CAN SHEEP CONTROL LEAFY SPURGE WITHOUT COMPROMISING EFFORTS TO RESTORE NATIVE PLANTS?

This thesis includes two studies on the ecological effects of using sheep to control Euphorbia esula (leafy spurge). While sheep can effectively reduce E. esula when used as a long-term management strategy, little is known about their impacts on native plants. The efficacy of sheep grazing for restoration depends both on its potential for controlling undesirable plants and its ability to promote native species that provide key ecosystem goods and services. I investigated the effects of sheep grazing on native plants at both seed and mature-plant stages. To assess impacts on seedling establishment, I measured density of forb and graminoid seedlings in 15 grazed and 15 control (un-grazed) plots; after grazing, plots were treated with one of five experimental seeding treatments that varied by season and density of seed applied. To assess sheep impacts on abundance of mature plants, I measured change in percent stems grazed (pre- to post-grazing) of perennial forbs in 55 plots (including five controls) in an E. esula -invaded area with remnant native plants. Grazed plots had significantly fewer graminoid seedlings than un-grazed ones (28 vs 61/plot, respectively), but forb seedling density did not vary significantly. Mean change in percent stems grazed was higher for non-native than for native forbs (70% vs 23%, respectively), indicating that sheep preferentially grazed non-natives. However, sheep also consumed native forbs when they were abundant. Thus, appropriately timing grazing and careful monitoring of consumption is critical to reduce impacts on native plants. I also conducted a pilot study of the role of sheep as dispersal vectors for plant seeds. I measured seedling germination rates in feces collected from sheep used for E. esula control. Density and species of germinants were recorded from pots with soil enriched with fecal samples from 13 time periods as well as control pots with only soil. A total of 125 seedlings germinated from the feces (80 forbs and 45 graminoids), including five non-native (three forbs, two graminoids) and two native (forb) species. Forb germination rates were highest in late summer samples, while graminoid rates were higher in early-summer and ones. Overall, sheep fecal dispersal favored non-native plants