Geology topics

Daniel G. Milchunas

Publications and source records attributed to Daniel G. Milchunas.

3 recordsLinked to original sources

USGS2018

Change in dominance determines herbivore effects on plant biodiversity

Herbivores alter plant biodiversity (species richness) in many of the world’s ecosystems, but the magnitude and the direction of herbivore effects on biodiversity vary widely within and among ecosystems. One current theory predicts that herbivores enhance plant biodiversity at high productivity but have the opposite effect at low productivity. Yet, empirical support for the importance of site productivity as a mediator of these herbivore impacts is equivocal. Here, we synthesize data from 252 large-herbivore exclusion studies, spanning a 20-fold range in site productivity, to test an alternative hypothesis—that herbivore-induced changes in the competitive environment determine the response of plant biodiversity to herbivory irrespective of productivity. Under this hypothesis, when herbivores reduce the abundance (biomass, cover) of dominant species (for example, because the dominant plant is palatable), additional resources become available to support new species, thereby increasing biodiversity. By contrast, if herbivores promote high dominance by increasing the abundance of herbivory-resistant, unpalatable species, then resource availability for other species decreases reducing biodiversity. We show that herbivore-induced change in dominance, independent of site productivity or precipitation (a proxy for productivity), is the best predictor of herbivore effects on biodiversity in grassland and savannah sites. Given that most herbaceous ecosystems are dominated by one or a few species, altering the competitive environment via herbivores or by other means may be an effective strategy for conserving biodiversity in grasslands and savannahs globally.

Nature Ecology & Evolution↗

USGS2013

Grazing effects on plant community succession of early- and mid-seral seeded grassland compared to shortgrass steppe

Questions: Grazing may speed or slow secondary succession, and the direction may depend on seral stage and relative tolerance of native perennial grasses compared with annual invasive species. How does grazing affect succession where undisturbed communities have a long evolutionary history of grazing by native herbivores and are tolerant to livestock grazing? Location: Shortgrass steppe, North American Great Plains, Colorado (40°49′N, 104°46′W), USA. Methods: Ungrazed and grazed early-seral (4–6 yr) and mid-seral (18–20 yr) seeded fields (Conservation Reserve Program) and traditionally grazed native steppe were compared for effects on plant composition in relation to changes expected from regional succession models. Results: Recovery towards undisturbed native shortgrass steppe for early- and mid-seral communities, respectively, was 19% and 36% for total vegetation cover, 5% and 21% for planted native species, 10% and 88% for non-planted native perennial grasses, only 0.2% and 13% for short grasses, and overall dissimilarity in community species compositions was 97% and 68%. In general, grazing effects were neutral or most often not significant in all years and/or were small in overall community magnitude. The early-seral community displayed more changes indicative of a slowing of succession with grazing (total vegetative and grass basal cover) rather than reducing invasive species (species targeted by timing of grazing), although drought had limited the establishment of grazing-tolerant short grasses. The mid-seral community showed more changes consistent with advancing successional recovery with grazing (overall community dissimilarity, forbs, planted native perennial grasses, tall grasses and target species). However, non-planted native perennial grasses responded negatively to grazing in the mid-seral community and positively in native shortgrass steppe where outside seed would originate. Conclusions: Grazing effects on particular functional groups and species were not the same across seral stages, were mixed in terms of speeding or slowing succession, and were generally not large at the community level. Evolutionary history of grazing may serve as a general guide but decisions on whether to graze successional grasslands may best be made after assessing whether tolerant perennial short grass species are significant components. Monitoring may then be necessary to determine species responses in particular community matrixes and effects on subsequent immigration of non-seeded native perennial species.

Colorado↗

USGS2011

Allelopathic cover crop prior to seeding is more important than subsequent grazing/mowing in grassland establishment

The effects of grazing, mowing, and type of cover crop were evaluated in a previous winter wheat–fallow cropland seeded to grassland under the Conservation Reserve Program in eastern Colorado. Prior to seeding, the fallow strips were planted to forage sorghum or wheat in alternating strips (cover crops), with no grazing, moderate to heavy grazing, and mowing (grazing treatments) superimposed 4 yr after planting and studied for 3 yr. Plots previously in wheat had more annual and exotic species than sorghum plots. Concomitantly, there were much greater abundances of perennial native grass and all native species in sorghum than wheat cropped areas. The competitive advantage gained by seeded species in sorghum plots resulted in large increases in rhizomatous western wheatgrass. Sorghum is known to be allelopathic and is used in crop agriculture rotations to suppress weeds and increase crop yields, consistent with the responses of weed and desired native species in this study. Grazing treatment had relatively minor effects on basal and canopy cover composition of annual or exotic species versus perennial native grass or native species. Although grazing treatment never was a significant main effect, it occasionally modified cover crop or year effects. Opportunistic grazing reduced exotic cheatgrass by year 3 but also decreased the native palatable western wheatgrass. Mowing was a less effective weed control practice than grazing. Vegetative basal cover and aboveground primary production varied primarily with year. Common management practices for revegetation/restoration currently use herbicides and mowing as weed control practices and restrict grazing in all stages of development. Results suggest that allelopathic cover crop selection and opportunistic grazing can be effective alternative grass establishment and weed control practices. Susceptibility, resistance, and interactions of weed and seeded species to allelopathic cover species/cultivars may be a fruitful area of research.

Rangeland Ecology and Management↗