摘要:

Wild radish (Raphanus raphanistrum) and ragweed (Ambrosia artemisiifolia), the dominant producers during initial succession in an old field, were labeled with32P in several trophic—transfer studies. Feeding relationships were determined by correlating the population dynamics and32P uptake by consumer species with the phenological changes in producer components and isotope distribution in plant tissues. The relative magnitude of consumer pathways was estimated by using trophic—transfer indices. The first year of old—field succession was characterized by rapid phenological changes in plant—organ availability with related shifts in consumer trophic relationships. The food web of wild radish, the early summer dominant producer, changed rapidly over time resulting in successive pulses of leaf, pollen, and seed feeders. Grasshoppers were the major leaf feeders. Aphids consumed internal juices within stems while flea beetles were concentrated and feeding solely on nutrient—rich seed tissue. Leafhoppers and tarnished plant bugs also consumed internal fluids during wild radish development. The high densities and consumption by plant bugs and aphids, the major consumers of wild radish, resulted in a low food—web diversity for wild radish. Predator—prey relationships during early summer were governed mainly by the aphid infestation. Young ragweed plants and the litter and seed crop of wild radish were major food—web bases in midsummer. Generalized herbivores such as grasshoppers and crickets utilized radish seeds and ragweed tissue during this period. A second phase of specialized feeders entered the old field as ragweed became available. Food—web diversity was highest at this time when many species were present and at low densities. Ragweed was the dominant producer late in the growing season. Some herbivores utilized ragweed leaves while several species of plant bugs became abundant and were consuming the nutrient—rich fluids being channeled into pollen tissue. Plant bugs were the major ragweed herbivores. The importance of these plant bugs resulted in a decline in food—web diversity near the end of the growing season. Most consumers decreased in numbers and32P activity as ragweed died back in September. Ragweed litter, however, supported crickets and other components of the detrital food web over the fall season. Wild radish and ragweed were the major food—web bases during the first year of succession. Over 90% of the arthropods sampled in the old field were food—web components of these producers. Many of these consumers were specialists and sucking forms which peaked in density and32P activity as nutrient—rich fluids were entering their specific food sources. The synchronal occurrence of consumer population peaks and successive nutrient pulses in host—plant organs thus appears to maximize the energy utilization by these consumers while minimizing interspecific competition for sites of trophic transfer. Little foliar damage was evident for wild radish or ragweed because of the importance of herbivores with sucking mouthparts. Tracer studies were thus critical in evaluating the role of both producers as food—web bases. The similarity of results in two replicate studies confirmed the repeatability and accuracy of tracer techniques for trophic—transfer analysis. However, temporal aspects of food—web structure should be considered in future studies where rapid changes in plant phenology are present.

ISSN:0012-9615    

DOI:10.2307/1942156

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY

摘要:

The net primary productivity of the pine species and lesser vegetation (herbaceous ground cover) plus turnover rates between the litter layer and soil organic matter were investigated in young loblolly and slash pine plantations in Beaufort County, North Carolina. Estimations of the aboveground tree biomass and production by components were made by applying multiple regression, weight—prediction equations developed from harvest data (15 slash pine and 41 loblolly pine trees) to tree dimensions from 28 permanent, undisturbed plots. Optimal plot size was determined by time—cost analysis. Total biomass for slash pine ranged from 5.2 mt/ha and 9.9 kg/tree (age 4) to 31.7 mt/ha and 29.8 kg/tree (age 8), and loblolly pine ranged from 4.2 mt/ha and 4.7 kg/tree (age 4) to 107.6 mt/ha and 82.7/tree (age 12). Net production for slash pine was greater than loblolly pine and ranged from 3.9 mt/ha per year and 8.0 kg/tree per year (age 4) to 12.3 mt/ha per year and 11.6 kg/tree per year (age 8). Loblolly pine produced from 4.3 mt/ha per year and 4.8 kg/tree per year (age 4) to 4.1 mt/ha per year and 4.3 mt/ha per year (age 8). Net primary production of loblolly pine was 22.3 mt/ha per year and 18.8 kg/tree per year at age 10 and 11 and dropped to 18.4 mt/ha per year and 13.6 kg/tree per year at age 12. The decrease in production rate of loblolly pine at age 12 was correlated with a decrease in branch needle biomass as canopy closure progressed. Five root systems were excavated, and percentage of aboveground tree biomass was used to calculate root biomass and production. Canopy closure also caused a decrease in net primary production of the lesser vegetation from 1.8 mt/ha per year in the open stands to 0.2 mt/ha per year under closed stands. Simultaneously, the ground—litter component increased from 3.7 to 5.8 mt/ha through litter fall, and the turnover rate to the soil decreased. Soil organic matter accounted for 88% (102.0 mt/ha) to 52% (124.0 mt/ha) of total plot biomass in the youngest and oldest stands, respectively. Caloric analysis was done on all plant material and ground litter. The results are presented as net primary production and turnover rates (109cal/ha per year).

ISSN:0012-9615    

DOI:10.2307/1942157

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY

摘要:

The reproductive cycle, energy budget, and relationships between rates and energetics of vitellogenesis, lipid deposition and depletion, and changes in lean carcass tissue were examined in Desmognathus ochrophaeus salamanders from northeastern Ohio. Oviposition begins prior to emergence in mid—April, reaches a peak in mid—May, and continues in greatly reduced frequency into September. Lipids in the carcass and ovarian fat bodies apparently are used for maintenance and vitellogenesis during winter dormancy (mid—October to mid—April) and after emergence in the spring when food reserves are scarce. Carcass lipids, lean carcass tissue, and perhaps fat body lipids are used for maintenance during brooding. A typical female oviposits in mid—May, replaces depleted lipids and lean carcass tissue by late August, and produces intermediate—sized follicles before winter and full—sized follicles during April and early May. Energy ingested, egested, assimilated, and lost through respiration was examined in gravid females, spent females, and males acclimated to four temperature—photoperiod regimes: (1) 5°C, LD 10:14; (2) 5°C, LD 14:10; (3) 15°C, LD 10:14; and (4) 15°C, LD 14:10. Size, sex, reproductive condition, and temperature significantly influence energetics in D. ochrophaeus. Photoperiod does not affect metabolism significantly at either temperature. There is an inverse exponential relationship between body weight and weight—specific rates of ingestion, egestion, assimilation, and respiration. Gravid females, actively depositing yolk in follicles, show higher O2consumption rates than recently spent females and males at 15°C, but not at 5°C when yolking rates apparently are reduced. Rates of O2consumption are significantly higher at 15°C than 5°C. At 15°C gravid females, spent females, and males lost 43.2, 3.60, and 3.84 cal g—1day—1metabolic heat; at 5°C they lost 1.92, 1.92, and 1.68 cal g—1day—1, respectively. Salamanders cease to feed at 5°C. At 15°C gravid females, spent females, and males ingested 21.0, 37.8, and 23.8 cal g—1day—1and egested 2.7, 5.1, and 2.9 cal g—1day—1, with assimilation efficiencies of 87.2%, 86.3%, and 88.2%, respectively. The estimated annual energetics of a typical female are: ingestion = 2,940 cal; egestion = 378; assimilation = 2,562; respiration = 1,104.4; and secondary production = 1,457.6. Reproductive activities (vitellogenesis = 807.5 and brooding maintenance = 430 cal) cost 48.3% of a female's annual energy flow (A = 2,562 cal).

ISSN:0012-9615    

DOI:10.2307/1942158

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY

摘要:

The ecology and life history of Desmognathus ochrophaeus inhabiting wet rock faces at high elevations near Highlands, North Carolina, were studied. Comparisons were made with rock—face populations in Great Smoky Mountains National Park and with woodland populations at Rabun Bald, Scaly Mountain, and Blue Valley in the Blue Ridge Mountains. The reproductive cycle in rock—face populations is annual; spawning occurs mostly in July and August, and females begin to emerge from brooding as early as late August. The new spermatogenic cycle begins in May and June. Animals on rock faces average 40—45 cm in movements between successive captures and appear to have a limited range. Animals displaced to the base of the rock face and higher on the rock face can return to their home range. Population density is high (up to 25 individuals/m2), diurnal activity is nearly as high as nocturnal activity (correlated with subdued body colors), and at least a moderate level of activity occurs during warm winter periods. Animals mature at 2 years of age as in woodland populations, but at a smaller size, and grow more slowly thereafter. Small body size is not due to paucity of food on the rock faces. The high population densities on the wet rock faces indicate that they are highly favorable habitats for D. ochrophaeus. Differences between rock—face and forest—floor populations are attributable to modified selective pressures in the two different ecological settings. Variation among rock—face populations is attributed to the intensity of selection for small body size and cryptic coloration, the size of the rock face, and variation among forest—floor populations from which they were derived. Since rock—face populations are linked genetically to surrounding forest—floor populations of D. ochrophaeus, they cannot be considered examples of taxonomic variation, but rather of ecotypic variation.

ISSN:0012-9615    

DOI:10.2307/1942159

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY

摘要:

A method for quantitatively evaluating the successional relationships which exist between species was tested on data from Menominee County, Wisconsin. The method is based upon an examination of the changes in interspecific association which occur when successively larger diameter classes of one or both species are used as the basis for the calculation of an association index. With this method it is possible in most cases to determine which species tend to replace which others. In addition, by averaging replacement tendencies in various ways, index values can be arrived at which express the degree to which a particular species may be characterized as pioneer or climax. The results of the analysis agree well with other studies on succession in these forests. This method should prove useful in elucidating successional relationships, especially in areas where these are not well understood.

ISSN:0012-9615    

DOI:10.2307/1942160

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY

摘要:

Collards were grown at Ithaca, New York, in two experimental habitats: pure stands and single rows that were bounded on each side by diverse, meadow vegetation. The arthropods associated with these plants were sampled on 20 dates over a 3—year period. The status of the herbivore species was measured by their rank in biomass in each sample. The two most prominent species, Phyllotreta cruciferae and Pieris rapae, maintained high status throughout the investigation, but another important species, Brevicoryne brassicae, was absent for an entire season. Pit feeders usually formed the most important herbivore guild. Nevertheless, the guild spectrum, which describes the functional structure of the fauna, varied widely in time and space. The size distributions of species and of individuals were both highly skewed toward the smaller sizes. Herbivore loads, the mean biomass of herbivores per 100 g of consumable foliage, were consistently higher in the pure stands. Moreover, herbivore loads varied significantly with season in each experimental habitat. Both the number of herbivore species and the diversity of the herbivore load were greater in the diverse habitat. Biomass was more heavily concentrated among the prominent herbivores in the pure stands; increased dominance, rather than differences in species richness, appeared to be the major cause for the lower herbivore diversity in this habitat. The diversity of predators and parasitoids was higher in the pure stands. Most of the abundant species found on collards shared a similar narrow range of hosts. As a result the species in this core group of herbivores and parasitoids were regularly associated with each other. Predators and the less abundant herbivores tended to be less specialized and served to link the collard association with the surrounding community. Plant—arthropod associations are representative of component communities, well—integrated systems that form portions of larger compound communities. This distinction facilitates the analysis of community structure. Microclimates and the effectiveness of "enemies" did not appear to differ sufficiently in the two experimental habitats to account for the observed differences in the herbivore load. The results suggest a new proposition, the resource concentration hypothesis, which states that herbivores are more likely to find and remain on hosts that are growing in dense or nearly pure stands; that the most specialized species frequently attain higher relative densities in simple environments; and that, as a result, biomass tends to become concentrated in a few species, causing a decrease in the diversity of herbivores in pure stands.

ISSN:0012-9615    

DOI:10.2307/1942161

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY