The Science of Birds - Biogeographic Realms and Their Unique Birds

Episode Date: October 13, 2022

This episode—which is Number 62—is all about the major geographic divisions among birds across the planet.Why are bowerbirds found only in Australia and on the islands of New Guinea? Why are the birds you see in India so different from those in China? Does North America have any unique, endemic bird families?Questions like these fall within the domain of biogeography. Biogeography is the study of where living things are found and why they’re found there—both in the present and the past. Today, we’ll be sketching out the big picture of what types of birds are found where across the world map.By learning the basics of bird biogeography, you can get a deeper appreciation for the uniqueness of bird communities in different parts of the world. ~~ Leave me a review using Podchaser ~~Links of InterestWhat Are The 7 Realms of Biogeography? [VIDEO]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 exploration of bird biology for lifelong learners. This episode, which is number 60, is all about the major geographic divisions among birds across the planet. Why are Bower birds found only in Australia and on the islands of New Guinea? Why are the birds you see in India so different from those in China?

Does North America have any unique, endemic bird families? Questions like these fall within the domain of biogeography. Biogeography is the study of where, living things are found and why they're found there, both in the present and the past. Today, we'll be sketching out the big picture of what types of birds are found where across the world map. By learning the basics of bird biogeography, you can get a deeper appreciation for the uniqueness of bird communities in different parts of the world. And for any bird you happen to be looking at through your binoculars, biogeography can give you a better sense.

of how that bird fits into the epic story of life on earth, against the backdrop of drifting continents, changing climates, sea levels and mountain chains rising and falling, and the constantly shifting distributions of ecosystems. I've told you before that I love me some biogeography. I really do. Almost as much as I love pizza or coffee. So I'm super excited to talk about this stuff.

The specific focus for today will be the topic of biogeographic realms. Biogeographic realms are the largest chunks of real estate we can divide Earth's land surface into, specifically with respect to the distributions of living things. We touched on this topic back in episode 46, which was about Wallace's line. In that episode, I was telling you about the boundary between two biogeographic realms in Indonesia. In today's episode, we're zooming out to look at the entire planet, at how it's divided into a number of distinct and marvelous realms of avian diversity. Birds don't care.

about the political boundaries that we humans get all worked up about. They cross our borders all the time without a second thought and without a passport of any kind. Birds laugh in the face of the border patrol, and they fly right over our silly walls and electric fences. To a biologist, the question of which birds are found in, say, France might not be all that interesting, because France is just a thing that humans invented. Sort of like democracy, Disneyland, and parkour.

Parkour, parkour! But that same biologist might be very keen to learn why the birds on the subcontinent of Europe are similar to those of Northern Africa. They might ask questions like, How has the distribution of birds between Europe and Africa been affected by the Mediterranean Sea? To better understand birds and to conserve them, We need to identify the borders that matter to them, the natural barriers. What limits the movement of birds from one part of the world to another?

Each of the world's roughly 6 to 8 biogeographic realms is defined by its natural barriers. We're talking mountain ranges, oceans, deserts, glaciers, and so on. The Mediterranean Sea is an example of a geographic feature that creates a border. border. It's a challenging barrier to cross, even for many birds. Other examples of a border slash barrier include the Andes Mountains in South America and the deserts of Western North America. Biologists have been able to define different biogeographic realms because the collection of species living in each realm has its own distinct character or flavor. A certain je ne se co if you will.

Let's say I blindfolded you and dropped you from a helicopter into an undisclosed wilderness location. After I fly off in the chopper, you take the blindfold off, curse my name, and then start pushing your way through thick

stands of bamboo on a steep mountainside. Would you have a rough sense of where you were if you soon ran into a red panda, a snow leopard, and some monkeys, some macaques to be more specific? Sure you would. You'd know this combination of plants, animals, and topography places you in Asia,

most likely somewhere in the foothills of the Himalayan mountain range. The place has its own distinctive character. Now, if a person were to walk for days or weeks traversing a biogeographic realm, they're eventually going to come to a zone where the floor, flora and fauna change, either abruptly or more gradually. In any case, this is the border, the transition between two neighboring realms. Transitions like this were first described by biologists back in the 1800s. Those guys came up with the concept of biogeographic realms.

We'll come back to them in a moment. But first, we need to look into the past if we're going to make sense of all this stuff. the plants and animals living in each biogeographic realm have a long evolutionary history together as in millions of years these organisms have been traveling companions on that particular landmass coexisting and co-evolving for a long long time each of these realms exists today because in one way or another, it was isolated from the rest of the world for millions of years. During that long period of isolation, the species living in each realm went off on their own weird and wonderful evolutionary journeys. They were on a separate path, disconnected from the rest of the world. Of course, some realms

are still isolated today. Australia, for example, has been and continues to be largely isolated because it's surrounded by vast oceans. The birds, other animals, and plants of Australia are very different from those in other parts of the world. So to understand biogeographic realms and the birds that live in them, we have to consider not only the natural barriers that exist around the world today, but also the barriers of the past. Some barriers lasted for millions of years, but then gradually disappeared. India, for example, was an island for a long, long time. Tectonic forces pushed it slowly north until it eventually smashed into Asia. So the ocean used to be a border on all sides of India, including the north. But that northern barrier disappeared

about 50 million years ago as the two land masses became one. But at the same time, the collision of India and Asia created a new and highly effective barrier, the Himalayas. You know, where I left you for dead after tossing you out of my helicopter? You can see how biogeographic realms make sense only when we consider the movements of the continents, as they've been riding around on Earth's slow-moving tectonic plates. Any time a continent became isolated for tens of millions of years, the independent evolution of its flora and fauna resulted in distinct lineages. But when two lonely continents meet up and smash together, the isolation is broken.

Plants and animals then have more freedom to mix between the continents. This is like what happened with India colliding with Eurasia. A similar thing happened with South America joining North America after a long period of isolation. There are other examples, but you get the idea. Today, some of these stitched-together land masses still harbor their distinctive variety of critters because the faunal exchange is ongoing. There hasn't been enough time for complete mixing, even though the borders are becoming fuzzier over time.

Physical or climatic barriers can also slow the mixing process. Okay, you still with it? me? This has been my rambling way of explaining the basic concepts of biogeographic realms. I hope it's making sense. The concept of biogeographic realms was first proposed in the 1850s by a British guy named Philip Sclater. Sclater divvied up the land's surface of Earth into six realms. These were specifically avian realms, since Sclater was an expert ornithologist, and birds were his jam. But Sclater's realms also made sense for many other animals and plants.

So even to this day, we still recognize some of the biogeographic realms he described over 150 years ago. Then along came the biologist Alfred Russell Wallace. Besides being famous for independently coming up with the theory of evolution by natural selection, Wallace is considered sort of the grandfather of biogeography. In the 1870s, Wallace expanded Sklater's concept of the biogeographic realms to include not just birds, but also other animals and plants. Wallace incorporated the idea of shared evolutionary history into his definitions of the realm. Other biologists continued to mess around and tweak the maps of Sclater and Wallace

until everyone came to a sort of shaky consensus, a consensus about the number and locations of the planet's biogeographic realms. For the last 50 years or so, the following realms have been taught to us in high school and college classrooms everywhere. the Nearctic, Palearctic, Neotropical, Afro-Tropical, Indo-Malayan, Australasian, Oceanian, and Antarctic realms. Do any of those sound familiar? I hope so. Some relatively recent research has helped to further refine and enrich our understanding of the biogeographic realms. In particular, there was a study published in 2013 in the journal Science.

The paper's title is An Update of Wallace's Zogeographic Regions of the World Zogeographic regions slash realms are pretty much the same thing as biogeographic realms The prefix Zoh just means we're specifically

talking about animals, right? Anyway, the researchers in this study did something pretty cool. They created a new map based on data from thousands of bird, mammal and amphibian species, not just data on the geographical distributions of these animals, but also from the evolutionary relationships among them. The researchers incorporated the genetic evolutionary differences between all these critters. Their analysis included an amazing

10,074 bird species. The end result of this 2013 study was the unveiling of a new set of biogeographic realms. The new map wasn't a total shocker since there's a lot of similarity between the new realms and the old school realms. But there are some interesting new details. And I'd say the important thing is that because this next generation map of realms is based on data from DNA, it's grounded more solidly in evolutionary theory, which I think is super duper cool. I'll put this next-gen map in the show notes for this episode on the Science of Birds website.

How about we hit fast forward on the tape machine now to skip to the part about the actual realms themselves? Let's talk about what makes them unique in terms of their birds and all that jazz. We're about to travel

the globe to look at the avian fauna in each of the biogeographic realms. It's going to be a whirlwind tour, so hold on tight. I'll mostly stick to the traditional versions of the realms, but I'll sprinkle in some updates about the next-gen realms from that epic 2013 study. Let's start with the realm that contains more bird diversity than just about anywhere else, the neotropical realm. This is a paradise for birds, and therefore a paradise for the humans who love birds.

The neotropical realm includes the entirety of South America, Central America, and the Caribbean. Its northern extent is kind of fuzzy, but it's somewhere in central Mexico. Tropical forests are the dominant ecosystem

across much of this realm. But it also includes the mighty Andes Mountains, vast grasslands and savannas, and the cool temperate zone at the southern end of the continent. As I mentioned, the continent of South America was isolated for a long time, for about 50 million years. That was plenty of time for the birds of South America to evolve into many distinct families.

Only when the itthmus of Panama. Ah, this is like my nemesis, this word, itthmus. Only when the itthmus of Panama formed about 10 million years ago did South America come into direct contact with North America. Then it became much easier for birds. from the two continents to mix. Even today, the neotropical realm is home to many bird families that are found nowhere else. In other words, these birds still haven't spread to other parts of the world. There are something like 35 to 40 bird families endemic to the neotropics. Some outstanding examples are, in no particular order,

toucans, oven birds, puff birds, new world barbets, Cottingas, ant birds, tanagers, Rias, Tinnamu's, and the Watson. The neotropical realm is also the stronghold of the New World parrots and the hummingbirds. The map of the next-gen realms from that 2013 study identified a brand new one, the Panamanian realm. This includes the northern edge of South America, all of Central America, and the islands of the Caribbean. Traditionally, those regions were all part of the larger neotropical realm. Bird families unique to this newly recognized Panamanian realm include Toadies, Spendalases, Chatt Tanagers, and the Cuban Warbler.

These birds have evolved on various islands in the Caribbean. Now we head into North America, into the Nearctic realm. Neo means new, right? So neotropical refers to the New World Tropics. Nearctic refers to the northern part of the New World. The enormous continent of North America was, for a long time, joined with Eurasia. But the two landmasses split apart about 60 million years ago as the North Atlantic Ocean form. However, North America and Eurasia have continued to have some connections on and off again ever since.

One of the strongest connections has been across the Bering Land Bridge between Siberia and Alaska. So there's been a lot of time for birds to move back and forth between these continents. And that means we find representatives of many bird families all across the northern temperate latitude. North America is vast and has an enormous diversity of ecosystems. We Americans sometimes think we have the best of everything, the best national parks, the best plastic surgeons, and hands down, the best selection of breakfast cereals at the grocery store. But seriously, we do have many amazing natural wonders. You know what we don't have much of? Unique bird families. That's what. The New Arctic realm has only one

endemic family. That's right, just one. Pusa dramedy. And the funny thing is, the family pusadramidee contains a grand total of one species. The olive warbler, Pusidramus tiniatus. Ever heard of the olive warbler? In the U.S., you'd have the best luck of finding one in the coniferous forests of Arizona or New Mexico. Pretty crazy, huh? This lack of avian endemism at the family level reflects the fact that North America was never all that isolated from the rest of the northern hemisphere. It was never as lonely and disconnected as South America was back in the day. Heading west, crossing the Pacific Ocean, we land in Eurasia. We are now in the Palearctic realm.

The traditional map of this realm includes all of Asia. North of the Himalayas, but not including southern China and southeastern Asia. The Palearctic also includes the Middle East, Europe, and Northern Africa. It's the largest biogeographic realm, by far. The far northern latitudes of the Palearctic realm are covered in boreal forests and tundra. Elsewhere, we find lots of temperate forests and expansive deserts. The story here is similar to the region. that of the Nearctic. There aren't many unique endemic bird families in the Palearctic realm

because of the long history of mixing with North America and other parts of the world. The Palearctic has just four endemic bird families. They're all songbirds, and each family has just one species. These are the bearded reedling, wall creeper, Chivalski's pink tail, and the hypocholius. If you consider both the Palearctic and the Neary Arctic together, which is sometimes called the whole Arctic, there are several bird families found only in these northern lands. The loons, grouse, ox, and wax wings are birds found only in these two biogeographic realms. The map from that 2013 study split the OG Palearctic realm into a few pieces. Two new next-gen realms were identified, the Sino-Japanese realm and the Sahara Arabian realm.

As you can imagine, the Sahara Desert is a major barrier to the movement of birds and just about every other living thing. So maybe it's no surprise that there's a distinct biogeographic realm to the south of the Sahara. This is the Afro-tropical realm. The journey of the African continent sort of parallels that of South America. Africa was isolated, surrounded by oceans on all sides for many millions of years before it finally connected with Eurasia. Many birds and other organisms in sub-Saharan Africa are still isolated today. The Mediterranean Sea, the deserts of the Middle East, and the Sahara are all major barriers.

There are dozens of endemic bird families in the Afro-tropics. Some great examples are ostriches, Turacos, African barbets, mousebirds, ground hornbills, sugar birds, and the shoebill. For a birder, Africa offers a chance to see many wonderful bird lineages like these. You'll find them nowhere else. Various biologists have pointed out that math Madagascar is worthy of its own biogeographical realm, and that was true for the researchers who created our 2013 next-gen map of realms. Some people have called Madagascar the eighth continent.

It became its own island, completely disconnected from Africa and India, about 90 million years ago. So the plants and animals there have evolved in isolation for an incredibly long time. Today, Madagascar is second only to Australia for having the most evolutionarily distinct animals. The Madagascan bioregographic realm is home to several endemic bird families. These are the Messites, ground rollers, Malagasy warblers, and the ascites. Acetes? I'm not sure how to pronounce that one. Vangas are one special group of birds in Madagascar. The family Vangadi isn't endemic to the island.

but one group of vangas here is remarkable. Their ancestors arrived in Madagascar about 25 million years ago. Then evolution kicked into high gear. The vangas diversified into a bunch of species that look wildly different. Their bill shapes, habitats, and lifestyles are so varied that it's hard to believe these birds are all in one family. The vangas of Madagascar represent a beautiful example of adaptive radiations. A more widely known case of adaptive radiation is Darwin's Finches of the Galapagos Islands.

Leaving Madagascar and heading east again, we come to the Indo-Malayan realm. This spans India, Southeast Asia, and the western part of the Indonesian archipelago. Tropical and subtropical ecosystems dominate this realm, which is rich in by, biodiversity, and shockful of amazing birds. There are about 10 endemic bird families in the Indo-Malayan realm. For example, there are the Asian barbets, leaf birds, tree babblers, and their allies, and the bristlehead of Borneo. Biologists have long recognized that

the far-flung islands of the Pacific Ocean are probably worthy of being treated separately as their own realm. Its name is the Oceanian Realm. But there's been disagreement about which islands should actually be included in this realm. Sometimes it includes all the islands north and east of New Guinea and the Solomon Islands. But in the updated version of the Oceanian realm in the 2013 analysis, New Guinea is included. This is interesting because New Guinea is intimately associated with Australia and has been considered part of the Australasian realm for well over a hundred years. In any case, this is a fascinating part of the world because the countless scattered islands have been little evolutionary factories

for producing new species. As far as I can tell, however, there aren't many bird families that are found only in this realm. One of my favorite birds, however, does fall into this category, the cagoo of New Caledonia, Rincoquitos Jubatis. This flightless gray bird with a wild crest on its head is the only member of its family. I was lucky enough to see some wild cagos a few years ago when I spent some days exploring New Caledonia. It was seriously a dream come true to see this special bird. Some other non-endemic bird specialties of the Oceanian realm are the birds of paradise, Bowerbirds, and Hawaiian honey creepers.

That is for the oceanian realm as newly defined if it includes New Guinea. Australia became an enormous island starting between, 50 and 30 million years ago. Before then, it was connected to Antarctica. Australia has been drifting north on its own ever since. The tectonic plate it sits on eventually collided with the Pacific plate. That caused the formation of New Guinea, which rose high out of the sea. In the classic definition of the Australasian realm, it's composed of Australia, New Zealand, and the eastern part of the Indonesian archipelago, including New Guinea. The long histories of isolation in Australia and New Zealand

let evolution run wild, with little input from the rest of the world. Not only did we end up with the many amazing marsupials of Australia, we also got some unique bird lineages. There are dozens of endemic bird families in Australia. Some highlights include the cassowaries and emu, lyre birds, cockatoos, honey eaters, and partolotes. Meanwhile, New Zealand became a world dominated by birds, including the Kiwis, Moas, and New Zealand parrots. Like Madagascar, biologists sometimes designate New Zealand as its

own realm, or it gets lumped into the Oceanian realm. The Australasian realm is the most evolutionarily distinct of all. On a first-time visit to this, the area, just about any bird you see is going to be distantly related to any other bird you've seen in other parts of the world. Believe it or not, soon after I record and edit this episode, I'll be taking a long series of flights to Australia. It isn't my first visit, but I'm super excited. I can't get enough of the amazing birds and other natural wonders of the Australasian realm. Last and most definitely least, we come to the Antarctic realm.

The massive continent of Antarctica is so inhospitable to most living things that biologists sometimes forget to include it when talking about biogeographic realms. For example, that 2013 update to the realms didn't even include Antarctica in the analysis. But since we're talking about birds, we have to. give Antarctica serious props for being home to so many penguins. Penguins are absolutely amazing. They seem to approve of Antarctica, so I suppose we can let it have its own biogeographic realm. That concludes our brief tour of the world's biogeographic realms. We only talked about the birds in each of them, so maybe we should call them the

Ava Geographic realms. Ava Geographic. Ooh, did I just invent a new word? Let me check. Dang. No, it already exists. It's not in the dictionary, but it looks like a few random people on the internet have used the word Ava Geography.

Oh well. Anyway, there is much, much more we can talk about for each of the biogeographic realms. Maybe I'll make separate episodes for some of them down the road. And each realm can be subdivided into biogeographic regions or ecoregions. So one realm can include a bunch of biologically distinct regions. That 2013 study identified 11 biogeographic realms. But in the case of birds, the researchers were able to divide those realms into 19 biogeographic regions. As just one example, most of Mexico is its own region,

and that region is nested within the Nearctic realm. You can see that there's much more we can geek out over with this stuff. But for now, I'll leave you with one more thing to think about. Earlier, I was talking about barriers, mountain ranges, deserts, etc. Well, barriers aren't absolute. A barrier to one type of animal might not matter at all to another type of animal. For example, any kind of dry land is a serious barrier to a fish. Or, similarly, you might stop in your tracks if you see a no trespassing sign on a fence. Whereas I see that as a challenge, an invitation for mischief. I climb right over that fence. Don't tell me where I can't go.

Birds are an interesting case because they can fly. Some of them fly from one end of the earth to the other in just a matter of weeks or even days. It seems like very few things would act like a barrier to a flying bird. You'd think that birds would have been flapping around all over the globe for millions of years, such that by now their diversity should be completely homogenized across the planet. But that's not the situation. Isn't it interesting that much of the world's bird diversity is still deeply divided into biogeographic or, as I like to say, Ava Geographic realms? This tells us that things like oceans and deserts have been and continue to be imposing barriers to at least some birds.

There was actually a recent study that looked into this. It was published in 2020 in the journal Global Ecology and Biogeography. The researchers ran statistical analyses on birds as well as mammals and amphibians. Their goal was to identify the most significant barriers for each of these animal groups. The researchers looked at the boundaries between all the next-gen-gen-biogographic realms as defined in that 2013 paper. The results of this more recent study showed that the most significant barriers for birds are, one, sharp transitions in rainfall seasonality, and two, mountain ranges. So for millions of years, birds have been less likely to move across places where, for example,

a rainforest quickly transitions to arid savanna or desert, where there are dramatic changes. changes in vegetation and levels of humidity. Just because birds can fly, it doesn't mean they can fly anywhere they want like it's no big deal. They may have zero respect for our human borders, but they still have to obey natural laws when it comes to crossing the borders between the biogeographic realms. Thank you for being curious about birds. and nature and all this biogeography stuff. You listened all the way to the end, so high-five to you.

I hope you enjoyed our speedy journey through the realms. If you get educational and or entertainment value from the Science of Birds podcast, please consider supporting me on Patreon. You can do that by popping over to my Patreon page at patreon.com slash science of birds. You can always shoot me an email if you have something you'd like to share with me.

Maybe you have a thought about the podcast. Or you want to list off another 200 things that America is best at, including hamburgers, military robots, expensive clothing for house cats, and divorce. Whatever you'd like to share, my email address is Ivan at Scienceofbirds.com. You can check out the show notes for this episode, which is number 62, on the Science of Birds website, science ofbirds.com.

This is Ivan Philipson sending you good vibes. Now go find some birds.