By Wayne Heideman
Do you remember what it was like on your first day of kindergarten, college, or work? You were placed into an environment that was different than the one you had known previously, whether it was because of the people, place, or experiences. Around 3.1 million years ago, this is what North America and South America, provided they had feelings, would have felt when they became connected via the Panama isthmus. Not only did it connect two continents, but it connected the organisms that inhabited the two continents.
Through the new connection, organisms travelled into the opposite continent where there were organisms that they had not seen before. This dispersion across continents is known as the Great American Biotic Interchange (GABI) (Smith and Klicka, 2010). The GABI allowed organisms to disperse but there are a few puzzling details about it. First of all, there was a greater north to south dispersion than south to north. Second, birds remained in their home continents until the GABI, even with the adaptation of wings. Some of these details can be better explained with some knowledge of the area 3.1 million years ago.
Today, over fifty percent of mammals that originated in North America are found in South America. On the other hand, only about ten percent of South American mammals are found in North America. Birds show a similar pattern. Twelve North American bird families dispersed into South America, with only 3 families constrained to tropical conditions. Sixteen families of South American birds invaded North America but 14 of those families are restricted to the tropical region in Central America (Smith and Klicka 2010).
Why were North American mammals and birds able to inhabit and disperse across South America but not vice-versa? Habitat conditions may be a factor. The habitats of Central America and South America were different during GABI than they are today. During the time of connection, the Isthmus of Panama and South America was largely composed of savannas. The savanna stretched from th
e Great Plains to Brazil allowing for a wide array of organisms to cross over.
The North American savanna species, however, were better adapted to colonization than their South American counterparts. This is largely because of glaciation. The repeated glaciations in the northern hemisphere gave the North American species an edge because they were used to differing warming and cooling conditions. South America was warmer than North America, and so South American species were not as likely to proliferate during colder periods in North America. As time got closer to present day, the savanna was over taken by the rainforests we have now, which also allowed for another exchange, this one more in the South American species’ favor (Fig. 1) (Webb 1991).
The adaptation of wings in a bird allows it to travel vast distances across a landscape. Even with this adaptation, there doesn’t seem like there was too much interchange between North and South America. As stated before, the habitat did have some interference with bird dispersal. Birds that live in tropical regions tend to not like to fly over large bodies of water. Flying over the ocean becomes out of the question as some tropical birds don’t fly over large lakes.
The map in Fig. 2 can also provide some insight on avian dispersal. Map A shows the amount of species richness in bird families present before the GABI and Map B shows the result of the GABI. Almost the entirety of North and South America are highlighted with at least 75 different species.
More insight on avian dispersion can be seen in the graph in Fig 3. The GABI (3.1 million years ago) is seen as having a peak of avian dispersal. This graph shows how individuals (red), species (blue), genera (yellow), and families (gray) diversified with the GABI. Also, it should be noted that there are two peaks, one perhaps with the savanna habitat and the other associated with the tropical habitat.
In another study, passerines were collected and tested for genome analysis. The 4 bird families chosen were present in both North and South America. Two of the families were tropical based (Thamnophilidae and Dendrocolaptidae) and two were generalist based (Icteridae and Thraupidae). All four of these families crossed the Isthmus of Panama after it had been fully connected, even with their varying life styles. It was found that even with differing life styles, they still crossed the Isthmus of Panama around the same time.
Birds, although not as prominent in the fossil record, provide great insight on diversification and colonization. Many papers, including some cited here, say that the North American mammal species readily crossed into South America and proliferated. Species were exchanged between the two continents, but the North American species tended to “win” more often. This however, may have been different for birds, because migratory behavior allowed them to avoid the limitations of colder temperatures in the north. The majority of the South American bird species still reside in the tropics of North America, even though they were successful colonizers of the North.
Hawkins, B.A., Diniz-Filho, J.A.F., Jaramillo, C.A., and Soeller, S.A. 2006. Post-Eocene climate change, niche conservatism, and the latitudinal diversity of New World birds. Journal of Biogeography 33: 770-780.
Smith, B.T. and Klicka, J. 2010. The profound influence of the Late Pliocene Panamanian uplift on the exchange, diversification, and distribution of New World Birds. Ecography 33: 333-342.
Webb, S.D. 1991. Ecogeography and the Great American Interchange. Paleobiology 17(3): 266-280.
Weir, J.T., Bermingham, E., and Schluter, D. 2009. The Great American Biotic Interchange in birds. Proceedings of the National Academy of Sciences of the United States of America 106(51): 21737-21742.