The Wallace Line, Part 2

of Wallace. He's a man of many adventurers and many opinions, best known today for being the other guy who discovered evolution by natural selection. He came up with a slightly different version of the theory of natural selection around the same time Darwin did, just some sort of differences of emphasis. Basically, Zarwin's writings ultimately proved more influential in convincing his peers on the reality of common descent and on articulating the

mechanisms by which species evolved. Wallace was also something of a celebrity at his time. It's not like one of those tragic cases of somebody, you know, somebody else who also came across a great idea but was just like

totally forgotten. Wallace was kind of a celebrity, especially because of the quality of his writing, and that would be in books like The Malay Archipelago, published in eighteen sixty nine, in which he vividly described his earlier travels in that region of the world, including lots of very well observed biological and cultural detail. And we read a bunch of selections from that book in the last episode, and they are kind of it's somewhat magical to read.

where and why. Wallace is considered a very important founding figure in biogeography, with one of his most enduring observations being what is now called in his honor, the Wallas Line, an invisible boundary passing in the ocean between the islands of originally Borneo and Sulawesi, and even more astonishingly in the tiny narrow strait between Bali and Lombach, which marks the westernmost boundary of a lot of characteristic Australasian fauna marsupials,

cockatoos and things like that. So we are back today to talk more about Wallace and the Wallas Line. Now, one thing I think we've got to do before we move on any further is just a bit of clean up, because, as I mentioned in the last episode, our model of the Wallas line has undergone some major revisions since Wallace

first proposed it in the mid nineteenth century. I'm not going to do a whole detailed play by play of the shifting history of the line, but for a brief summary of developments, I was just looking at the abstract of a paper called Wallace's Line WALLACEI and Associated Divides and Areas History of a tortuous tangle of ideas and labels. This is by Ali and Heene, published in the journal

Biological Reviews in twenty twenty one. So the authors go through the concept which we discussed in the last episode, the idea of a faunnel boundary being basically a place where you find animals mainly of one sort on one side of the line and animals mainly of a different sort on the other side of the line. And this

raises questions how did it end up like that? The authors write that the earliest dividing lines in this region were considered quite sharp, especially the line Wallace drew in eighteen sixty three, and this was based on looking at the distribution of land mammals and some birds. This is the line we've been talking about so far, passing between Bali and Lombach, with Asian animals found west from the west coast or from the eastern coast of Bali and

Australasian animals running east from Lombak. This particular theoretical boundary proved very influential and it got you know, reproduced in a lot of texts. It was baked into maps and plates and stuff all over so like it had a big cultural footprint, and within the discipline of biogeography it came to be seen as something of a fixed marker in nature, maybe like the tree line on a mountain.

But over time it also became obvious to experts in the field that there were a lot of exceptions to the Wallace line, especially a lot of Asian fauna found east of the so called line. And this would not have been very surprising to Wallace himself, I think, who argued that biogeographic boundaries were all to some extent permeable, But it might have been more surprising to people who got a you know, a map printed with a line on it that says animals do not cross this line. Yeah.

By later in the nineteenth century, many biogeographers had started to think about zones instead of life. For example, there is now a biogeographic region known as Wallace Sea. I've also heard it pronounced to Wallasia, so Wallasia or Wallacea named after again Alfred Russell Wallace. This region was sort of proposed in the nineteen twenties, which is a faunal transition zone where you essentially have Australasian fauna to the east of the zone, Asian fauna to the west of

the zone, and more of a mix within. Though the western boundary of Wallacea is still basically Wallace's line, passing between Bali and Lombach and in between Borneo and Sulawesi. So we're actually here creeping a bit back toward that gradual transition idea of animal ranges that we talked about in part one. Though some of the divisions you see, especially between Bali and Lombach, are still actually quite striking, So Rabbi included for you to look at in our outline.

Here an illustration from one of the papers I was looking at, the region highlighted in red, and all the islands within that is what is generally considered Wallacea today.

the one he finally settled upon in nineteen ten. Its path around Sulawesi was transferred from the west to the east of the island, ideally Huxley's divide, And that's referring to Thomas Henry Huxley, who proposed a modification to this to the where the line goes. Hucks divide eighteen sixty eight should carry his name rather than Wallace's. The latter never accepted the proposition. Lydacer's line of eighteen ninety six ought to be labeled the hil Prin Lydacer line in

recognition of Angelo Hilprin's eighteen eighty seven contribution concerning transition zones. Ideally, Wallacea should correspond to its original nineteen twenty four description,

which incorporated the Philippine Islands bar the Palawan group. Notably, though a smaller form introduced by Darlington in nineteen fifty seven used frequently from nineteen ninety eight onwards, in which all of the Philippine Islands are excluded, is entrenched within the recent literature, but this is often without evident justification. It should also be recognized that the reduced meaning southern Wallacea area was effectively defined by hilprint in eighteen eighty seven,

but was then labeled the Austro Malaysian transition zone. So it's a mess. This is one reason why I've really enjoyed researching for the episodes. But like when I was reading all this stuff for the last episode, I was just like, ah, I can't.

of a mess. There have been a few details that have persisted, but there has not been clear or consistent agreement across time on where to place the boundaries, whether it's a line or a zone, to call those lines and zones, and what exactly they mean. So you might be wondering based on that, like, well, if you know, if we can't agree on what we're talking about, is the idea of a faunal boundary here just useless or

is it nonsense? I think the answer is no. I mean, for one thing, some of this confusion is historical, Like there have been general trends in how the idea is getting refined over time. So there's nothing wrong with that.

I mean, obviously ideas get updated. But some of the confusion is also because we're trying to figure out what exactly is being proposed here, and so like you have gotten away from the idea of a simple boundary line on a map that you know, where you have one thing on one side and one on another and more

into say the idea of a transition zone. And it is absolutely still safe to say that there is something very interesting with reference to biogeography that's happening in the islands between Borneo and Bali to the west and Papa New Guinea to the east, let's say, and this, this boundary line or transition zone can tell us a lot about the history of life on the surrounding continents in the history of the Earth itself.

ocean at all. And also the environment on the two islands is very similar, so it seems like you would expect that the animals find on one to be the same as the ones you'd find on the other, right, right, So to summarize sort of the original mainstream explanation, and there will be some additions to this as we move on. In this episode, Wallace understood the cause of the apparent faunnel boundary between Bali and Lombach to be a result

of geological history. This is a core insight of biogeography. Studying where animals live can tell us not only about the animals, but also about the land, and there are actually really awesome examples of this. If you get into the way paleontology has helped interact with the geohistory of Earth like fossil organisms from hundreds of millions of years ago that appeared to live only in Africa and South America. Huh. One example of this, there's this early Permian reptile called Mesosaurus,

the sort of shaped like a little crocodile. It lived exclusively in freshwater lakes and rivers almost three hundred million years ago, and its remains have only been found in southern South America and southern Africa. Wait a minute, If it only lived in fresh water like lakes and rivers, how did it give from Africa to South America and

as far as we can tell nowhere in between. And then you've got other examples like this, like there's a Triassic land based therapsid called Syno Nathas, also found in the fossil record of South America and Africa, among a few other equally baffling locations like Antarctica. These examples showed, along with lots of other evidence that we have now,

that these two land masses used to be one. That you know, the range was one continuous range hundreds of millions of years ago, and these animals died, their remains were posited, they were quickly buried, they became fossilized, and then the land split apart and the Atlantic Ocean in between was formed. So that's one example of how biogeography can tell us not just about the animals, but also

about the land. Coming back to Wallace. At the time Wallace lived, plate tectonics was not yet an accepted theory. In fact, it wasn't even really proposed in a form that would be recognizable to us until I think, like the nineteen tens around them, and it wouldn't be widely

accepted by geologists until the nineteen sixties. So Wallace in his contemporaries did not know that the continents and land masses of the Earth moved around, splitting and rejoining on the timescale of tens or hundreds of millions of years. But Wallace did know that the the surface of the Earth could change drastically due to other factors, and one

of those was climate. Climate patterns determine Earth's average temperature and weather, and in instances long ago, when Earth's surface was colder and more of Earth's water was locked up in polar ice caps, sea levels were lower much lower.

As a result, many areas that are now ocean were once dry land, and many parts of the world that are now islands surrounded by shallow seas were once continuous land masses, allowing land animals to cross freely between them, which is how you end up with large land mammals like tigers and elephants living on what appeared to be remote islands. In most cases, they didn't swim there, certainly

not in stable breeding populations. They crossed on land at a time when there was a land bridge, and for the most part, Wallace correctly identified that many islands east of the line of the Wallas line were once joined as a single land mass with Australia, this land mass now known as the paleocontinent Sahul, with islands west of the line once being part of a more continuous land

mass with Asia, now known as Sunda. However, just because islands are close to each other does not mean a land bridge always forms between them when sea levels drop. What Wallace did not know for sure, but what later proved perfectly consistent with his observation of the faunal boundary, is that between Bali and Lombach there is a deep

ocean trench. The Strait of Lombach is not wide again, it's just a few kilometers wide at the narrowest point, but it is very deep, meaning that even when sea levels were at their lowest, a land bridge between the two islands never formed, and thus there was much less opportunity for land based animals to colonize one island from the other. The reason for the existence of this deep ocean trench, by the way, brings us back once again

to plate tectonics. This trench is the meeting point between two plates where one is undergoing subduction, meaning it's being driven down underneath the other plate at the meeting point.

Subduction zones tend to create deep ocean trenches along with other geologic activity like earthquakes and volcanic eruptions, so plate tectonics also it explains that the deep ocean trench that keeps the islands apart and never forming a land bridge, but it Plate tectonics also helps explain why there were such different collections of animals on Sunda and so Whul.

To begin with, they were once separated by much more ocean than they are now Sahul And again, this is modern day Australia and New Guinea and some other associated islands.

This was one part of the same land mass as what is now Antarctica, and over tens of millions of years in isolation from the Afro Eurasian and American land masses, this ancient continent experienced its own distinct branch of evolution, giving us, for example, marsupials instead of the placental mammals that you would find on the rest of the globe.

It was only a little more than thirty million years ago that sa who will actually broke away from the land mass that would become Antarctica, and so who will? After that, slowly drifted north and then collided with the continental plates of Southeast Asia, giving us the arrangement of islands that we see today in Indonesia and the Malay Archipelago. And some of the islands we see in that archipelago

were created by the collision. They were raised up. But again at the place where the plates collide we get a subduction zone and a deep ocean trench, which means even though the islands are now close, land bridge is never formed between Bali and Lombach, So even though they have been close, now for millions of years. There is

relatively little interchange of large land animals back and forth. However, there is some interchange, and as we've discussed already, all faunnel boundaries are to some extent permeable, and studying those crossover examples can tell us even more about how this boundary works. I'm going to come back to that in a bit, but Rob, I think you also had some good stuff about how this interacts with plate tectonics.

crystallized accepted form. At the time, a leading approach to understanding all this was a theory of continental extensionism, which Radical by Nature author James T. Costa describes as land bridges on steroids. So you know, we've been talking here about the concept of land bridges and how they work, and again they're certainly real. The Isthmus of Panama is a current land bridge, lost land bridges once connected India and Sri Lanka, and a land mass known as the

Basian Plaine once connected Tasmania to mainland Australia. And of course we've also talked about the now submerged north Sea land mass of Doggerland.

local examples of geological distribution, that's all good. The problem with continental extensionism as it was used at the time was it leaned heavily on this as the prime, or even the only way that many forms of fauna had spread, employing actual and likely examples of land bridges, you know, sunken land masses and shallow seas, but then adding in other necessary lost land masses and land bridges as required to fill in the gaps, often with little or no

actual evidence, and even dipping into lost continent myth making and pseudoscience in order to get the job done.

know it to another. How did they do it? There had to be a land bridge, but not just any land bridge. There had to be a lost continent right there in the middle.

conspiracy theory, and of course fiction. You can look to various examples such as the fiction of Roberty Howard the fiction of Lynn Carter. You'll find Lumiria showing up as a location for various fantastic magical adventures.

of the scientific mainstream. Again, he was very interested in spiritualism and defended spiritualism and so forth, So it's not a stretch to imagine that he, you know, wouldn't have strong objections to some of these concepts, which again at the time were not like firmly wrapped up in a lot of occultist dreaming, and was more firmly within the realm of possible scientific explanations for the world.

to explain everything that we saw. In fact, Darwin favored the permanence of ocean basins and continents in his calculations, so not to say he thought everything was set in stone. You know, he recognized that there would be regional uplift and subsidence, for example. This was a part of a key part of his understanding how reefs and atolls formed.

We've talked about that on the show before. But you know, he was not keen on the idea that there were lost continents that would have connected one land mass to another and didn't think they were necessary. More the point, and this is all interesting because everyone here had some really good ideas going on, but those ideas had to ultimately be overclocked to make up for what we had yet to settle on regarding continental drift and plate tectonics.

So again, land bridges are a local slash regional reality, but you can't apply them globally. It's not a solution to every problem in the distribution of fauna.

of floatation in action. But then you have the now extinct Saint Helena Olive of Saint Helena Island in the South Atlantic. The olive, as listeners might remember, possibly became established there by dropping off of an albatross. So it's one of these things that was extremely unlikely to occur, but unlikely rare events do occur over the course of geological time.

are suddenly allowed to move into each other. They're allowed to freely colonize one another's regions. Examples of this usually occur when a barrier to travel between two places is suddenly removed, and suddenly here would be a relative modifier suddenly on geologic time. One example of this is the American interchange, where land and freshwater animals from North and South America were suddenly able to migrate across into each other's territory. After Rob you mitched this earlier. The idea

of the Panama is isthmus. Oh Man, that word is so hard to say isthemus for me, It's difficult. After the Panama Isthmus was raised by volcanic activity, so that came up from the seafloor. And previously there had been a saltwater barrier between North and South America for some time.

But you have this volcanic activity. Now there's a land bridge in Central America that happens between two and three million years ago, and now there's all this exchange of life forms across the barrier or across the former barrier. Another one of these great biotic mingling events is the Indo Australian Interchange, which arose when the Australian and Eurasian techtonic plates smashed together, forming some islands in between within the region we were talking about earlier known as Wallacea.

Now we've already talked about how there are some barriers to exchange along this connection point. The extent of that barrier to cross colonization was the original observation of the Wallace line. It's like what Wallace observed is, hey, there's not a lot of animal species going across here. Unlike the American Interchange. There is not and was never a

full land bridge between Sunda and Sahul. But as Wallace also observed, all boundaries are permeable, and we now know a good bit of exchange does happen across this line, and there are some more faunally mixed transition zones in between in the islands of Wallacea. But the authors of this paper point out something interesting. Biotic interchanges are rarely symmetrical. They're rarely totally even. Usually you have more life flowing in one direction across the new corridor than flowing in

the other direction. Why would that be? Why the asymmetry. Some answers lie in characteristics of the organisms. Maybe some organisms are just more adaptable, they're better at dispersing into new environments. Maybe they're better at evolving more quickly and changing what they're adapted to. But some answers, on the other hand, might lie in characteristics of the land and environment on either side of the new corridor, or within the corridor itself. What if something about the geography favors

dispersal one way but not the other. And to be clear here, the authors do not argue that Wallace was wrong about the deep ocean trench in the Lombock Strait being a barrier to dispersal. It is, as it does stop a lot of exchange, and they note that the only mammals which seem to have successfully crossed that boundary. Naturally are bats and rodents. Big mammals seemingly do not cross. But of the animals that have crossed, one thing that

is clear is which way the asymmetry goes. Over the last twenty million years, way more animals from Asia have successfully crossed the barrier and made their way into New Guinea and onto the Australian continent. Relatively little dispersal has gone the other direction, from Australia and New Guinea to

the west into Asia. Why would that be well, The authors argue, based on analysis of a huge data set and a sophisticated computer model, that the answer has to do with the climate of the islands within and on either side of Wallacea, especially how wet or dry those islands are, and the precipitation tolerance of the animals that

evolved on either side. So to quote the authors quote here, analysis of more than twenty thousand vertebrate species with a model of geoclimate and biological diversification shows that broad precipitation tolerance and dispersal ability were key for exchange across the deep time precipitation gradients spanning the region Sundanian meaning Southeast Asian lineages evolved in a climate similar to the humid quote stepping stones of Wallacea, facilitating colonization of the Sahulian

meaning Australian continental shelf. By contrast, Sahulian lineages predominantly evolved in drier conditions, hampering establishment in sunda and shaping faunal distinctiveness. So, in other words, the islands in the transition zone here are typically full of tropical rainforests like you would find further up on the Malay Peninsula and the Asian Maine land. New Guinea, on the same tectonic plate with Australia, is

also dominated with tropical rainforest. So animals adapted to Asian tropical rainforests would find generally familiar and tolerably wet climates throughout most of the islands that formed these stepping stones leading to Australia and New Guinea, and once they reached Australia there would be different conditions. The continent would be much drier, but they could have more evolutionary time to adapt by colonizing the islands along the way or by

landing in the friendly climate of New Guinea. Meanwhile, animals adapted to the dry climate of Australia would have a quite difficult time finding hospitable conditions along the humid stepping stones, and once you get to the Asian mainland, it's still just humid and dominated by tropical rainforest. So because the transitional islands in between were more easily tolerated by the Asian animal, more Asian animals flowed into the land masses

of the Australian Plate than the other way around. And I thought that was really interesting because it I don't know if it often occurs to us to think when we think about like animals dispersing from one region into another, we think about the sort of end stage reach, like they start here and then they end up here, But the place that they're having to move through as they migrate also plays a big role in whether that migration can take place at all.

people crossing one way or another. I mean, in reality, there would be a lot of things influencing this, but you could imagine that which side of the highway you have to drive on on the highway going in between might determine a lot about who decides to make the journey more often, like if you got to be if you got to be on the left side of the road, I can imagine a lot of Americans saying, I'm not driving that far on the left side. I don't know

how to do that. So if we were driving on the left side, maybe it would be a motorway and not a highway. I'm not sure.

triangle shaped. It's located between the Pacific and Indian Oceans and encompasses portions of two biogeographic regions, so the Indonesian Philippines region and the Far Southwestern Pacific region. If it looks like anything, it looks more like a rough sketch of a fish.

forty coral species that we know of. So just a rich hot bed of marine biodiversity, and there's a reason that snorkelers and divers from around the world seek it out. There's just breathtaking abundance there. I've never experienced snorkeling like this before, and you know, unless I go back there, I'm not sure I will again.

many other marine invertebrates as well. Additionally, there's the Indonesian through flow, which moves massive amounts of water from the Pacific to the Indian Ocean, and so it plays a crucial role here as well. It'll transport some larvae, but also acts as a barrier to others, influencing the genetic flow between populations of coral on either side of the line.

So some coral species have wide Indo Pacific ranges and can be found on both sides, but others follow distribution patterns that are more in line with some of the terrestrial animals that are impacted by the Wallas line. And we also see this play into distinct species. For instance, there are two distinct subpopulations of the Indo Pacific leopard shark, and they are separated by the Indonesian through flow current,

which again coincides roughly with the Wallas line. Here. For instance, the Missoul Foundation, which runs the Missoul resort in raja Ampat. There's they are part of a Star Project Reshark program to help re establish the eastern subpopulation of the species in raja Ampat. Meanwhile, there's another program Star Project Thailand focused on the western variety. So one variety of Indo Pacific leopard shark on one side and then another variety on the other side.

raja Ampat. So they had to reach out to various aquariums to get surplus eggs from into Pacific leopard sharks of the right variety, and one of the aquariums is actually the Georgia Aquarium here in Atlanta. Uh So it was entirely I don't know for certain, but it was entirely possible that some of the the the baby leopard sharks that I that I saw in the tanks that were then going to be gradually re produced into the wild. Perhaps we're descendants of eggs that were produced all the

way back home in Atlanta. The circle of life, Yeah, circle of life with leopard shark eggs, you know, taking international flights for days across the Earth, which in a way is entering because it brings it back into the like the vast way that humans end up moving around. And of course we've talked before in the show about how the movements of humans has of course been another huge factor in the way other organisms are intentionally and

unintentionally spread around the world. Yeah, of course, but occasionally

we get strategic about it. We attempt to fix things that we had broken before, because you know, there's a reason that sharks are missing, as we've discussed before, or shark populations are greatly decreased in various areas, and it's because we have hunted them, we fish for them, we have feared them, and so forth, and in doing so, you know, we end up damaging the ecosystem that they were a part of, because those apex predators are essential

to the overall structure of things. And also, as we've discussed in the show, their place is fragile.

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