29th Annual Meeting and Symposium of the
Desert Tortoise Council, February 20-23, 2004
Abstracts

PLENARY ADDRESS

15 Years of Research on Disturbance Effects in Desert Tortoise Habitat

Matthew L. Brooks
U.S. Geological Survey, Western Ecological Research Center Las Vegas Field Station, 160 N. Stephanie St., Henderson, NV 89074

There are many reasons why the Mojave population of the desert tortoise (Gopherus agassizii) is a Federally threatened species. It has been directly affected by illegal collecting, and by mortality caused by disease, predation, and crushing by vehicles and livestock. Indirect effects primarily occur via habitat loss associated with human disturbances. Although my research over the years has not focused on the desert tortoise per se, the results have direct implications for management of desert tortoise habitat. Below I summarize some of the major findings of projects I have been involved with over the past 15 years.

When I started my research in the late 1980s, the deserts were thought to be one of the least invaded habitats in the United States. The primary alien species recognized as potential threats to desert tortoise habitat were the grasses Bromus rubens, Schismus arabicus and S. barbatus, and the forb Erodium cicutarium. These and other alien plants can have a wide range of negative ecosystem effects within desert tortoise habitat and elsewhere (Brooks and Pyke 2001; Brooks and Esque 2002; Klinger et al. in review). We specifically know that these species can compete with and reduce the productivity of native annual plants within the Mojave Desert (Brooks 2000a), especially when alien plant growth is enhanced by increased nitrogen availability (Brooks 2003). We also now have hard evidence that both Bromus spp. and Schismus spp. can promote fire where it may not have otherwise occurred (Brooks 1999a), and that fire frequencies have gradually increased in the Mojave Desert during the 1980s and 1990s (Brooks and Esque 2002), partly due to increased dominance of alien annual grasses (Brooks and Esque 2002; Brooks and Minnich in review). The positive effects of alien grasses on fire frequency, and the subsequent positive effects of fire on alien grass dominance (Brooks 2002; Brooks and Matchett 2003), have shifted fuel structure and fire regimes outside of their natural range of variation (Brooks et al. 2003), leading to a loss of desert vegetation in some regional hotspots (Brooks and Esque 2002; Brooks and Minnich in review). This process is generally referred to as the invasive plant/fire regime cycle (sensu Brooks et al. in press), and is similar to the more widely recognized problem of Bromus tectorum converting native shrublands to alien annual grasses in the Great Basin (Brooks and Pyke 2001). Although fire is thought to be historically infrequent in the Mojave Desert, its frequency, intensity, and ecological effects likely varied among vegetation zones (Brooks and Minnich in review). Unfortunately for the desert tortoise, the lower elevation zones that comprise its habitat are among the most sensitive to damage from fire. Direct mortality from fire, and indirect effects from loss of cover sites and native forage due to fire and competition from alien plants, all pose significant threats to the desert tortoise (Brooks and Esque 2002).

Within a desert subjected to multiple uses, some are bound to conflict. For example, a recent study indicates that intense grazing near livestock watering sites can reduce the amount and diversity of perennial plant cover and native annual plant forage (Brooks et al. in review), potentially reducing the quality of desert tortoise habitat. In this study from the west-central Mojave Desert, native plant cover, species richness, and plant structural diversity were lower within increasing proximity to livestock watering sites, whereas alien plant cover was higher close to watering sites. Interestingly, the alien forb Erodium cicutarium and the alien grass Schismus spp. increased with proximity, whereas the alien annual grass Bromus madritensis ssp. rubens decreased with proximity to watering sites, suggesting that not all alien species respond the same to grazing gradients, probably because of competition among the aliens themselves. In addition, all significant effects were focused within 200 m of the watering sites, suggesting that restoration of discontinued watering sites should focus within this zone.

The prevalence of vehicle routes may also reduce the quality of desert tortoise habitat. At the Dove Springs Open Area in California, the amount of area subjected to disturbance related to off-highway vehicle (OHV) recreation increased from 7% in 1965 to over 30% in 2001 (Matchett et al. 2004). The rate of route proliferation at this site has recently tapered off, but is still increasing and is spatially correlated with washes and utility corridors. Cover and species diversity of the vegetation, and density and diversity of the soil seedbank, both declined with increasing density of OHV tracks at this site (Brooks et al. in preparation). In general, alien species were more abundant where OHV track density was high, and native species were more abundant where OHV track density was low. Biomass and species richness of alien annual plants are positively correlated with density of dirt roads (Brooks 1998; Brooks and Berry in review), and alien plants such as the new invader Brassica tournefortii have higher stem densities (Berry and Brooks in preparation), biomass, and seed production (Brooks and Trader in preparation) on roadsides than areas away from paved roads. Fire frequency is also associated with proximity to roads (Brooks and Esque 2002).

Fenced exclusion of human disturbances such as OHV recreation and livestock grazing can benefit desert tortoise habitat. Plant cover, biomass, species diversity, seedbank biomass, and dominance by native species were all higher inside compared to outside the Desert Tortoise Research Natural Area, approximately 20 years after it was established to protect tortoise habitat and 10 years after fencing was completed (Brooks 1992; 1995; 1999bc; 2000b). Nocturnal rodent density and diversity (Brooks 1995), plus lizard and bird abundance and diversity (Brooks 1999b), were higher inside the fenceline, whereas abundance of Lepus californicus was higher outside.

We now know that well over 100 other alien species occur within desert tortoise habitat (Kemp and Brooks 1998; Brooks and Berry 1999; Brooks and Esque 2002), some of which are currently expanding from the edges of the desert tortoise range (e.g. perennial grasses, Brooks and Esque 2000). Cooperative Weed Management Areas in San Bernardino and Clark counties have been established to help manage these alien plants at regional scales and across jurisdictional boundaries. A prioritization tool is now available to help rank species based on their potential ecological threats (Warner et al. 2003), and a decision-support tool has been developed to help manage species that may specifically alter fire regimes (Brooks in press). Recommendations for control of alien annual grasses are currently available (Brooks 2000cd), although their management at landscape scales is probably futile without effective biocontrol agents which have yet to be identified. In contrast, species such as Brassica tournefortii which is now in the process of invading are potentially amenable to mechanical or chemical control, and methods are currently being evaluated for their cost effectiveness (Brooks et al. unpublished data). Although there is still much to learn about the ecology and management of invasive plants and fire in desert tortoise habitat, we are in much better shape to do so now than we were 15 years ago.

Brooks, M. L. 2003. Effects of increased soil nitrogen on the dominance of alien annual plants in the Mojave Desert. Journal of Applied Ecology. 40:344-353.

Brooks, M.L.2002. Peak fire temperatures and effects on annual plants in the Mojave Desert. Ecological Applications 12:1088-1102.

Brooks, M. L. 2000a. Competition between alien annual grasses and native annual plants in the Mojave Desert. American Midland Naturalist 144:92-108. 

Brooks, M. L. 2000b. Does protection of desert tortoise habitat generate other ecological benefits in the Mojave Desert? Pages 68-73 in S.F. McCool, D. N. Cole, W. T. Borrie, and J. O'Laughlin (eds.), Wilderness Science: In a Time of Change conference, Volume 3: Wilderness as a Place for Scientific Inquiry, Missoula MT, May 23-27 1999. RMRD-P-15-VOL-3. 

Brooks, M. L. 2000c. Bromus madritensis subsp. rubens (L.) Husnot. Pp. 72-76. In C. Bossard, M. Hoshovsky, and J. Randall (eds.). Invasive Plants of California's Wildlands. University of California Press. Berkeley, CA.

Brooks, M. L. 2000d. Schismus arabicus Nees, Schismus barbatus (L.) Thell. Pp. 287-291. In C. Bossard, M. Hoshovsky, and J. Randall (eds.), Invasive Plants of California's Wildlands. University of California Press. Berkeley, CA.

Brooks, M. L 1999a. Alien annual grasses and fire in the Mojave Desert. Madroño. 46:13-19. 

Brooks, M. L. 1999b. Effects of protective fencing on birds, lizards, and black-tailed hares in the western Mojave Desert. Environmental Management 23:387-400. 

Brooks, ML 1999c. Habitat invasibility and dominance by alien annual plants in the western Mojave Desert. Biological Invasions. 1:325-337. 

Brooks, M. L. 1998. Ecology of a biological invasion: alien annual plants in the Mojave Desert. PhD Dissertation, University of California, Riverside. 186 pp. 

Brooks, M. L. 1995. Benefits of protective fencing to plant and rodent communities of the western Mojave Desert, California. Environmental Management 19:65-74. 

Brooks, M. L. 1992. Ecological impact of human disturbance on the Desert Tortoise Natural Area, Kern County, California, 1978-1992. M.A. Thesis. California State University, Fresno. 51 pp.

Brooks, M. L. and K. H. Berry. In review. Dominance and environmental correlates of alien annual plants in the Mojave Desert. Journal of Arid Environments. 

Brooks, M. L. and K. H. Berry. 1999. Ecology and management of alien annual plants in the California deserts. CalEPPC News (California Exotic Pest Plant Council Newsletter). 7(3/4):4-6. 

Brooks, M. L. and C. M. D'Antonio. 2003. The role of fire in promoting plant invasions. In M. Kelly (ed.). Proceedings of the California Exotic Pest Plant Council Symposium. Vol. 6: 29-30. 

Brooks, M. L. C. M. D'Antonio, D. M. Richardson, J. Grace, J. J. Keeley, DiTomaso, R. Hobbs, M. Pellant, D. Pyke. In press. Effects of invasive alien plants on fire regimes. BioScience. 

Brooks, M. L., and T. C. Esque. 2002. Alien annual plants and wildfire in desert tortoise habitat: status, ecological effects, and management. Chelonian Conservation and Biology 4:330-340. 

Brooks, M. L. and T. Esque. 2000. Alien grasses in the Mojave and Sonoran deserts. Proceedings of the 1999 California Exotic Pest Plant Council Symposium 6:39-44. 

Brooks, M. L., T. C. Esque, and T. Duck. 2003. Fuels and fire regimes in creosotebush, blackbrush, and interior chaparral shrublands. Report for the Southern Utah Demonstration Fuels Project, USDA, Forest Service, Rocky Mountain Research Station, Fire Science Lab, Missoula, Montana. 17pp. 

Brooks, M. L, J. R. Matchett, and K. H. Berry. In review. Effects of livestock watering sites on plant communities in the Mojave Desert, USA. Journal of Arid Environments. 

Brooks, M. L. and J. R. Matchett. 2003. Plant community patterns in unburned and burned blackbrush (Coleogyne ramosissima) shrublands in the Mojave Desert. Western North American Naturalist 63:283-298. 

Brooks, M. L., J. R. Matchett, T. C. Esque, and J. F. Weigand. In preparation. Vegetation Responses to Off-highway Vehicle Disturbance at the Dove Springs OHV Open Area, California. Report prepared for the Bureau of Land Management, California State Office, Sacramento, California. 

Brooks, M. L. and R. A. Minnich. In review. Desert bioregion. In Sugihara et al. (eds.) Fire in California Ecosystems. University of California Press.

Brooks, M. L. and D. Pyke. 2001. Invasive plants and fire in the deserts of North America. Pp. 1-14 In K. Galley and T. Wilson (eds.), Proceedings of the Invasive Species Workshop: The Role of Fire In the Control and Spread of Invasive Species. Fire Conference 2000: The First National Congress on Fire, Ecology, Prevention and Management. Miscellaneous Publications No. 11, Tall Timbers Research Station, Tallahassee, Florida, USA. 

Brooks, M. L. and M. Trader. In preparation. Patterns of Sahara mustard (Brassica tournefortii) abundance along desert roadsides. 

Berry, K. H. and M. L. Brooks. In preparation. Invasion of Sahara mustard (Brassica tournefortii) along an eastern Mojave Desert roadside. 

Kemp, P. and M. L. Brooks. 1998. Exotic species of California deserts. Fremontia 26:30-34. 

Klinger, R., M. L. Brooks, and J. Randall. In review. Fire and invasive plants. In Sugihara et al. (eds.) Fire in California Ecosystems. University of California Press. 

Matchett, J. R., L. Gass, M. L. Brooks, A. M. Mathie, R.D. Vitales, M. W. Campagna, D. M. Miller, and Weigand, J. F. 2004. Spatial and Temporal Patterns of Off-highway Vehicle Use at the Dove Springs OHV Open Area, California. Report prepared for the Bureau of Land Management, California State Office, Sacramento, California. 17pp. 

Warner, P. J., C. C. Bossard, M. L. Brooks, J. M. DiTomaso, J. A. Hall, A. Howald, D. W. Johnson, J. M. Randall, C. L. Roye, M. M. Ryan, and A. E. Stanton. 2003. Criteria for Categorizing Invasive Non-Native Plants that Threaten Wildlands. Available online at www.caleppc.org and www.swvma.org. California Exotic Pest Plant Council and Southwest Vegetation Management Association. 24 pp.