The number of immigrant families in South America rose (Marshall, 1988) with the arrival of North American families. This happened when the southern continent was at peak of familial diversity and appearance of the Isthmus Bridge. Likewise, North America experienced a sharp rise (Larry et al., 1982) in the number of immigrant families from the south.
When comparing the relative survival of North and South American interchange species, one should distinguish the true dispersants as well as the number of potential dispersants (Marshall, 1988). The larger geographic area of North America is likely to have more fauna as compared to the South American dispersants (Marshall, 1988). This aspect too has to be kept in mind when studying the symmetrical interchange. About 60% of the North American genera (Marshall, 1988) that moved to South America experienced considerable diversification after their arrival. An excellent example of a true dispersant from North America to ‘south America is the elephant-like gomphothere Cuvieronius, (Marshall, 1988) which gave rise to pseudodispersants. Just because a species only moved from their native continent does not make them a true dispersant (Marshall, 1988).
The total surface area of North America is much greater than that of South America, and the bigger continent enjoyed % greater generic diversity (Marshall, 1988). This is the reason there are more potential dispersants from the North as compared to South (Marshall, 1988). When one compares the number of true dispersants and the size of the source faunas, the interchange can be seen to be balanced (Marshall, 1988). It is the later explosive variation of true dispersants in South America that alter the equilibrium theory (Webb, 1991) and make it asymmetric. North America is unique and asymmetrical, as well as competitively superior to South America (Marshall, 1988). Moreover, the bigger continent is a land of extreme weathers and has already experienced many invasions (Marshall, 1988). The fauna that existed in North America at the start of the interchange were already survivors of numerous earlier invasions (Woodburne, 2010), and already equipped with an evolutionary advantage. The immigrants from North America (Webb, 2006) did not compete directly with South American natives but simply came and occupied unfilled space. Of the few South American mammals that colonized the North American continent, such as armadillos, porcupines, opossums, only the porcupines (Marshall, 1988) have moved extensively into the north. The sloths and glyptodonts remained in the southern areas and are now extinct everywhere (Marshall, 1988). On the other hand, there are many North American mammals’ species that colonized South America and included bears, raccoons, rabbits, mice, foxes, weasels, cats, and more (Marshall, 1988). Mastodons & horses have become extinct on both continents while tapirs & camels are alive in South America (Marshall, 1988).
The northern groups diversified greatly once they colonized South America, and many of these families have more species in the south as compared to the north (Marshall, 1988). A good example is the larger number of species of wild dog and cat in South America (Marshall, 1988), even though they moved here only about a couple of million years ago. To sum up, studies show that a majority of the South American mammal species today derive immigrant lineages from the north (Webb, 2006). The major reason behind is the greater diversification of the northern migrants post-migration (Webb, 2006). This may also have been the reason behind the extinction of several native South American species (Lessa, Valkenburgh& Fariña, 1997). Among other animal groups, a reverse trend was shown by reptiles and amphibians and one good example is Toads and treefrogs (Marshall, 1988) that are now widely distributed in North America and migrated from South America. South America is also referred as the birds continent (Weir et al., 2009) and house more than third of all the world's species. The birds fauna is highly distinctive and extremely rich (Weir et al., 2009). However, as the fossil record of birds is weak, it is not sure how these forms evolved, or the Great American Interchange had any role to play. DNA sequences (Weir et al., 2009) do make it certain that an enormous part of the South American birds fauna is originated and diversified natively (Weir et al., 2009). Nevertheless, there is sufficient evidence that the modern birds fauna of South America arrived from the north (Weir et al., 2009). Although they diversified heavily within South America, their derived lineages can be traced back to North America (Weir et al., 2009).
The notion that North American immigrants outcompeted their South American counterparts held several views extinction (Lessa, Valkenburgh& Fariña, 1997). It is their higher rates of colonization, greater resilience to extinction, and superior diversification exhibit a strong association with the probability of extinction (Lessa, Valkenburgh& Fariña, 1997). As per the biogeographic theory, the source fauna and the other fauna that is receiving the immigrant should be proportional in size. There were considerable differences in the kind of species, the size of the geographical areas involved and the climate factors too had a role to play (Woodburne, 2010). The species from the north were more resilient as compared to their southern counterparts, and these factors led to an imbalance and difference in turnover rates (Woodbourne, 2010).. A unique aspect of the Great American Interchange is the significant diversification of the North American secondary immigrants into the southern continent that led to long observed asymmetry in the exchange dynamics (Webb, 1991) between the two continents.
Another theory implies that there were ongoing changes in South American land created new habitats and this led to the extinction of some native fauna (Lessa, Valkenburgh & Fariña, 1997) before and during the interchange. North American immigrants naturally took advantage of timely immigration and adapted well to the new habitats and opening of adaptive zones (Larry et al., 1982). There is another view that takes into consideration all the above aspects to explain the imbalance for the interchange. The biological superiority (Larry et al., 1982) of the northern taxa explains their higher success in the south after the interchange.
Conclusion
Generally speaking, the Great American Interchange is looked upon as a kind of evolutionary competition, where North America won (Woodburne, 2010). This is because the fauna living in the North continent had adaptations that were better suited to the change and competition (Woodburne, 2010) as compared to South American fauna. The fauna from the South were heavily adapted to tropical weather and were not as strong as the fauna of the North (Woodburne, 2010), who were used to a more climate-diverse continent. The interchange is highly significant and visible from both aspects of stratigraphy and nature (Woodburne, 2010). The exchange gave a chance to amphibians, arthropods, flightless birds (Marshall, 1988) reptiles and even freshwater fish to migrate. The fossils in the Isthmus have helped to shed light in the past and to study as to what happened in the past can prepare us better as to what might happen in the future (Woodburne, 2010).
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