The Science of Birds - The Biodiversity of Birds

Episode Date: September 22, 2020

Episode: 3SummaryBirds are an incredibly successful and diverse group of animals. In this episode, you’ll get an overview of the origin of avian species diversity and its present-day distribution across the planet.I offer some definitions of ‘biodiversity’ with respect to birds. We’ll discuss why this diversity is important.Links to Some Things Mentioned in this EpisodeClements Checklist of the Birds of the WorldResearch Citations‍Genetic research leading to the species split between the Winter Wren and Pacific Wren (Toews and Irwin. 2008. Molecular Ecology)Errors and ClarificationsClarification - 07:25 - I say that the Clements Checklist has 10,721 bird species. This is true. However, this number includes about 160 extinct species. So this means there are 10,561 extant (i.e. living, non-extinct) bird species on the planet today.Link to this episode on the Science of Birds websiteSupport the show

I am continually amazed by the diversity of birds. As a bird-watching tour leader, it's my job to travel the world to find as many bird species as possible. Even so, I've seen less than half of them. Thousands more are out there waiting to be admired. The variety of shapes, colors, behaviors, and ecological roles of birds is mind-boggling. Just when I think I've seen the most outlandish or beautiful species, either in the flesh or in the pages of a guidebook, I come across another. one that's even more impressive. They just keep coming. Even hearing the names of birds can light

up the imagination. Horned screamer, marbled frog mouth, shining sunbeam, pearly-eyed thrasher, yellow-eared spider hunter, Tibetan snowfinch, rusty laughing thrush, rock-loving sastikula, blue jewel babbler. Wow. Believe it or not, I don't actually want to see all the bird species of the world. Sure, I'll keep traveling and leading tours to see new and wonderful birds, and I'll be happy to see my list of species grow. But I really like the feeling that there will always be more avian diversity out there, more than I can ever experience in my lifetime. Hello and welcome. This is the Science of Birds. I'm your host, Ivan Philipson.

The Science of Birds podcast is a down-to-earth discussion, a fast, fascinating topics in the wonderful world of bird biology. This is ornithology for lifelong learners, bird nerds, and those who are simply bird curious. This episode is all about the biodiversity of birds. The roadmap of where we're going today is. First, we'll talk about when birds first started to diversify on the planet, then we'll do an overview of their present-day diversity. We'll follow that with a discussion of what biodiversity is. Then we'll consider how bird diversity is distributed around the world. And lastly, we'll touch on why all this diversity is important.

The first birds evolved between 150 and 200 million years ago, when dinosaurs were running amok across the planet. Birds are, of course, dinosaurs themselves. You can learn all about that by listening to episode one of this podcast, if you haven't already. In any case, birds began their long history back in the Mesozoic era and have, over many millions of years, multiplied into a dizzying array of species. Sixty-six million years ago, an asteroid smashed into Earth, and the resulting destruction killed off almost all of the dinosaurs.

That epic catastrophe is technically called the Cretaceous Paleogene extinction. I'm just going to call it the Big Extinction for simplicity. Life on Earth before and after that event were wildly different. The Big Extinction makes a stark dividing life. line in the fossil record for birds, and pretty much all other forms of life. Before the extinction, there were many different primitive bird species. The earliest gliding and flying birds were expanding, diversifying into different forms alongside their cousins, the standard-issue non-flying dinosaurs.

Things were pretty good, and the future seemed bright. But then the asteroid hit, and the party came to a screeching halt. From the fossil record, it seems clear that many bird lineages were extinct. extinguished in one fell swoop by the big extinction 66 million years ago, and avian diversity crashed. Amazingly, though, several bird lineages survived. One of those was the ratite lineage, a group of birds that is still alive and kicking today. These are the ostriches, Kiwis, Reyes, and a few others, most of which are flightless. When the big extinction happened, the ratites had already been around a while as a distinct group. Two other lineages that survived the firestorm are

quite familiar to us, ducks and chickens. Their ancestors, anyway. The two present-day taxonomic orders, and seraphormes and galiformes, existed before the extinction. And seraphormes includes ducks and geese and the like, while galiformes includes chickens, pheasants, quail, etc. So chicken little could have actually been around 66 million years ago, and he would have been 100% correct when he cried, the sky is falling. The only other lineage of birds that we know survived the big extinction was the one that ultimately gave rise to the rest of the living birds. If you exclude the ratites and birds in the Anceriformis and Galaformis orders, you are left with a big group known as Neo-Avies.

95% of the world's living birds are in this group. The ancestor of today's Neo-Avians was a survivor of the extinction. There has been a debate among paleontologists and other scientists about the timing of when the earliest birds began. branching off, first with the ratites, then the Galaformis, and Anceriformis. I won't get into it, but I can say that the evidence we have from both fossils and genetic data indicates that the bird lineages I just mentioned were each distinct from each other before the big extinction. Diversity in birds was at a low point 66 million years ago, just after the asteroid impact. But when conditions around the planet slowly settled into a new

normal, life recovered, and birds hit the ground running, or, well, they hit the air flying? An explosion of diversification in birds began. Countless ecological niches occupied by other dinosaurs and the flying terrosaurs were suddenly vacant, free for the taking. As you know, nature abhors a vacuum. Birds began to fill the vacancies, adapting to take advantage of untapped resources. They evolved into new species and then into whole new, dramatically different lineages. Most of the modern bird groups can be traced back to those Wild West days just after the big extinction. This relatively rapid diversification from a few ancestors is known as an adaptive radiation. A similar thing happened to mammals as they diversified after the extinction.

If you want to learn more about the origins of birds, remember to check out episode one of the podcast. Now, let's jump in our time machine and go back to the future, by which I mean today. Let's consider the diversity of birds as it is today. There are about 10,000 bird species. 10,000 is a nice round number. It's easy to remember. If someone pulls you aside at a fancy cocktail party and asks you fervently, how many bird species are there in the world, you'll be able to escape any crippling embarrassment because you can give them the answer with confidence.

10,000. However, between you and me, that isn't the exact number of bird species. It's actually a bit more than that. Also, the several authoritative organizations who keep track of such things report slightly different numbers. In my opinion, the most accurate and up-to-date resource is the Clements Checklist of Birds of the World, maintained by the Cornell Lab of Ornithology. That's the gold standard. The Clements checklist is used by the American Birding Association and is used as the basis

for eBird, which is far and away the best app for people like you and me to track our bird observations. Today, the Clemens checklist has 10,721 bird species listed, but that could change a bit later this year, since the checklist is updated annually. Why would the number of species change? Rarely these days, a completely new bird is discovered in some remote corner of the world. That's always a cause for celebration. And of course, the number of species can go down due to extinction. Boo. A more common situation is when scientists, using genetic or other data such as plumage or

bird song, discover that what we thought was one species is in fact two. This happened in 2010 when, based on genetic data, the winter wren in North America was split into two species, the Pacific Wren and the Winter Wren. Or the opposite thing can happen. What we thought were two species can get lumped together as one, if careful scientific analysis suggests that they should be. We'll have to chat in another podcast episode about what makes a species a species and how new species form in birds. I don't blame you if you'd rather just remember that the number of bird species is about 10,000 instead of exactly

10,721. The latter number will change next year anyway, probably. For comparison, there are approximately the same number of living reptile species, about 10,000. I guess I should say non-avian reptiles because birds are, in the evolutionary sense, reptiles, even though we classify them separately. Anyways, there are approximately 8,000 amphibian species and 6,400 mammal species. Now remember that when we classify organisms scientifically, we have that good old system of taxonomic ranks, domain, kingdom, phylum, class, order, family, genus, and species. Birds fall into the class avies. Those 10,721 bird species are currently divided into about 2,500 genera, genera is the plural of the genus, 257 families, and 41 orders. The taxonomic

rank of family is, to me, a helpful unit of organization. Birds that all belong to a particular family, often share many recognizable features. For example, the family Eccipatrity includes all the world's hawks, eagles, and kites. These birds have strong hooked bills, grasping feet with long talons, broad wings, etc. If I see a bird with those features, even if I don't know the species, I can probably place it correctly in the Exipatriti family. That way, I can narrow down my search for its identity. I'll flip to the appropriate section of my field guide or bird app and go from there.

As I said, there are about 250 bird families, and that, I think, is a manageable number of groups to have rolling around in my head. Not that I have them all memorized, mind you, but maybe someday. Some families contain lots of species. They're very diverse. For example, there are mega diverse families centered in the tropical parts of South America. There are 349 hummingbird species in the family trochilidae. The tyrant flycatchers of the family Tyrannity, however, have the hummers beat with a whopping 422 species.

Tyrannity is actually the most speciose, in other words, species-rich bird family in the world. At the other end of the spectrum, there are depopurate families with only a few species, or even just one lonely species. When a family or genus includes only a single species, it means, that species has no close relatives. That family or genus in this case is said to be monotypic, having only one type. A monotypic bird family that comes to mine is that of the sunbidern, which lives along forested streams in central and South America. The sunbidern has no close relatives and is the only member of the family Euripigidae. Species diversity and family

diversity, these wax and wane over the millennia and over millions of years. If we look at the diversity of life on earth for the last 500 million years, there is an overall upward trend, an increase in diversity. This can be seen with birds, too, over the last 50 million or so years. But mass extinctions, as well as the less dramatic but continual losses of individual species, have pumped the brakes, so to speak, slowing the increase in diversity. Taken as one big group, birds seem to be more diverse today than they have ever been, more or less. But that's not necessarily true for each order or family of birds looked at individually. Families that are species poor today, like the sunbidern family, might have been much more diverse in the past.

Some of them had a glorious golden age that is long behind them. We've been talking about diversity in birds mostly in terms of species richness, but the term biodiversity is a bit more inclusive. One fairly succinct recent definition of biodiversity is variation of life at all levels of biological organization. If we go down to the smallest microscopic level, the ultimate source of all biological variation is mutation. Mutations are rare changes in DNA sequences that happen when there are goof-ups in DNA replication. Natural selection then acts upon individual variation among individual birds, say, which explains. expresses underlying genetic differences, including those mutations.

This leads to diversity among individuals, populations, species, and all the way up to ecosystems. To keep things simple, let's continue thinking of avian biodiversity at the species level. Now, imagine a distinct region, perhaps a large island. It's a nice place, with lots of palm trees and, thankfully, no mosquitoes or obnoxious squirrels. Let's say we do a big survey and find all the bird species on the island, which means we get a great measure of their biodiversity there. If there are 300 bird species on the island, why is it 300? Why is it that number and not 1,000? Or why not 25?

The answer is complicated. Many factors influence the biodiversity of a given place. The overall pattern is that species increase in a region through either evolution or immigration and that they decrease by extinction. Biodiversity is the balance of these. And these processes, in turn, depend on things like the region's geographic area, any natural barriers within and around the region, climate variation, etc. Evolution, immigration, and extinction are often acting in the present day,

but the way these have unfolded over the region's entire history is really what has shaped the biodiversity we find there. As I said, complicated, but fascinating if you ask me. We're on a slippery slope here, heading into a discussion of biogeography. I love biogeography, but let's save that for another podcast episode. Instead, let's look at the way avian biodiversity is distributed around the globe. Birds live on all seven continents in a vast majority of the world's ecosystems, including the open ocean and the frozen wastes of Antarctica.

We have our 10,000 bird species. We have all this spectacular diversity. But it isn't distributed uniformly across the planet, nor is it distributed randomly. Instead, bird diversity follows some interesting patterns that we can detect. One of the strongest patterns is the latitudinal diversity gradient, also called the global diversity gradient. This describes how the highest levels of biodiversity, the greatest numbers of species, live in the tropical regions of the world, and diversity decreases as you move away north or south from the equator, towards the poles. That is the latitudinal diversity gradient. It applies to birds and many other forms of life. Why does this exist? Short answer, we don't know. It's not entirely understood.

It could be that since the tropics receive more sunlight, there is more energy to produce vegetation, and that ends up supporting more individual animals and somehow more species. Or it could be that the mix of temperature and rainfall in the tropics is more agreeable to life in general, and that allows for more species to exist there. Or, since tropical forests have been one of the most stable ecosystems for millions of years, extinctions might be relatively less frequent there, allowing more species to stack up over time. It could be a combination of these and other factors.

I think that many scientists would agree that several forces are interacting to create this strong latitudinal gradient. There are several regions that stand out as major hotspots of bird diversity. It should be no surprise that these are in the tropics. If we look at a map of species richness across the planet, we can see that the Amazon Basin, just east of the Andes in Peru, Ecuador, and Colombia, is a major hotspot for birds. And northern South America, in general, is super species rich for birds.

But we can also look to East Africa. Africa, which has a high diversity of birds, as does Southeast Asia. Looking at the maps of these places, you'll see that mountains seem to be associated with this high level of diversity. It may be that when there is topographic diversity in a region, more habitats are available for birds and therefore more species can live there. Related to that idea is that, in addition to the latitudinal diversity gradient, there is, in many places, an elevational diversity gradient.

This isn't as strong a pattern as the latitudinal gradient, and is even less understood. But in many places there are more bird species at lower elevations than at high elevations. In other places, there is a maximum of diversity at mid-elevations, meaning that you have a certain number at lower elevations, then you find more and more species as you climb to mid-elevations, and then the number drops back down as you keep climbing to the mountain peak. Another important concept here as we look at global bird diversity is endemism. A bird is endemic if it is found only in one region.

For this term to make sense, we must first clearly define the area we're talking about, either a country, a continent, a region, an island, an island archipelago, whatever. Once we've defined the area, we can say whether or not a species is endemic to it. Isolated islands have many, many examples of endemic birds. The Galapagos Islands are perhaps the most famous. Darwin's Finches, the flightless cormorant, the Galapagos penguin, those are all endemic to the Galapagos. They're found nowhere else in the world.

A variety of factors contribute to why a species might be found only in one place. It could be that this species was once very widespread and has gone extinct everywhere but that one place. Or it could be that the species evolved in that place and never spread to anywhere else. That's typically the case on isolated islands. Collectively, islands around the world harbor lots of unique endemic species. That's another global pattern of avian diversity. So why is the biodiversity of birds important?

Well, first we have to ask, important to whom? Important to humans? To the birds themselves? I'm pretty sure they think that their own diversity is important. But why should human beings care about diversity in birds? Yes, this is one of those philosophical questions that we can get lost in for hours. Let's not do that. Let's try to answer the question pragmatically and also from a self-centered perspective.

Birds serve us by performing important jobs in the world that benefit us. Birds act as pollinators and seed dispersers for many plants that are valuable to us, such as trees that produce building material, fruit and other foods, and medicine. The more bird species there are in the world, the more birds. more plant species they can help propagate. Birds also provide the service of regulating pests and diseases by eating insects and other invertebrates. Some species are scavengers, cleaning up carcasses that would otherwise pile up. The more bird species there are in the world, the more ways they can help reduce pests and disease.

And these days, you have people like me and my tour guests who love to travel the world to see new and wonderful birds. Birdwatching is at the heart of a vibrant ecotourism industry that provides sustainable, nature-friendly jobs for many thousands of people across the globe. Bird diversity itself is the main attraction for many, many travelers. Those travelers support lots of economic growth. Now, if we consider the good of more than just our own species, as we should, we see that bird diversity is also important because birds play integral role. in their ecosystems. They provide ecological balance. Many regulate the populations of the small animals they eat. In some places, like remote islands, birds are the top predators. Many bird species also serve as prey for other wildlife. They are connected to countless other

organisms in these ways. Some birds can be considered keystone species. One example is woodpeckers in a forest ecosystem. Woodpeckers have strong bills that that they use to chisel out nest cavities in trees. Numerous other bird species and mammals depend on woodpecker cavities. And those cavities also help to start the decomposition of a dead tree, ultimately helping to recycle the nutrients in the tree

back into the soil. So when woodpeckers aren't present, a lot of things change in a forest, mostly for the worse. I could go on and on about the importance of bird diversity. Besides all the things I just mentioned, there is the cultural, aesthetic, and spiritual importance of birds to us humans.

The world has been filled with a sparkling wild diversity of birds since our kind first stood up on two legs in eastern Africa. Birds have been our constant companions for hundreds of thousands of years. They are the voices of nature, and they are the wild animals we can most easily connect with anywhere on earth. But as you probably know, birds are in trouble. Since the year 1500, humans have wiped. out 182 species. We'll never see the giant moas of New Zealand again, or the Carolina

parakeet, or the Imperial woodpecker. You might think that 182 species lost over 500 years isn't so bad, but currently there are at least 1,200 bird species that are likely to go extinct in the coming decades because of what we are doing to them. That's about 11% of all bird species. And that's not counting the many species that are in decline across the world, even if they aren't facing imminent extinction. We are losing bird diversity much faster than it is being replaced by evolutionary and ecological mechanisms. You're listening to this podcast, so perhaps I'm preaching to the choir here. We still have a chance to save the vast majority of our wonderful bird species. Each of us should do what we can to help these amazing creatures and

the ecosystems that they live in. And that is it for this episode, my friends. Thanks so much for being here with me. Sorry to end on a slightly solemn note, but you can't really talk about diversity without running into the reality of the current biodiversity crisis. In any case, I hope you learn something here today and that you feel inspired to help birds in whatever ways you can. If you love birds and want to learn more about their biology and their conservation, please

subscribe to this here podcast, The Science of Birds. You can also see the show notes for this episode on the website at scienceofbirds.com. I would be delighted if you signed up for the email newsletter, which you can do right there on the website, scienceofbirds.com. I'm Ivan Philipson and I'll catch you next time. Ciao.