The San Simon Valley of southeastern Arizona maintained high plant diversity over much of the past 30 years. However, during the mid-1990's, this plant community underwent a dramatic change associated with the sustained irruption of the invasive winter annual plant, Erodium cicutarium. Coincident with the increase of this invasive plant, the winter annual plant diversity dramatically declined, species composition shifted, and the seed-eating rodent community changed. This research utilizes physiological, ecosystem, community and theoretical approaches to disentangling the various environmental factors hypothesized to have led to the dominance of E. cicutarium, and its major impacts on the local ecosystem. These environmental factors include recent changes in climate (decreased winter rainfall and more frequent drought), pollution in the form of nitrogen deposition (from agricultural fields and nearby copper smelters) and changes in the rodent community (from dominance by a kangaroo rat to dominance by a pocket mouse). Results from field manipulations and growth chamber studies will form the basis of mathematical models to predict changes in the annual plant community and in the functioning of the ecosystem under future climate and nutrient enrichment regimes. These models should elucidate the roles of human drivers of change in this ecosystem. Results from this work will inform mitigation and management strategies for plant invasions in arid ecosystems, and should aid efforts to restore native plant communities. This project also has an important research training component for undergraduates and graduate students and a postdoctoral fellow. It will support the training of women and minorities in a scientific discipline where they have been underrepresented.
We studied a community of annual plants in an area of Chihuahuan Desert in southeastern Arizona. This area was originally dominated by a diverse group of native winter annual plant species, but recently a nonnative species from Eurasia began increasing in abundance. Prior to the mid 1990's, the native species showed strong fluctuations in abundance without major trends. Beginning roughly in 1997, the previously rare alien species, Erodium cicutarium, began increasing to a roughly steady level of 80% of the annual plant community. At the same time, the native species underwent dramatic changes. Most decreased greatly in abundance, but some species that previously had been rare started becoming common. Our aims were (1) to see if the rise of the alien species was the cause of the changes in the native plant community, and (2) to try to understand the causes of the increase in abundance of the alien species. We removed the alien species from a series of plots in the field, and showed that the alien species has strong negative effects on most of the native species. However, we found also that one native annual plant species, Astragalus nuttallianus, suffers greatly when the alien species is removed and seems to depend on it. Using data from the field site collected over more than twenty years we were able to show that the more common native species were all uniquely affected by yearly fluctuations in the weather, and that the alien species is affected differently than the average of the native species. This result suggests that changes in the weather might have caused the alien species to increase in abundance and for the native species to be decline. However, although the weather does vary greatly over time, there was no strong trend in the right direction that might have contributed to the dramatic changes that we observed in this annual plant community. We collected air samples from our study site and found that our site receives higher than expected atmospheric deposits of nitrogen compounds, which can arise from pollution, and fertilizer and pesticide use in nearby agricultural fields. To understand the likely effects of excess nitrogen we conducted a large field experiment over two years in which we added nitrogen to some plots, and removed it from others. To further develop our understanding of the effects of the weather, we combined the nitrogen modification experiment with watering treatments to mimic the rainfall patterns that can occur in different seasons. We confirmed that the various species are affected in their own peculiar ways by environmental factors, suggesting differences between species that would allow them to coexist under the right mix of environmental factors in space and over time in the field. We also confirmed that the alien species shows a positive response to soil nitrogen, and is able to take more advantage of it than most native species. Surprisingly, we found circumstances when the native species Astragalus nuttallianus is the dominant when we would have expected the alien species to dominate. Controlled laboratory studies confirmed the results from our field experiments that each species has a unique way of responding to the environment. We hope that further analysis of the data will reveal a picture of those conditions that allow the alien species to dominate. The potential that excess fertilizer or pesticide applications have facilitated the dominance of this community by an alien species, by adding excessive nitrogen compounds to the atmosphere, emphasizes need for care in their application. The high sensitivity of all species to environmental factors, including the alien species, in this study emphasizes a need to be concerned about global climate change, which has the potential to greatly modify natural processes, and bring about major changes in vegetation. This research project was also an important education and training exercise. Several undergraduate students were heavily involved in the work, and many seemed to find it inspirational. Two students worked on their own research subprojects, and gained a more detailed understanding of the scientific process by designing their own studies, analyzing their data and presenting the results to an audience. A laboratory technician working on these projects gained valuable experience that facilitated her entry into a graduate school program in biology. The postdoctoral fellow on the project was a Native American woman, and the training that she received has helped her move on to an assistant professor position.
