ECOLOGY: The Ups and Downs of Island Life
the Sverdrup Basin, Arctic Canada, which is now known to contain thick sections of Permian and Triassic rocks. Sections at the margin of the basin, where some erosion is documented to have occurred, show an abrupt carbon isotope drop at the boundary. Sections in the center, which seem to record continuous deposition and a thicker boundary layer than most other sections globally, show a more gradual decline over about 3 m, after a period of relative stability that is not well resolved elsewhere.
... resolved elsewhere. Although the sections thus provide important detail on the extinction record and perhaps a period just before it, detailed dates are not yet available to calibrate absolute rates of change. -BH Adult hair follicles sustain repeated cycles of hair loss and regrowth. Stem cells reside within a small niche, called the bulge, located in the upper part of the hair follicle. These stem cells are responsible for driving this cycle of growth and can repopulate follicles and surrounding epidermis damaged by wounding. However, the first hair follicles in mice arise from the developing epidermis, not from preexisting bulge stem cells, and these first-time CREDITS (TOP TO BOTTOM): (TOP LEFT) JACQUES DESCLOITRES, MODIS LAND RAPID RESPONSE TEAM, NASA/GSFC; (TOP RIGHT) DAVID HOSKING/ALAMY; MARVIN DEMBINSKY PHOTO ASSOCIATES/ALAMY EDITORS'CHOICE GEOLOGY Weighing Sverdrup Sediment The huge extinction at the end of the Permian period (~252 million years ago) is marked globally by a large drop in the carbon isotope ratio ( 13 C/ 12 C) of organic matter preserved in sedimentary rocks. Low values persist for millions of years after the extinction, in part marking the delayed recovery of Earth's biota. In addition to providing a marker of the event, the magnitude and abruptness of the drop are important constraints on the likely cause(s) and pace of the extinction. Many sections show a relatively abrupt decrease, though some of these may be partly compressed by erosion; the Permian was a time when most of the land masses were assembled together in one supercontinent (Pangea), and sea level fluctuated markedly. Yet some sections seem to show a more complicated or gradual decrease. Grasby and Beauchamp describe several isotope records preserved across The equilibrium theory of island biogeography, developed in the 1960s by MacArthur and Wilson, has been the principal reference point for ecologists investigating how the dynamic processes of colonization, speciation, and extinction affect biodiversity in insular habitats. However, the theory has been less successful when applied to longer-term evolution on oceanic islands, where geological dynamics such as erosion come into play. Whittaker et al. have developed a general dynamic model (GDM) that takes account of the humped trend in an oceanic island's carrying capacity over its entire life span. The model generates predictions about the biotic properties (species diversity, rates of speciation and extinction) of oceanic islands that fit snugly with data from oceanic archipelagoes, including the Hawaiian islands and the Galápagos (shown above) among others. By unifying evolutionary and ecological time scales and processes in a single theoretical framework, the GDM adds to the understanding of island biology. -AMS J. Biogeogr. 35, 977 (2008). Going, going, gone: extinct coral. hair follicles do not exhibit the conspicuous bulge that only becomes apparent some weeks after birth. Nowak et al. have analyzed the origins of the hair follicle stem cell niche in developing mice. The stem cells of the bulge are in fact established much earlier than previously suspected and begin to form in the embryo. These cells, which are characterized by expression of the transcription factor Sox9, not only contribute to the formation of the initial hair follicle, but also give rise to the adult bulge stem cells that are responsible for the maintenance of the hair follicle itself. Ablation of Sox9 left the embryos without bulge cells, and the mice never grew any hair and did not have any sebaceous glands. Moreover, the skin did not repair epidermal wounds well when Sox9 was missing. These results implicate Sox9 in establishment of the hair follicle stem cell population and show that early stem cells can contribute to skin morphogenesis before assuming their role as adult stem cells. -PJH Cell Stem Cell 3, 33 (2008). PHYSICS Every Little Second Counts Part of the appeal of nuclear magnetic resonance phenomena is that the two-level system being manipulated (nuclear spin up, or nuclear spin EDITED BY