Season 3
2. Past, Present, and Future | How Fame Changed MacArthur’s Warblers
Transcript
Title: Past, Present, and Future
How Fame Changed MacArthur’s Warblers
Transcript:
On an early morning in late June, Schoodic Institute’s Catherine Schmitt and I stood on the side of a busy road on Mount Desert Island. As cars rumbled by in pursuit of a parking space and chance to see a lighthouse, College of the Atlantic student Fiona Young explained her research process for the 2024 field season.
{Young}: “So it's a snapshot method, so I'll stand by a point like this one, usually there's less cars. And listen for three minutes. And then after three minutes, I record the location of all the birds using bearing and distance, which is estimated. And, yeah. And then I repeat that for all the points…”
Starting at five AM and surrounded by spruce and fir trees, she pushes through the evergreen forest with a GPS and compass in hand. She stands at one of her seventeen sites throughout the forest and attentively listens for specific species of warblers and her identification skills are wicked impressive. During our roadside conversation Fiona’s head was on a swivel, noting each bird that sang around us.
{Young}:“So there’s a black and white warbler and there’s a Black-Throated Green warbler, the Black-throated Green warbler is in the genus that I’m recording Setophaga but the Black and White warbler is not. That’s also different from the genus that Macarthur had been studying because it was reclassified.”
That MacArthur Fiona mentioned is Robert MacArthur, a man who walked these same woods almost seventy years ago and who- despite his short life- changed the field of ecology.
Sea to Trees is brought to you by Schoodic Institute at Acadia National Park. I’m Trevor Grandin. In this episode we’ll learn about one of ecology's most influential studies that happened right here in Acadia, the man behind the study, and how the research of the past is influencing the research of the present.
In the summer of nineteen-fifty six Yale PHD student Robert Macarthur stood in a spruce-fir forest in Acadia National Park. As he peered to the sky, streaks of yellow and orange darted through the treetops. Warblers- like feathered gymnasts- jumped from branch to branch, dangled upside down, and leaped in pursuit of insect prey.
MacArthur’s birdwatching was more than a hobby. He was listening and looking for five specific species of warblers that occurred in mature spruce and fir forests – the Cape May {Cape May Song}, Yellow-rumped {Yellow Rumped Song}, Black-throated Green {Black-Throated Green Song}, Blackburnian {Blackburnian Song}, and Bay-breasted warblers {Baybreasted Song}.
All of this birdwatching was spurred by a theory Macarthur was eager to explore – that theory was called Gause’s principle, also known as the principle of competitive exclusion. Michael Kaspari from the University of Oklahoma explains it.
{Kaspari} “Gauss’ law basically said or suggested that species have to be different in an ecological way, and how they forage, where they nest, what their predators and parasites are, they have to be different in a profound way in order for them to coexist together. Basically, it was one of the first theories of what we now call biodiversity.”
For example, in the early nineteenth century, gray squirrels were introduced into the United Kingdom. At the time, the UK already had a dominant squirrel species- the red squirrel. After the gray squirrels' introduction, they spread rapidly. Red squirrel populations plummeted. Because these two squirrels are relatively similar in their ecological niche, competitive exclusion underscores why they can’t exist together. Red squirrels are endangered throughout much of the United Kingdom.
When the theory was first introduced at the turn of the twentieth century many ecologists and researchers were skeptical about its validity. Gause's law was often only supported in theoretical or lab based settings, rarely observed in natural ecosystems. In fact, some species and ecosystems run counter to the competitive exclusion idea all together.
The popular “paradox of the plankton” describes the situation that many ocean plankton find themselves in- a plethora of species all eating the same food in the same environments. Somehow, they get along just fine, coexisting without competition taking hold. Prior to the nineteen-fifties, researchers also pointed at the Eastern warblers as a special case in the conversation surrounding competitive exclusion.
{Kaspari} “The warblers of the east were just utterly gorgeous, utterly captivating. They literally looked like Christmas tree ornaments. They were just painted in different ways… And so MacArthur would hear from other ornithologists and say all the Gauss, that's crazy. That's because look at the Warblers, five things that look are just colored differently, but seem to be doing exactly the same thing. So this is an exception.”
MacArthur, a talented birder himself, saw these warblers as a perfect subject to test the mechanics of Gause’s principle. For two field seasons he sat in a folding chair in the spruce/fir forests of Mount Desert Island and watched the trees. In a relatively low tech field MacArthur went about his research in a way birders and ecologists could fully understand. With a pair of binoculars in one hand and a stopwatch in the other he took note of species numbers, where they were in the spruces, how much time they spent there, and what they were doing.
Cape May…top of the tree… terminal zone… foraging Yellow-rumped… bottom of the tree… base zone… calling
Slowly but surely MacArthur’s hypothesis started to take shape. He thought that the warblers were employing niche partitioning when they foraged for food. The birds were dividing the trees into their own ecological niches. Each species spent a bulk of their time foraging in their own specialized parts of the tree, a practice that aligned them with the ideals of competitive exclusion.
Some species preferred to forage in the very tops of the trees where the new growth was happening. Others spent most of their time at the very bottom, picking through the lowest branches and scratching through the underbrush. Despite living and coexisting in the same cafeteria and often eating the same food, these birds were sitting at different tables saving them the trouble of major competition.
In the October nineteen-fifty eight issue of Ecology MacArthur published his “Population Ecology of Some Warblers of Northeastern Coniferous Forests” in and shifted the field of ecological science. That might sound like hyperbole, but the reception of his paper was one of raucous applause. MacArthur had taken a relatively well-known, partially-disputed theory and went about testing it in a clear and concise manner.
That clarity is owed in part to his undergraduate and graduate schooling. MacArthur, a mathematician at heart, injected statistics and measurable data into a field that, at the time, was dominated by purely observational study.
Dr. Kaspari explains it’s the difference between deduction and induction. Ecology had often been all about induction, creating stories and explaining the world through pure observation. MacArthur went about his work in a very deductive way, taking assumptions about the world, creating predictions based on those assumptions, and collecting data with those predictions in mind. He brought ecology much closer to the hypothesis based research we see today.
{Kaspari} “And what MacArthur did was say, ‘Well, if Gauss is going to be useful to us, we need to be able to dissect that hypothesis, that verbal hypothesis, literally one sentence into its component parts; put mathematical relationships in there, extract predictions, and then go off and test and that is really a much better definition of what the MacArthurian paradigm turned out to be… So he really did bring that toolkit in a way that I think that lifted everybody up and although people don't talk about much about MacArthur’s warblers anymore, except as kind of an iconic study, most ecologists have now incorporated some of his basic fundamental mathematics into the field.”
The finding of niche partitioning in warblers and the incorporation of experimental design in his study weren’t the only reasons for MacArthur’s success. His writing style created an interesting journey of discovery unmarred by jargon and statistics.
And choosing Acadia National Park as a field site tied MacArthur to the park’s science history, a rich and vibrant lineage that reaches all the way back to well before the park’s founding and continues to influence research initiatives to this day. Enter Bik Wheeler, wildlife biologist in Acadia National Park. His introduction to MacArthur’s study happened like many other young undergraduates.
{Wheeler}: “I first formally read it when I was in undergraduate school. I think that it's one of those formative studies that's been so ubiquitous in science literacy, at least in my era, where I think that I was exposed to it far before I knew it even existed. And so thinking back I know I probably did have a high school textbook that had, like the diagram and I probably did have middle school lessons that were indirectly referencing these types of theories that came about from this one sort of origin seed.”
So when it came time for his masters thesis Bik chose to revisit MacArthur’s warblers. In the years since the publishing of“Population Ecology of Some Warblers…” the study hadn’t been directly replicated. Some tried to apply the same methods onto different theories, some applied the same theory onto different species in different locations, some even replicated the study close by to the original site, but no one had directly replicated the study at the same place with the same birds. And replication is a major part of the scientific method.
{Wheeler}: “You know there's not a lot of glory in repeat studies that confirm findings. But really those are the unsung heroes, because those are the ones that really allow us to settle in understandings and so if I had whatever commodity that's important to people, let's say citations or something, if I had a million citations that I could just give out to papers, I would do all of the repeat method papers that didn't get the proper exposure that they deserve, because what's the story there? Okay, they did the same thing. They found the same thing. You know, let's not do a press release about that. But that's the important stuff.”
Although direct replication was the hope, one big, recent scientific change would make it difficult.
Science flashback!
The year is twenty-eleven and you’re at a meeting of the American Ornithological Union.
The organization has just decided to reclassify the scientific names of some warblers, big deal.
The reclassification was spurred by new DNA research that revealed some warblers were more closely related than first thought. So, what does the reclassification look like?
It consolidated and created entirely new genuses. The changes were wide ranging but one modification seemed to be more influential than the others. Before the twenty-eleven change, the genus Dendroica, home of the wood warblers, had twenty-seven individual species in it. Dendroica had some of the most well known warblers in its genus like the yellow-rumped and cape-may warbler. Also before the change, the genus Setophaga had one lone species- Setophaga reticula, the American Redstart.
Based on DNA data these two genuses were merged to better articulate the relationship between them. Every wood warbler was now reclassified.
End flashback!
Apart from ornithologists and bird watchers, who cares? Why does the reclassification matter to Bik’s study? Well, the five birds that MacArthur was observing in the 1950s had been part of the genus Dendroica. Now in twenty-fourteen, Dendroica was no more and the new genus was even bigger. There were birds breeding in MacArthur’s research area that were even more closely related than he knew at the time leaving a large piece of the niche partitioning story untold. Maybe these unaccounted for birds played a part in the resource hierarchy that MacArthur wasn’t seeing.
Not only were the bird names different, but the birds themselves were different. Some species didn’t breed in that forest anymore and new ones had entered. On top of that, changes in forest composition turned the study site, once dominated by white spruce, into a mostly red spruce forest. All these changes led Bik to reconsider the phrase “repeat study.”
{Wheeler}: “I don't call it a repeat study, because we're talking about different trees. We're talking about different warblers. We are talking about the same soil. That's what we've got. We’ve got different times. We do have the same methodology, and so we have some consistencies, but we do also have so much that's changed. And so it's really a revisit and looking at what's changed, and trying to understand those changes, both from an ecological species interaction perspective and also the context of that social component.”
During his study, Bik observed four warbler species. Two carried over from MacArthur’s study, the yellow rumped and black throated green warbler, and two were unique to his study, the Magnolia Warbler (Magnolia song) and Northern Parula (Parula song). Bik’s field work took place during the breeding seasons of twenty-fourteen and twenty-fifteen. Using MacArthur’s same methodology Bik was looking at different birds among different trees. And after the field work, Bik’s data illustrated that. The data pointed to a breakdown in partitioning. That means the warblers weren’t holding to the distinct foraging sections MacArthur had observed. {Wheeler}: And so what I was finding was a lot more overlap in what zones that the warblers were using. because even in the original study, the sampling was from multiple trees, and it was creating zonations of each tree and understanding how much overlap or partitioning there was between species utilizations of that resource. And what I saw repeating the same methodology was a much higher overlap, sort of like a collapse in that partitioning, if you will.”
Maybe the partitioning played a bigger role with the populations and forest makeup MacArthur was observing but as certain warbler species stopped coming back and the trees changed Gause's Law lost its hold. Which begs the question, how are there so many warbler species? What are the factors that allow so many different warblers to coexist with similar needs? Bik has theories.
{Wheeler}: “It's like a really hard job to be a warbler. There's not too many of them, basically, right? Because for them to have to partition resources, to not be able to coexist, to have competition between species be so intense that they can't coexist, there needs to be limited resources and so many warblers. I do think that resources are becoming more and more limited, but also warblers are becoming more and more limited.”
Interrupting Bik for just a moment, it is really hard to be a warbler. The decline in bird populations comes from many different angles. Habitat loss and degradation spurred by human development, in both North and South Americas, is constricting breeding and wintering areas. And more erratic and sustained temperature swings are shifting bird ranges creating new interspecies interactions that historically haven’t occurred. Climate change is also contributing to something called phenological mismatch. When earlier springs cause migration patterns and resource abundance to become out of sync causing warblers to arrive at stop-over and breeding grounds well after peak food collecting time causing them to fly further without food, risking exhaustion, predation, or limited breeding.
But it’s not all doom and gloom. Initiatives and intentional conservation efforts are yielding positive results. Some bird populations, like waterfowl and wetland birds, are holding steady or rising thanks to policy changes and international partnerships to protect them and their habitat, showing that changes can be made. Back to Bik and his explanation of why there are so many warbler species.
{Wheeler}: “There's also a few foundational differences, one being interactions between species, is also, by definition, less than interactions within species. And so we get a lot of these territorial interactions and species and interspecies and intraspecies dominance hierarchies. So there's a lot at play as far as social interactions.”
“And then, you know, it's just a really tough life to be a migratory, neotropical migrant, to have to make these migrations twice a year, to have to be in all of these different landscapes and utilize those resources, and then raise the young. So it's, it's kind of this amazing challenge that we have any warblers at all, let alone warblers that are competing so intensely that they're out competing each other.”
So if a landscape and habitat can change that drastically in only sixty years, what will it look like after Bik’s study and how will the data change?
That’s where Fiona Young reenters. As an upcoming senior at College of the Atlantic, Fiona was looking for something to do in the summer of 2024. Her criteria were simple, outside and with birds. Her professor mentioned that the ten year mark of Bik’s field season was coming up and how cool it would be if someone could recreate it once again to get an updated idea of breeding numbers. Fiona said, “Sign me up.”
{Fiona}: “I said, okay, and the three of us met and talked about it. And definitely made me nervous, because, you know, it has this whole hype around it, but it was, it was really cool to feel like I was following a chain of scientists doing really cool stuff.”
From early June to mid-July, Fiona stood in the forest listening to birds five hours a day two days a week. Now that her field season is over and she’s been able to give a cursory glance at her data, Fiona’s preliminary findings complement and differ from Bik’s twenty-fourteen study in interesting ways. Overall, different species are breeding. Three of the four breeding species that Bik observed in twenty-fourteen carried over into Fiona’s study and only two out of MacArthur’s five species were still there meaning some species aren’t stopping in these woods at all and once again different warblers are taking their place.
But Fiona’s experience recreating MacArthur’s study went beyond the data and statistics. As she started her research, the early morning bushwacking seemed like a less than ideal way to spend her last summer as an undergraduate. Unsurprisingly, those feelings faded and a greater appreciation came to the surface.
{Young}: The first few times I went out, to be honest, I had a hard time. I mean, it's early, there's a lot of places where you [are], physically, pushing through things. And even though I was like, ‘Yes, this is something I like doing. I'm doing it for the birds. They're beautiful. It's nice to be outside.’ But, you know, I wish I wasn't pushing around sticks in the woods right now. And I think that that attitude kind of subsided as I did it more and on the very last survey, it was a really cool feeling, because I realized, ‘Oh, I feel kind of sentimental about this place.’
Maybe that’s the feeling that MacArthur had as he was pushing through the woods looking up at the warblers overhead. A sense of sentimentality for such a unique ecosystem. From MacArthur in nineteen-fifty six, to Bik Wheeler in twenty-fourteen, to Fiona Young in twenty-twenty four. A scientific history anchored in the same Acadia forest by some of the same birds. A scientific chain of discovery trying to explain the natural world and its beauty.
For his warbler paper MacArthur won awards and accolades continuing a string of influential research he’d been conducting throughout his academic career. In his book Song of the Dodo, David Quammen describes MacArthur’s prospects as a young researcher.
{Quammen}: “Young MacArthur was a hot prospect. He possessed the right combination of talents and ambitions to make a big impact at that particular time in that particular field. He was formidably bright, restless, almighty, curious, innovative, and mathematically proficient. He loved the natural world, especially birds, but he had no interest in spending his life as a descriptive field naturalist. He cared about ideas and deep mechanisms, about order and explanation, not just about creatures and landscapes. He was eager to change the very character of ecology.”
And looking back MacArthur ended up doing just that. Shortly after his warbler paper was published he met Edward O. Wilson, an ecologist and ant specialist who MacArthur became great friends with. Working together Wilson and MacArthur published a cornerstone piece of research in nineteen-sixty seven called The Theory of Island Biogeography. It hypothesized that islands maintain an equilibrium of plant and animal species through extirpations, or local extinctions, and new introductions.
After graduation, MacArthur’s academic career was spent as a professor at the University of Pennsylvania and Princeton working closely with collaborators and colleagues. In nineteen-seventy two, MacArthur learned that he had renal cancer and his work went into hyperdrive. He compiled his life’s work into the book Geographical Ecology: Patterns in the Distribution of Species and published it later that year. Robert MacArthur died soon after at the age forty-two.
A year after his death the Ecological Society of America awarded MacArthur the Eminent Ecologist award and his friend E.O. Wilson wrote the acknowledgement. He included one of MacArthur’s own quotes:
MacArthur was a scientist’s scientist, to whom gifted biologists of all ages turned for advice and encouragement. “Doing science,” he {MacArthur} wrote, “is not such a barrier to feeling or such a dehumanizing influence as it is often made out. It does not take the beauty from nature. The only rules of {the} scientific method are honest observations and accurate logic. To be great science it must also be guided by a judgment, almost an instinct, for what is worth studying. No one should feel that honesty and accuracy guided by imagination have any power to take away nature’s beauty.”
MacArthur’s work changed the field of biogeography and set a new course for what ecology could look like. Ecology, having relied heavily on the cataloging and observation of natural history, was blooming into the predictive and theory driven discipline it had the potential to be. Those contributions live on through the work of Bik and Fiona as well as professors and instructors like The University of Oklahoma’s Michael Kaspari.
{Kaspari}: “So I use MacArthur mainly to show what a multivariate, multitudinous mind can do to advance a whole field with one paper, which is largely fieldwork from the library and from a lawn chair, doing what ecologists love to do, and that is, watch critters. I think it's a lovely story by someone who we lost at 40 years old and again, one of the great minds in the field and it's always good to show students. Look at this. He did this as a graduate student. You know, you don't have to be around for a long time to make a big contribution. You just have to have a different perspective.”
It’s easy, in science, to get wrapped up in the newest, shiniest discovery. As an ever changing and evolving discipline, theories and research are always being overridden, tooled with and reworked. Not often does a piece of research completely alter the protocols and norms of a field of study and even less often does that research come from a small stand of trees in a national park. In a world that’s hurdling forward by leaps and bounds, looking into the past for a little help could uncover and explain the present we live in, as long as we’re willing to listen.
At the end of our visit with Fiona Catherine asked her,
{Schmitt}: “Do you have a favorite site?” {Young}: “I think J8 might be my favorite site, because it's closest to the cliffs. And the other day, I saw scoters there, and you can hear the bells. And it's really nice to come out of the woods after two hours and just look at the ocean for a second, and we go ‘ahhh.’” {Schmitt}: “Can we go there?” {Young}: “Yeah we can.” {Schmitt}: “Do you have time?” {Young}: “I do have time.”
*walking and brushing through branches*
We pushed through spruce branches, wadded into smatterings of sheep laurel, and vaulted over the trunks of long-dead trees. After about ten minutes of trudging south,
{Schmitt}: “Yeah, I can see why you like this site.” {Young}: “You kind of just have to stop for a second.”
We stood and listened.
Join us for the next episode as we move from the past to the present where we’ll try and understand the human disturbances affecting critters all throughout Acadia National Park. From the late-night amphibian pedestrians traversing the park's roads,
{Trevor}: “Ope, here’s another guy! Or is it a worm?” {Marisa}: “He’s like, ‘I’m a worm!’”
to the furry mice that scamper along its trails.
{Brittany}: “Bag please, bag please, bag please, bag please, bag please! These guys can turn really easily if they get that hind leg on you.”
Thank you for listening to Sea to Trees, a podcast from Schoodic Institute at Acadia National Park. Acadia National Park is on the traditional lands of the Wabanaki, People of the Dawn. This show was made by Trevor Grandin, the Cathy and Jim Gero Acadia Early Career Fellow in Science Communication. Catherine Schmitt is our senior editor. The cover art was created by Sarah Luchini.
Special thanks to Dr. Michael Kaspari, Bik Wheeler, and Fiona Young for sharing their expertise with us. A very big thank you to David Quammen for providing voice over. Many thanks Laura Sebastianelli and the Schoodic Notes project for the bulk of the bird sounds. The cape may warbler call was recorded by Gregory Budney and is stored in the Cornell Lab of Ornithology Macaulay Library.
As a nonprofit partner of the National Park Service, Schoodic Institute inspires science, learning, and community for a changing world. To learn more visit schoodicinstitute.org.
Episode Description:
In 1956, Robert MacArthur sat in a spruce-fir forest of Acadia National Park and tried to understand the truth behind warbler diversity. How could there be so many different species coexisting, when theory seeks to crown “one warbler to rule them all?” Learn about MacArthur’s study, how it changed the field of ecology, and the scientists revisiting his work over half a century later on Season 3 Episode 2 of Sea to Trees.
Reference Links:
Knowing Your Warblers: Thoughts on the 50th Anniversary of Macarthur by Michael Kaspari = https://www.allaboutbirds.org/news/spruce-woods-warblers-revisited-60-years-later-the-cast-of-characters-has-changed/
Spruce-Woods Warblers Revisited: 60 Years Later, the Cast of Characters Has Changed by Irby Lovette = https://www.allaboutbirds.org/news/spruce-woods-warblers-revisited-60-years-later-the-cast-of-characters-has-changed/
Schoodic Notes – Bird Sounds of Acadia = https://schoodicnotes.blog/
In 1956, Robert MacArthur sat in a spruce-fir forest of Acadia National Park and tried to understand the truth behind warbler diversity. How could there be so many different species coexisting, when theory seeks to crown “one warbler to rule them all?” Learn about MacArthur’s study, how it changed the field of ecology, and the scientists revisiting his work over half a century later on Season 3 Episode 2 of Sea to Trees.