ISSN:0012-9658    

DOI:10.2307/1933653

出版商:Ecological Society of America    

年代:1970

数据来源: WILEY

摘要:

In interspecific competition between various strains of Drosophila melanogaster and a vermilion mutant strain of D. simulans in continuous populations, heterogeneity among replicate populations and among generations within populations has been observed when the fitness of the melanogaster strain was apparently similar to that of simulans vermilion. Larval and adult numbers vary widely among generations, so that differential effects of population density on fitness components of the two species could be an important factor contributing to the observed heterogeneity. The effects of variation in larval density and in proportions of the two species on viability, sex ratio, developmental period and adult body weight have been determined for the Oregon—R—C wild type strain of melanogaster and the standard vermilion strain of simulans. Thirty treatment combinations of larval density (six levels) and species proportions (five levels–two single species, three mixed) were repeated three times at intervals of 1 week. This variable of time had highly significant effects on all components. Results observed in mixed cultures generally were not predictable from results in single species cultures, particularly at high densities. The general effects of density on both species were similar, but there were significant differences between the species that would cause differences in competitive outcome at different densities. Further, the different components showed different responses to density. Species frequency also had significant effects on each component, but these effects were often different for the two species and were different for each component. As the outcome of competition within a generation was both density and frequency dependent, and given as well the significant effects of time, variation among populations and among generation within populations can be expected. The dynamics of interspecific competition in continuous populations will not be simply predictable.

ISSN:0012-9658    

DOI:10.2307/1933654

出版商:Ecological Society of America    

年代:1970

数据来源: WILEY

摘要:

The Red—tailed Hawk population on a 60—mile2(156—km2) area in central Alberta was studied during 1965—68. This migratory population was present from early April to September each year. Breeding red—tails occupied open and forested habitats with equal facility, taking prey species from all cover types associated with nesting sites. Mammals, chiefly Richardson's ground squirrels and snowshoe hares, comprised 66% of total prey biomass taken by red—tails; waterfowl comprised 18%. Representation of meadow voles in the red—tail diet reflected annual variations in vole densities, but there was no consistent relationship between snowshoe hare and Ruffed Grouse densities and their utilization by red—tails. Numbers of adult red—tails resident on the study area varied little from year to year, and nesting success was not directly related to changes in cyclic hare and grouse populations. Red—tails took between 1.9 and 7.0% of the adult hares, and 0.3—0.8% of the juveniles on the study area during mid—April through August, 1966—68. Estimates of red—tail predation on Ruffed Grouse over this same period were 1.4 to 5.1% of adults, and 0.9 to 2.2% of juveniles. Calculated losses of Richardson's Ground squirrels to Red—tailed Hawks during 1967 and 1968 ranged from 22 to 60% in three populations. Losses of adults were especially high, and may have exceeded 90% in one instance. Adult male ground squirrels were significantly more vulnerable than adult females.

ISSN:0012-9658    

DOI:10.2307/1933655

出版商:Ecological Society of America    

年代:1970

数据来源: WILEY

摘要:

Stems of seven young loblolly pines on the Savannah River Plant Reservation, South Carolina, were each inoculated with 3.28 x 103μCi of radiostrontium. Accumulation of radiostrontium in foliage continued for 13 months after inoculation, when a maximum of 1.27 x 103μCi/tree was attained. Radiostrontium concentration was consistently higher in lower crown foliage than in upper crown foliage, and higher in second—year needles than new needles. There was little evidence of basipetal translocation of radiostontium to roots. About 2% of the maximum foliage burden of radiostrontium was released by leaching during the 16 months following inoculation. Radioactivity of rainwater falling through crowns was best correlated with needle radioactivity, but was also inversely correlated with intensity of precipitation. Transfer of leached radiostrontium to the ground by rainwater flowing down stems was only 23% of that transferred by rainwater falling through tree crowns. Twenty per cent of the maximum foliar uptake was released by needles falling from the trees. The inoculated pines accumulated three times more radiostrontium in foliage than they released by needle fall and leaching. After 16 months 94% of the radiostrontium released from the inoculated trees was in litter and only 4.6% in mineral soil. Radiostrontium transferred to soil was generally confined to the top 2.5 cm where feeder roots are most abundant, but less than 10% of the radionuclide accumulated in soil was in roots. Accumulation in soil and roots closely approximated input from needle fall and foliar leaching, indicating that there was negligible input of basipetally translocated radiostrontium and negligible output due to leaching to subsoil, runoff, blowing away of litter, or recycling. The radiostrontium burden of a pine forest might be reduced considerably by removal of litter after contaminated needles have fallen.

ISSN:0012-9658    

DOI:10.2307/1933656

出版商:Ecological Society of America    

年代:1970

数据来源: WILEY

摘要:

Seven species of heteromyid rodents varying in size from 8 to 114 g were studied in the laboratory. Starved rodents husked four kinds of large seeds (spinach, sunflower, squash and pumpkin) and were timed at the task. Geometric mean husking times varied from 3.1 to 70.1 seconds, depending on the seed and the rodent. Husking time was inversely proportional to an animal's body size. However, despite their slowness, smaller animals appear to be more efficient at dealing with these large seeds because they have a smaller metabolic drain. This conclusion is based on the assumption that gross caloric need increases linearly with (body weight).75. The authors defend the assumption and thus believe that smaller animals have a competitive advantage while husking seeds. These rodents might engage in a limited amount of habitat and resource selection based on their relative seed—husking abilities. However, the order in which the various species should prefer these seeds is quite similar, so resource allocation based on husking speed differences would seem of little potential importance in maintaining competing heteromyids in a state of coexistence. Previous investigators have shown that seed selection occurs in some cases, but many cases are also known in which little or no selection is practiced.

ISSN:0012-9658    

DOI:10.2307/1933657

出版商:Ecological Society of America    

年代:1970

数据来源: WILEY

摘要:

The two crayfish, Orconectes virlis and O. immunis, have similar ranges but are ecologically isolated within these ranges; the former species inhabits streams and lake margins, the latter inhabits ponds and sloughs. Field and experimental data suggest that the stream species, O. virilis, is excluded from ponds by summer drying and periodic low oxygen periods of those ponds. It is not as competent a burrower as O. immunis nor is its as tolerant of low oxygen levels. Neither current nor substratum excludes the pond species, O. immunis, from the stream. But both field and laboratory observations strongly indicate a competitive exclusion of O. immunis by O. virilis. The latter was demonstrated to be intrinsically more aggressive in interspecific contacts, and in other experiments, evicted O. immunis from crevices in the substratum.

ISSN:0012-9658    

DOI:10.2307/1933658

出版商:Ecological Society of America    

年代:1970

数据来源: WILEY

摘要:

A new approach is developed for the estimation of insect population densities which eliminates the necessity of counting every individual per sample. Although the method was developed for populations of corn rootworm eggs, it seems as applicable to other organisms featuring contagious distributions. Estimation by the proposed method is based upon the proportion (p) of samples containing at least t insects each, where t, termed threshold density, can be any positive integer specified. For example, if t is 1, counting is entirely unnecessary since the only information required is the proportion of samples in which the organism is present. The prediction equation which links mean density to the variables ° and t must be estimated beforehand from a series of samples extending over a wide range of population densities. Expressing the model in terms of logarithms premits the estimation of its parameters by linear least squares. The study indicates that there is an optimal threshold level associated with every population density and specified allowable error. Estimation near these optima is apparently more efficient than conventional estimation involving complete sample enumeration. By enlisting a two—stage sampling scheme, one should consistently be able to approach the optimum threshold and thereby produce estimates comparable in efficiency to direct counts but free from much of the labor.

ISSN:0012-9658    

DOI:10.2307/1933659

出版商:Ecological Society of America    

年代:1970

数据来源: WILEY

摘要:

Data on the thermoregulation of Anolis limifrons in Panama suggest that at least some tropical species are less warm adapted than previously thought. Anolis limifrons has a lower minimum voluntary temperature, eccritic temperature, and critical thermal maximum than a majority of the temperate zone lizards thus far studied. Thermoregulatory adjustment is by behavioral mechanisms as indicated by observations on basking and orientation. A seasonality in the preferred temperature is indicated by field data from the wet and dry seasons. It is suggested that the lower preferred temperature in the dry season is an adaptation to conserve water or minimize water loss. Anoles (A. limifrons and A. frenatus) in colsed canopy forests are less warm adapted than the grassland A. auratus, whereas an ecotone species (A. tropidogaster) is intermediate in its thermoregulatory characteristics. Differences in dermal morphology in three species of Panamanian anoles are suggested to be an adaptation to varied capacities of heat gain.

ISSN:0012-9658    

DOI:10.2307/1933660

出版商:Ecological Society of America    

年代:1970

数据来源: WILEY

摘要:

Primary productivity of reef—building algae was studied by putting samples from the reef in a closed system and measuring oxygen exchange in the light and in the dark. Gross productivity determined for 32 samples in full sunlight had a mean value of 0.048 mg O2cm—2hr—1. Photosynthesis was found to increase with the logarithm of light intensity up to 1,000 ft—c and was constant between 1,000 and 8,000 ft—c. Rates of gas exchange in flowing water showed no correlation with water velocity but were greater than rates in still water. Daily patterns of photosynthesis were calculated for populations of calcareous algae living on the submarine faces of the windward sides of atolls. During most of the daylight hours light is probably not a limiting factor for photosynthesis in these populations. Calculated productivity of various calcareous algal zones indicates that these do not contribute significantly to overall reef production on atolls of the northern Marshall Islands. Island reefs are less productive than previously studied inter—island reefs.

ISSN:0012-9658    

DOI:10.2307/1933661

出版商:Ecological Society of America    

年代:1970

数据来源: WILEY

摘要:

Seasonal variation in food of the beaver (Caster canadensis Kuhl) was examined on the Mackenzie Delta, Northwest Territories, Canada. Leaves and, to a lesser extent, growing tips of willow (Salix spp.) were the main food items during July and August; during the remaining 10 months food consisted of the bark of willow (76%), poplar (Populus balsamifera) (14%) and alder (Alnus crispa) (10%). Protein: calorie ratios in the diet were approximately 40 and 8 mg/cal during those two periods respectively. The northern beaver has adapted to low energy availability characteristic of winter by storing food in the autumn and again in the spring, and by intrinsically lowering food intake during the winter. It has adapted to seasonal variation in protein availability by utilizing high—protein willow leaves almost exclusively when they are available. The heavy dependence upon willow, and its resultant removal from the community, can impart a high degree of instability to northern beaver populations.

ISSN:0012-9658    

DOI:10.2307/1933662

出版商:Ecological Society of America    

年代:1970

数据来源: WILEY