»Ê¹ÚÌåÓýappÏÂÔØ

Introduction

Image Details

The Bureau of Land Management oversees 10,600 hectares (26,000 acres) of Eastern Mojave desert near Las Vegas, Nevada, as a special area to receive desert tortoise translocations. This site is characterized by valleys of desert scrub and mountain ranges that extend through southern Nevada and southeastern California. Translocated Mojave desert tortoises (Gopherus agassizii) were released at this protected site in an effort to increase tortoise populations. Both winter and summer precipitation are critical for the survival and establishment of numerous annual and perennial plants that provide forage and cover for this threatened species.

In 2011, the US Fish and Wildlife Service enacted the Revised Recovery Plan to increase population densities of the desert tortoise. This plan calls for actively increasing native plant cover and reducing cover of invasive annual species such as Arabian schismus (Schismus arabicus) that dominate the landscape, have a low nutritional value for desert tortoise diet, and increase fire frequency to the detriment of native shrubs and forbs.

Key Issues Addressed

Passive establishment of perennial and annual vegetation is limited by feral horses and domestic burros that forage on native vegetation, competition with invasive plant species that increase fire frequency, and soil moisture limitations for germination. Reductions in native vegetation cover resulted in Mojave desert tortoise habitat degradation. Specifically, less perennial shrub cover is available to provide thermal refugia, and  remaining annual plant communities have decreased forage quality. For several years, research did not link desert tortoise population size with improvements to habitat and forage quality, which limited the development of successful active revegetation strategies designed to benefit desert tortoises.

Project Goals

• Increase perennial and annual plant density to improve desert tortoise habitat and forage quality
• Use a controlled field experiment to test watering, seeding, and herbivore exclusion treatments on native plant density
• Identify active revegetation strategies that improve habitat quality

Project Highlights

Microhabitat Creation: Below the canopy of perennial plants, litter and soil nutrients accumulate creating ‘Fertile Islandsâ€� that enhance the passive recruitment of multiple annual forage species and provide desert tortoise thermal refugia.

• Baseline Analysis: A vegetation survey was conducted prior to the experiment to characterize the plant community composition and density of native perennial and annual plant species.
• Multifactor Experiment:A 14-hectare split-split plot experiment was established in 2013 to test the effect of whole plot watering (present or absent), subplot seeding (no seeding, bare and pelletized seeds), and sub-subplot fencing (present or absent) on the establishment of perennial and annual species.
• Herbivore Exclusion: Medium and large-sized herbivores and burrowing animals were excluded with thirty 10-meter by 10-meter wire fences (1-meter tall) located within the experimental blocks. Not only did collaborators test for the effect of herbivore exclusion on seedling establishment but also tested for the interaction between watering, seeding and herbivore exclusion treatments.
• Native Plant Seeding: Seeds for four perennial and annual species were collected from two eastern Mojave Desert seed zones in Nevada and California. Seeded Subplots were sown with bare seeds (non-pelletized) or pelletized seeds coated in Gro-Coat ®. Perennial cheesebush and winterfat shrubs were seeded at  densities of 5,000 and 1,700 seeds per m², respectively, to increase plant cover. Desert globemallow, a perennial species, and desert plantain, an annual species, were seeded at a density of 13,000 and 5,300 seeds per m², respectively, to increase forage.
• Watering Events: Municipal water from the city of Las Vegas was transported in a water truck to the experimental site. In 2013, five whole plots were watered with 20,000 L (average depth of 0.5 mm) of water at five different time intervals starting the day after seeding and the last watering event added 2.5 mm of water to the surface.
• Vegetation Monitoring: Plant density, frequency and cover measurements were collected at 3, 10, 12 and 20-months post-treatment to account for the short and long-term effects of  treatments.

Lessons Learned

Image Details

• The pre-treatment plant community was characterized by:
  • Perennial Plants:
    • Creosote bush (Larrea tridentata)
    • White bursage (Ambrosia dumosa)
    • Cheesebush (Hymenoclea salsola)
    • Winterfat (Krascheninnikovia lanata)
    • Desert globemallow (Sphaeralcea ambigua)
    • Littleleaf ratany (Krameria erecta)
    • Big galleta grass (Pleuraphis rigida)
  • Annual Plants:
    • Non-native Arabian Schismus (Schismus arabicus)
    • Desert plantain (Plantago ovata)
    • Shaggy fruit pepperweed (Lepidium lasiocarpum)
    • Pebble pincushion (Chaenactis carphoclinia),
    • Devil’s spineflower (Chroizanthe rigida),
    • Broadfruit combseed (Pectorcarya platycarpa)
• In all vegetation inventories, the density of seeded annual plants was greater than that of seeded perennial plant species.
• Treatments did not impact the density of the invasive Arabian schismus and were unable to increase winterfat seedlings on plots.
• In spring 2013 (3 months post-treatment), irrigation treatments led to increased native plant species richness (2.9 species per m² compared to 1.7 species per m²)
• Treatments with pelletized seeds resulted in higher desert plantain density than bare seeds.
• Seeding and fencing plots led to increased non-seeded annual plants in fall 2013 (10 months post-treatment). Furthermore, the interaction between pelletized seeds and fencing treatments led to the highest increase in desert plantain density (39 plants per m²) 10 months post-treatment.
• Though initially successful, seeding experiments may have only short term effects for some species. Cheesebush and desert globemallow seedlings were observed 10-months after treatments with varying seed treatment effects: seeding with bare cheesebush seeds and pelletized globemallow seeds results in higher plant densities. However, cheesebush and globemallow were absent from inventories 20-months post-treatment.
• For other plant species, treatments may have long term effects. In fall 2014, there remained a positive effect of fencing and seeding on desert plantain cover and frequency.

Next Steps

• Monitor desert tortoise population size after active habitat restoration
• Test for the effects of improved forage quality on desert tortoise population size at the patch, home range, and landscape level
• Identify an irrigation regime that  stimulates germination
• Identify techniques that lead to successful perennial shrub establishment 

Funding Partners

Resources

• Abella, S. R., and Berry, K. H. (2016). â€�.â€�&²Ô²ú²õ±è;Journal Of Fish & Wildlife Management7(1): 255-279. doi:10.3996/052015-JFWM-046
• Abella, S. R., et al. (2015). â€�.â€�&²Ô²ú²õ±è;Journal Of Fish And Wildlife Management 6(2):278-289.

Contact

• Scott R. Abella, Assistant Professor, University of Nevada-Las Vegas, [email protected]

Case Study Lead Author

• Andrea Lopez, CART Graduate Research Assistant, University of New Mexico

Suggested Citation

Lopez, A. (2018). “Desert Tortoise Habitat Restoration Methods for the Eastern Mojave.â€�&²Ô²ú²õ±è;CART. Retrieved from /project/desert-tortoise-habitat-restoration.