A Systematic Review of Burro Grazing Effects on Mojave Desert Vegetation

1. School of Life Sciences A Systematic Review of Burro Grazing Effects on Mojave Desert Vegetation Scott R. Abella scott.abella@unlv.edu http://faculty.unlv.edu/abellas2/

2. Burro sightings in Lake Mead NRA, 1991-20051 Yellow lines = aerial surveys NEVADA ARIZONA 1Map provided by GIS division, Lake Mead NRA

3. Scattered literature, contradictory • Need more focused conclusions than “burros have negative impacts” • Trivial in the sense of removal because of status as exotic species or as a domestic livestock • Addressing questions of past effects, precisely what has been affected and to what extent important for mgt. Rationale

4. Methods • Science Direct, Agricola, Biological Sciences, Google Scholar, Journal of Range Management • Key words: burro, feral ass, grazing, Mojave, vegetation, impact, effect • Douglas & Hurst (1993) annotated bibliography, Lake Mead NRA 1994 Environmental Impact Statement • Examined all references therein

5. Criteria for Inclusion • Must be conducted in Mojave Desert or in adjacent transition containing Mojave species • Must provide some quantitative data on burro diets, forage, or effects on plants • Published/unpublished, but for unpublished, must be in report w/ methods • > 50 documents; 22 meet criteria

6. Classification of Studies 1. Diet analysis – 9 studies 2. Utilization – 6 studies 3. Control/impact comparisons – 3 studies 4. Exclosure – 3 studies 5. Forage analysis – 6 studies

7. Diet Analysis – 9 Studies 7/9 studies characterized > 1 year Range 11-54 spp in diets; mean = 33

8. Diet Analysis - Graminoids Only

9. Diet Analysis Plantago ovata, Bromus rubens, perennial grasses, Atriplex, Ambrosia dumosa, Sphaeralcea ambigua

10. Preferred: Elymusmultisetus, Achnatherumhymenoides, PleuraphisjamesiiNot preferred: Artemisianova, Ephedra, Larrea tridentata, Ambrosia dumosa

11. Utilization – 6 Studies • Disadvantages – hard to measure if plants removed entirely; no controls unless plants caged; only shrubs measured • Conflicting results: e.g., Ambrosia utilized in 2 studies, not in another • Ephedra not utilized in 3 studies • As expected, utilization heavy near water

12. Exclosures – 6 Studies 2/3 studies sample only 1 exclosure No pre-treatment data

13. Control-Impact – 3 Studies 2/3 studies unreplicated Match site equivalency

14. Forage Analysis – 6 Studies • One study concludes that Plantago ovata rich in protein, P, B-carotene, and is highly digestible • Few studies matched forage characteristics to actual consumption, or analyzed species not expected to be good forage

15. Main Conclusions • Replication, randomization, study/ experimental design rigor, and publication rates overall low • Nevertheless, several consistencies – perennial grasses and certain forbs heavily consumed; most shrubs eaten only when no other available alternatives • Variety of approaches useful, and to measure spatial and temporal variation

16. Further Research • Enhance basic understanding of veg. change in absence of burro grazing • Measure potential longer term changes in the past • New emerging questions – e.g., does burro grazing interact with desert fires? • Useful opportunity to better understand grazing in desert ecosystems

17. PUBLIC LANDS School of Life Sciences Scott R. Abella University of Nevada Las Vegas scott.abella@unlv.edu http://faculty.unlv.edu/abellas2/