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Researchers then developed “exclosure” studies, in which they fenced an area off from the surrounding forest and compared changes between the areas outside and inside the fence over time.

An exclosure situated near a pond, on low ground producing a luxuriant growth of big bluestem and western wheat grass, has supported voles in 1948, 1949, 1952 and 1953.

Also, the ungrazed exclosure had greater yield and a thicker mat of debris than the grazed short grass surrounding it and was thus a relatively good habitat, although it did not compare favorably with the lowland type.

An upland exclosure containing only short grasses also supported a few voles in 1953.

The unexpected presence of voles in the short grass exclosure was probably due to two factors.