In the autumn of 1948 the population of woodrats was far below the level it had attained in 1947 or earlier, but the rats were still abundant and distributed throughout a variety of habitats. Almost every part of the woodland was occupied by at least a sparse population. Also, many rats lived beyond the limits of the woodland proper, in such places as deserted buildings, thickets, roadside hedges, and tangles of exposed tree roots along cut banks of gullies. All these situations are characterized by providing abundant cover, a limiting factor for this woodrat. In 1947, when the population of woodrats was especially high, plant succession on the wooded parts of the Reservation may have been near the optimum stage for the rats. For some 80 years, since the time the land was first settled and prairie fires were brought under control, woody vegetation has been encroaching into areas that were formerly grassland. About 1934 the University changed its policy with regard to treatment of the tract that was later made the Reservation. Up to that time, most of the area had been used as pasture and subjected to heavy grazing, but several fields had been fenced and cultivated. Under the new policy the hillsides and hilltop edges with open stands of various deciduous trees were enclosed with stock fences and protected from grazing. Successional trends were greatly altered. Woody vegetation, already favored by protection from the prairie fires originally important in the ecology of this region underwent further development as a result of protection from browsing. Thickets of shrubs and saplings sprang up throughout the woodland, forming a dense understory layer beneath the discontinuous canopy of the relatively scattered mature trees. The composition and density of the undergrowth varied markedly in different parts of the woodland. The parts that were formerly most open acquired the most dense understory. Blackberry, honey locust, osage orange, and prickly ash formed in places thorny tangles almost impenetrable to humans. This thicket stage reached its peak in density in the middle to late forties coinciding approximately with the time of maximum abundance of the rats. In the past eight years, under continued protection from burning, cutting and browsing, the forest has developed further; sizable trees 20 feet or more high and up to eight inches in trunk diameter have grown from seedlings during the period of protection. An almost continuous canopy of foliage has developed, shading the understory and thinning it by killing shrubs and saplings. In those situations where the canopy is most dense, as on north slopes having stands of young hickory averaging twenty feet high, the understory is now largely lacking, but in other situations, particularly on south slopes, the understory thickets are still dense. On the whole, however, habitat conditions have become less favorable for the woodrat.