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School of Life Sciences. A Systematic Review of Burro Grazing Effects on Mojave Desert Vegetation Scott R. Abella [email protected] Burro sightings in Lake Mead NRA, 1991-2005 1

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School of Life Sciences

A Systematic Review of Burro Grazing Effects on Mojave Desert Vegetation

Scott R. Abella [email protected]

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Burro sightings in Lake Mead NRA, 1991-20051

Yellow lines = aerial surveys



1Map provided by GIS division, Lake Mead NRA

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  • 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.


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  • 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

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  • 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  • Published/unpublished, but for unpublished, must be in report w/ methods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

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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

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Diet Analysis – 9 Studies

7/9 studies characterized > 1 year

Range 11-54 spp in diets; mean = 33

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

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

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  • 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

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Exclosures – 6 Studies

2/3 studies sample only 1 exclosure

No pre-treatment data

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Control-Impact – 3 Studies

2/3 studies unreplicated

Match site equivalency

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  • 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

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  • 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

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  • 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

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School of Life Sciences

Scott R. Abella University of Nevada Las Vegas [email protected]