ISSN:0012-9658    

DOI:10.2307/1936427

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

A model of radiation, air temperature, and vapor density in the vegetation canopy, plant water relations, and photosynthesis, developed in the primary production research program of the U.S. Tundra Biome, IBP, is described and results for the years 1970 through 1973 presented. The model calculates daily courses of direct and diffuse solar radiation; infra—red radiation; wind; air temperature and humidity; leaf temperatures; convectional and transpirational exchange by leaves, stems, and dead material; leaf H_2O content, leaf water potential; leaf resistance to H_2O loss; internal resistance to CO_2 diffusion; and net photosynthesis. Climate varied from year to year: 1970 and 1971 were similar, 1972 was relatively warm and dry, and 1973 was relatively cold and wet. Plant parameters were obtained for Carex aquatilis, Dupontia fischeri, Eriophorum angustifollium, and Salix pulchra. Leaf area indices varied by species and by year. Leaf areas of all species were lowest in 1973. Of the incoming solar radiation ° 20% is reflected back and 32%—53% absorbed by the canopy, increasing with leaf area index. In the canopy 5—10 times more heat is lost by convection than by evaporation. Total seasonal vascular plant CO_2 uptake ranged from 400—627 g CO_2 m_G(—2) (subscript G refers to ground). In 1971 cuvette estimates for total seasonal vascular plant CO2uptake were 602 while the model predicted 627. Individual species incorporated about 4 g CO_2 m_G(—2) day(—1) at most. The daily CO_2 incorporation was larger than the downward CO_2 flux from the atmosphere, implying that soil respiration is a source of CO_2 for the vascular plants. Photosynthesis increased with solar radiation, air and ground temperatures, and air vapor density and decreased with increasing infrared radiation for the sky and root resistance to water uptake. Factors increasing transpiration without directly affecting photosynthesis tend to decrease photosynthesis because of the effect on water stress. Net photosynthesis is higher at the top of the canopy, in spite of more favorable temperatures within the canopy, because of light limitation. Total accumulated carbon dioxide, after subtracting growth costs of leaves, becomes positive late in the season and is highest at the lower levels because of the longer duration of leaf area these levels. Photosynthesis seems adjusted to maximize carbon gain under the most frequent conditions but not under all conditions or extreme conditions.

ISSN:0012-9658    

DOI:10.2307/1936428

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

This paper quantifies changes in the importance of ecological parameters associated with the development of intensive agriculture in midwestern USA. Changes in the important of 20 edaphic and 31 cover—type parameters were measured for three stages of development: (1) conversion of natural vegetation to pre—intensive agriculture (1833—1934), (2) agricultural intensification associated with chemical subsidies and mechanization (1934—1948), and (3) recent shifts in crop diversity and production (1934—1972). Between 1833 and 1934 all the natural upland vegetation, and two—thirds of the original marshlands were transformed. Of the original prairie surface, 92% was converted to a cropland—temporary pasture system by 1934. Much less of the original savanna was converted to cropland. By 1934 the alteration of savanna to cropland, permanent pasture, and oak forest was in the ratio of 4:3:2. A principal components analysis of the factors significant in the alternation of the natural vegetation indicated strong interactions with soil attributes of the first component. Prairie and cropland were bimodally distributed on the catenal gradient. The distribution of savanna and forest vegetation was related primarily to steep, shallow soil types. Intensification of agricultural production was accompanied by a reduction in the total cropland acreage, and forest cover increased by 10.6% in the 1934—1961 period. With agricultural intensification there was an increase in crop specialization. In fact, crop diversity declined by 35% between 1950 and 1965; since 1965 the tendency toward monoculture increased only slightly. Furthermore, the trend toward monoculture coincided with a shift toward large—stature plant forms. The models of canonical correlation indicated that with agricultural intensification the moisture—related variables became more important while those related to fertility and catena did not. The dependence of intensive agriculture on favorable sites and large energy inputs is discussed from the viewpoint of growing problems of energy supply and population.

ISSN:0012-9658    

DOI:10.2307/1936429

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

Two lizard species, the northern prairie lizard (Sceloporus undulatus garmani) from Reno County, Kansas and the northern sagebrush lizard (Sceloporus graciosus graciosus) from Washington County, Utah, were used to test four hypotheses and one assumption related to the theory of r— and K—selection. The northern prairie lizard is short—lived, matures early, and has a high reproductive effort (r—strategist) while the northern sagebrush lizard is long—lived, has delayed maturity, and a low reproductive effort (K—strategist). One of the assumptions of the r— and K—selection theory is that competition for food is more intense for K—strategists than for r—strategists. Given this assumption, greater food availability for the prairie lizard was hypothesized to result (1) a higher level of body lipids, (2) a higher rate of lipid utilization, (3) a lower percentage of ingested energy available for metabolism, and (4) and expenditure of less energy per offspring than in the sagebrush lizard. Total lipid levels in the two species collected before and after hibernation indicated that prairie lizards had significantly higher lipid levels than sagebrush lizards during both collection periods. A comparison of the amount of body lipids lost and the amount of egg lipids gained during vitellogensis of the first clutch suggested that more lipids are being utilized for egg production in prairie lizards than in sagebrush lizards. Sagebrush lizards apparently are better adapted physiologically to lower food levels since they had lower rates of lipid utilization during starvation studies and they extracted more energy usable for metabolism from ingested food than prairie lizards. Although both species expended about the same amount of energy on a given clutch of eggs, prairie lizards produced more offspring per clutch and therefore expended less energy per offspring. Prairie lizards produced as many as three clutches per season as compared to two clutches per season for sagebrush lizards, and changes in length—lean weight relationships for the two species over a given season suggested that the additional clutch of eggs produced by prairie lizards may be contributing to the higher mortality observed in this species. Indirect evidence, based on precipitation levels and insect biomass at the two study sites and mouthgapes of the two species, supported the assumption that more food was available to the prairie lizards than the sagebrush lizard. Higher precipitation levels (indicating greater insect biomass) occurred at the prairie lizard collection site and this species had a smaller mouthgape index, indicating greater specification in utilization of prey sites.

ISSN:0012-9658    

DOI:10.2307/1936430

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

Two species of salamander, Plethodon cinereus, and Plethodon hoffmani were used in experiments designed to test the hypothesis that food is the object of competition. Plethodon hoffmani occurs in the Appalachian Ridge and Valley Province where it appears to exclude the closely related and contiguously allopatric P. cinereus. Detailed field observations and a laboratory experiment confirm Jaeger's (1972) hypothesis that the food of salamanders could be limited in availability during dry weather and suggest that the cause of the low availability is the reduced mobility of the salamanders rather than a reduced food supply. However, because stomach content data and behavioral observations indicate that salamanders do not forage when the surface is dry, it was reasoned that food competition could occur under only two restricted conditions: (1) when more than one animal is isolated under damp refugia, such as rocks and logs, during dry weather, and (2) when many animals emerge onto the surface to forage when wet weather follows a prolonged dry period. Empirical evidence suggests that salamanders do not clump in refugia as the habitat dries out and that they do not surface simultaneously to forage after a dry period. Thus, even though food is periodically unavailable, the staggered feeding schedule prevents the density of salamanders from ever reaching competitive levels. Interference mechanisms are not ruled out as mechanisms of the observed spacing patterns. Competition for space is suggested as an alternative to the food competition hypothesis.

ISSN:0012-9658    

DOI:10.2307/1936431

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

Invasion of western juniper into vegetation dominated by mountain big sagebrush and perennial bunchgrass on the Owyhee Plateau of southwest Idaho appears to be directly related to cessation of periodic fires. Evidence from adjacent climax juniper stands indicates that fires were frequent for at least several hundred years preceding white settlement. Fires have been much less frequent during the past century due to active fire control, development of roads and other fire barriers, and reduced fuel because of heavy grazing and a shift towards decreased precipitation. Physical and biotic factors affecting the establishment of juniper, seed dispersal mechanisms, and the fire history of the study area were investigated. Results indicated that range condition as such had a negligible effect on juniper establishment. Juniper seedlings became established most readily on areas supporting well—developed herbaceous and shrubby vegetation. Seed dispersal was primarily localized, and accomplished by gravity and disturbance by animal trampling. Abundant evidence of fire in the form of charred stumps and fire scars on living trees was found throughout the study area. Old juniper stands are confined to rocky ridges where understory vegetation is sparse and fires less intense. Juniper was apparently kept out of the denser vegetation of deeper soils by more intense fires. Most herbaceous and shrubby species survived this treatment due to greater tolerance to fire, or rapid reproduction from seed.

ISSN:0012-9658    

DOI:10.2307/1936432

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

The activity patterns, body temperatures, microhabitat characteristics and feeding behavior of larvae of two sphinx moths which inhabit the Mojave desert were investigated. Hyles lineata occurred in association with and fed upon a variety of desert annual plants; Manduca sexta was associated with only one plant species, Datura metalloides, a perennial. Body temperatures of H. lineata were relatively constant over a wide range of air temperatures; body temperatures of M. sexta approximated the air temperature for most of the day. Elevated T_b was associated with exposure of the larvae to direct solar radiation and high ground temperatures. Hyles lineata regulated T_b by orienting with respect to solar radiation and exploiting thermal heterogeneity in its microhabitat. Manduca sexta was shielded from solar radiation and thermal reradiation by its foodplant and never voluntarily spent time on the ground. Feeding and locomotor activity in both species were reduced during periods of both high and low air temperatures but were not related to time of day. Upper lethal temperature in saturated air was 45 degrees C for both species. Feeding rates were strongly dependent on body temperature in both species. Hyles lineata is a generalist, utilizing a variety of energy resources; the occurrence of M. sexta in the desert may depend on the presence of a single plant species, the jimson weed.

ISSN:0012-9658    

DOI:10.2307/1936433

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

Permanent 20—yr—old quadrats located in five plant communities in the University of Florida's Welaka Conservation Reserve in Putnam County, Florida, USA were inventoried to determine successional changes. The five communities included mesic hammock, xeric hammock, bluejack oak sandhill, turkey oak sandhill, and scrub. The reserve has been protected from fire and no fires had occurred on the study area for at least 40 yr. The quadrats were analyzed in terms of changes in species composition, number of stems, and growth. Increased similarity in species composition to the mesic hammock was found in the xeric hammock, bluejack oak sandhill, and turkey oak sandhill. Tree species composition changed most in the turkey oak sandhill, which gained six tree species and lost none, followed by the xeric hammock, the bluejack oak sandhill, the mesic hammock, and the scrub which exhibited no species change. A successional scheme for the xeric plant communities of north—central Florida was constructed. Rates of succession were also determined. It is proposed that the bluejack oak sandhill, turkey oak sandhill, and xeric hammock communities are changing toward a form of mesic hammock. Although the scrub was not changing in species composition, the individual trees were growing in number and size and the community may possibly become a xeric hammock. The successional scheme was derived for a group of ecosystems under man—induced fire suppression and thus is not necessarily true for undisturbed conditions in which natural fires play an important role. As the amount of land uninfluenced by man's management practices decreases, successional patterns as observed in this study will become the predominant forms.

ISSN:0012-9658    

DOI:10.2307/1936434

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

Between latitudes 9 degrees and 59 degrees in North America, the House Sparrow, Passer domesticus, shows progressive increases northward in energy expenditures for temperature regulation, existence metabolism, reproduction, daily energy budget, permanent residency, and mobilizable energy. Energy requirements are greater in the winter than in the summer for temperature regulation, existence metabolism, daily energy budget, and mobilizable energy. Seasonal acclimatization to winter temperatures brings lower rates at equivalent temperatures in existence metabolism. However, latitudinal adaptation at colder northern latitudes brings higher rates at equivalent temperatures in existence metabolism. The number of broods produced per year is inversely proportional to the energy cost per brood, and the energy expended by the species for reproduction is approximately the same at all localities. The reduction in work capacity at high ambient temperatures increases southward, but is greater in desert climates with large daily and seasonal fluctuations in temperature than in the tropics with their uniform climates. The species may be limited in dispersal into desert regions by daily maximum temperatures that approach or surpass its upper limit of tolerance, but is capable of dispersing through the Central American tropics into South America. The species is limited northward by low temperatures at which it is unable to mobilize sufficient energy for existence and/or reproduction except where it makes use of artificial man—made shelters and food supplies. These variations and adjustments noted between geographic populations of the House Sparrow probably apply to bird species generally.

ISSN:0012-9658    

DOI:10.2307/1936435

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

The vegetation patterns on the undulating topography of the Mima Mounds, Washington, USA exist on at least three scales. At the smallest scale the distribution of individuals is determined primarily by microtopographic features which provide small, but significant habitat variation. An Intermediate scale is dependent upon exposure and elevation. Mounds alter drainage and insolation to create moisture gradients to which all species respond. The largest scale involves the shift of species distributions with respect to mounds as one moves from the prairie to a zone of Pseudotsuga menziesii invasion. Conditions become more mesic as a consequence of reduced insolation and wind in this ecotone, resulting in an increase in mesophytes and a decrease in xerophytes. Both ordination and classification methods are applied in this study. The analyses are complementary and provide insight into the nature of factors controlling vegetation. A method of detailed mapping is proposed which involves the description of a small sample of the area to be mapped, preliminary clustering, the application of discriminant analysis to perform reallocation of samples, and the construction of a key to the resultant associations. The key is used in the field to assign samples to the proper association.

ISSN:0012-9658    

DOI:10.2307/1936436

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY