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West Bishop, CA, United States

Wiens D.,White Mountain Research Station | Slaton M.R.,U.S. Department of Agriculture
Biological Journal of the Linnean Society | Year: 2012

Following publication of On the Origin of Species, biologists concentrated on and resolved the mechanisms of adaptation and speciation, but largely ignored extinction. Thus, extinction remained essentially a discipline of palaeontology. Adequate language is not available to describe extinction phenomena because they must be discussed in the passive voice, wherein populations simply 'go extinct' without reference to process, specifics, effects, or causality. Extinction is also described typically in terms of its dynamics (including rate or risk), and although correlative variables enhance our ability to predict extinction, they do not necessarily enable an understanding of process. Yet background extinction, like evolution, is a process requiring a functional explanation, without which it is impossible to formulate mechanisms. We define the mechanism of background extinction as a typically long-term, multi-generational loss of reproductive fitness. This simple concept has received little credence because of a perception that excess generation of progeny ensures population sustainability, and perhaps the misconception that the loss of reproductive fitness somehow constitutes selection against reproduction itself. During environmental shifts, reproductive fitness is compromised when biotic or abiotic extremes consistently exceed existing norms of reaction. Subsequent selection will now favour individual survival over reproductive fitness, initiating long-term negative selection pressure and population decline. Background extinction consists typically of two intergrading phases: habitat attenuation and habitat dissolution. These processes generate the relict populations that characterize many species undergoing background extinction. © 2011 The Linnean Society of London. Source

Wiens D.,White Mountain Research Station | Allphin L.,Brigham Young University | Wall M.,500 N College Avenue | Slaton M.R.,U.S. Department of Agriculture | Davis S.D.,Pepperdine University
Biological Journal of the Linnean Society | Year: 2012

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. Source

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