Comparative Ecology of Pinyon Mice and Deer Mice in Mesa Verde National Park, Colorado

by Charles L. Douglas

Not all associations of the pinyon-juniper woodland support large numbers of P. truei. Pinyon-juniper woodland having a ground cover of Poa fendleriana, and no shrubs, supports few mice; the woodland on Wetherill Mesa near Long House is an example. Juniper-pinyon woodland having a Purshia tridentata understory also supports only a few mice. Such areas occur on the southern ends of the mesas and are characterized by widely-spaced trees and little ground cover—a reflection of the relatively low amounts of precipitation received by the southern end of the park.

P. truei was not found in grasslands on Navajo Hill, or in meadows at the southern end of Moccasin Mesa. The old burned areas on the northern end of Wetherill Mesa and on Morfield Ridge now support numerous grasses and shrubs, but P. truei appears not to live there.

P. truei tends to avoid stands of sagebrush, or grasslands, lacking pinyon or juniper trees. P. truei may venture into such areas while feeding. This species is found in thickets of Gambel oak and in areas with an overstory of mixed shrubs only when a living pinyon-juniper canopy is present, or when a woodland adjoins these areas.

Rocky terrain apparently is not a requirement for P. truei, since much of the pinyon-juniper woodland that is free of rocks supports large numbers. Optimum habitat, however, had a rocky floor. In such places, rocks probably are of secondary importance, whereas the shrubs and other plants growing on rocky soils are important for food and cover. Rocks likely provide additional nesting sites, and allow a larger population to live in an area than might otherwise be possible.

In Mesa Verde the deer mouse, P. maniculatus, prefers open areas having dense stands of grasses, or brushy areas adjoining open terrain. This species lives in stands of big sagebrush; in grassy areas having an oak-chaparral or mixed-shrub-overstory; and in grasslands without shrubs, such as on the southern end of Moccasin Mesa. Pure stands of sagebrush did not support large numbers of mice unless there was additional cover nearby in the form of shrubs or oak brush.

Optimum habitats for P. maniculatus were on Navajo Hill, in the burned areas on Morfield Ridge, on the northern end of Wetherill Mesa, and in the grassy areas near the entrance of the park. The trapping areas in the first three mentioned had heavy growths of grass and an overstory of shrubs.

Some individuals of P. maniculatus ventured into pinyon-juniper woodland and entered traps. Such animals usually were found in places having a heavy understory of sagebrush, or in disturbed places within the woodland.

P. maniculatus, but not P. truei, was taken in the arid pinyon-juniper-bitterbrush stand on the southern end of Wetherill Mesa. P. maniculatus also was present, in about equal numbers with P. truei, in a pinyon-juniper-muttongrass stand north of Long House. Both of these localities supported only a few mice.

P. maniculatus is found more frequently in pinyon-juniper woodland when the population density is high, and when such woodlands adjoin grasslands or sagebrush areas. As mentioned earlier, P. truei and P. maniculatus occur together in ecotonal areas between the forest and grassy or brushy areas. In Mesa Verde the deer mouse inhabits exposed grassy areas that have mostly shrubs in the open canopy.

P. maniculatus is the first to colonize areas that have been burned; this species invades such areas as soon as primary successional vegetation becomes established. It can be stated that in general, P. maniculatus will be found in the harsher, more arid habitats. If the habitat is so inhospitable that only a few mice can survive there, P. maniculatus will be present. P. truei apparently requires the more moderate conditions found in the pinyon-juniper forest, and this species does not venture far from the edge of the forest.