Identifying the causes and consequences of species decline is essential to ensure that species persist into the future. Desmodium cuspidatum is a nitrogen-fixing perennial legume that has experienced dramatic population losses in the last 30 to 40 years in New England though declines have not been documented elsewhere in its range. This project explores causes and consequences of this decline by studying the role that nitrogen (N) deposition may have played in the decline of a nitrogen-fixing species, and incorporates demography, reproductive biology and genetic diversity to provide a synthetic understanding of the current status of this species from which to guide future management decisions. In chapter one, I show that nitrogen deposition could contribute to the decline of a nitrogen-fixing plant species through changes in its ability to outcompete non-fixing species. Under high levels of N deposition, a non-fixing species had more aboveground biomass, used applied N better than D. cuspidatum and N fixed by D. cuspidatum. In addition, D. cupsidatum produced significantly more nodules when grown with the non-fixing species. Chapter two describes trends in demography over a five year period, whereby populations in New England have remained relatively stable in size, survivorship of all individuals has been high, but recruitment rates have been low. Low recruitment is attributed to low reproductive success. I show that both pollen and resources limit reproductive success in New England populations. Germination rates are unlikely to explain low recruitment as germination rates for D. cuspidatum in the field were similar to those of a common Desmodium species. Chapter three describes greenhouse experiments that show that germination rates in D. cuspidatum were highest when seeds were sown on the soil surface, or were scarified and sterilized and that large seeds had higher germination rates than small seeds. Patterns of genetic diversity of New England populations are described in Chapter four and were similar to patterns of diversity observed elsewhere in the species range.