摘要:

The objective of this study was to evaluate for a 6‐yr period the effects of a twice‐over rotation grazing system (ROT) and a season‐long grazing system (SL), and compare these effects with long‐term grazing exclosures (NG) in terms of (1) species composition and basal cover, (2) aboveground net primary production (ANPP) and aboveground N uptake (ANPP‐N), (3) rates of litter and root decomposition and N release, (4) soil N mineralization and immobilization, (5) aboveground C and N flow, and (6) grazing intensity (GI) and animal performance. The study period included the drought of 1988. No major differences were found in ANPP and ANPP‐N among treatments, but there were important seasonal variations. An average of 72% of ANPP and>82% of ANPP‐N occurred by mid‐June. There were no differences among treatments in terms of decomposition and N release rates from litter and root biomass, or in soil N mineralization. Grazing, however, reduced the amount of C and N immobilized in standing dead and litter and the flow of C and N from standing dead to litter to soil organic matter. The NG and ROT treatment were more similar in this regard when compared to the SL treatment, and their similarities increased after the drought of 1988. There were no consistent differences in GI between the ROT and SL treatments. Before 1988 GI averaged 21% but in 1988 and 1989 GI increased to an average of 49% as a result of the drought and its aftereffects. Cumulative animal performance was similar under both grazing treatments but there were significant seasonal variations. Species composition was more responsive to grazing than were C and N flows. The differences in this case were found between the grazed and NG treatments but not between the two grazing treatments studied. There were no broad patterns of change in total plant basal cover as a result of grazing patterns or drought. Changes in species composition were highly dependent on range site. The most consistent pattern involved Bouteloua gracilis, which had higher relative cover in the grazed treatments than in the NG treatment. Results from this study indicate that in the grasslands of western North Dakota (1) the recommended stocking rate may be too conservative, (2) rotation grazing may allow for higher stocking rates than season‐long grazing without a major impact on animal performance, (3) rainfall is more important than grazing or grazing systems in the control of the ecosystem‐level variables measured, (4) species composition is affected by drought and grazing (but not by grazing systems) but the responses are highly dependent on range site, and (5) drought and grazing tend to increase the relative composition of warm‐season grasses and forbs.

ISSN:1051-0761    

DOI:10.2307/2269567

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

In most terrestrial ecosystems, the amount of substrate entering the soil from plant litter production is only sufficient to meet the maintenance requirements of soil microorganisms, allowing for no net annual growth. However, the rising atmospheric CO2concentration has the potential to alter such a balance by increasing plant litter production, and hence the amount of substrate available for heterotrophic metabolism in soil. In a recent experiment, we observed that greater belowground plant litter production at elevated atmospheric CO2significantly increased the biomass of soil microorganisms in both rhizosphere and non‐rhizosphere soil. Because soil microorganisms differ in their ability to convert substrate into biomass, we hypothesized that greater plant litter production at elevated CO2should shift community composition as fungal populations increase in response to greater substrate availability. We used a molecular technique, phospholipid fatty acid (PLFA) analysis, to gain insight into the composition of soil microbial communities beneath Populus grandidentata growing at ambient and twice‐ambient atmospheric CO2. PLFAs extracted from rhizosphere and non‐rhizosphere soil were derivatized and identified using gas chromatography and mass spectrometry. After one growing season the proportions of bacterial, actinomycetal, and fungal PLFAs were not significantly influenced by elevated atmospheric CO2in either rhizosphere or non‐rhizosphere soil. However, clear differences were present between microbial communities in rhizosphere and non‐rhizosphere soil. Although enhanced belowground plant litter production under elevated atmospheric CO2increased the biomass of soil microorganisms, we have no evidence to suggest that such an increase occurred through a shift in community composition, at least in the short term.

ISSN:1051-0761    

DOI:10.2307/2269568

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Concentrations and fluxes of Cl‐and Na+in bulk precipitation and throughfall in an inland, acidic peatland (Alton Bog, Maine, USA) reveal the influence of canopy structure on the deposition rate of road‐salt and nutrient‐bearing aerosols. Alton Bog borders a four‐lane highway that is salted in winter. In open areas of the peatland steep gradients occurred in both Cl‐and Na+influxes in bulk precipitation and in surface‐water concentrations within ≈ 200 m of the highway. In winter, wooded sites had consistently higher atmospheric influxes of Cl‐(up to 4X) than did treeless sites at the same distance from the highway. These results were consistent with expectations based on studies of dry deposition in nonpeatland areas. The increased influx of Cl‐at wooded sites implies a significantly higher total influx of nutrients in peatlands where trees occur (especially unevenly spaced trees of mixed heights) than in peatlands with no trees. The effect is likely to be especially important in peatlands that receive all or most incoming nutrients from the atmosphere

ISSN:1051-0761    

DOI:10.2307/2269569

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

The leaves of plants in nature are commonly subjected to damage from a wide variety of agents, including herbivory, air pollutants, and simple physical damage. Despite the attention paid to damage effects on living plants, the potential effects on the quality of litter derived from damaged leaves has not been considered. We used controlled laboratory assays of decomposition to show that both ozone (0.2 mL/m3, 4 h) and mite damage, but not ultraviolet radiation (UV‐B) exposure, to living leaves of cottonwood plants resulted in a decrease in decomposition rate of litter derived from damaged leaves. Decomposition rates were ≈ 50% slower for litter from damaged plants, and there was a twofold increase in the refractory fraction. Contrary to expectation, there was a negative relationship between rate of decomposition and litter nitrogen content. Our finding of slow decomposition of high‐nitrogen litter is explained by a general mechanism whereby cellular damage causes increases in complex phenolic material. Such materials can lead to reductions in decomposition and binding of available nitrogen. We suggest that this mechanism can translate a common occurrence, damage by a diversity of processes, into long‐term and possibly large‐scale alterations in detritus processing.

ISSN:1051-0761    

DOI:10.2307/2269570

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

The armyworm (Pseudaletia unipuncta) was used as a model host insect to explore the influence of agricultural landscape structure at two spatial scales on larval parasitoid species richness and rates of larval parasitism in southern Michigan. First, within fields, we compared parasitoid communities in maize fields near, and distant from, a hedgerow edge. Second, we replicated these studies within a complex landscape (agricultural fields of small size embedded in a landscape with abundant hedgerows and woodlots) vs. a simple landscape (agricultural fields of large size embedded in a landscape with few hedgerows and woodlots). The structural differences between the simple and complex agricultural landscapes were characterized by analysis of aerial photographs and digital land‐use data. After landscape analysis, three maize fields from each area were selected for the experimental studies. The complex landscape contained fields that were 75% smaller, had 63% more perimeter of wooded field edge per hectare of field area, and had 81% more field edge in wide hedgerow than fields in the simple landscape. Fields in the simple landscape had 74% and 53% more field edge in herbaceous roadside and crop‐to‐crop interfaces, respectively, than did fields in the complex landscape. In the six selected maize fields, third and fifth instar P. unipuncta were released individually onto maize plants 5 m and 90 m from a hedgerow edge. Larvae were recovered after 4‐5 d and reared in the laboratory to record parasitoid emergence. Parasitoid species diversity was similar in both landscape types (simple landscape: four species; complex landscape: five species). Mean percentage parasitism was significantly higher in the complex landscape than in the simple landscape (13.1% vs. 2.4%) but was not affected by the location within fields (near hedgerows vs. distant from hedgerows) in either landscape type.

ISSN:1051-0761    

DOI:10.2307/2269571

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

We used mark‐recapture data to measure and model movement by the adult stage of the cinnabar moth Tyria jacobaeae (Lepidoptera, Arctiidae). Our objectives were: (1) to standardize the description of cinnabar moth movement and thereby provide a more reliable basis for comparison and extrapolation, and (2) to screen the potential influence of gender, physical condition, time, spatial location, and spatial heterogeneity in the environment on movement parameters. Recapture rates depended on wing wear (low 16%, high 11%) but were independent of behavior immediately after release (mobile 15%, immobile 14%), gender (male 13%, female 16%), or location in the study area (central 15%, peripheral 15%). Thus, mortality and/or emigration was greater for individuals with high wing wear compared to those with low wing wear. Moths were aggregated at the southwest margin of the study area both at initial capture and recapture. The direction of movement was biased in north and south directions, perhaps because forests on the east and west boundaries created a corridor channeling movement. The movement observed within a single generation did not conform to simple diffusion, which predicts that the mean square displacement (MSD) increases linearly with time. Instead, observed movement rates varied over time, and the relationship between MSD and time varied by gender, smoothly decelerating in males and erratically increasing in females. Observed movement (represented by the frequency distribution of displacementsxfor each timetwith time measured at a daily resolution) was adequately approximated by a Weibull distribution, for which estimates of the shape and scale parameters were calculated. Male distributions were more frequently mounded, associated with higher values of the shape parameter α, and the scale of male distributions shifted sooner to longer distances, associated with higher values of the scale parameter β. The Weibull parameters varied by sex and time, suggesting that males were initially more mobile than females, but the magnitude of the difference varied over time. The speed and direction of individual movement were independent of the moth's spatial location within the field. We conclude that (1) movement speed depended on gender and time, while movement direction was related to spatial heterogeneity in the environment, and (2) much of the disagreement about movement rates among studies of the cinnabar moth can be related to researchers taking measurements and drawing inferences on different scales of observation, and therefore arriving at conflicting results.

ISSN:1051-0761    

DOI:10.2307/2269572

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

The red sea urchin, Strongylocentrotus franciscanus, is a conspicuous member of subtidal communities in the north Pacific. Within the last decade, this ecologically important species has been exposed to intense harvesting for the first time ever. Analysis of population census data suggest that harvestable size urchins have rapidly declined in shallow regions while catch‐per‐effort and landings data suggest that divers have maintained high landings by exploiting more distant and difficult fishing areas, including deeper areas. We present a size‐structured model for the red sea urchin both to estimate what previous levels of harvesting mortality were and to explore what effect future harvesting strategies might have on population trajectories. Using population census data, we explore three models: one that would result in an equilibrial population size in the absence of fishing, one that includes positive density dependence (an Allee effect), and one that incorporates realistic variability in recruitment. Our principal findings are that annual fishing mortality levels that best fit the observed census data in the past (1984‐1993) were 0.38‐0.49 and represent a 70‐90% decrease in the survivorship of harvestable urchins in the years of fishing. Under a variety of fishing strategies, 100‐yr projections indicated that the inclusion of an Allee effect or variability in recruitment could drive the harvestable population to<50% of present estimated population size at relatively low annual fishing mortality values. Our simulations also indicated that, although a yearly fishery would have higher yields, a rotational fishery would maintain populations at a level less likely to cause irreversible decline. The wealth of empirical evidence that sea urchins are an important component of nearshore communities should encourage management strategies that emphasize long‐term population viability.

ISSN:1051-0761    

DOI:10.2307/2269573

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

To produce reliable estimates of the size or vital rates of a given population, it is important that the boundaries of the population under study are clearly defined. This is particularly critical for large, migratory animals where levels of sustainable harvest are based on these estimates, and where small errors may have serious long‐term consequences for the population. Once populations are delineated, rates of exchange between adjacent populations can be determined and accounted/corrected for when calculating abundance (e.g., based on mark‐recapture data). Using satellite radio‐collar locations for polar bears in the western Canadian Arctic, we illustrate one approach to delineating wildlife populations that integrates cluster analysis methods for determining group membership with home range plotting procedures to define spatial utilization. This approach is flexible with respect to the specific procedures used and provides an objective and quantitative basis for defining population boundaries.

ISSN:1051-0761    

DOI:10.2307/2269574

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

The stable (Martes zibellina), a little‐known forest carnivore of the Asian taiga, has undergone reductions in distribution and abundance that have been attributed, in part, to habitat changes resulting from forest management. Like its circumboreal sibling species, the boreal forest martens (M. martes, M. melampus, and M. americana), the sable has been hypothesized to select for forest attributes associated with late successional stages, and to respond negatively to reductions in the amount of physical structure near the ground, especially the loss of large tree boles. The sable now is endangered in China. To investigate how forestry practices in northern China, which emphasize extensive selective cutting of large‐diameter larch (Larix gmelini), affect behavioral preferences of sables, and to test for hypothesized ecological similarities among the boreal forest martens, we studied habitat selection by sables in the Daxinganling Mountains, Heilongjiang, People's Republic of China, for three winters. We sampled topographic and vegetative features of three study sites by systematically located plots. We studied sable selection of habitats while traveling by following sable trails in snow, and while resting by locating sables with telemetry. Sables selected for (preferred) sites with dense tree canopy, mixed tree species dominants, and high densities and diameters of larch tree boles. Sables were more selective of habitat for resting than for traveling, like other martens (M. americana). Sables were indifferent to attributes of hardwood trees (Betula sp.) in their habitat, but preferred mixed tree species stands to single species stands. The sable responds negatively to the absence of tree canopy, of large live trees, and of coarse woody debris, and resembles other boreal forest martens in its habitat associations. The sable may be a useful model for examining the responses of vertebrates to forestry practices in boreal east Asia. Conserving late successional stands of conifer forest appears crucial to the persistence of the sable in China.

ISSN:1051-0761    

DOI:10.2307/2269575

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Merriam's Turkeys (Meleagris gallopavo merriami) are associated with ponderosa pine (Pinus ponderosa) forests in the western United States, but are not native to the ponderosa pine forest of the Black Hills, South Dakota. The Black Hills population was established by transplanting birds from New Mexico and Colorado between 1948 and 1951. Despite being outside its original range, this population provides a unique opportunity to assess mechanisms of habitat selection because the age of the population is known and literature indicates that it is more productive than other populations. We studied microhabitats of Merriam's Turkeys in the Black Hills, South Dakota between 1986 and 1991. We found few differences in microhabitats among diurnal time periods or between sexes. Cluster analysis of variables at turkey microhabitats indicated two groups, broadly interpreted as summer and winter microhabitats. Winter microhabitats of turkeys had less understory vegetation and more overstory cover than random sites, which in turn had less understory and more overstory cover than summer microhabitats. Both random sites and winter microhabitats had higher basal area of ponderosa pine than summer microhabitats. Summer microhabitats had trees with the largest dbh. Random sites had more small and large woody debris than sites used by turkeys. Tree density at random sites was more than two times greater than at winter microhabitats and more than three times greater than at summer microhabitats. Turkeys preferred southern exposures during winter. Production of pine seed, a major food item of turkeys, differed among years. There was a strong relationship between abundance of pine seeds and microhabitats selected by turkeys. Basal area of microhabitats between October and March was positively correlated with annual ponderosa pine seed production. Abundance of ponderosa pine seeds at turkey microhabitats during this period was at least four times the estimated average annual production. Management prescriptions for ponderosa pine of basal area ≤ 18 m2/ha will reduce winter habitat for turkeys. Summer habitats are more compatible with timber management goals for ponderosa pine in the Black Hills.

ISSN:1051-0761    

DOI:10.2307/2269576

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY