摘要:

The Late—Wisconsin spruce forest that characterized most of Minnesota (and the rest of the Middle West) differed from the modern Boreal Forest of Canada not only in the absence of pine but also in the apparent admixture of black ash, oak, and elm and in the presence of openings in which Artemisia was abundant. Close—interval pollen analyses at 3 sites in Minnesota show some of the details in the transformation of this spruce forest to pine or hardwood forests. At Kirchner Marsh in southeastern Minnesota, the spruce forest was replaced abruptly about 10 000 yr ago birch, ald and temperate hardwoods before pine (jack pine?) entered the region for a short period. At Wolf Creek in central Minnesota, pine arrived from the east virtually at the moment when spruce declined. At Lake of the Clouds in northeastern Minnesota, spruce forest had succeeded tundra about 10 000 yr ago, but simultaneously pine (jack pine?) and the temperate hard woods (especially elm) appeared in quantity. The spruce and hardwood components then declined about 9300 yr ago as pine (red pine?) increased to dominance during the next 400 yr. Charcoal counts for the 3 sites provide no support for the hypothesis that increased fire incidence explains the forest transformation from spruce to pine. Climatic change must instead be the primary cause. The apparent openness of the spruce forest can be attributed to the continued presence of the wasting ice sheet just to the north, which favored a dry and windy climate, perhaps with relatively mild winters. As the ice sheet withdrew, its influence decreased. Differential rates of tree immigration as well as progressive leaching of upland soils may have been additional factors effecting changes in forest composition.

ISSN:0012-9615    

DOI:10.2307/1942569

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Juvenile Sarotherodon mossambicus in Lake Sibaya, South Africa, grow rapidly and maintain good body condition, but adults grow slowly, show extensive marasmas and appear to suffer from severe malnutrition. The cause of this malnutrition was sought by comparison of juvenile and adult diets. Juveniles feed on benthic detrital aggregate from nearshore sand terraces, whereas adults feed on benthic detrital aggregate in deeper offshore waters. Chemical analysis of 39 benthic detrital aggregate samples collected from juvenile and adult feeding areas showed that detrital aggregate from these 2 areas did not differ in caloric content (° =0.97 J/mg organic), organic content (° = 43.0%), soluble carbohydrate content (° = 16.0% of organic), total carbohydratecontent (° = 45.7% of organic). Protein content ranged from 1.8 to 14.2% of organic matter and decreased with increasing depth. Assimilation efficiencies determined experimentally for juveniles feeding on benthic detrital aggregate were: energy 42%, organic matter 37%, soluble carbohydrate 36%, total carbohydrate 35%, and protein 46%. A large proportion of the organic matter assimilated was nonliving amorphous detritus. Ratios of digestible protein to digestible energy calculated for the 39 samples show a highly significant inverse relationship to increasing depth. These ratios indicate that juvenile diets contain adequate protein to support good growth, but adult diets would be expected to result in malnutrition due to protein deficiency. Nutritional constraints imposed by low protein levels in detrital aggregate account for the characteristic nearshore distribution of detritivorous fishes and the paucity of exclusively detritivorous fish species.

ISSN:0012-9615    

DOI:10.2307/1942570

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Phytophagous insect larvae (Lepidoptera and Tenthredinoidea) in the leaf—chewing guild were sampled from accessible parts of all the 18 species of common woody plants in a mature upland forest in New York State, in early and in late June. Among the procedures used to describe patterns of associations between plants and insects are principal components analysis and the clustering techniques of numerical taxonomy, whereby the faunistic similarity among plant species is described. We introduce a weighting factor for cluster analysis that weights a character (in this case the density of an insect species on a plant) in proportion to its deviation from the norm. During June, both the abundance and variety of insects declined on most plant species, which vary greatly in both these aspects of their fauna. Insect species that are either quite highly specialized or generalized in diet appeared to be prevalent; those with intermediate diet breadth were less so. Clustering plant species by faunal similarity revealed some clusters of taxonomically related species, but the correspondence between faunal similarity and taxonomic affinity is not strong. Few plant species have a highly distinct, specialized fauna. The complexity of distribution of insects over plants indicates that insects are responding to many factors that differentiate plant species.

ISSN:0012-9615    

DOI:10.2307/1942571

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Three small streams located at Coweeta Hydrologic Laboratory, North Carolina, USA, on an old field watershed, a pine plantation watershed, and a hardwood forest watershed were investigated to determine effects of watershed perturbation on K and Ca dynamics in the stream ecosystems. Data collected included measurements of litterfall inputs, large particulate organic matter and benthic organism standing crops, large particulate organic matter and organism drift, and insect emergence. We used85Sr and134Cs to estimate detritivore ingestion and elimination rates of Ca and K, respectively. We found that watershed perturbations had altered stream inputs and caused accompanying changes in the stream fauna. Our results indicated that the perturbed streams had less efficient physical processing of allochthonous inputs, but greater biological utilization of inputs. The streams exhibited high resilience to perturbation with complete recovery limited by the recovery rate of allochthonous inputs.

ISSN:0012-9615    

DOI:10.2307/1942572

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

The mode of pollination was studied in 2 dwarf mistletoes, Arceuthobium douglasii and A. strictum, from Utah and northern Mexico. Insect visitations to the staminate flowers of A. strictum were common. Visits were found to be rare, however, to the pistillate flowers and to either sex of A. douglasii. Pollen was found to be consistently liberated by the wind from the well—exposed anthers. Air—borne pollen concentration decreased rapidly with distance from the source. No inferences, however, could be made from the data as to dispersal distance. The paucity of insect visitations, the abundance of air—borne pollen, and the moderate to high seed set combined with the absence of apomixis (in A. douglasii) were considered evidence for an anemophilous pollination syndrome. The genus as a whole appears to be anemophilous based on the uniformity of the reproductive morphology. The floral characteristics of Arceuthobium were reevaluated with respect to their suggested role in wind pollination.

ISSN:0012-9615    

DOI:10.2307/1942573

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Throughout the southeastern United States the hardwood Nyssa sylvatica (sensu lato) is distributed along a soil moisture gradient from upland sites, which are never flooded, to floodplains, which are periodically flooded and drained to permanently flooded swamps. Population differentiation with respect to flood tolerance and related physiological attributes was investigated using 1—year—old seedlings grown in a greenhouse from seed collected along this gradient. Upland plants were very intolerant of flooded soils. Their root systems deteriorated, root respiration rates dropped and, after a year under such conditions, survival was poor and those that did remain were greatly stunted and had accumulated large concentrations of many nutrient elements. In contrast swamp plants were quite tolerant of flooded soils. Upon flooding, parts of the orginal root system were lost but new roots were initiated which had an increased capacity for alcoholic fermentation. Many of these new roots were more succulent, larger in diameter, and less branched than drained roots. Such succulent roots however were only a temporary response to short—term flooding; plants flooded for a year did not have such roots, rather the root system superficially resembled drained roots. Concomitant with this return to drained—like roots was an increase in internal oxygen transport to the roots and a drop in alcoholic fermentation. Floodplain plants under drained conditions allocated less biomass to roots than to shoots and had high respiration rates, traits similar to upland plants. Under flooded conditions they initiated new roots with medium respiration rates, allocated less biomass to roots than to shoots, significantly increased oxygen transport to the roots and had high survival, traits similar to swamp plants. Thus, the floodplain population produced a distinctly flood—tolerant phenotype; but not nearly as tolerant of flooded conditions as the swamp phenotype. Floodplain plants differed from swamp plants in transporting less oxygen to the roots under drained conditions, initiating fewer succulent—type roots and not accelerating alcoholic fermentation upon flooding and after a year under flooded conditions having less total biomass, less oxygen transport to the roots and a greater accumulation of Fe and Mn in the roots. The floodplain plants apparently have been selected to be similar to upland plants under drained conditions and swamp plants under flooding and one consequence of this is that their tolerance of flooded conditions is intermediate. It is argued that one of the more important trade—offs in adapting to flooded conditions is that high internal oxygen transport carries with it a ‘cost’ in terms of excessive water loss under water stress conditions.

ISSN:0012-9615    

DOI:10.2307/1942574

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

An intensive study of the zooplankton community of Mirror Lake, New Hampshire, was undertaken over a 3—yr period. Our objectives in the lake study have included measurements of a number of attributes of the zooplankton community that integrate structure and function at the ecosystem level; among these are dispersion, biomass, productivity, respiration, and nutrient cycling. Eight species of rotifers and 3 species of cladocerans were successfully cultured. Generation time for planktonic rotifers was °8—10 days (17°C). The effect of higher food levels on rotifers was to shorten generation time and to increase brood size. In cladocerans, high food levels caused an increase in length and brood size. A curvilinear relationship existed between zooplankton community respiration and temperature in Mirror Lake. Mean monthly zooplankton community respiration ranged from 96.0 kg C/ha/mo in June of 1969 to a low of 20.5 kg C/ha/mo in April of 1970. Over a 3—yr period, respiration was 79.9% of assimilation. The 0 to 4.5—m strata (°epilimnion) contributed 68.5% and 46.5% of the annual zooplankton production and biomass. Zooplankton community production ranged from 22.3 kg C/ha/yr to 29.3 kg C/ha/yr with a 3—yr mean of 25.2 kg C/ha/yr. The annual zooplankton biomass ranged from 1.4 to 2.6 kg C/ha with a 3—yr mean of 2.0 kg C/ha. A linear relationship was found to exist between net phytoplankton and zooplankton production in various lakes of the world. Ecological efficiency apparently increases with the trophic status of the lake. It is recommended that the term ecological efficiency be refined to include both autochthonous and allochthonous inputs of reduced carbon into the lake. Rotifers assume a major role in intrasystem nutrient cycling and energy transfer within the lake ecosystem. Of the total amount of P incorporated into the organic matter of zooplankton community each year, 33.5% is assimilated in rotifer tissue. The annual turnover rate of P by rotifers is 30.9 and is high compared to crustaceans (10.1). Copepods comprise 55.4% of the total zooplankton biomass. However, the copepods, with their slow growth over an entire year, represent only 19.3% of the zooplankton production, while rotifers account for 39.8% of the zooplankton production annually in Mirror Lake. Also, evidence is presented that rotifers play a major role in energy transfer in lakes of varying trophic status (oligotrophic to eutrophic).

ISSN:0012-9615    

DOI:10.2307/1942575

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY