Limits to range expansion in the native annual legume Chamaecrista fasciculata.

Abstract

Species range limits are determined by historical (e.g., range expansion), ecological (e.g., biotic interactions) and genetic (e.g, gene flow) processes, but comprehensively understanding the relative role of these processes in limiting any single species‟ range has been elusive. This research is timely for understanding species‟ responses to climate change. The goal of this research was to examine the processes that limit the range of the native annual legume Chamaecrista fasciculata, by integrating ecological-genetic field studies and population genetic laboratory studies. In Chapter 1, I investigate the extent to which C. fasciculata is in demographic range edge equilibrium at its western and northern range edges, and the effect of biotic interactions at these range edges. I find that C. fasciculata fitness is reduced to zero when planted beyond the western and northern range limits, indicating it is in equilibrium with its range. Neighbors increase early-season survival, but decrease seedpod production. The goal of Chapter 2 was to examine if the mutualism between C. fasciculata and its associated rhizobia was disrupted beyond the range edge, potentially limiting range expansion. The results demonstrate that compatible rhizobia are nearly absent beyond both range edges, which may limit range expansion. In Chapter 3, I ask how the habitat where C. fasciculata establishes may change with range shifts. I conclude that habitat type influences C. fasciculata fitness, but the outcome depends on both the substrate and competitive environments. Finally, in Chapter 4, I use population genetic methods to gain insight into the history of range expansion, population structure and gene flow. Population genetics indicate that the edge populations have reduced genetic diversity compared to the southernmost interior population, and are highly differentiated from each other. However, there is little evidence for contemporary gene flow between populations at the scale investigated. Overall, this work suggests that ecological-genetic or metapopulation dynamics are likely to be involved in setting the northern and western range limits. Further, it highlights the value of integrated approaches to studying species‟ range limits.