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Twenty-Third Annual Meeting and Symposium of the
Desert Tortoise Council, April 3-5, 1998
Abstracts
STUDENT PAPER
Status of Alien Annual Plants and their Environmental Correlates in Desert Tortoise Habitat
Matthew L. Brooks1 and Kristin H. Berry2
1University of California, Riverside, Riverside CA 92521
Mailing address: 41734 South Fork Dr. Three Rivers CA 93271
2U.S. Geological Survey-BRD, Box Springs Field Station, Riverside, CA 92507
To describe the current status of alien annual plants in desert tortoise habitat, we measured above-ground live annual plant biomass at 8 sites in the Fremont-Kramer, 10 sites in the Ord-Rodman, and 16 sites in the Superior-Cronese Desert Wildlife Management Areas designated in the Recovery Plan for the Desert Tortoise (Mojave Population). These sites respectively represented the western, southern, and central Mojave Desert regions. During the high-rainfall spring of 1995, three alien species accounted for 66% of the total annual plant biomass, Bromus madritensis subsp. rubens, Erodium cicutarium, and Schismus spp. Measurements made during a year of low rainfall produced even higher values, suggesting that 66% may be at the low end of the interannual range. The two other alien species that were sampled, Bromus trinii and Bromus tectorum, each accounted for less than 1%. In contrast to biomass, alien species represented only 5 of 130 (4%) of all annual species that were collected. These data suggest that number of alien species (richness) may be a poor indicator of the extent and impact of plant invasions, and that alien biomass is likely a better measure.
We also evaluated correlations of alien annual plant biomass and species richness with disturbance, soil nutrients, and native plant diversity. In general, alien biomass and richness were positively correlated with variables in the first two categories, and negatively correlated with variables the last category, but analysis of individual species and covariance among species was necessary to minimize confounding variation. The most significant variables were density of dirt roads and frequency of fire, annual rainfall, and native annual plant diversity. Land managers in the Mojave Desert should assess land use practices based on individual alien species, not on alien plants as a whole, and should consider the effects of alien species on each other when evaluating these relationships.
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