Ecology of a biological invasion: Alien annual plants in the Mojave Desert.

摘要

Alien plants affect the integrity of native plant communities and ecosystems worldwide, but their effects have rarely been studied in the Mojave Desert. Alien annual plants comprised only 5% of the total number of annual plant species in this region, but two alien annual grasses, Bromus madritensis subsp. rubens (Bromus rubens) and Schismus spp., and one alien forb, Erodium cicutarium, accounted for 66% of total annual plant biomass during a year of high rainfall. To describe the ecology of these species, I evaluated their correlations with various environmental factors and tested these correlations experimentally.; Biomasses of all alien species were positively correlated with disturbance, but their correlations with plant productivity variables varied, indicating that environmental associations should be evaluated separately for each alien species. Species richness, density, and biomass of native seedlings were high where alien abundance was naturally low and where the density of alien annual grasses was experimentally reduced, indicating that aliens outcompeted natives. Alien species richness and biomass were generally higher than natives where soil nutrients were relatively high, and addition of nitrogen increased the richness and biomass of aliens and decreased that of natives. Alien annual grasses contributed more than alien or native forbs to the frequency and cover of fine fuels in the summer. Primarily Bromus rubens and Schismus spp. carried flames across the landscape, but only Bromus rubens produced flame lengths and temperatures sufficient to ignite perennial shrubs.; Dominance of alien annual plants may be limited by minimizing road densities, urban sprawl, and rates of atmospheric nitrogen deposition, or by closing areas to livestock grazing and off-highway vehicle use. Reducing dominance of alien annual grasses will reduce the frequency and size of fires, producing significant economic and ecological benefits. Monitoring roadsides and washes for new species and targeting for eradication species with the greatest potential for negative ecological impact may allow land managers to control the invasion of new alien species. Much can be learned from Bromus rubens, Schismus spp., and Erodium cicutarium that will help land managers predict the effects of new alien species and prevent them from becoming established in the Mojave Desert.