Increasing demands on arid and semiarid ecosystems, which comprise onethird of Earth's terrestrial environment, create an urgent need to understand their biodiversity, function, and mechanisms of change. Sagebrush (Artemisia) steppe, the largest semiarid vegetation type in North America, is endangered because of losses to agriculture, excessive grazing, and invasive species. Establishment in 1950 of what is now designated as the Idaho National Engineering and Environmental Laboratory (southeastern Idaho, USA) created the largest existing reserve of this extensive vegetation type. We used cover, density, and frequency data for vascular plants sampled on 79 permanent plots nine times during 45 years to (1) assess long-term changes in abundance and distribution of major species and life forms, (2) assess changes in species richness and plot similarity, and (3) test the hypotheses that plant cover and stability of cover are positively associated with species richness and that invasibility is inversely related to native plant cover and richness.From 1933 through 1957 the area was subject to severe drought, with annual precipitation exceeding the long-term mean only four times. Cover of shrubs plus perennial grasses was 18% in 1950, and the vegetation was heavily dominated by sagebrush. Perennial grass cover was only 0.5%. With elevated precipitation after 1957, shrub cover increased to 25% by 1965, and by 1975 cover of perennial grasses had increased 13-fold. Subsequent fluctuations in cover did not track precipitation closely. Cover and density of major species were often out of phase, and correlation analyses indicated lags of 2-5 yr in responses of species or functional groups to precipitation. Aggregate species richness of the area has not changed appreciably, but richness of shrubs, perennial grasses, and forbs per plot steadily increased from 1950 to 1995. Vegetative heterogeneity also increased, with mean similarity among plots declining from 72% to 40%. Plots having higher species richness tended to maintain higher levels of cover and to vary less in cover relative to their mean level, indicating links between species richness and function. Abundance of nonnative species was negatively correlated with cover, but not with richness of native species. Thus, adequate cover of native species can render these semiarid communities more resistant to invasion. Maintaining richness and cover of native species should be a high management priority for these ecosystems.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Ecology. SUMMARY(1) Cover is among the most widely used measures of abundance of plant species because it is not biased by the size or distribution of individuals. This study compared cover estimates obtained by line interception, point interception, and cover-class estimation from 136 sample lines located systematically in sagebrush steppe in western U.S.A.(2) Line interception estimates of shrub cover were significantly higher than those obtained by point interception; estimates of 'bare ground and litter' by point interception were higher than those of line interception by the same amount. These offsetting differences resulted from the assumption, used in line interception sampling, that shrubs completely cover the areas within the outlines of their canopies. Otherwise, point interception and line interception estimates were very similar.(3) Cover-class estimation provided reliable estimates only for the dominant shrub species. Typically, estimates for grasses and other species with small or rare individuals were high in comparison with the other techniques. This is a consequence of the assumption that cover values are uniformly distributed about the mid-points of the cover classes.(4) Point interception achieved about the same degree of precision as line interception in one-third less sampling time. Point interception is the most efficient of the three methods where estimates for most of the species in a community are needed.(5) Optimal combinations of numbers of lines and numbers of sample units per line in relation to the time involved for a particular level of precision were investigated. In general, increased precision was achieved by sampling more lines rather than more sample units per line. * Present address: 34 Pinewood, Topsham, Maine 04086, U.S.A.

Abstract.Nutrient dynamics of large grassland ecosystems possessing abundant migratory grazers are poorly understood. We examined N cycling on the northern winter range of Yellowstone National Park, home for large herds of free-roaming elk (Cervus elaphus) and bison (Bison bison). Plant and soil N, net N mineralization, and the deposition of ungulate fecal-N were measured at five sites, a ridgetop, mid-slope bench, steep slope, valley-bottom bench, and riparian area, within a watershed from May, 1991 to April, 1992.Results indicated similarities between biogeochemicalproperties of Yellowstone grassland and other grassland ecosystems: (1) landscape position and soil water affected nutrient dynamics, (2) annual mineralization was positively related to soil N content, and (3) the proportion of soil N mineralized during the year was negatively related to soil C/N.Grazers were a particularly important component of the N budget of this grassland. Estimated rates of N flow from ungulates to the soil ranged from 8.1 to 45.6 kg/ha/yr at the sites (average = 27.0 kg/ha/yr), approximately 4.5 times the amount of N in senescent plants. Rates of nitrogen mineralization for Yellowstone northern range grassland were higher than those measured in other temperate grassland ecosystems,possibly due to grazers promoting N cycling in Yellowstone.

Increasing demands on arid and semiarid ecosystems, which comprise one‐third of Earth's terrestrial environment, create an urgent need to understand their biodiversity, function, and mechanisms of change. Sagebrush (Artemisia) steppe, the largest semiarid vegetation type in North America, is endangered because of losses to agriculture, excessive grazing, and invasive species. Establishment in 1950 of what is now designated as the Idaho National Engineering and Environmental Laboratory (southeastern Idaho, USA) created the largest existing reserve of this extensive vegetation type. We used cover, density, and frequency data for vascular plants sampled on 79 permanent plots nine times during 45 years to (1) assess long‐term changes in abundance and distribution of major species and life forms, (2) assess changes in species richness and plot similarity, and (3) test the hypotheses that plant cover and stability of cover are positively associated with species richness and that invasibility is inversely related to native plant cover and richness. From 1933 through 1957 the area was subject to severe drought, with annual precipitation exceeding the long‐term mean only four times. Cover of shrubs plus perennial grasses was 18% in 1950, and the vegetation was heavily dominated by sagebrush. Perennial grass cover was only 0.5%. With elevated precipitation after 1957, shrub cover increased to 25% by 1965, and by 1975 cover of perennial grasses had increased 13‐fold. Subsequent fluctuations in cover did not track precipitation closely. Cover and density of major species were often out of phase, and correlation analyses indicated lags of 2–5 yr in responses of species or functional groups to precipitation. Aggregate species richness of the area has not changed appreciably, but richness of shrubs, perennial grasses, and forbs per plot steadily increased from 1950 to 1995. Vegetative heterogeneity also increased, with mean similarity among plots declining from 72% to 40%. Plots having higher species richness tended to maintain higher levels of cover and to vary less in cover relative to their mean level, indicating links between species richness and function. Abundance of nonnative species was negatively correlated with cover, but not with richness of native species. Thus, adequate cover of native species can render these semiarid communities more resistant to invasion. Maintaining richness and cover of native species should be a high management priority for these ecosystems.

Naturalness is a scientific concept that can be evaluated and quantified Intactness or integrity of ecosystems can be defined and assessed in similar ways. Three indices of naturalness are proposed: ( I ) the degree to which the system would change if humans were removed; (2) the amount of cultural energy required to maintain the functioning of the system as it currently exists; and (3) the complement of native species currently in an area compared with the suite of species in the area prior to settlement. These indices are complementa y and provide a conceptual framework for evaluating naturalness. The latter two can be quantifed