摘要:

The circulation of phosphorus in 200—liter aquaria, continuously supplied with tap water, was analyzed by means of analytical and tracer methods. This open—system design ultimately resulted in a steady—state system whereby the influx of phosphorus to various biological compartments was equaled by a corresponding outflow. This model of phosphorus circulation is believed to apply to small lakes with extensive littoral vegetation during summer stratification, but contrasts with models proposed from other tracer studies on such lakes. The other models are considered to be feasible, but unlikely and based on incorrect analyses of data. Previous estimates of the rate of phosphorus circulation based on tracer analyses are reanalyzed and appear to be of the correct magnitude. The 200—liter system developed two communities of attached and planktonic organisms, with the former rapidly removing a large percentage of the phosphorus from the latter. With this removal any direct relationship between the phosphorus concentration of the open water and the influx concentration was obscured. A major means by which phosphorus was removed from the open water was shown to be trapping of particles by the community associated with the sides of the aquaria. The rate of such removal varied widely, depending on the extent of "littoral" growth and the nature of the particle. These results suggest that a multicompartment analysis of nutrient circulation is significantly more realistic than a simplistic assumption of homogeneity. Considering succession to be the total change in physical and biological conditions over prolonged time, this open—system design developed from a one—community, few—species system into a two—community, several—species system. The rate of nutrient circulation greatly increased with time and developed into a steady state, not an equilibrium system which depended on a continual phosphorus influx to maintain the concentrations and circulation rates.

ISSN:0012-9615    

DOI:10.2307/1942228

出版商:Ecological Society of America    

年代:1972

数据来源: WILEY

摘要:

The foraging ecology of the asteroid Leptasterias hexactis was studied at five rocky intertidal areas in the San Juan Islands, Washington, for 2—2¾ years. Feeding activity during high tides reaches a high point in mid— to late summer (July and August) and a low point in the winter (January). Three probable factors selecting for this cycle are (1) reproduction, virtually immobilizing the females, during the winter; (2) low food availability in winter compared to summer; and (3) violent winter storms. Persistently reduced low tide feeding during all but autumnal night low tides indicates that some light—related factor(s) select for a tidal cycle of feeding also. Factors believed responsible are desiccation stress and possibly predation by visual predators. A graphic model of foraging behavior is presented. Analysis of the diet of Leptasterias by both numbers and calories of prey consumed indicates that this starfish is a food generalist. Moreover, numbers of prey consumed, when compared to prey available, indicate those prey chosen selectively and those which are avoided. Lastly, calories consumed compared to calories available indicate why particular prey are chosen. Many factors are involved in prey choice including ease of capture, relative sizes of predator and prey, and caloric yield of the prey. Actual availability is defined as that portion of available prey (i.e., prey physically present) which could be consumed by a predator. The factors affecting prey choice, coupled with prey and predator distribution, abundance, and behavior must be considered when determining actual availability of prey. Actual availability coefficients are computed and suggest that (1) hard—to—capture prey are least available; (2) Leptasterias is more selective in summer and autumn, when food is more abundant; and (3) prey are more available to large starfish. Calculations suggest that, under normal conditions, Leptasterias would be severely energy—limited in winter if it foraged at rates similar to those of spring or summer, or both. Thus, food limitation may be partially responsible for reduced feeding in winter. Comparison of numerical patterns of consumption of energy—rich and energy—poor prey indicates that energy—rich prey are most important except in summer when energy—poor prey become the most important prey, presumably because of their tremendous numerical increase and domination. Comparisons between the foraging strategy of this starfish and theoretical models previously proposed support some predictions but not others, indicating that complex natural systems require more comprehensive and precise models. Predictability of resources in time and space probably determines the degree of resource specialization or generalization in animals.

ISSN:0012-9615    

DOI:10.2307/1942229

出版商:Ecological Society of America    

年代:1972

数据来源: WILEY

摘要:

A study of the macrobenthos and its environment in the Pamlico River estuary, North Carolina, included measurements of salinity, temperature, and dissolved oxygen, as well as the particle size and concentrations of organic matter of the sediments. Salinity ranged from 1% to 20%, and temperature from 5° to 31° C. Anoxic conditions in the bottom water over—lying the deeper portions of the estuary occurred in the late summer. Two distinct sediment types were found: a sand substrate in the shallow area and a silty clay substrate in the deep area of the estuary. A preliminary sampling program in July 1968 showed that the macrobenthos was characterized by both low diversity and low density because of unfavorable environmental conditions of salinity and sediment. The statistical sampling design for this study was stratification of the estuary and equal random allocation of samples. Only 18 species were found, and the Shannon—Weaver information diversity index (H) showed low values of 0.66—1.77. In addition, low density values (maximum number of organisms at any station was 629/m2) further indicated that stressing salinity conditions and unstable sediments are factors that make this estuary unfavorable for the benthos. The absence of benthic life in much of the deeper portions of the estuary correlated directly with anoxic conditions in these areas. On the basis of trellis diagrams, the estuary was divided into three areas: (1) and oligohaline zone dominated by Rangia cuneata and Nereis succinea; (2) a mesohaline zone with a Macoma balthica—Heteromastus filiformis—Nereis succinea association; and (3) a polyhaline zone at the mouth of the estuary characterized by a Macoma phenax—Mulinia lateralis—Glycera dibranchiata association. A seasonal sampling program, with transects located in these communities, showed vast seasonal changes in species composition, range of distribution, density, and community delimitation caused by larva set in the estuary. A total of 36 species was found during this 1968—69 sampling program. Although a few species, such as Rangia cuneata and Nereis sucinea, showed relatively little change in densities and range of distribution, most of the benthos underwent great fluctuations due to recruitment from new set. Settling periods in the fall and spring added large numbers of juveniles to the benthos, not only in the deeper portions where re—establishment of life was found after the late fall mixing destroyed the anoxic conditions, but also in the shallow areas. Due to this recruitment, the distribution of communities in the estuary changed so drastically during the year that a great homogeneity was found throughout the estuary during the winter and spring sampling periods. Ho ever, the communities noted in the preliminary sampling reappeared in the summer. Thus the macrobenthos undergoes significant seasonal cycles in the Pamlico River estuary.

ISSN:0012-9615    

DOI:10.2307/1942230

出版商:Ecological Society of America    

年代:1972

数据来源: WILEY

摘要:

Three contiguous ecosystems, an upland oak forest, marginal fen, and cedar swamp, were analyzed along a gentle topographic gradient on the Anoka Sand Plain in east—central Minnesota. The organic structure and aboveground primary productivities of these forests were compared with similar data for other natural ecosystems of the sand plain. Living, aboveground biomass totals of the oak, fen, and swamp forests were 124.6, 98.8, and 159.6 mt/ha, respectively. Aboveground, net annual productivities were 8.9, 7.1, and 10.3 mt/ha, respectively, which compared well with other forest data for the region and with a predicted value based on actual evapotranspiration. Several measures of structure were compared in the analysis. Species richness and equitability were greatest in the marginal fen where biomass and productivity were the least. Richness and equitability were lowest in the oak forest. The highest biomass—to—production ratio was found for the Thuja swamp, although it had the highest annual production. Apparently the high production in the swamp was achieved with a large mass of relatively inefficient foliage concentrated in Thuja occidentalis. The percentages of primary production entering detritus pathways were 52, 62, and 48 for oak, fen, and swamp, respectively. Total detritus, including dead boles, branches, and forest floors, was large in relation to living biomass in all three forests, but was especially important in the fen and swamp systems where deep forest floors had accumulated. Total detritus was 61%, 331%, and 577% of aboveground biomass in the oak, fen, and swamp, respectively. All data were produced in caloric as well as weight units.

ISSN:0012-9615    

DOI:10.2307/1942231

出版商:Ecological Society of America    

年代:1972

数据来源: WILEY

摘要:

Water economy and thermal relations of plethodontid salamanders were studied in the laboratory and in the field. Laboratory measurements included behavioral responses in temperature and relative humidity gradients, rates of dehydration and dehydration at various relative humidities and soil—moisture levels, and determination of critical thermal maxima by rapid controlled heating to a definite endpoint. Salamanders were acclimated to a combination of three temperatures and two photoperiods: 5°C, 16 hr of light alternating with 8 hr of darkness (LD 16:8); 5°C, (LD 8:16); 15°C (LD 16:8); 15°C (LD 8:16); 25°C (LD 16:8); and 25°C (LD 8:16). Three series of experiments were conducted on 20 populations representing 14 species. Critical thermal maximum increased with an increase in acclimation temperature indicating that the salamander's heat resistance was readily altered by its previous thermal history. Salamanders selected a definite range of temperatures and did not merely avoid extremes in the thermal gradient. Thermal preferenda were relatively stable for each species and were not significantly affected by either acclimation temperature or photoperiod. Rate of dehydration was dependent upon body size, drying power of the air, and ambient temperature. Interspecific differences in dehydration rates appeared to be related in part to differences in size. Species composed of small individuals lost weight faster than species made up of large individuals. As the vapor pressure deficit increased the dehydration rate increased. Dehydration was more rapid at higher temperatures. Rehydration rate increased as the percentage of weight loss due to dehydration increased and was more rapid at higher temperatures. Salamanders absorbed water from soil when soil—moisture tension was as high as 2.8 atm at 25°C. All species absorbed water from unsaturated soil at similar rates that were dependent upon soil—moisture content. Salamanders in the humidity gradient responded positively to differences in the moisture content of the air, and all but one species were more than 70% successful in selecting the highest relative humidity available in the gradient. Interspecific differences were apparent, but were not always correlated with habitat preferences. Plethodon glutinosus did not show any specific adaptation in its thermal responses or water relations that would account for its widespread distribution in the eastern United States. Plethodon ouachitae and P. caddoensis apparently have survived in the Ouachita Mountains due to favorable microhabitats and their ability to burrow deep beneath talus—covered slopes during hot and dry summers. Surface activity of P. caddoensis is limited in the summer by hot and dry conditions in its microhabitat.

ISSN:0012-9615    

DOI:10.2307/1942232

出版商:Ecological Society of America    

年代:1972

数据来源: WILEY