Effects of Cattle Grazing and Environmental Factors on

Effects of Cattle Grazing and Environmental Factors on Seed Production in Dropseed Dominated Grasslands of the Chihuahuan Desert AMP recipient: Iris De La O Conservation Ecology Project Mentor: Martha J. Desmond NMSU, Fish, Wildlife and Conservation Ecology

Overview • • Introduction to Chihuahuan Desert Grasslands Desertification and Woody Encroachment Chihuahuan Desert Grasses and Grazing Winter Grassland Birds Problem and Importance Hypothesis and Predictions Methods Expected Outcomes

Chihuahuan Desert Grasslands • Of all the Great Plains regions, the Chihuahuan Desert is the most heat-stressed and arid system with • Historically few native ungulates therefore • Grass species evolved without heavy grazing pressure. • Dominated by two perennial grass species- tobosa (Pleuraphis mutica) and black grama (Bouteloua eriopoda ). Robb Hitchcock

Desertification and Woody Encroachment • Has dramatically altered southwest desert grasslands in the past 200 years. • Results in: – increased shrub cover – decreased canopy cover of grasses – increased resource heterogeneity • Attributed to – climatic change – fire suppression – atmospheric CO 2 concentrations – removal of native herbivores (black tailed prairie dog, pronghorn antelope – overgrazing by livestock.

Chihuahaun Desert Grasses and Grazing • Long-lived Black grama stands transition to short-lived perennials such as three awn and dropseed. • Dropseed produce abundant seed important to wintering grassland birds. • May be preferentially grazed during growing seasons for their higher forage value during this period • Livestock have influenced grass distribution, diversity and abundance.

Chihuahuan Desert Grasses and Grazing • Methods (most common) – Year-round contiguous – Best-pasture system • Intensity – the percent use of palatable forage by the forage species • 20% (light) • 40% (moderate) • 60% (high) – Guidelines for semidesert grasslands recommend 30 -40% utilization – Utilization as high as 45 -60% has occurred in this region

Winter Grassland Birds Migratory. Breed and winter different places • Entirely granivorous (eat seeds) during their non-breeding period. • Must consume seeds every 2 -5 seconds to meet metabolic needs • Abundance and distribution is strongly linked to diversity and abundance of seed resources. • Of any North American avian assemblage, grassland birds have exhibited the largest and most consistent population declines

Problem and Importance • No studies have investigated effects of cattle grazing on seed production in semi-desert grasslands. • Effects of grazing regimes and stocking densities on seed production in desert grasslands are little understood. • Seed production in relation to usual grazing regimes and recommended stocking rates must be evaluated. • Better methods of grazing management for this grassland type can be recommended. • Ensure the success and continuation of granivorous bird communities in the desert southwest.

Hypothesis and Predictions • Hypothesis: Seed production will be related to stocking rates within pastures – Prediction: Seed production will be higher on non-grazed versus grazed plots. – Prediction: A treatment effect will be observed with lower seed production on plots with moderate compared to low stocking rates. Conservation Commission of Missouri

Methods • Four pastures with two grazed and two ungrazed 25 m 2 plots each have been selected for a total of 16 plots. • Pasture selection is based on current and past stocking rates and accessibility. • An automated weather station was centrally located within each pasture. • Plant cover and species composition were estimated for each of the 16 plots using the line-intercept method (Cook and Stubbendieck 1986) during august

Methods: Seed Traps • Using the design of Pulliam and Brand (1975), twenty seed traps were evenly spaced in a 5 x 4 grid within each plot. • All traps were a minimum of 20 m from the exclosure edge • Seed traps were not set within 100 m of watering areas. • Temporary grazing enclosures made with two strands of barbed wire were established around seed traps in grazed areas once the traps had been placed, to prevent cow/calf pair disturbances.

Methods: Samples • Seed traps were emptied every 2 weeks from September 15 -January 30. • Seed samples are sorted through by hand under microscopes. • Seeds are separated and organized into individual baggies and placed in a small envelopes. • Seed sample filter and associated debris are returned to the original envelope. • Seed quantity and type are recorded on data sheets and backed up on disk.

Lab Responsibilities • Sort seed samples • Organize database • Conduct initial analysis – Grazed vs. Non-grazed • Seed Abundance • Seed Composition

Expected Outcomes • Seed production will differ between grazed and ungrazed grids, being lower on grids with moderate compared to low utilization. • Differences in seed production may be weather dependent with greater differences between treatments in moderate to wet years. Plants need rain to produce seed • As no other studies of this kind have been done, further predictions are difficult to make.

Acknowledgements • Martha J. Desmond, Associate Professor and Mentor, Fish, Wildlife and Conservation Ecology Dept. • USDA Seed Lab for help with seed identification • Elena Tsentas, Research Assistant