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Of spring migrations, and the lily pad problem

Bill Meredith

In an ecology course I took in graduate school, the professor defined extreme changes in animal populations (technically called irruptions or crashes) as "increases or decreases severe enough to be noticed by untrained observers." That definition came back to mind early last month when the Baltimore Sun ran a front-page article about the decline in the number of birds seen in the spring migration. The people who recorded the decline were trained observers… ornithology professors, biologists with the U. S. Fish & Wildlife Service, experienced volunteers from the Audubon Society, etc…. but the editors of the paper must have thought it was significant enough to be of interest to the untrained observers among their readership. In fact, it was just the most recent of several articles on this subject that have appeared in the popular press lately. Bird populations are in trouble.

Spring has always been a time of anticipation for birdwatchers, and it is becoming a time of anxiety as well. The first week of May marks the peak of the spring migration in this region, and it is the custom of the Audubon Society to select a day, usually around the first weekend in May, when birders get up before dawn and try to see how many birds they can find in one day. Looking back over old notes, I have some vivid memories. The best is of May 2, 1959, when John Richards and I logged 103 species in one day within 10 miles of Emmitsburg. It was an unforgettable experience; the trees were so full of birds that I couldn’t focus my binoculars on just one, and there was such a cacophony of chirping that we had to concentrate to make out individual songs.

I know deep down I won’t see the equal of that again, simply because of aging eyes and ears, but I keep trying. This year I went to Michaux Park with a group from the Gettysburg Audubon Society, which includes some competent observers. We found about 40 species in the park, and I ended up with 63 for the day… an adequate list, considering that I wasn’t out all day, but not enough to brag about. But I was concerned by the numbers of individuals. Unlike 1959, there was no cacophony of song. A hooded warbler here, an ovenbird there… a single blackburnian warbler, an Acadian flycatcher, and so it went. Even more worrisome was the absence of species that once were common. There were no wood peewees (I have heard one since that day, but the woods should have been full of them). A few years ago there would have been both black- and yellow-billed cuckoos; this year I haven’t seen or heard them. Bobwhites are also missing. We did see a few indigo buntings, but there should have been one in the top of every dead tree. Spring isn’t silent yet, but it’s not as noisy as it used to be.

There are probably a lot of reasons for the decline in numbers of migrating birds. Perturbation of weather patterns and toxic chemicals in the environment are important, but the most serious problem seems to be loss of habitat, both in the breeding grounds to the north, the wintering areas to the south, and along the migration routes. The newspaper article mentioned above focused on the coast of Louisiana where birds come ashore after flying across the Caribbean; expanding commercial development has destroyed much of the forest which used to provide a rest and re-fueling site for exhausted migrants. And the same problem persists all over the hemisphere. As human populations expand, less room is left for all other creatures.

When I was teaching, we used to go through a cycle in which, every few years, we had a meeting to review the general biology course for non-majors. In one such meeting we were asked to name the one concept we considered most important for those students to remember. I decided my choice would be the concept of exponential growth. It is difficult… probably impossible, in fact… to get that topic across to non-scientists in pure mathematical terms, but I always found most students could grasp it by way of the lily pad problem. This involves a pond in which there is one lily pad, which grows by dividing each day; in other words, each day, the number of lily pads doubles. In 30 days, the pond is completely covered with lily pads. The question is, on what day is the pond half full of lily pads? Answer: on the 29th day.

I thought this concept was important for my students to know because it describes so many processes in the real world, but most importantly, population growth. When a population starts growing, it seems innocuous at first, because the numbers are small; no one is concerned if it doubles in a given amount of time. But as time passes, larger numbers are multiplied in each time interval; and suddenly, we look around and there is no room left. One day, the pond is half full of lily pads, and there seems to be plenty of room; the next day, it is full.

In 1933, when I was born, there were just over two billion people in the world. In 1996, there were 6 billion; this year, we reached 7 billion. This is not just an abstract problem that exists in China and India. Montgomery County filled up; Frederick filled up; and look at the rate housing developments are popping up in Emmitsburg. The world is the pond; we are the lily pads. The birds are warning us. We may already be in the 29th day.

Read other articles by Bill Meredith