There are too many deer at the Ryerson Conservation Area. According to the best available estimates, the 550-acre preserve west of Deerfield could comfortably support about 10 of them. An aerial census taken last winter discovered 52.

The Lake County Forest Preserve District–which owns and manages Ryerson–has proposed dealing with the problem by shooting 40 of the deer, concentrating on does.

Suggesting that animals, especially Bambi’s relatives, need to be shot in large numbers is not a good way to win friends. Residents of Riverwoods, the sylvan suburb that adjoins the Ryerson preserve, are circulating petitions opposing the shoot. A local veterinarian has offered to inject the animals with birth-control chemicals as an alternative. Lake County newspapers have been running outraged letters filled with words like “barbaric.” Somebody has threatened the life of the man responsible for the decision, Dan Brouillard, who is the forest preserve district’s superintendent of conservation.

You may be wondering why too many deer is a problem. We establish preserves to protect nature; shouldn’t we be pleased that our efforts are succeeding? We hear so much bad news about cheetahs and California condors and sperm whales and African elephants, shouldn’t our hearts be warmed by the news that we don’t have to worry about the white-tailed deer?

Well, yes. We should definitely be pleased that white-tailed deer are doing well. It is certainly better to have too many deer than to have none at all. But our nature preserves are not deer parks. They are supposed to protect all the plants and animals that live in them. Deer populations larger than the carrying capacity of a preserve can very quickly destroy or drive out both plants and animals, reducing a diverse natural area capable of supporting large numbers of species to a weed zone scarcely distinguishable from the average vacant lot.

Ryerson is an old growth forest, the finest extant remnant of the sort of woodlands that once dominated the east banks of our rivers. The river provided a measure of protection from fires moving east in front of westerly winds, so species such as sugar maple, American elm, basswood, and various ashes could thrive.

Ryerson is famous for its spring wildflower display. In May, the white blossoms of the large-flowered trillium carpet the ground–or at least they did until a few years ago. The dramatic decline in trillium numbers in the past three or four years is the most visible early warning that deer numbers are getting out of hand. Lately too, a browse line has been developing. Browse lines mark the highest point off the ground deer can feed. Below them, only unpalatable plants can grow.

The destructive process has only just begun at Ryerson, so the proposed kill by the Lake County FPD is an attempt to deal with the problem before serious, permanent damage is done. I’m going to devote two columns to this question. This week, I will concentrate on the effects of large deer populations on the plants and other animals in preserves. Two weeks hence, I will discuss the alternatives available for dealing with excess deer.

All plants and animals live in communities. They are parts of ecosystems, and the continuing survival of individual species depends on the continuing survival of the system. The best way to save a rare species–the timber wolf, for instance–is to protect the forest and tundra ecosystems that provide the wolf with what it needs to live. The best way to protect a wildflower, like the white trillium, is to protect the deciduous forests where it thrives.

This insight is fundamental to the science of ecology, but it took time for it to penetrate the art of applied ecology called land management. Land management used to be game management. Managers concentrated on single species that were of particular interest to hunters. How many pheasant can we get to grow here? How many Canada geese can we produce?

In the interest of producing more Canada geese, managers actually chopped down forests to plant cornfields for goose food. They still drain marshes, just as the nesting season is getting under way, in order to plant forage crops to feed migrating ducks in the fall.

Thinkers like Aldo Leopold challenged this approach, and in the past couple of decades, systems rather than individual species have become the central concern. John Fitzpatrick, chief ornithologist at the Field Museum, told me flatly that “if you don’t think in terms of systems, you can’t be a conservationist.”

The goal of the systems approach is balance, the sort of balance that would exist naturally if we had not altered the environment so drastically. In the Illinois of 1788, too many deer would have been a local and temporary phenomenon. Excess animals could move in any direction they chose in search of someplace with fewer competitors.

Some animals would have been pushed out of prime habitat. Forced into land with less shelter, they would have been easy prey for wolves and pumas. The wolves and pumas would have responded to the rich food supply by having more offspring, providing more hungry mouths to eat deer.

The top predators are gone from Illinois and there is no hope of bringing them back. Our deer populations are confined to tiny islands of nature surrounded by subdivisions and industrial parks. When they try to migrate in search of less crowded land, they get hit by cars. Collisions with vehicles are one of the largest causes of mortality among our local deer. In the six years from 1981 through 1986, such collisions in Cook County jumped from 171 to 469 per year, and in Lake County, the increase was from 74 to 250, and a total of 47 people were hurt. Despite this highway carnage deer populations have kept increasing.

The white-tailed deer is a very adaptable animal. It is a browser rather than a grazer, which means it eats the leaves, stems, buds, and shoots of broad-leaved plants, including trees and shrubs, rather than grasses. Deer also enjoy fruits and seeds, from apples to acorns, and they are very fond of green corn.

Their reproductive potential is high. If they are getting enough to eat, does can begin producing fawns when they are only a year old, and they can have as many as three young a year. The whitetail is a polygynous animal–the bucks collect the largest harems they can defend from rivals–so every doe can be impregnated every year.

In an experiment conducted in Michigan, deer were confined in a woodland surrounded by a 12-foot fence. With ample food and no predators, the herd increased in six years from 6 animals to 212. Two hundred twelve deer can eat a whole lot.

The future of Ryerson, if some of the deer are not removed, can be seen in the sad recent history of the Ned Brown Preserve in Elk Grove Village. The preserve–a part of the Cook County Forest Preserve system–lies along Salt Creek. Its 3,700 acres hold several kinds of wild communities, but at its heart is the 440-acre Busse Woods Nature Preserve, an oak-hickory grove that has occupied this land since before settlement.

Busse Woods was dedicated as a nature preserve in 1965. Its selection was based on the richness of its plant community, and it was especially famous for its spring flower display, a display that has now almost completely vanished.

Jim Witham and Marty Jones, biologists working for the Illinois Natural History Survey, have been studying the deer of Cook, Lake, Du Page, and Kane counties since 1983, looking for, among other things, ways to control populations in the absence of hunting. For them Busse Woods and the Ned Brown Preserve became a laboratory, a place to study the effect of heavy deer populations on natural systems.

Busse Woods’ deer problems began with the development of the surrounding land. Witham and Jones studied a series of aerial photos of the Elk Grove area and plotted the change from agriculture–which dominated the land as recently as 1949-to homes, businesses, and industry. The deer population explosion began as soon as the preserve was cut off from other open land, which was also the moment when corn and other crops disappeared as potential food sources.

In 1983, 400 deer lived at Ned Brown. Most of them lived north of Higgins Road, which divides the preserve into two unequal parts, with the heaviest concentrations in and around the nature preserve. The animals were showing the effects of their crowding. They tended to be small and undernourished, but they continued to reproduce. The huge fall acorn crop came at just the right moment to put the does in good condition for breeding despite the semistarvation of the rest of the year.

Witham and Jones began removing animals from Ned Brown while simultaneously studying the effects of the deer on the plant life. Since 1983, they have removed 320 animals. Fifty-two of these were moved to new homes on U.S. Army land near Joliet. The rest were shot. The deer population is now about 75 animals, which Witham and Jones think is about right.

But the plants still carry the record of the deer infestation. Look at the edge of a typical Illinois woodland, at a place where it borders a road, for example. You will see a continuous curtain of green from the top of the tallest trees to the ground. The trees have very deep crowns at places like these, and below them, shrubs and herbs struggle to grab as much of the light as they can.

The woodland edges in the north half of Ned Brown don’t look like this. They have the tall trees, but below the trees, at a height of between four and five feet, the green just stops. The line is as neat as YOU could cut with a hedge clipper. This is the browse line, the upper limit of feeding by deer. Below this line, almost nothing grows except a few stunted buckthorn shrubs. Buckthorn is an alien weed nearly worthless as deer food, but the Ned Brown deer got so hungry they ate it.

They also ate all the spring wildflowers, even species like false solomonseal, which they usually find unpalatable. The once grand spring display is gone from Ned Brown. In all Marty Jones’s sample plots and transects, only one white trillium can be found. Other spring flowers–white trout lily and woodland phlox, for example–grow only in a deer exclosure, a fenced-in area the deer can’t get at. The exclosure is also the only place you can find healthy, vigorous hickory saplings.

Shrubs have disappeared too. The Illinois Natural Areas Inventory surveyed Busse Woods in 1976. According to their report, American hazelnut and downy arrowwood–both favored wildlife foods–were among the more important shrubs of the forest understory. After five years of looking, Witham and Jones have been unable to find a single individual of either species.

Overall reproduction of woody plants is also down. In ’76, the inventory counted an average of 5,400 to 5,600 stems of shrubs and small trees per hectare. In a three-year study beginning in 1985, Witham and Jones found 2,040 stems per hectare the first year. By the third year, the totals had fallen to 1,770 stems per hectare, and among woody plants less than one inch in diameter at breast height, there were more dead stems than live ones.

And the deer aren’t just going after the little trees. In one particularly hard winter, they were driven by starvation to stripping bark from elm trees. Among 253 slippery and American elms the deer had girdled, over half died.

While the deer were destroying the botanical diversity of Busse Woods, they were also harming its animal life. Reduce the numbers of plant species and you almost inevitably reduce the number of insect species, the basic food source for many birds, reptiles, and small mammals.

Birds of the forest understory, species like wood thrush, veery, and ovenbird–all of which currently nest at Ryerson–divide the environment very finely. Slight differences in nesting and feeding preferences allow these three small, primarily insectivorous species to live side by side without competing. When the deer eat the understory, they remove the shelter these birds need, creating a uniform habitat that pushes different species into an unnatural competition with each other. Inevitably, one or more species disappear from the preserve. And with the understory gone, the soil becomes warmer and drier; ground-dwelling insects escape these conditions by moving deeper, thus reducing the food supply for any remaining birds.

Deer have a right to live, but they have no more right to destroy a nature preserve than we do. Their population needs to be brought into balance with the available resources before they wreck the richest ecosystems that remain in our state.