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Many exotic species negatively affect native species and alter ecosystem function. Erodium cicutarium, an exotic annual plant, can attain high densities, but little is known about its effects on native plant communities. We first examined patterns of abundance of E. cicutarium and native annuals over a 16-year period at a long-term study site in southeastern Arizona. In years of high E. cicutarium abundance, the correlations between the abundance of E. cicutarium and native annuals in small-scale local communities were typically negative, suggesting a competitive interaction. To further examine the interaction between E. cicutarium and native annuals, we conducted a short-term field experiment at the same location using plots that contained pairs of quadrats. One quadrat in each pair was subjected to E. cicutarium removal during the winter of 2003-2004 while the other served as a control. At the end of the growing season, E. cicutarium removal quadrats contained significantly higher abundance and richness of native annual plants. However, control plots contained significantly higher abundance of all annuals due to the presence of E. cicutarium. Thus, in the single growing season examined, while E. cicutarium appears to suppress the diversity and abundance of native species, its presence significantly increases community productivity. (c) 2006 Elsevier Ltd. All rights reserved.