The biogeographical consequences of previously separated continents merging

According to the theory of plate tectonics or previously named continental drift first proposed by Alfred Wegener in 1912, the continents of the world are constantly moving and re-arranging themselves by the action of great convection currents in the Earth’s mantle. This causes large landmasses to split into smaller pieces, slide past each other and merge together. There are many consequences to these events; more importantly to this essay, there are massive biogeographical implications.

There are three main types of environmental barriers that inhibit species spreading by causing major environmental discontinuities. These include oceans and seas, mountain ranges and large deserts. Australia is completely surrounded by water and therefore is biogeographically isolated from other landmasses. The extensive deserts of North Africa and the Middle East effectively separate Africa from Europe and Asia; in a similar way the high Tibetan plateau of which the Himalayas are the southern fringe separates South East Asia and India from the rest of Asia. Cox and Moore 1993).

The Americas are independent from the rest of the world by the surrounding oceans however are to some extent currently biogeographically separate from each other. However when the Panama Isthmus first united them there was a great interchange of species, which will be discussed later. Although the plates move very slowly, only about 5 – 10 cm a year (Cox and Moore 1993), over geologic time, life would be greatly affected. At one point in geologic history all the continents were united as one super-continent known as Pangaea about 260 million years ago.

This came from the uniting of Euramerica, Siberia, Gondwana and two portions of what is now China. At this point all land flora and fauna were joined by land and could potentially colonise the entire world causing the spread of certain species and extinction of other species by competition. However there would have be a massive dry area inland of the super-continent with extremes of weather that may have experienced annual temperature changes of 50-60oC, which no reptile could sustain and very difficult for other species to survive (Cox and Moore 1993).

This would be the last time all the land in the world would be joined. After this, the continents have moved and split and slowly formed the world today. The continents of today are separated to an extent that they can be classified and scientists have split it up into zoogeographical realms. The first to do this was Alfred Russell Wallace in 1876 who organised them as follows; Nearctic (North America); Neotropical (South America); Palearctic (Europe and Asia); Ethiopian, sometimes described as Africotropical (African); Australasian (Australia) (J. C. Briggs 1987).

This is of course a generalisation as there are highly endemic large islands to be considered like New Zealand, Madagascar and Hawaii. Some of the continents that have recently merged include Africa with Eurasia, North America with South America and India with Asia. The joining of the Americas was very recent, just over 5 million years ago either in the late Miocene or early Pliocene. It occurred as South America had been independent and was moving west towards the great oceanic trench of the South Pacific; as it reached the trench, it moved over the zone where oceanic crust was being drawn down into the earlier crust.

This then caused a lot of volcanic activity forming the Andean mountain chain. More importantly the westward movement of South America meant that the Central American link could be completed (Cox and Moore 1993). Evidence comes from the fossil remains of two families from each continent being found in the other continent dating to this time. These species could have crossed via an island chain that may have existed. However more likely they crossed when the Great Pliocene uplift occurred raising the Andes from 2000 to 4000m above sea level, and the Panama land bridge was completed as well.

This meant that there could be a great interchange of species between the two continents; though South America’s topography had now completely changed as a vast, lofty mountain chain had quickly sprung up on its western coastline. These mountains now stopped much of the moist air that used to sweep across from the Pacific across South America bringing moisture inland. The old sub-tropical areas became drier and turned into savannahs and semi-deserts, and the existing savannahs became deserts. The interchange of species has been studied in detail by American palaeontologists.

Mammals provide the best fossil record for studying the interchange that occurred (J. C. Briggs 1987). Prior to the interchange, each continent had 26 families of land mammals, and 16 of each dispersed into the other continent when Isthmus of Panama was completed about 3 – 5 million years ago in the late Pliocene or early Pleistocene. From North America, 29 genera of mammals dispersed south including shrews, rodents, felids, canids, bears, mastodont elephants, tapirs, horses, peccaries, camels and deer. From South America, several genera including the ground sloth, armadillo, porcupine and caviomorph rodent dispersed Northwards.

After these initial transactions anteaters, llamas and opossums followed 15 million years ago. (Cox and Moore 1993) The type of fauna that interchanged between the continents were more adapted to the savannah type environment implying that the Panama Isthmus was of similar environmental characteristics. However in the late Pleistocene tropical rainforest developed across the land bridge and acts as a barrier for the species that once crossed the connection of the continents few millions years previous (Cox and Moore 1993).

This is believed because of the current disjunct distribution in species that crossed the connection, as they are found in both North and South America but not near the Panama Isthmus. It appears that the North American species that dispersed have been far more successful than those that radiated from South America. In North America 21% of the living land mammal fauna originated from South America, however 50% of living land mammals in South America derived from North America. The most successful mammal would be the cricetid rodents that have diversified into 45 genera in South America (Cox and Moore 1993).

The reasons for the North America fauna success is mere speculation however it is thought that due to the fact that North America was part of Laurasia for a long time after South American split from Gondwanaland and remained a separate continent, meant that the North American species had been through millions of years of competition with species of Laurasia and were therefore fitter in Darwin’s terms and could compete very well with the South American species which had been competing merely amongst themselves for a long time. Widely ranging species, abounding in individuals, which have already triumphed over many competitors in their own widely extended homes will have the best chance of seizing on new places, when they spread into new countries. ” Charles Darwin, The Origin of Species, 1859 The consequences of this continental linkage were a great loss of South American flora and fauna; in particular very few living mammals today of South America are the original species from the early Cenozoic. However there was also a flourishing of the well-adapted North American species as a result of the fusion.

This result advocates the general assumption that continental fusion slows down evolution and brings previously separated species into competition and therefore causing major extinctions. This is emphasised by the current high diversity in species today as opposed to 245 million years ago in the age of Pangaea where diversity was low as all species were in competition. The separate realms encourage allopathic speciation, which debatably is the most important form of speciation. North America has also had interchanges of flora and fauna with Asia over time. Maps of the late cretaceous have differed.

Some show a large gap in between North America and Asia like the maps of Smith and Briden (1977). However there is evidence to show that at intervals there was a land connection allowing interchange of species between the two continents. Barron et al. (1981) drew maps of this period with a connection between the continents and Fujita (1978) described a series of collisions with some smaller plates between Siberia and North America. The first record of the link was of mammalian genera fossils (multituberculate and placental) suddenly found in North America in the late cretaceous presumably from Asia (Webb 1985).

Animal migrations seemed continuous through the tertiary and the popular view is the land connection persisted for a long time until the Palaeocene where the connection seemed to break and rejoin periodically. Some of the Faunal migrations to North America from Asia included three genera of mesonychids in the mid-Palaeocene; three genera of large herbivores representing the order of Pantodonta. Three families of edentates, notoungulates, and xenungulates were believed to have migrated from South America to North America then to Asia via the Beringia link.

The early Eocene saw the appearance of new genera in North America from Asia including the orders of Tillodonta, Rodentia, and Pantodonta. A little later more faunas immigrated across the bridge including tapiroids, omomyid primates and hyaenodontids to name a few (J. C Briggs 1987 quoting Gingerich 1985, Simons 1960, Webb 1985). Following this, the connection must have been severed as towards the mid-Eocene, North American fauna became substantially more endemic. Into the Oligocene the most traceable groups to Asiatic origin to immigrate were Castoridae, Anthracotheriidae and Tapiridae.

Then in the early Miocene a further 16 genera established themselves including species of cats, bears, pronghorn antelopes, beavers, and flying squirrels (J. C. Briggs 1987). In the mid-Miocene, migration into North America seemed to have slowed, however towards the end of the Miocene, a renewed stock of immigrants moved across including several large carnivores, large ungulates and small herbivores (J. C. Briggs 1987). In the Pliocene another burst of migration occurred as 72 new Genera appeared in North America, most of which was probably Asiatic origin (Savage and Russell, 1983).

More periodical linking of the continents seemed to have occurred during the Pleistocene, however these are believed to be the result of global cooling and ice sheets connecting the land as opposed to continental drift being the culprit, although some previous connections may also be attributed to this type of connection. It is important to make a note that there were many more interchanges of animal species between North America and Asia since the cretaceous not mentioned and numerous interchanges of plant groups not mentioned also.

Notably the interchanges were much more often migrants travelling from Asia to North America, as opposed to vice versa. Reasons for this maybe, North America simply had many unoccupied niches, as species diversity was simply low. Alternatively the migrants travelling from Asia were biologically fitter as they derived from the super-continent of Laurasia. By this time Laurasia would have a wealth of strong species as the fusion of India into Asia had occurred supplying strong species as well as the Fusion of South East Asia with Laurasia.

The current situation shows a result of these interchanges as increasing North American species diversity as Asia was a rich source of highly evolved fauna flora. Also due to the non-permanent connection that occurred meant the two continents remained ecological individual yet dispersed species between each other. The most recent collision would be Australiasia and South East Asia, although no land bridge has developed, a chain of islands has formed allowing some species to migrate between the realms set out by Wallace in 1976.

Wallace’s line is based upon faunal distribution and fits nicely, however the line is not quite as fixed for botonists. Many plant groups have interchanged a long several islands from both realms as seeds are dispersed much easier than animals. However there may have been Gondwanaland and Laurasian interchanges long before the plate embedded itself in South East Asia, as South Tibet, Burma, the Thai-Malay peninsula and Sumatra may rifted from Northern Australia during the late Jurassic and drifted North to collide with Asia possibly in the early cretaceous (Rendel Williams 2002).

The examples given in this essay describing the merging continents are not perfect as in none of the detailed examples fully fuse (yet) like in the case of the formation of Pangaea. The example of the North America and South American faunal interchange did show the biologically destructive effects of continental merging as many South American species were lost to the more dominant North American invaders.

However the other detailed example depicting the interchange of species between North America and Asia showed a more positive side of possible mergence as species diversity increased in North America by the invasion of more Eurasian fauna and flora. The Asia/North America example is however poor as the two continents did not fuse but a land bridge formed periodically to help species disperse without completely removing the environmental barrier that once separated the realms and therefore forming disjunction’s of many species that made the trip.

The continuous severing of the Bering strait prevented the continuous environmental conditions required for the realms to unite as one at the cost of many weaker species. A general conclusion would be to say that consequences of continental merging depend highly upon which continents are being discussed but generally anytime two landmasses meet, there will be a massive interchange of species and interspecific competition will take place on a large scale, where Darwin’s theory of ‘survival of the fittest’ will be of most importance.