Changing the face of science in Acadia National Park: warblers, coexistence, and hypothesis-driven ecology

By Juliet Nagel, Univ. Maryland Center for Environmental Science

This article, and others in the series "Parks in Science History", was written by a graduate student at the University of Maryland. The articles highlight the roles that national parks have played in the history of science and, therefore, the world's intellectual heritage. More articles and videos will be produced in the future.

Yellow Rumped Warbler

COURTESY SETH BENZ

In the 1950s, a young man was walking through the woods. Many species of colorful warblers flitted among the spruce trees surrounding him. He saw Blackburnian warblers with flame-colored heads, Myrtle warblers with bright yellow patches, bay breasted warblers with bright chestnut caps, and more. All these different species in the same trees!

These warblers all have a similar diet and are about the same size. Thus, some ornithologists at that time concluded they all shared the same ecological niche, or role in the environment. But was that possible? If they were all looking for the same foods in the same places, surely one of them would be better than the others. If one species was faster, or better at spotting its insect prey, it would out-compete the others. Then only one kind of warbler would be living in these woods. He decided to figure it out.

Robert H. MacArthur

That young man was Robert H. MacArthur. His work, including this research done in Acadia National Park, would help change the way that ecologists conduct research. Born in 1930 in Toronto, Canada, he originally studied math. He earned both his Bachelor’s and Master’s degrees in Mathematics. He then switched to Ecology for his PhD at Yale University.

After finishing his PhD, MacArthur went on to become a professor at the University of Pennsylvania and later a professor at Princeton University. There, he co-authored The Theory of Island Biogeograhy with E. O. Wilson. MacArthur’s research appears in nearly every Ecology textbook published since the 1960s. Despite a short career (he only lived to 42 years), his works have been cited (referred to in other scientific studies) over 54,000 times!

Coexistence

At the time of this walk through the woods, MacArthur was just beginning his PhD. He wanted to answer the question of coexistence: how do all these warbler species live together? He decided the best way to find out was to watch them. He chose a patch of spruce woods behind Bass Harbor Lighthouse in Acadia National Park, and set about to do just that. Yet, as anyone who enjoys birdwatching can tell you, following a warbler through the woods is not that easy to do! An excerpt from his paper gives a glimpse into the difficulty of keeping up with these tiny birds:

“…a large number of hours watching resulted in disappointingly few seconds of worthwhile observations.”

Nevertheless, MacArthur persevered. He recorded the activities of five warbler species, watching where in the tree each bird went, and measuring the time spent there. As his observations began to add up, a remarkable picture emerged. The birds were avoiding competition with other species by using different parts of the trees!

• Cape May warblers preferred the outer branches in the upper levels of the tree.
• Blackburnian warblers used the higher parts of the tree, but fed closer to the trunk and also searched lower than Cape May warblers.
• Bay-breasted warblers preferred the middle section of the tree, foraging all the way inside towards the trunk.
• Black-throated green warblers also used the middle section, but stayed out away from the trunk, using the middle to the tips of the branches.
• Myrtle warblers searched for their insect food down near the ground, in the lowest branches.

Hypothesis-driven Ecology

Ecology is the study of how organisms interact with each other and their environment. Before MacArthur’s research, almost all papers in the journal Ecology were observational. They reported on something the author had seen, or described some aspect of natural history. MacArthur’s paper marked a major turning point in how the science of ecology was conducted. He used something that most scientific research does today: hypotheses tested with statistics.

MacArthur’s strong background in math gave him the knowledge and experience he needed to use statistics. He could have simply reported that he observed Cape May warblers at the tops of trees. Instead, MacArthur went beyond this. He formulated a hypothesis: Warblers were using different foraging strategies to avoid competition from other species. He then used statistics to test if this was true. He was able to conclude that the time each species spent in the sections of a spruce tree was significantly different from one another. In addition, through statistics he discovered other behavioral differences. For example, black-throated green warblers hovered significantly more often than other species. This hypothesis-driven research marked a turning point in ecology, and it happened in Acadia National Park!

Blackburnian, Cape May, and bay-breasted warblers no longer breed in Acadia’s spruce forests. Instead, many other species are now living in these forests.

Photo courtesy of Josh Robert @joshwrobert. Used with permission.

Warblers in Acadia Today

Nearly 60 years later, MacArthur’s research is still valuable. It provides a clear picture of the warblers present in Acadia in the 1950s. Current-day researchers can investigate whether the species composition changed over time. Bik Wheeler, a graduate of College of the Atlantic, recently repeated MacArthur’s study for his Masters of Science degree. Some of the same species are present, though not all. Blackburnian, Cape May, and bay-breasted warblers no longer breed in Acadia’s spruce forests. Instead, many other species are now living in these forests. These include American redstarts, common yellowthroats, black-and-white warblers, magnolia warblers, and yellow warblers.

Science done within Acadia is relevant to many park visitors, as it provides information about what they can experience on their visit, such as which birds they are likely to see. Visitors can walk in MacArthur’s shoes, perhaps becoming inspired to ask questions and explore nature’s mysteries themselves. Park managers can use MacArthur’s and Wheeler’s research to document changes in natural resources over time. These studies also serve as a reminder that the little headland at Bass Harbor is worth protecting as a place where a walk in the woods can lead to a scientific epiphany.

MacArthur’s classic scientific paper was about more than warblers. It developed niche theory (ideas about the different roles that species play in a community) and advanced the field of ecology into hypothesis-driven methods. It is still influencing research done today. All this came about because of a walk in the woods, a simple question about coexistence, and the determination to find an answer. And it all happened in a wonderful location to conduct the research: Acadia National Park.