The role of climate in the invasion of an exotic species and its impacts on a Chihuahuan Desert ecosystem

In recent years, extensive areas of the Chihuahuan Desert have undergone dramatic shifts in vegetation structure and composition due to invasions by both native and alien plant species.  We have observed a similar invasion of a previously rich community of winter annual plants in the San Simon Valley of southeastern Arizona.  We will present a 20-year dataset that documents changes in plant community composition and biodiversity in conjunction with the invasion of an exotic Eurasian species, Erodium cicutarium.  E. cicutarium was nearly absent from the plant community prior to 1995, and has recently increased from a minor presence to strong dominance over many areas of the valley, with relative abundance ranging from 80 to 96% of the total annual plant community.  Along with the wide-spread irruption of E. cicutarium, total native annual plant abundance has dramatically declined.  Although there are indications that N deposition may be an important factor, various climatic conditions, including timing and amount of precipitation, may play a strong role in driving the observed community dynamics through its effects on timing of germination and growth.  Preliminary analyses indicate that total seasonal precipitation influenced the number of species present, but not E. cicutarium or total annual plant abundance.  Total annual plant abundance was zero after a small amount of late-season precipitation, despite considerable precipitation that occurred early in the growing season.  Plant invasions change vegetation structure and composition, with potential for major impacts on biodiversity and ecosystem functioning.  Deciphering the link between climate and plant community dynamics will aid in our understanding of exotic plant invasions and their continued success in arid and semi-arid ecosystems.