The Science of Birds - Wallace's Line: Where Two Bird Worlds Collide

Episode Date: February 18, 2022

This episode—which is Number 46—is about  a special place in the Malay Archipelago where two bird worlds collide. This region lies between Southeast Asia and Australia, between the Indian and Pacific oceans. And it’s crowded with about 25,000 islands, of all sizes.Specifically, we’ll be looking at a geographic feature called Wallace’s Line. More generally, today’s episode will touch on the topic of biogeography.~~ Leave me a review using Podchaser ~~Link to this episode on the Science of Birds websiteSupport the show

Hello and welcome. This is the Science of Birds. I am your host, Ivan Philipson. The Science of Birds podcast is a lighthearted, guided exploration of bird biology for lifelong learners. This episode, which is number 46, is about a special place in the Malay Archipelago where two bird worlds collide. This region lies between Southeast Asia and Australia, between the Indian and Pacific oceans, and it's crowded with about 25,000 islands of all sizes.

Specifically, we'll be looking at a geographic feature called Wallace's Line. More generally, today's episode will touch on the topic of biogeont, This is an intentionally, shamelessly short episode. It's bite-sized, so you can listen on your break and then get back to work. Time's a wasting, so let's go. My other gig, when I'm not producing this podcast, is organizing and leading birding tours on occasion, to various destinations around the globe. People who sign up for these tours often are excited to visit far away exotic lands, to see birds unlike the ones they find back home. Not just different species, but also birds representing entirely novel families. Let's say you live in

New York, for example. You sign up for a trip, then you get on a plane, fly across the Atlantic ocean, and land in South Africa. Instead of seeing familiar and And, let's face it, boring birds like cardinals and warblers, you're greeted by sunbirds, weavers, and waddle eyes. Now we're talking. And instead of bears and moose, you see baboons and elephants. It's an entirely different world. And hey, just kidding about those birds being boring. All birds are wonderful creatures that deserve our admiration. I really, really believe that. Anyway, we totally expect to find such wildly different groups of birds and other animals, not to mention plant life, when we travel thousands of miles across a vast ocean to the other

side of the planet. But what if you traveled only a few miles from one island to another and found the same level of dissimilarity? Well, that's exactly what Alfred Russell Wallace famously discovered back in the mid-1800s when he explored the neighboring islands of Bali and Lombok in the Malay Archipelago. The birds and other animals on the two islands seemed to belong to different worlds. Wallace wrote that when looking at the birds of the entire archipelago, quote, The contrast is nowhere so abruptly exhibited as on passing from the island of Bali to that of Lombok.

In Bali, we have barbets, fruit thrushes, and woodpeckers. On passing over to Lombach, these are seen no more, but we have abundance of cockatoos, honeysuckers, and brush turkeys, which are equally unknown in Bali and every island further west. The straight here is 15 miles wide, so that we may pass in two hours from one great division of the earth to another, differing as essentially in their animal life as Europe does from America. End quote. Alfred Russell Wallace is the British naturalist who, independently came up with the theory of natural selection at about the same time as Charles Darwin. Hopefully you've heard of Mr. Wallace. He was an adventurous naturalist who collected specimens

of animals and plants, both to make a living and in the name of science. After traveling in the Amazon basin for four years, he spent eight years exploring the melee archipelago. In all his island hopping, Wallace noticed that sharp contrasts in species exist at many places across the region. Now, I know that all you and I really care about are birds, but I probably should mention that the differences in mammals across the archipelago are even more striking. To the west, you have mammals with an Asian affinity, like tigers, monkeys, and rhinos. But to the east, the furry critters are mostly of Australian origin, kangaroos and other marsupials and such. Wallace eventually wrote up and published his observations in the early 1860s.

He produced a map of the Melee Archipelago with a bold red line threading its way through the islands, between Bali and Lombach, between Borneo and Sulawesi, and trailing off to the east of the Philippines. According to Mr. Wallace, the red line marked the division between two biological realms. Today we call the one to the west the Indo-Malayan realm, and the one to the east is the Australasian realm. Like any respectable scientist, Alfred Russell Wallace knew better than to name this line after himself. It wasn't like he said, This boundary that I and I alone have discovered, this border between two natural kingdoms that are so unalike, I think we will all agree that it is a good line.

nay, it is, dare I say, a most excellent line, the finest of lines. And so I shall call it Wallace's line, and forever shall it be known as such. No, it was another biologist Thomas Huxley, aka Darwin's Bulldog, who later named the line after Wallace. So the line exists. Great. What's the big deal here? The big deal has to do with why the line exists. exists. How do we explain it? The physical environment across the melee archipelago doesn't change all that much from west to east. The entire region sits within a relatively narrow band of tropical latitudes. So there aren't things like major differences in temperature, rainfall, or habitat to explain Wallace's line. What if you consider the distances between the islands and you also factor in

the abilities of birds and other animals to get around. In other words, they're dispersal abilities. Does Wallace's line mark the location of a major barrier to dispersal? That's a very reasonable hypothesis. But on the face of it, there are some problems. Consider the following. West of the line, despite there being a bunch of isolated islands over there, the bird and mammal species are relatively homogenous across the islands. Those Asian critters don't seem to have had much trouble island hopping from, say, Sumatra to Java. The same thing is true to the east side of Wallace's line. Creatures of the Australian sort appear to have been bouncing around all over there, on and between islands like Sulawesi, Lombok, and New Guinea. So dispersal abilities

don't seem to be the limiting factor. These birds and mammals have, seemingly, been able to disperse over water between islands. And it's not like the distances between neighboring islands across Wallace's line are significantly larger. They don't seem to be great enough to explain the sharp biological divide, to explain why there isn't more mixing of animals from both sides. Something else has to be going on, right? I'm just spitballing here, but what if there's some kind of invisible force field, you know, like those invisible fences for dogs, where the dog gets an electric zap by its collar if it crosses the line, or maybe, and just hear me out, maybe Wallace's line was created by the god of the sea, Poseidon himself, for some inscrutable reason known only

to him. Who are we mere mortals to question his purposes anyway? How dare we? In reality, it turns out that Alfred Russell Wallace was a smart dude. Surprise, surprise. He actually offered a compelling explanation for his line when he first published his discovery. And he was, more or less, on the money. The key to understanding how Wallace's line came to exist is that we have to look back in time, way back. We already concluded that if we look at how the island of the melee archipelago are distributed today, which is just as they were 170 years ago when Wallace was tramping around, bird and animal dispersal abilities don't seem to offer the answer. But what if we look back many thousands of years? Then things get really interesting.

During the Pleistocene epic, what we call the ice ages, sea level was sometimes hundreds of feet lower than it is today, because so much of the water would get locked up. as ice, right? The seas surrounding many of the islands today are shallow, mostly less than 50 meters or 165 feet. Back when sea level was much lower, the shallow seas drained away, exposing the continental shelf between the islands. The islands became connected by dry land. In other words, they ceased to be islands. The land west of Wallace's line was all continuous with the Asian mainland. Birds and other animals could disperse freely across this region,

even if they were flightless, like tigers, lizards, and snails. Similarly, low sea levels east of Wallace's line connected New Guinea and Australia as one large landmass. But the ocean is much deeper between these Asian and Australian continents, in the area marked, more or less, by Wallace's line. The channel is so deep that it was a formidable, marine barrier even during the ice ages. Beasts like tigers and rhinos never crossed from west to east and marsupials never crossed from east to west. They would have had to swim a long way. They'd end up

getting their fur all wet and salty. I guess those mammals would just amble out onto the beach, look at the ocean, say nope, and turn around. Most birds can fly, of course, so they tend to be better at dispersal than the poor earthbound mammals. But even among the birds, there are major differences in dispersal ability. Strong flying birds like pigeons could easily cross the marine barrier dividing the Ice Age versions of Asia and Australia. We find birds like this on both sides. But for some other birds, even that distance of only 15 miles over the deep water straight between Bali and Lombok was too far. That was the case for weak flying birds like the chicken-like megapodes of Australia and the pheasants of Asia. So it turns out after all that dispersal abilities and the distances

between land masses do matter. They do explain Wallace's line. It marks the location of an ancient dispersal barrier between two continents. Because of that barrier, Asian and Australian animals have been mostly confined to their own continents. They've been evolving on separate trajectories for maybe 50 million years, even on those occasions when low sea levels gave them a bit more room to roam. Mr. Wallace's original explanation included a discussion of sea level changes, the depth of the seas around the islands, and animal dispersal abilities. So he had reasoned out most of the story, which is pretty impressive, given that this was in the mid-1800. We can point to some characteristic bird families found on either side of Wallace's line.

On the Australian, or technically what we call the Australasian side, we have the aforementioned megapodes. Those are birds like the various species of scrub fowl and brush turkey. There are also the honey eaters, cockatoos, fairy wrens and bower birds. Most famous of all, perhaps, are the 42 bird of paradise species. I look forward to doing a podcast episode on those guys. As you probably know, they're totally amazing and totally out of control.

Hopping over Wallace's line to look at the Asian or Indo-Malayan side, we find a lovely selection of bird families like the pheasants. bull bulls, leaf birds, tree babblers, and woodpeckers. Hornbills, too, are characteristic of this region. However, a few species, like Blythe's hornbill, have managed to sneak over onto the Australasian side. Cheeky buggers, those hornbills. In the many decades since Alfred Russell Wallace described his line, other biologists have studied the animal and plant distributions across the Malay Archipelago. Some of them have modified or tweaked Wallace's line on the map to account for new data.

Other researchers have drawn entirely new lines through the region. So it's not like Wallace's line is some kind of absolute dogmatic truth. It's just a helpful explanation or model for what we think is going on in the archipelago. We've updated or changed our understanding of this model with new observations. Because this is science, yo! Also, we now know that a bunch of islands in central Indonesia, including Sulawesi, have always been separated from the mainland and from each other by deep water. Even during the Pleistocene, when sea levels were low,

these islands weren't part of Asia or Australia. Today, they have relatively low species diversity, because birds and other animals have found it relatively hard to reach them. And these islands are home to more of a hodgepodge of birds, the ones with strong dispersal abilities from both Asia and Australia. These are strong flyers. Not surprisingly, very few land mammals are present. This region to the east of Wallace's line and west of New Guinea has its own name.

We call it Wallasia. Not to be confused with the historical region in Romania called Wallachia, home to Vlad the Impaler, aka the real-life inspiration for Dracula. People get Wallachia and Wallasia mixed up all the time. Luckily, the islands of Wallacea don't have any vampires, but they do have a relatively high number of endemic species, because of all the isolation and such. Endemic birds here include the Maluccan Woodcock, white-tipped monarch, and Taliabu Masked Owl.

Pretty much everything we've been talking about today falls under the larger topic of biogeography. This is the study of where organisms live on the planet, how their distributions have changed through time, and why we find them where we find them. As I think I've said before on the podcast, I love me some biogeography. It's fascinating stuff. Scientists working in the field of biogeography, like Mr. Wallace, have mapped out where all the world's animals and plants are found, in the present, but also in the past to some extent. One big pattern that has emerged from all this research is that, at the broadest scale, the land surface of Earth is divided into about eight bioreographic realms. These realms contain organisms that have been evolving side by side for millions of years,

while being isolated from the organisms in other such realms. If you live in North America, like me, you live in the New Arctic realm. We're the ones with the Cardinals, warblers, moose, and bears. The Afro-Tropical realm has the sunbirds and elephants. Every one of these biogeographic realms has its own flavors of birds and other life forms. But you and I care only about birds, as we've already made clear. Wallace's line and Wallasia lie along the world's most sharply defined boundary between any two bioregographic realms, the realms of Indomalea and Australasia.

This boundary is just too obvious for someone not to notice it eventually. But Alfred Russell Wallace was the first to formally describe this remarkable feature and the first to give a scientific explanation for its existence. Thanks for taking a little mental excursion to the islands of Indonesia with me today. Unless, of course, that's where you live. I guess in that case, you didn't go anywhere, did you? Well, wherever you are, I hope you enjoyed this short episode. A deep thank you to my supporters on Patreon,

my friendly wrens, helpful hornbills, and awesome osprey. And if you're listening and you don't know what I'm referring to, maybe you should hop over to my Patreon page at patreon.com forward slash science of birds. Take a look around, see if you feel inspired to become a patron. Who knows? A big shout out to my newest patron, Seng, from Australia. Welcome and thank you, Sange.

I'm generally reachable by email if you have something you'd like to share with me. A comment about the show or maybe you want to tell me what biogeographic feature you'd like to be named after. For me, I'm thinking Ivan's Badlands or Ivan's Festering Swamp, something of that nature. In any case, my email address is Ivan at Science of Birds.com. You can check out the show notes for this episode, which is number 46, on the science. of birds website, scienceofbirds.com. I'm Ivan Philipson, and I hope you're happy and well and that you're surrounded by birds. Cheers.