摘要:

Changes in land use over the past two centuries have caused a significant release of CO2to the atmosphere from the terrestrial biota and soils. An analysis of this release is based on amounts of organic carbon within an ecosystem following changes such as harvest of forests; it is also based on rates of changes, such as conversion of forest to agriculture, deduced from agricultural and forestry statistics. A model is used to calculate the net amount of carbon stored or released each year by the biota and soils of 69 regional ecosystems. Some of the changes, such as afforestation, the growth of harvested forests, and buildup of soil organic matter, result in a storage of carbon; others, such as harvest of forests and increase in pasture and agricultural areas, result in a loss of carbon to the atmosphere. According to this analysis, there has been a net release of carbon from terrestrial ecosystems worldwide since at least 1860. Until °1960, the annual release was greater than release of carbon from fossil fuels. The total net release of carbon from terrestrial ecosystems since 1860 is estimated to have been 180 x 1015g (a range of estimates is 135—228 x 1015g). The estimated net release of carbon in 1980 was 1.8—4.7 x 1015g; for the 22 yr since 1958 the release of C was 38—76 x 1015g. The ranges reflect the differences among various estimates of forest biomass, soil carbon, and agricultural clearing. Improvements in the data on the clearing of tropical forests alone would reduce the range of estimates for 1980 by almost 60%. Estimates of the other major terms in the global carbon budget, the atmospheric increase in CO2, the fossil fuel release of CO2, and the oceanic uptake of CO2, are all subject to uncertainties. The combined errors in these estimates are large enough that the global carbon budget appears balanced if the low estimate for the biotic release of carbon given above is used (1.8 x 1015g released in 1980) with the higher estimates of oceanic uptake. If higher estimates for biotic release are used, then the carbon budget does not balance, and the estimates of oceanic uptake or of other factors require revision.

ISSN:0012-9615    

DOI:10.2307/1942531

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Habitat use characteristics of all bird species occurring in the lower Colorado River valley were studied. Seventy—two transects, each through homogeneous stands of vegetation and together representing all riparian habitat types occurring in the area, were censused three times monthly for 2 yr (1977, 1978) to provide distributional records for all species. From the census data we divided the transects into three groups for each bird species each season: transects where the species was recorded in both 1977 and 1978, transects where the species was absent both years, and transects where the species occurred in only one of the years. We used discriminant analyses to classify the habitat on each transect as suitable or unsuitable for each species, using a set of eight species composition measures, four foliage density measures, and foliage height diversity (FHD). Mean habitat discriminability of all species varied from a low of 86% in summer to a high of 91% in late summer. Most species showed extensive variability among seasons in both the degree of habitat selectivity and the criteria used in habitat selection. This variability reflected both changes in population structure of the species and responses to changing resource and climatic pressures. Ecological attributes of the species were examined to determine which factors shaped the habitat use patterns of each. Feeding guild played only a small role in both the degree and criteria of habitat selectivity, except for highly specialized feeders such as nectivores and frugivores. Species with restricted distributions were usually habitat specialists, but species with widespread distributions included both habitat generalists and specialists. During spring and summer nonresident, short—stay species showed greater habitat selectivity than did extended—stay species, but during nonbreeding seasons both short—stay and extended—stay species showed equivalent habitat selectivity. That both the mix of habitat generalists and specialists and the habitat selectivity of individual species varied seasonally implies that the organization of habitat use of the community is the dynamic product of many interacting factors. The importance of individual habitat attributes was also examined. Species showed preferences for particular habitat attributes significantly more frequently than they avoided habitat attributes. FHD was the most frequently important variable in differentiating areas used from areas not used. Proportions of honey mesquite, cottonwood, and willow in the vegetation were also frequently important variables in habitat selection by avian species, but usually in conjunction with FHD. Foliage density characteristics were less frequently significant, but for species where these measures were important, they tended to be the primary habitat selection criterion. In summer and late summer, habitat selection criteria of individual species seemed independent of the selection criteria of the other species in the community. In other seasons there was evidence of species interactions in these habitat selection patterns. These empirical findings for an entire avian community over all seasons are an advance in our understanding of how habitat use patterns of communities are organized. The extensive seasonal variability in habitat use by avian species and variability of distributions of individual species between years imply that studies of community organization can best be done with large—scale, long—term studies.

ISSN:0012-9615    

DOI:10.2307/1942532

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Canopy interception data from 105 grassland stands in the Serengeti National Park in Tanzania and the Masai Mara Game Reserve in Kenya were used to evaluate community organization within an ecosystem defined by the annual movements of large herds of nomadic ungulates. Other studies tested hypotheses about factors influencing species composition and diversity of the communities. The herbaceous vegetation was principally C4grasses, which commonly contributed over 90% of total abundance. Ninety—nine stands were classified into 17 communities, using a numerical clustering method. Sixteen communities were principally perennial grasses; one was dominated by an annual species. Short grasslands predominated below 700 mm mean annual rainfall and in heavily grazed sites on hilltops at higher—rainfall locations. Much of the savanna understory at intermediate rainfall levels was medium—height Themeda triandra grassland; six variants were recognized by clustering. Tall grasslands dominated by Hyparrhenia filipendula occurred as a savanna understory in locations with>900 mm of annual rainfall. Two ordination axes revealed a shortgrass continuum that graded into a medium—to tallgrass continuum, and an outlying floodplain tall grassland dominated by Echinochloa haploclada. The latter and the short grasslands dominated by Andropogon greenwayi were very different in species composition from the other communities. The first ordination axis was related to grazing intensity, and the second to soil texture. Spatial heterogeneity was an important attribute of the grasslands from the level of local stands to the level of landscape regions. Spatial pattern, rather than point diversity, was the major vegetation property correlated with α—diversity. Higher α—diversity was accompanied by low dominance concentration and more clumped species distributions. Individuals of rare species were more aggregated than members of common species. β—diversity also was a significant feature of the vegetation, leading to pronounced γ—diversity and a variety of different grassland types in local areas. Coenoclines were evident only in more arid locations. γ—diversity was associated with topographic and geological heterogeneity. Animals, soils, the overlying tree canopy in savannas, and fire influence α—diversity through their contribution to creating spatial pattern. Animals have particularly complicated effects, in addition to defoliating areas. Grazing is a major environmental factor affecting the grasslands, but it is a composite factor interacting in complex ways with other environmental factors.

ISSN:0012-9615    

DOI:10.2307/1942533

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

This study uses descriptive data and field experiments to examine a seasonal succession between two planktonic grazers (Daphnia rosea and Daphnia pulicaria) in a small Vermont lake. Each year Daphnia populations hatched from resting eggs during May and both species produced numerous parthenogenetic generations before the appearance of males and ephippia in late September. During the 2 yr of study, D. rosea had an apparent advantage in spring and early summer, but was rapidly replaced by D. pulicaria during late summer and autumn. These shifts in the species balance were associated with reversals in reproductive success. Moreover, the species with the higher birth rate also tended to have a lower mortality rate. An analysis of population structure, reproduction, and mortality suggests that inter— and intraspecific competition, not fish predation, limited population growth. Field experiments in both small (3.8—L) and large (12 000—L) enclosures demonstrate that the demise of D. rosea during late summer was a direct consequence of competition with D. pulicaria. However, the outcome of field experiments was sensitive to seasonally changing conditions. During spring and early summer, when D. rosea had a reproductive advantage at low densities in the lake, it was also the stronger competitor at high densities within experimental enclosures. The shift in species advantage during summer seems associated with a changing resource base. In spring the food for grazers was composed mostly of highly edible flagellates, while "resistant" algae (mostly gelatinous greens) were predominant during late summer and autumn. Grazing experiments and observations on gut contents suggest how D. pulicaria was favored by this change in food conditions. Grazing experiments with a radioactively labeled flagellate (Chlamydomonas) show that the two Daphnia species had very similar feeding rates on this alga. In addition, direct observations on the composition of gut contents demonstrate that they ingested virtually identical proportions of common phytoplankton species. However, experiments with labeled natural algae and observations on the quantity of ingested algae indicate that D. pulicaria was more effective in utilizing "resistant"algae.

ISSN:0012-9615    

DOI:10.2307/1942534

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY