Glacial Till
Glacial Till
From A Place on the Glacial Till: Time, Land, and Nature Within an American Town
by
© 1997 by Oxford University Press, Inc. All rights reserved.
Used with permission.
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Editor's note: A Place on the Glacial Till is a quiet and lovely appreciation of the natural wonders waiting to be revealed in one's own backyard. Biologist Thomas Fairchild Sherman has taken to heart Thoreau's observation that a single pond or stream carries in it the entire world - he takes us to his hometown of Oberlin, Ohio, and invites us to share in the richness of the surrounding land and the life it has sustained since it was first formed. Sherman discovers both the poetry and the science in all the elements - animal, vegetable, and mineral - of this land, which at different times over the millennia has been locked in glacial ice and covered by sea water. In his fascinating exploration of this microcosm, Sherman offers a vivid reminder that "the most common things of our world are also the most miraculous."
Two great themes run through the ecology of the living world: the flow of energy and the recycling of materials. The energy of life is captured by the chlorophyll of plants - the green film that intercepts the sunlight and awakens the earth from a moon-like silence into the humming busyness of a summer's day. The honeybees bustling about my garden, the darters dashing in the little pools of Chance Creek, and the warblers flitting from branch to branch through the woods at Kipton Creek - all have their energy from the sun, passed onto them by channeled electrons in the chloroplasts of plants.
The sunlit plants could grow without the animals, but not the animals without the plants. Plants have coevolved with animals and learned to depend on them for pollination and seed dispersal. A woodland flower may coat its seeds with nutritious "elaiosomes" to induce ants to carry them to their nests - distributing future flowers around the woods. But the elaborate ornamentations of ecology do not alter the primary fact that the energy flux of the forest originates in the green leaves of its plants.
As the energy flows, materials must be recycled. Growth alone would soon cease for lack of essential nutrients, were plants unwilling to give their substance back to the earth and air. The air we breathe contains only about .03 percent carbon dioxide, an amount so small we do not notice it, but the photosynthesis of plants is utterly dependent on it. Green plants respire and return a certain amount of carbon dioxide to the air from the sugars that they have produced, but for the recycling of much of their materials, plants are dependent on fungi and microorganisms. The mushrooms that spring up after a rain from the fallen leaves and logs on the forest floor - the red chanterelles and deadly amanitas - are the fruiting bodies of hidden masses of fungi digesting and recycling the plant material. Animals speed the recycling by foraging on plants and on the fungi and microorganisms. A mushroom is often infested with beetles and flies that are consuming it, and in the soil, nematodes and earthworms work on the fungi, microbes, and plant remains. In principle, however, the other kingdoms of life could manage the recycling by themselves. Neither the ecological flow of energy nor the recycling of materials requires the presence of animals.
The biological character of a land, therefore, starts with the plants rather than with the animals. The evolution of land animals in the Devonian period was dependent on the establishment of a terrestrial flora. When F. Lucy Braun classified the ecology of eastern North America, she studied the distribution of trees, not mammals. In a woodland, the canopy trees are the first to intercept the sunlight, and they establish the character of the land and life below. In Braun's classification, northern Ohio is (or was) part of the beech-maple forest. No one has ever tried to call it a "mouse-squirrel-wolf region," because the specific animals have a less general effect than the canopy trees on the overall ecology of the land. Nevertheless, a red squirrel carries pine cones around, ants distribute the seeds of the spring flowers, and deer browse the lower limbs of trees. Every animal has an effect on the land.
Thoreau knew all this, from his walks around Concord. But the first in North America to record how land, plants, and animals affect one another through time was a young geologist-biologist named Henry Chandler Cowles. As a geology student at Oberlin, Cowles had learned how landforms change spontaneously with time. He brought this dynamic viewpoint to his graduate studies in botany at the University of Chicago in 1893. He reasoned that if landforms are changing by natural processes, so must the vegetation of the land, as the latter depends on the former. He came to see also that the relations between land and flora are interactive: that as changes in the land affect the vegetation, so too the vegetation affects the way in which the land changes.
As a place to study the vegetation of a dynamic landscape, Cowles chose the sand dunes at the southern end of Lake Michigan. Because the surface level of Lake Michigan has fallen somewhat in postglacial times, the dunes farther from the present lake shore have had a longer time to develop than those closer to the shore. Changes in the dune structure with distance from the shore could therefore be viewed as changes over time - so that one locale could display all at once the landform changes that have happened on a lake shore over a long time.
Cowles saw that the vegetation changed with distance from the lake shore, from no anchored plants at all at the water's edge, to beech-maple woods inland. The lower beach, which was washed by summer waves, had no plants. A middle beach, which was free of waves in summer but washed by higher water in winter, had (in summer) a few rooted plants; these were annuals (which could grow from seed and produce new seed in one summer season), and they were succulents (which could withstand the dry conditions of the summer beach). An upper beach, which was free of waves in both summer and winter, allowed the growth of biennials and perennials - plants that live more than one summer season. A succession of stages of vegetation could thus be seen in moving from the water's edge toward the first dunes.
With the formation of dunes, the vegetation changed still further. But this was not a simple one-way relation. Cowles saw that the geologic process of forming dunes by wind-driven sand was in fact vegetation-dependent: The formation of dunes required the presence of perennial plants to catch and hold the blowing sand, and the ultimate shape, area, and height of the dunes depended on the specific plants involved. Dunes formed by Elymus grass (wild rye) were small in area, as this grass does not spread rapidly by rhizomes, while those formed by Ammophila and Agropyron grasses, which do spread readily via rhizomes, were more extensive. Once dunes were stabilized by grasses or other herbs, woody plants could begin to grow, and because their roots could grow deeper, the dunes could become higher. Hence in succession from the lake shore (and in age), grass dunes would be replaced by dunes with willows, then those with cottonwoods or pines (adapted to dry conditions), and finally, as soil developed and moisture increased, by woods of beech and maple trees.
Cowles's study, published in 1899, became one of the foundations of the new science of ecology. Having shown that plants change with the age of a dune landscape, Cowles was confident that the animals must change as well, because they are dependent on the plants. In 1913, Cowles's student Victor Shelford recorded the way that the Indiana dunes showed succession of animals as well as plants. Shelford found, for example, that grasshoppers, digger wasps, and sand spiders are prevalent in the beach grass and cottonwood dunes close to the lake, but are not found at all in the wooded areas farther inland; while sowbugs, earthworms, and wood snails are prevalent in the beech-maple woods, but are absent from the lake-front dunes. The ants and tiger beetles found near the lake are different species from the ants and tiger beetles found in the woods.
The Cowles-Shelford studies of our Great Lakes dunes serve as a model of how the land, with its plants and animals, will tend to change with time. Geologic processes and the lives of flora and fauna are all linked together in an interactive and evolving community of nature. Whenever a new factor is introduced, whether by natural occurrence or by human intervention, we can expect many animals and plants to be affected, but it is impossible to predict exactly how the changes will unfold.
Thomas Fairchild Sherman taught biology at Oberlin College for thirty years. He has been a wilderness guide and has led biological field studies in the United States, England, and Canada.
Endlinks
Review: Place on the Glacial Till: Time, Land, and Nature Within an American Town - a short review of the book from the Electronic Green Journal.
Geology Fieldnotes: Indiana Dunes National Lakeshore - provides an overview of the geology of the Indiana Dunes from the National Park Service.
The Need to Know Library: Ecology and Environment Page - an extensive list of Internet resources related to ecology and the state of the environment.
Resources for Educators and Students - a collection of ecological resources including issues, facts, careers, and journals from the Ecological Society of America.
Succession and Restoration: How Ecosystems Respond to Disturbance - an overview of succession and the role of disturbance with Internet resources.You may purchase this book (cloth, 224 pp.) directly from: