We describe a 15-year study of the loss of reproductive fitness and population decline in Adenostoma sparsifolium, a rosaceous shrub endemic in the fire-prone chaparral vegetation of southern California (USA) and adjacent northern Baja California, Mexico. Our studies of background extinction concentrated on small relict populations occurring in the eastern Santa Monica Mountains where reproduction is genetically compromised by uniquely high rates of embryonic/endosperm abortion (97–99%) resulting largely from self-pollination in highly heterozygous populations. Environmental factors further reduce reproductive fitness. The relatively few viable seeds produced are not well adapted to survive wildfires that are a regular (approximately 21 years) occurrence in chaparral. Seedling recruitment after burning is rare and any established seedlings ultimately die from the annual 4–9-month summer droughts typical of Mediterranean climates. Adult mortality is manifest from wildfire (approximately 6%) and occasional multiple-year droughts (approximately 15%). Given the virtual absence of new post-fire reproduction and a low but persistent rate of adult mortality, slow population demise resulting in background extinction is inevitable. We posit that A. sparsifolium is ecologically ‘out of place’ in the present chaparral environment and appears best adapted to a moister climate with summer rains and few wildfires that prevailed before the increasing aridity and warming from mid-Holocene to the present. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105, 269–292.