Fall 2003

Studying Deer Balance

Over the last two decades, land managers have developed a number of strategies to determine how to bring deer back into balance with the ecosystem. One of the most extensive programs is in DuPage County, where ecologists combine detailed computer models of local deer populations with extensive monitoring of browse trends to determine how many deer each preserve can sustain.

To arrive at a rough idea of deer population density, DuPage County ecologists conduct an aerial census of deer in each preserve during the winter. They then estimate how many more will be born in the year by noting for the previous three years the number of pregnant females culled, the number of fetuses per doe, and the age of the deer that were culled. Next they estimate deaths likely to occur, based on natural mortality rates for fawns and adults. Ecologists use all of this data to estimate total population levels for the year, and compare it to their target density for a healthy herd, curently 20 deer per square mile.

To measure deer damage, scientists have long put up exclosures to keep deer out of certain areas, in order to compare plant growth and species composition outside the exclosure to that inside. About three years ago, DuPage County ecologists expanded their studies to include other methods. In one, researchers set out lines through the woods, called transects, and study the plants in the vicinity of these transects in early spring. In another, they conduct regular "meander" searches in the woods for evidence of browsing. In both, scientists calculate the percentage of browse on the plants in the study area.

"If we observe significant browse from our methods, then we apply to remove all the deer that our [population] model recommends," said Animal Ecologist Scott Meister. By the same token, he said, if observed browse is light, the ecologists can recommend removing fewer deer or even none.

When the data warrant it, professional sharpshooters thin deer herds. The goal is to maintain deer numbers before permanent damage to the ecosystem occurs. It's a constant vigil, and biologists say that sometimes they are just holding their own against the deer.