Page 12 - Black Range Naturalist, April 2020

separated into grassland habitats for Ord’s and shrub- dominated habitat for Merriam’s. When the livestock industry took over, much of the western U. S. grasslands were rapidly converted to shrub and small tree
savannas. Populations of both medium-size kangaroo rats increase with good summer monsoon rains, but when the summer is dry, as it was last year, reproduction of the kangaroo rats is
curtailed and population numbers decline. Since rodents are an important food base for a variety of predators other species are affected as well.
The Banner-tail
Kangaroo Rat2
behaves
differently from
the other two
species, includes
a variety of plants
in the diet, and
stores seeds and
hay in the burrow system. In the study to examine the effects of rodents on grasses (drop-seeds, black grama, several other grasses, and several forbs [herbaceous plants]), we utilized a one-hectare plot that had not been grazed by cattle in more than 30 years. The enclosure was a four-strand barbed wire fence with a chicken wire mesh barrier fastened to the barbed
grasslands. They are more abundant in areas with large mesquite than in grasslands with few or no shrubs. Spotted ground squirrels feed on green vegetation, seeds, and insects. Mesquite seed pods are an attractive food source for these squirrels. Spotted ground squirrels are not found in creosotebush communities, probably because of the paucity of suitable foods.
Cactus Mice are most abundant in the rocky foothills of desert mountains. They occur in lower numbers in creosotebush habitats on bajadas and may expand their range into basin shrublands during periods of successive wet years. These small, nocturnal mice eat seeds, fruits, flowers,
green vegetation and insects. During dry periods,
many of the foods needed by these mice for reproduction and lactation are not available. Cactus Mice populations decline rapidly during dry years and gradually increase in numbers during years with average or above average rainfall. Populations of Cactus Mice track year-to-year weather patterns, which results in large fluctuations in numbers of mice per unit area.
For the Western Harvest Mouse, the northern Chihuahuan Desert provides few suitable habitats
because this mouse prefers thick grass or shrub cover for foraging
A large Banner-tail Kangaroo Rat mound that is approximately 6 feet in diameter and elevated more than one foot above the surrounding vegetation (photo by Vic Crane).
wire. The chicken wire kept
jackrabbits and desert cottontails
from accessing the grasses. The
holes in the chicken wire allowed
kangaroo rats and pocket mice to
enter the exclosure. Within one
month after establishing the plot, we
found tillers that had been cut by
rodents. By the end of the growing season (August) up to 35% of the grass and other green foliage had been consumed. The August peak coincides with the reproductive peak and therefore the rats may be obtaining critical nutrients from the grasses. In addition to nutrients and some preformed water, kangaroo rats may be obtaining 6-MBOA, which is an estrogenic phytochemical that occurs in new growth grasses. This chemical has been found to boost reproductive activity in Ord’s Kangaroo Rat.
Spotted ground squirrels are the only rodents that are active during daylight hours. Spotted ground squirrels may be found in desert grasslands and in mesquite-
A pocket mouse (photo by Dr. David Lightfoot)
10
and nesting. In the last century or more, thick grass habitats are widely scattered and mostly consist of tobosa grass swales (low areas that receive overland flow in some large rains) and margins of dry lakes. Published studies of rodent populations in the Chihuahuan Desert report Western Harvest Mice from thick tobosa grass areas and from areas where Lehman’s love grass filled in spaces between grama grass tussocks. Western Harvest Mice are good climbers and are known to climb into low shrubs to obtain
food. These mice are omnivorous. They eat seeds, insects, fruit, and green vegetation. Population numbers of this species increase during wet periods and drop to exceedingly low numbers during dry periods.