The Isthmus of Panama is a narrow land bridge connecting the tip of South America to the base of North America. This land bridge began as a series of small islands, known as the Panama Arc, which were gradually uplifted. This uplift, combined with the expansion of the Greenland ice sheet led to the formation of a solid land bridge in place by 2.8 million years ago.
The establishment of the Isthmus of Panama had profound global effects on both climate and biodiversity. The climate was already in a period of cooling. As mentioned in a previous post, the opening of the Drake Passage resulted in the thermal isolation of Antarctica allowing for glaciation over the landmass. Ice sheets had also begun to spread across the northern hemisphere too under a continually cooling climate, most notably in Greenland.
The Isthmus resulted in the reorganisation of global thermohaline circulation. The shallowing of the seaway between North and South America and the eventual separation of the Pacific and Atlantic oceans in this area led to the formation of the gulf stream which introduced warmer waters into the high latitudes of the Northern hemisphere. The subsequent evaporation of these warmer surface waters aided deep water formation in the North Atlantic, as well as introducing moisture into the Northern hemisphere. It has been hypothesised that this increased moisture in the atmosphere paired with incremental changes in obliquity was necessary for further ice-sheet growth in Northern hemisphere. This suggests a threshold for ice-sheet growth. The idea of threshold values relating to changing climates and ocean acidification has already been explored, there could be an opportunity for further research in the area for ice sheet expansion and stability.
The Isthmus of Panama also had monumental effects on the biosphere. The land bridge allowed faunal interchange between the two Americas for the first time in millions of years, this is known as the Great American Interchange. There had actually been earlier instances of interchange between the continents, probably through island hopping or simple dispersal across the water. Examples of these early immigrations include sloths (9Ma), terror birds (5Ma) and glyptodonts (3Ma). However, after the establishment of the Isthmus of Panama, immigrations became much more frequent. Prior to the land bridges formation, both North and South America had been mostly isolated, North America periodically connected to Asia via western Alakska throughout the Cenozoic, but South America had been completely isolated since its fragmentation from Africa during the break-up of Gondwana during the Mesozoic. As a result, South American fauna included a bizarre variety of animals seen nowhere else on earth including a splendid array of marsupials, xenarthrans and monotremes, all of which were capable of growing to immense sizes such as the giant armadillos (glyptodonts), oppossums (Thylophorops) and sloths (Megatherium).
Immigration between the two continents was bidirectional with deers, horses, gomphotheres, sabre-tooth cats and canids travelling from North to South America, and sloths, birds and capybaras travelling from South to North America. Whilst migration between the two continents was fairly even, the success of the respective faunas was very different. South American fauna found it hard to establish themselves in North America and failed to compete with the native species. North American fauna on the other hand found great success in the Southern hemisphere and regional extinctions of native South American species through direct competition with invasive North American fauna and through predation.
The invasion and success of North American fauna has similarities with modern invasive species. Rates of species invasion are apparently higher than in any other time in history. Climate change has been shown to increase species invasion rates and results in ranges shifts. Thus the changing modern climate could increase the rates of species invasion and rates of regional extinction of native species. For instance, aquatic freshwater ecosystems are particularly vulnerable directly and indirectly to invasive species and subsequent habitat alteration.
Interestingly (or worryingly), invasive species may even be heading over water barriers traveling on plastic debris. Many species are capable of surviving long enough on plastic rafts to reach new land masses and colonise.
Perhaps it should ring an alarm for us all that climate change does not just mean melting ice sheets and rising temperatures, but it also has implications that we were previously unaware of, such as species invasion. Who knows what other issues we are ignorant to that are affected by global warming?
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