Interactive Effects of Black-Tailed Prairie Dogs and Cattle on Shrub Encroachment in a Desert Grassland Ecosystem

<div><p>The widespread encroachment of woody plants throughout the semi-arid grasslands in North America has largely resulted from overgrazing by domestic livestock, fire suppression, and loss of native large and small mammalian herbivores. Burrowing-herbivorous mammals, such as prairie dogs (<i>Cynomys</i> spp.), help control shrub encroachment through clipping of shrubs and consumption of their seedlings, but little is known about how this important ecological role interacts with and may be influenced by co-existing large herbivores, especially domestic livestock. Here, we established a long-term manipulative experiment using a 2 × 2 factorial design to assess the independent and interactive effects of black-tailed prairie dogs (<i>Cynomys ludovicianus</i>) and cattle (<i>Bos taurus</i>) on honey mesquite (<i>Prosopis glandulosa</i>) abundance and structure. We found that, after five years, mesquite abundance was three to five times greater in plots where prairie dogs were removed compared to plots where they occurred together or alone, respectively. While both prairie dogs and cattle reduced mesquite cover, the effect of prairie dogs on reducing mesquite abundance, cover, and height was significantly greater than that by cattle. Surprisingly, cattle grazing enhanced prairie dog abundance, which, in turn, magnified the effects of prairie dogs on mesquite shrubs. Mesquite canopy cover per hectare was three to five times greater where prairie dogs and cattle were absent compared to where they occurred together or by themselves; whereas, cumulative mesquite height was two times lower on sites where prairie dog and cattle occurred together compared to where they occurred alone or where neither occurred. Data from our experimental study demonstrate that prairie dogs and moderate grazing by cattle can suppress mesquite growth, and, when their populations are properly managed, they may interact synergistically to significantly limit mesquite encroachment in desert grasslands.</p></div

Mesquite height response to experimental treatments.

<p>(A) Effect of treatments on average mesquite height, and (B) cumulative mesquite height. Treatments had minimal effects on average mesquite height (A) but large effects on cumulative height (B) (+P+C = prairie dogs and cattle present; +P-C = prairie dogs present; -P+C = cattle present; -P-C = both species absent)</p

Prairie dog abundance.

<p>Number of prairie dogs (mean ± SD per ha) on each treatment, based on number of animals that occurred on each plot. +P+C = prairie dogs and cattle occurred together; +P-C = prairie dogs only occurred; -P+C = cattle only occurred; -P-C = both prairie dog and cattle were absent</p

Mesquite canopy cover response to experimental treatments.

<p>Average mesquite canopy cover (mean ± SD) per hectare on each treatment. Canopy cover was strikingly five-fold greater in plots where neither species was present compared to where they occurred alone or together (F STAT, P < 0.0004). (+P+C = prairie dogs and cattle present; +P-C = prairie dogs present; -P+C = cattle present; -P-C = both species absent)</p

Black-Tailed Prairie Dogs, Cattle, and the Conservation of North America’s Arid Grasslands

<div><p>Prairie dogs (<i>Cynomys</i> spp.) have been eliminated from over 95% of their historic range in large part from direct eradication campaigns to reduce their purported competition with cattle for forage. Despite the longstanding importance of this issue to grassland management and conservation, the ecological interactions between cattle and prairie dogs have not been well examined. We address this issue through two complementary experiments to determine if cattle and prairie dogs form a mutualistic grazing association similar to that between prairie dogs and American bison. Our experimental results show that cattle preferentially graze along prairie dog colony edges and use their colony centers for resting, resembling the mutualistic relationship prairie dogs have with American bison. Our results also show that prairie dog colonies are not only an important component of the grassland mosaic for maintaining biodiversity, but also provide benefits to cattle, thereby challenging the long-standing view of prairie dogs as an undesirable pest species in grasslands.</p></div

Small-scale experiment.

<p>Cattle grazing record’s percentage by foraging zone and season v.s. foraging zone area cover %. (a) Bonferroni confidence-interval results (P <0.001) are given by the following signs: “<b>+</b>” indicates preference (utilization above availability); “<b>=</b>” indicates random (utilization equal to its availability); and “–” indicates avoidance (utilization below its availability); (b) image of a typical prairie dog colony in the Janos grasslands, showing vegetation height and cover increasing away from the colony center (1) towards the colony edge (2) and the surrounding climax grasslands (3).</p

Large-scale experiment.

<p>Cattle record’s percentage by vegetation type, activity, and season. a) Total time cattle utilized the different vegetation types v.s. vegetation type % cover; b) Grazzing records by vegetation by season v.s. vegetation type cover %; c) Grazzing records by vegetation x season v.s. vegetation type cover %. d) Grazzing records by vegetation x season v.s. vegetation type cover %. Bonferroni confidence-interval results (P <0.001) are given by the following signs: “<b>+</b>” indicates preference (utilization above availability); “<b>=</b>” indicates random (utilization equal to its availability); and “–” indicates avoidance (utilization below its availability); and “<b>*</b>” indicates vegetation types with significant differences in utilization across seasons.</p

Spatial pattern of the three classes of cattle activity relative to the vegetation/ground cover types through seasons.

<p>Colored dots indicate cattle locations by activity across all weeks during each sample season.</p