The Wallace Line, Part 2

In this episode of Stuff to Blow Your Mind, Robert and Joe discuss the faunal boundary line between Asia and Australia known as the Wallace Line and the British naturalist it was named for, Alfred Russel Wallace.

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Episode Transcript

Speaker 1 (00:03):
Welcome to Stuff to Blow Your Mind production of iHeartRadio.

Speaker 2 (00:12):
Hey A, you welcome to Stuff to Blow your Mind.
My name is Robert Lamb.

Speaker 3 (00:16):
And I am Joe McCormick. And we're back with Part
two in our series on the nineteenth century British naturalist
Alfred Russell Wallace and on the Wallace Line, the faunal
boundary in the Malay Archipelago that bears his name. If
you haven't heard part one yet, I would recommend you
go back and listen to that one first. But in
part one we started off with a general character sketch

(00:39):
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,

(01:00):
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

(01:23):
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

(01:44):
are kind of it's somewhat magical to read.

Speaker 2 (01:47):
Yeah. Absolutely, again, you can see why this book was
such a success, and it was read by other naturalists,
but also just members of the general public who are
interested in the topic, interested in far away lands and
in the net.

Speaker 3 (02:00):
Yeah. So we read some selections from the chapters where
he goes exploring, initially against his will, by the way,
on the islands of Bali and Lombach. And this included
everything from these evocative descriptions of the land itself, talking
about the terrorist agriculture and the way the fields are
irrigated and all that, two discussions about the geographic distribution

(02:22):
of ghosts to complaints about how hard it is to
do science when everything smells putrid and discovered in ants.

Speaker 2 (02:29):
Right now, To be clear, it was putrid smelling because
of all the birds skinning gsty This is not a
general comment on the region or the people's there. This
was his nest.

Speaker 3 (02:40):
No, no, it was not like Lombox smells bad. It
was my room smells bad. Yea yeah. Also we talked
about the concept of biogeography, the study of what lives,
where and why. Wallace is considered a very important founding
figure in biogeography, with one of his most enduring observations

(03:01):
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,

(03:23):
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

(03:45):
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.

(04:07):
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

(04:28):
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

(04:49):
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

(05:11):
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

(05:33):
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.

(05:53):
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

(06:17):
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

(06:39):
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.

Speaker 2 (07:01):
Yeah, and on this map here it looks like a
big old heart, like a big old Valentine. They even
colored it.

Speaker 3 (07:05):
Red Happy Valentine's Day. You get some marsupials.

Speaker 2 (07:09):
Yeah, and I was in the eastern part of this
very region.

Speaker 3 (07:12):
Okay. So Ali and Heeney write that in the last
decade before their paper, even more new regions and boundary
modifications have been proposed, and the authors paint a truly
headache inducing picture of the historical understanding of all this.
As they say in their title, it is a tortuous
tangle of ideas and labels. For example, I'm just going

(07:35):
to read one section of their abstract to give you
an idea quote. Wallace's eighteen sixty three line is not
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

(07:57):
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,

(08:18):
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.

(08:42):
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

(09:03):
just like, ah, I can't.

Speaker 2 (09:05):
Yeah, I mean it's again, It's it's not like if
you take a kangaroo and have it walk over the
Wallas line, it will explode. Like, so, there's no it
becomes very difficult to actually test out some of these things.
It's based on a number of observations and factors.

Speaker 3 (09:22):
Well, these modifications absolutely are based on empirical observations. I mean,
it's not just people kind of jousting around about absolutely nothing.
Like there are reasons based on the different kinds of
fauna that have been observed and the underlying theory about
like the geologic causes and so forth. But unfortunately, the

(09:42):
theoretical definition of the Wallace line and Wallace are kind
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

(10:04):
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

(10:26):
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

(10:50):
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.

Speaker 2 (11:06):
Yeah, and this is what they were trying to figure
out obviously in the late nineteenth century. But so we'll
be discussing here they didn't have all the information they
needed in order to really understand and to make what
we would think of as a modern theory as to
why it was like this.

Speaker 3 (11:25):
That's right, and so that brings us to the question
of what causes the boundary. We raised this in the
last episode, but didn't really have time to answer it.
What causes the faunal boundary between the islands of say
Bali and Lombok. Why do you get mostly one set
of animals on Lombach and a fairly different set on Bali.

(11:45):
Even though the strait between these two islands is just
a few dozen kilometers wide, it's like it's barely any
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

(12:08):
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

(12:30):
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.

(12:56):
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

(13:18):
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

(13:40):
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

(14:02):
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

(14:25):
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

(14:48):
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

(15:10):
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

(15:34):
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

(15:56):
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

(16:22):
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

(16:43):
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

(17:05):
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

(17:27):
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.

(17:48):
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,

(18:11):
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

(18:35):
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

(18:56):
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

(19:19):
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.

Speaker 2 (19:29):
Yeah, and just getting a little bit into a little
more into just sort of the history of Wallace and
Darwin in trying to figure out why the Wallace line
seemed to be a thing, how this boundary worked, and
what were the factors behind it. Again, as you mentioned,
you know, we didn't have continental drift in a sort
of crystallized form until nineteen twelve, and we didn't have

(19:50):
plate tectonics until the nineteen sixties, again in its more
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

(20:11):
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

(20:32):
land mass of Doggerland.

Speaker 3 (20:34):
Ah. Yeah, we did a couple of episodes on that.
Go back and look them up. Yeah, sometime last year.

Speaker 2 (20:39):
So land bridges as an explanation for certain regional slash
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,

(21:02):
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.

Speaker 3 (21:21):
God love a lost continent.

Speaker 2 (21:23):
I mean, we all do. It's a it's a wonderful
concept in myth and in fiction and science fiction. But yeah,
this was especially the case when trying to connect the
dots across like Abystle Seas, so you'd have respected naturalists
of the days suddenly leaning into well, you know, the
idea of Lemuria, but even Atlantis.

Speaker 3 (21:43):
We may have also talked about them in a series
of episodes we did, I think starting with the Eltannan Antenna,
where we were talking about how people take anomalous underwater
imagery that kind of looks interesting and then kind of
run wild with it, deciding that oh, this thing under
water has got to be an alien antenna, and actually

(22:03):
it's probably a sponge, and this other thing has got
to be evidence of Atlantis, when actually it's probably a rock.

Speaker 2 (22:10):
That's right. And we definitely talked about Lemurria. This was
This one's a little different compared to Atlantis. It was
proposed in eighteen sixty four by zoologists Phillip Sklater to
explain the presence of lemur fossils, thus Lmuria on Madagascar
and in India, but not in Africa or the Middle East.
So you can you can see how Lumurria is very

(22:31):
much about filling in that gap, saying, these creatures had
to get from one side of the world as we
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.

Speaker 3 (22:45):
That's funny because it would be trying to explain, actually,
what is evidence for continental drift or plate tectonics.

Speaker 2 (22:51):
Right right, So initially it's just it was an idea
to Lemuria was just a way to hypothetically fill in
the blanks here continental extensionism to explain everything into other
Individuals like biologist Ernst Haekel, considered the idea that human
origins might tie to this lost continent as well. But

(23:12):
then in the late nineteenth century, occultists began to appropriate
the concept, and Lumiria of course dies as a scientific
hypothesis in the nineteen sixties, but refuses to die in occultism,
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

(23:35):
a location for various fantastic magical adventures.

Speaker 3 (23:38):
It's a great place to have a leather diaper, barbarian face,
some psychic monsters.

Speaker 2 (23:43):
Yes, so continental extensionism. This was a concept that Wallace
himself was rather taken with early on. Again, it seemed
a possible way to explain geological distribution of fauna and
explain some of what he was seeing. Though obviously, as
we've also discussed, he clearly wasn't opposed to viewpoints outside

(24:04):
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

(24:27):
realm of possible scientific explanations for the world.

Speaker 3 (24:30):
That's right, And again to clarify we talked about this
last time, but spiritualism is different than what people mean
when they say like I'm spiritual today. Spiritualism then meant
the belief that you could make contact with and communicate
with the dead or with spirits other than living humans,
and that kind of contact was exciting to a lot
of people who were in the sciences. They were like, hey,

(24:52):
look here's something we can study empirically. Let's you know,
let's study it, let's take notes.

Speaker 2 (24:57):
And this is another place where we kind of get
into the the butting heads of Darwin and Wallace, who again,
these two were not enemies. They rode to each other,
they seemed, you know, friendly with each other, and Wallace
looked up to Darwin. But Darwin really hated all of this.
He hated that naturalists would even flirt with the idea
of atlantis, and Darwin believed that over the course of

(25:19):
life's history on Earth, wind wing flotation, these were sufficient
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

(25:41):
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,

(26:02):
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

(26:25):
to every problem in the distribution of fauna.

Speaker 3 (26:29):
Right, Land bridges explain a lot, but they don't explain
how life forms got to Easter Island, or to Hawaii
for that matter.

Speaker 2 (26:36):
Right, right and wing, wind and flotation can get the
job done over the fullness of time, sometimes in remarkable ways.
But there are limitations to that as well. But there
are also some pretty amazing examples. I'm reminded of my
recent interview with Tom Lathan about his book Lost Wonders
regarding a number of species that wind up in rare,
far flung places, including both the Galapagos tortoise, which that's,

(27:00):
as we've discussed on the show before, that's an example
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,

(27:22):
but unlikely rare events do occur over the course of
geological time.

Speaker 3 (27:28):
Yeah.

Speaker 2 (27:28):
So Wallace did eventually come around to a different understanding
and wrote the eighteen eighty book Island Life, in which
he sets aside really any thought of lost continence in
favor of kind of a combo of both land bridges
and long distance dispersal via wind wing and floatation. And
so in this it brought his line of thinking closer

(27:49):
to Darwin's own, but ultimately leaning on a combination of factors.

Speaker 3 (27:54):
And again he was aware of the idea of past
sea level changes due to changes in how much of
Earth's water is locked up in ice.

Speaker 2 (28:02):
Absolutely and again those deep ocean trenches, though, would have
just been too deep to have been impacted by these
sea level drops, you know, I mean, sea level can
only drop so much and you're still going to have
to contend with trenches at that point.

Speaker 3 (28:25):
So I was looking for some more recent papers on
the Wallace line, and I came across one that I
thought was pretty interesting. This was a twenty twenty three
study published in the journal Science by Skills at All
called paleo Environments Shaped by Exchange of terrestrial vertebrates across
Wallace's Line. The authors in this paper start off by

(28:46):
establishing a concept called biotic interchange. This is what happens
when the flora and fauna of two previously separate regions
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

(29:09):
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

(29:29):
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

(29:50):
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

(30:11):
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

(30:32):
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,

(30:53):
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

(31:15):
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

(31:39):
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

(32:00):
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

(32:24):
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

(32:44):
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,

(33:05):
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

(33:28):
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

(33:52):
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

(34:15):
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

(34:39):
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

(35:03):
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

(35:23):
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.

Speaker 2 (35:32):
Hmm. Yeah, that's a great point.

Speaker 3 (35:34):
I was trying to think of a good analogy, and
this doesn't exactly work, but to grossly oversimplify. What if
there were a highway suddenly opened across the Atlantic Ocean
between Great Britain and the United States and you could
cross it by car, do you think you'd have more
people crossing one way or another. I mean, in reality,

(35:55):
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

(36:17):
the left side, maybe it would be a motorway and
not a highway. I'm not sure.

Speaker 2 (36:20):
Yeah, yeah, but I see what you're going for here,
Like the side of the car that you drive upon
is kind of like the environmental conditions of the passage. Yeah. Yeah.
And how conducive drivers on either side or going to
be to that passage.

Speaker 3 (36:34):
Yeah. It's not just how well adapted you already are
to the end point. It's like how well adapted you
are to each little step along the way.

Speaker 2 (36:42):
Yeah.

Speaker 3 (36:43):
And in the case of Wallacea, it proved easier for
the humid adapted Asian mainland fauna to to make the
journey into especially into New Guinea.

Speaker 2 (36:55):
Now, as we begin to reach the end of this episode,
I want to bring it back a little bit to
my travel experience in Indonesia where I actually visited these
stepping stones or some of these stepping stones we've been
talking about. So again, I was east of the wallace
line in Raja Ampat, which situated is also in an

(37:16):
area that is often called the coral triangle. Joe, I've
included a map of the coral triangle for you here.
This one isn't heart shaped and it's also not really
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

(37:41):
looks like anything, it looks more like a rough sketch
of a fish.

Speaker 3 (37:44):
Yeah, I guess see, it's a psilocybin triangle.

Speaker 2 (37:49):
Yeah yeah. And so why do we call it the
coral triangle? Well, because it is super rich in coral.
According to the Coral Reef Alliance, this region contains over
seven twenty six percent of the world's coral species and
thirty seven percent of reef fish species. So we're talking
about somewhere around five hundred of the eight hundred and

(38:11):
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.

Speaker 3 (38:30):
Well, as as I've said, I have no experience snorkeling myself,
but I'm still envious of what you got to see there.
But this does raise the question does the Wallis line
hold at all for underwater fauna. We know that it
holds to some extent for like land mammals. But you
might think intuitively, well, surely there would be no way

(38:53):
that the Wallas line would have any impact on say,
fish or other things that I can swim through. They
can just swim right across the strait.

Speaker 2 (38:59):
Right, you would think of the fish or just have
a free for all, and so they don't have to
obey the rules of the Wall's line, and neither do
the birds. But this is not the case. Despite the
fact that we're under the water here, we're still subject
to some of the boundary effects of deep ocean channels.
So these deep trenches seem to act as persistent barriers

(39:22):
that prevent the widespread migration of many coral larvae and
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

(39:42):
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

(40:03):
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.

(40:24):
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

(40:45):
on the other side.

Speaker 3 (40:46):
Wow, I mean, even though we've established that the line
is not impermeable and sometimes animals do cross, I don't know,
it's fascinating how just like how much of a barrier
there could be that's just totally invisible to us and
and even even affects creatures. You would think that it
that it couldn't like many underwater creatures.

Speaker 2 (41:06):
Yeah, that we can read it in the coral, we
can read it in the leopard sharks, and yeah, it's
it's fascinating. Yeah, just the power the powerful interplay of
division and mixing between rich ecosystems. Here. Another interesting thing
about the leopard shark program there is that they uh,
they were having to get you know, you wanted to

(41:27):
make sure that you were using the right variety of
into Pacific leopard shark to repopulate the region. There in
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

(41:51):
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

(42:16):
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

(42:39):
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,

(42:59):
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.

Speaker 1 (43:12):
At the top.

Speaker 3 (43:13):
Yeah, all right, Well, I have enjoyed this journey with
Alfred Russell Wallace.

Speaker 2 (43:18):
Yeah, like I said, a fascinating individual. If you want
to read more about him, that book Radical by Nature
by James T. Costa is quite good, and there are
a number of other books as well that have come
out in over the recent decades about his work. There's
kind of been a resurgence of interest in Alfred Russell Wallace.

Speaker 3 (43:36):
I enjoyed so much just reading Wallace's own writing in
the Malay Archipelago that I may want to come back
and plug that plum that maybe for some future topics.

Speaker 2 (43:45):
Yeah, yeah, Plus, yeah, he wrote many, many books on
a number of topics, so yeah, he may pop up
again in a future episode of Stuff Blear Your Mind.
All right, we're going to go ahead and close it
up there, but we'd love to hear from everyone out
there if you have thoughts about the Wallace line, about
some of the organisms that we've discussed here. Just a
reminder of Stuff to Blow Your Mind is primarily a
science and culture podcast, with core episodes on Tuesdays and Thursday,

(44:08):
short form episode on Wednesdays and on Fridays. We set
aside most serious concerns to just talk about a weird
film on Weird House Cinema.

Speaker 3 (44:15):
Huge thanks as always to our excellent audio producer JJ Posway.
If you would like to get in touch with us
with feedback on this episode or any other, to suggest
a topic for the future, or just to say hello,
you can email us at contact, stuff to Blow your
Mind dot com.

Speaker 1 (44:36):
Stuff to Blow Your Mind is production of iHeartRadio. For
more podcasts from My Heart Radio, visit the iHeartRadio app,
Apple Podcasts, or wherever you listen to your favorite shows.

(45:00):
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