As the residents of the Tri-county area await the return of the ferry between Yarmouth and Bar Harbor, I have been thinking about a recent discovery that I made about the inauguration of the first passage between these locations by the Bluenose Ferry in 1956, and its timing and proximity to a groundbreaking study in the field of ecology.
As many of you know, the Town of Bar Harbor is surrounded by the Acadia National Park. During the summer of 1956, a young graduate student, Robert MacArthur, a native of Canada, began a study of warblers in a patch of coniferous forest in the park. In 1958, the study was published under the title of “Population Ecology of Some Warblers of Northeastern Coniferous Forests” in the journal Ecology. The study laid the groundwork for several key theoretical concepts in ecology, established MacArthur as one of the founding figures in this field, and inspired many students to become ecologists after reading this remarkable study. Sixty years later, the paper is still required reading in introductory courses in ecology in many universities around the world.
MacArthur’s study addresses the question as to how a variety of warbler species of similar body and bill size, nesting in the same area, all pursuing the same insects as food in the same trees, can compete without affecting the survival of each other? In other words, what enables these warbler species to coexist?
His study was conducted in a mature coastal spruce and fir forest with trees 20 meters in height. The warblers present in this forest patch included Cape May Warbler, Blackburnian Warbler, Bay-breasted Warbler, Black-throated Green Warbler, and Yellow-rumped Warbler.
MacArthur divided the trees into zones and carefully observed where the warblers were foraging. He found that Cape May and Blackburnian foraged in zones near the top of the trees, Black-throated Green and Bay-breasted in zones in the middle portion of the trees, and Yellow-rumped in zones in the lower branches, and by flying to catch insects in the air. Thus, the birds avoided excess competition by focusing their foraging in specific parts of the tree, creating a niche for each species. The study provided a simple, yet brilliant illustration of how similar species can coexist by occupying different niches.
Currently, another graduate student, Bik Wheeler, is replicating MacArthur’s study in the same patch of forest in Acadia National Park. As reported in the magazine Living Bird, he found there are fewer tall coniferous trees in the patch, and the Cape May, Bay-breasted, and Blackburnian no longer nest there. Instead other warblers have established themselves; the Magnolia Warbler, the American Redstart, the Black-and-white Warbler, the Common Yellowthroat, and the Yellow Warbler. The Magnolia prefers small coniferous trees, the Redstart and Black-and-white more open, mixed coniferous and deciduous forests, and the Yellowthroat and Yellow brushy habitats. It appears that the forest has changed in this part of Acadia National Park over 60 years.
The warblers that have disappeared from MacArthur’s study area, the Cape May, the Bay-breasted, and the Blackburnian are the three species that depend the most on mature, old-growth coniferous forests. Their absence may indicate less availability of this type of nesting habitat in the park, in surrounding areas, or both. This assessment is further complicated by the fact that the Cape May and the Bay-breasted thrive during a spruce budworm outbreak such that their numbers vary in relationship to the abundance of this insect.
We have seen declines in these three warbler species here in Nova Scotia as well. The province has classified the breeding status of the Cape May as “vulnerable”, the Bay-breasted as between “vulnerable” and “apparently secure”, and the Blackburnian as “apparently secure”. The Atlantic Canada Conservation Data Centre defines “apparently secure” as “some cause for long-term concern due to declines or other factors”.
Just as the forest conditions have changed since the launch of the Bluenose ferry in 1956, so has the type of ferry to make the voyage, and the social and economic conditions in the Tri-county area. Given the importance of the ferry to our economy, we may also feel our status, like the missing warblers, is now “vulnerable”. Let us hope for, and work towards, changing the status of both of us to “secure.”