Past, Present, and Future | How Fame Changed MacArthur’s Warblers
Description
---
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“Populati
United States
