Zoogeography and the Sea : 2

Ancient Isolations, Distant Conjunctions & a Recent Connection

In this lecture we shall make an aquaintance with (1) Wallace's Line - the marked changeover between the Southeast Asian and Australasian faunas which takes place in the Indonesian archipelago; (2) the Great American Faunal Interchange which took place about 5 million years ago when the Central American Land Bridge was formed, joining North to South America;  and (3) the entry of humans into the New World via the Beringian land bridge formed between far eastern Siberia and far western Alaska during the recent glacial maximum.

Wallace's Line

We know from the study of fossils that, during the late Permian & early Triassic Periods (approx. 270-230mya), when the land-masses were all largely combined into the super-continent Pangea, there was very little regional differentiation of the faunas.  At these times, the world was dominated by a diverse array of so-called "mammal-like reptiles".  Most of these lineages died out by the middle Triassic - all except for those which eventually gave rise to the mammals themselves.

In the early Jurassic, around 190mya, the world became dominated by the ancestors of the dinosaurs.  These Jurassic dinosaurs, despite the fact that Pangea was beginning to split up during these times, gave rise to a globally-distributed array of dinosaur forms.

But as was outlined in the first lecture, the widening separations between pieces of earth's crust permitted independent evolutionary diversification to proceed, resulting in these separated continents supporting often widely differing biotas, especially in groups of organisms which largely diversified during the isolation phase. This widening became significant during the mid to late Cretaceaous (110-90mya) and the effects of this continental isolation are evident in the arrays of dinosaurs from different parts of the world at these times.  Of course, this great diversity of dinosaurs became extinct around 70mya, leaving the world for mammals and birds to fill the ecological vacuum.  Thus we would expect birds and mammals to show the clearest evidence of independent continantal diversification.

We also noted in the first class that the most peculiar (in the sense of being highly distinctive) continental biota is that of Australasia (meaning Australia itself plus associated islands such as New Guinea and Sulawesi), and this is explained by its long (and continuing) isolation from the rest of earth's terrestrial regions since well back into the Mesozoic.

Nowadays, however, parts of the Australasian crust are in close proximity to South East Asia - some parts only being separated by a few tens of kilometres of seaway.  Climatic conditions are very similar in these adjacent parts, and the vegetation has many features in common.  It was therefore a stunning (not to say a puzzling and rather mysterious) finding that Alfred Wallace made when he travelled from Bali to Lombok (only 24 km. apart - you can see one island from the other):  the assemblage of animals (Wallace initially noticed the birds) on Lombok were greatly different from those on Bali.

Bali's fauna is very similar to those of Indonesian islands further east (such as Java and Sumatra) while the fauna of Lombok has clear affinities with the fauna of New Guinea and Australia.  Wallace recognised this ocean strait between Bali and Lombok as a major faunal discontinuity, as striking as any that he had catalogued elsewhere in the world.  The discontinuity was all the more striking and surprising because there didn't really seem to be much of a barrier for many of the animals which clearly showed the pattern.

Later in his work in Indonesia, he showed that there was a similar discontinuity between the large islands of Borneo and Celebes (now called Sulawesi), and he therefore drew a boundary between what he called the Oriental Realm to the west and north, and the Australasian  Realm to the south and east.  This boundary has come to be known as Wallace's Line in his honour.  Others have drawn the line in slightly different places based on the study of other groups of organisms, but the primary point remains that in this region of more-or-less evenly-spaced islands there is a dramatic shift in the nature of the biota.

Rather than try to fix where "the" line between these two realms should lie, most modern biologists recognise that the whole Indonesian Archipelago region represents a zone of changeover, wherein the two faunas progressively replace one another.  In some groups, the main step is here, in others it is there;  other groups show no clear steps of abundance at all, but rather a steady diminution or increase of importance in the total biota.  The existence of this changeover zone represents dispersal of organisms from their centres of origin into new territories that eventually became accessible as the two great regions drifted closer.  You will find the changeover zone is referred to as Wallacea.

First we shall outline the nature of the faunal differences between these realms, and then we shall specify the modern explanation for their existence.  We have no time for a complete statement of the situation, and besides, many of the animals will be unfamiliar.  Suffice to say that on either side of Wallace's Line there are distinctive faunas of freshwater fish, amphibians (frogs & toads), reptiles, birds and mammals.  Here I shall list some of the birds and mammals.

Fauna of Oriental Realm:

Fauna of Australasian Realm:

 This dramatic phenomenon was very puzzling to 19th.-century biologists, and indeed provided a riveting focus of attention for zoogeographers for generations after Wallace's time.   It was not until the widespread acceptance of the idea of Continental Drift that a comprehensive explanation of the phenomenon was provided.  Now we know that Australia has had a completely independent evolutionary history from the rest of the world since it split from the rest of Gondwanaland during the late Mesozoic, and it has, like most of the rest of the southern continents, been separated from the great land mass of Asia since long before that.

Thus we now know that, during virtually the entire period when most terrestrial organisms, notably freshwater fish, birds and mammals, were undergoing their main evolutionary  radiations (lineage-formation), Australia was widely separated from Asia - indeed from most of the rest of the world.  It maintained contacts with Antarctica for a long time, but eventually virtually all terrestrial life was extinguished there.

The great distinctiveness of these biotas then (even though a little blurred now because of the geographical proximity of the great land masses of Australasia and S.E. Asia and the capacity of animals to disperse and migrate) is a striking testimony to the power of the idea of the historical evolution of both the planet and the biota.

If one looks at a map of this part of the world, there is nothing to be seen which would indicate the location of Wallace's Line.  But if one looks at a map which shows submarine contours and one locates the edge of the continental shelves at about 200m below current sea-level, then one sees the true extent of the ancient continents clearly:  Wallace's Line marks the deep-water channel which marks the outer (south-western) edge of the Sunda shelf which unites Borneo, Bali, Java and Sumatra to the mainland of southeastern Asia.  Australia, on the other hand is united broadly with New Guinea.

The deep water between these two large shelf areas (the Sunda and Sahul shelves) is what remains of the massive ocean barrier that for a period in excess of 50 million years kept life on Australia from that of Asia.

The Great American Faunal Interchange

Back in the Age of Dinosaurs (by which time the bird and mammal lineages had originated), the land that was to become North America split off from the great southern continent of Gondwanaland, and then 90 million years ago (well before the extinction of the dinosaurs), the island continent of South America separated from Antarctica. South America retained this isolated island status for about 85 million years, during the time when birds and mammals were undergoing great diversification, until the Panama Land Bridge was formed.

The processes which led to a dry-land connection between the two continents of North and South America are complex and only partly-understood.  However, one popular hypothetical explanation, consistent with all the data available so far, is as follows:

At that point began what is known as the Great American Faunal Interchange (GAFI).  This dramatic event amounted to a gigantic natural experiment in evolutionary biogeography, involving the effects of dispersal, evolution and extinction.  It permitted an encounter between two continental biotas that had been evolving in isolation for nearly 150 million years.

How distinct are these biotas?

The great bulk of the information about GAFI comes from the study of mammals, since they have an excellent fossil record.  Birds are also important, but their study is largely restricted to modern forms, since their fossil record is so poor.

Mammals originated from their reptilian ancestors in the earliest Mesozoic Era, about 200mya, well before the dinosaurs had come to dominate the globe.  At these times, the world's land-masses formed a super-continent called Pangea.  Thus, the very earliest lineages of mammals had access to all of the earth's habitats, where they began their own diversification.

The marsupial (pouched) mammals seem to have originated somewhere in the South America - Antarctica - Australia chain of continents in the mid-to-late Cretaceous, while the placental mammals probably originated in Asia around the same time and thereafter spread throughout Eurasia and North America.  Before the full separation of North and South America, it is evident that some placentals got into South America, while some marsupials got into North America (though they went extinct soon afterwards).

By the time of the extinction of the dinosaurs (about 65-70mya), South America had already become an island, and its mammals could evolve in splendid isolation from the rest of the world.  On the other hand, North America was in extensive, though intermittent, contact with Eurasia throughout much of the Cretaceous and most of the Cenozoic (70-15mya).

The mammals of present-day South America can be separated into three major assemblages which relate to the sequence of tectonic events shown above in the previous section:

What were the effects of the exchange?


Rather few kinds of South American mammal colonised the North American continent:  porcupines (rodent), armadillos (edentate) & opossums (marsupials) have contributed living species, though only porcupines have expanded extensively into the north;  ground sloths and glyptodonts (both edentates) did colonise as far north as the USA and survived until the last few thousand years,but they are now extinct everywhere.

On the other hand, many lineages of North American mammals colonised South America, 17 families in all:  rabbits, mice, foxes, bears, raccoons, weasels, cats, peccaries & deer still have close living relatives in N. America;  tapirs & camels are still alive in South America but they went extinct in their original home;  mastodons & horses went extinct on both continents (of course horses were re-introduced from the Old World in historical times).

About half of all species of today's South American mammal fauna derives from this influx of northerners.

Having colonised South America, many of these northern groups diversified greatly, so that nowadays, most of the families have more species in the south than remain in the north.  For example, there are more species of wild dog  and cat species in South America than on any other continent, even though they got there less than 5 million years ago.

In aggregate, as a result of the faunal exchange, about 50% of all South American mammal species derive from immigrant lineages from the north, while the corresponding figure for south to north migration is 10%, and the bulk of this effect is due to higher rates of post-migration diversification on the part of the northern migrants, which in turn may have led to some higher rates of extinction of native South American species..

Other animal groups

Among reptiles and amphibians, the pattern is rather the reverse of that shown by mammals:  that is, the majority of colonization was south to north.  Toads and treefrogs, for example, are now widely distributed in North America yet derive from South America.

South America is "the bird continent" with more species than any other continental region, and over a third of all the world's species.  This bird fauna, as well as being extremely rich, is highly distinctive, with 22 endemic families (not known anywhere else in the world).  Since the fossil record of birds is so poor in the main, we cannot be sure that all of these endemic forms evolved where they are found now, however, evidence from DNA sequences makes it almost certain that a very large part of the South American perching-bird fauna is authochthonous - that is, it originated and diversified in place.

Given that birds have considerable dispersal powers through flight, one would not expect to find the clear picture described above for mammals.  Nevertheless, it seems clear that a substantial fraction of the modern bird fauna of South America came down from the north.  When they arrived is unclear, but it seems evident that it antedated the complete dry-land connection by some long time in many cases.  Following their arrival, they diversified greatly within South America, and then many of these derived lineages re-invaded back into North America.  Thus, for example, most of the summer breeding migrant birds of North America have their immediate evolutionary origins in South America.

Altogether, it is abundantly clear that the biota of South America would have remained as highly-distinctive as that of Australia is today if it had not been for the elimination of the deep ocean barrier provided by the Tethys Seaway when it opened about 150mya.

This invasion and diversification experiment has had diverse results:  most mammals moved and diversified from a north-to-south movement, while in the other vertebrates it seems that most of the effect has been in the reverse direction.

The Peopling of the New World

The last two million years of earth's history has been marked by climatic instability.  After an extremely long period during which almost all of the world's surface enjoyed an equable climate (there were ancient polar forests on Canada's arctic islands during the Mesozoic - say 150mya) there recently ensued a time of strongly alternating periods of colder temperatures and what we think of as average temperatures.  This is the time of the Pleistocene glaciations.  The colder periods are known as glacials and the warmer periods as interglacials.  We are at present in an interglacial period which began about 10,000 years ago;  if the past pattern is anything to go by, we are probably approaching the end of it.

During the glacial periods, the snows that accumulate during the winters only partially melt during the summers.  Thus snow-packs build up, especially at higher latitudes and altitudes.  As the snows become deeper, they become compressed under their own weight, and become icy - in this way the massive montane and continental glaciers are built.  The continuing accumulation of enormous weights of ice at the higher latitudes and altitudes forces the ice to flow outwards and downwards, scouring the earth before and beneath.  During the last glaciation, such ice-caps covered up to 97% of Canada, to an average depth of about 2km.

These ice caps lock up an enormous volume of water, which ultimately must come from the sea.  At the last glacial maximum - about 18,000 years ago - enough water was trapped to lower the sea level globally by about 140m.  Depending on the submarine relief, such a sea-level drop made an enormous difference to the shape of continental coastlines, even to the extent of joining offshore islands to a larger mainland, as in the case of Britain being joined to Europe, or the Falkland Islands (Islas Malvinas) being joined to Argentina.  In one particularly dramatic instance, it resulted in the joining of two continents when the shallow sea separating far-eastern Siberia in Asia from Alaska in the Americas dried out forming a 1000km-broad land bridge called Beringia.

Though much of the arctic region was thus covered in ice-sheets during glacial times, because glacier development depends upon snow accumulation, glaciers were absent wherever it was too dry for signifcant precipitation.  This was the case in much of Siberia and also in the case of Beringia itself.  Thus, at these times, the animals living in the tundra of eastern Siberia had access to a new ice-free area the size of Texas which they duly colonized by creatures like the Woolly Mammoth.

Also living in Siberia were bands of people who lived off the rich mammal community which included, besides mammoths, various species of antelope and cattle and others, and naturally these people followed their food.  Thus Homo sapiens arrived in the New World.  The Siberian origin of American aboriginals is virtually certain, as is evidenced by archaological and extensive and detailed genetical information.

As the glacial period drew to a close, the glaciers retreated and the sea-level began to rise.  Around 10,000 years ago, the dry-land connection between the continents was broken and Beringia sank beneath the waves, leaving some people on the American shore.  Though at the glacial maximum the colonists were restricted to Beringia and adjacent Alaska, as soon as the glaciers began their retreat, a way southward was opened between the ice cap based on the Rocky Mountains of Yukon and British Columbia to the west and the enormous ice cap based on the Territories and Hudson Bay to the east. Through this narrow but rapidly widening corridor, humans made their entrance into the whole continent.

It is supposed that initially the human population was very small, but even modest levels of natural increase could easily provide for the population levels achieved by historical times. When they arrived, the first Americans encountered a very rich and diverse fauna, comparable in its character to that of contemporary Africa.  Shortly and suddenly after the appearance of humans, a large fraction of this diversity went extinct (about 70% of the large mammal genera seem to have disappeared in about 2000 years).  Scientists argue vigorously about the reason for these extinctions;  one of the strong contenders is what is known as the Pleistocene Overkill Hypothesis.

Although there are other theories about these extinctions, based on climatic factors, there is a global pattern which provides quite strong circumstatial evidence that humans are indeed implicated.  Thus, there are substantial similar extinctions in South America at around the time that humans would have reached there;  similarly, there is a much earlier series of extinctions in Australia (around 40,000 years ago), again coinciding with the appearnace of humans there.

Either way one looks at this, the elimination of the Bering Sea barrier probably had an enormous impact on the character of the New World biota.

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