Removing an invader: evidence for forces reassembling a Chihuahuan Desert ecosystem

Community reassembly is an important concept in community ecology, but in most circumstances cannot be observed and is hypothetical. Reassembly of communities is an observable process that can be examined effectively following species invasions. Our study system in southeastern Arizona has declined in diversity and abundance of native winter annual plants coincident with a sustained irruption of a nonnative invasive Eurasian species, Erodium cicutarium, and a major reassembly of the winter annual plant community. To understand the role of E. cicutarium in the reassembly process, we initiated a multiyear E. cicutarium removal experiment that varied the timing of removal. Species richness was higher as a result of removal, although this effect was variable over time. Surprisingly, the abundance of native species collectively was decreased by E. cicutarium removal. However, previously rare Astragalus nuttallianus appears to be facilitated by E. cicutarium, as demonstrated by higher density and relative abundance in the presence of E. cicutarium, and is now codominant. Excluding A. nuttallianus abundance, the remaining native species are collectively increased by removal of E. cicutarium. Although our results showed strong effects of removal, the timing of removal never significantly affected any plant variables. Our experiment implicates a major role for E. cicutarium in the reassembly of the native plant community. Our work suggests a broadening of perspective on processes structuring annual plant communities in arid regions, and calls for an integrated approach beyond the recent popular focus on the interaction between competition and the physical environment.