Significant Effect Of Density Research Articles

Overwintering hatchery-reared individuals of the soft-shell clam, Mya arenaria L.: a field test of site, clam size, and intraspecific density

Soft-shell clams ( Mya arenaria L.) are a commercially important intertidal bivalve in Maine, USA where they are managed as a common property resource. Ten million 8–12 mm clam juveniles are reared annually at the Beals Island Regional Shellfish Hatchery and are transplanted onto publicly-owned mudflats during the fall or early winter. The success of this strategy is unpredictable because of sporadic occurrence of winter ice formation and subsequent scouring of the intertidal zone. These events, in addition to other severe winter storms, can result in complete mortality of the transplanted seed. Transplanting hatchery-reared seed in the spring, therefore, is preferable, but until now there has been no economically effective technique to hold millions of soft-shell clam seed over the winter. A field experiment designed to test the interactive effect of clam size and density on survival was conducted during the winter of 1991–1992 at a sheltered and exposed site near Beals, Maine, USA. Three sizes of hatchery-reared clam seed (Small, X ̄ = 4.3 mm shell length [SL]; Medium, 8.2 mm SL; Large, 11.5 mm SL) were used. Clam density was approximate and depended on clam size (Small, 18 750 ind, 37 500 ind, and 56 250 ind; Medium, 11 000 ind and 22 000 ind; Large, 4500 ind and 9000 ind). Clams of each size/density combination were added separately to nylon window screen, zippered bags (aperture = 1.8 mm). Bags containing clams were housed in wooden-framed subunits (0.45 m × 0.45 m × 0.08 m) covered with a 12 mm extruded mesh netting. Subunits were arranged vertically in groups of 4–6 and were submerged 1 m above the bottom in shallow water (3–8 m) from November 1991 to April 1992. Mean survival of medium and large size clams at both sites was 97.7%. Significant density effects were detected for both size groups at the sheltered site, although the difference between treatment levels was less than 2%. Mean survival of small clams pooled across sites was 67.8%. At the sheltered site, clams in the uppermost level of the overwintering units had consistently lower survival rates than those nearer the bottom. Density effects for small clams were detected only at the sheltered site where animals held at the highest density had significantly higher survival rates than animals at either of the two lower densities (75.3% vs. 60.3%). Clam loss over the winter may be reduced by suspending seed in seawater instead of transplanting it to intertidal flats.

Effect of selection for shell coloration on growth rate and mortality in the Japanese pearl oyster, Pinctada fucata martensii

Effects of inbreeding on the growth rate and mortality are suggested on the basis of results of comparative growth experiments for a line of the Japanese pearl oyster, Pinctada fucata martensii, selected for white shell color in the prismatic layer. Growth in shell height, whole weight and mortality were measured using one inbred white cross and two hybrid white/brown crosses held in pearl nets under different densities. Analyses were conducted at the juvenile (3–12 months of age) and adult (12–20 months of age) stages. The growth rate was estimated by the differences between measurements at the start and after 4, 5, 6, 8 and 12 months in the juvenile experiment, and after 17 and 20 months in the adult experiment. Growth in whole weight and survival were significantly inferior in the inbred white crosses than in two hybrid crosses. With the exception of one case, there was no observation in which the performance of an inbred cross was significantly superior to that of the two hybrid crosses. There were significant effects of density on growth rate in weight, shell height and mortality.

Effects of Sediment Fertility and Initial Plant Density on Growth ofHydrilla verticillata(L.F.) Royle andPotamogeton nodosusPoiret

ABSTRACT In a six-week greenhouse investigation, variations in the growth of Hydrilla verticillata (L.f.) Royle and Potamogeton nodosus Poiret (- P.. americanus) were examined separately under high and low levels of sediment fertility and plant density. For both species, diminished biomass production and shoot lengths accompanied increased root-to-shoot ratios on infertile sediment. In corresponding density/fertility treatments, Potamogeton consistently produced greater total biomass and root-to-shoot ratios than Hydrilla. Shoot production was similar between species on infertile sediment, but on fertile sediment, Potamogeton produced nearly twice as many shoots as Hydrilla. High plant density, combined with high sediment fertility, resulted in a significant reduction in aboveground biomass of individual Hydrilla plants. However, at neither level of sediment fertility did the biomass of individual Potamogeton plants show significant density effects. Growth responses under nutrient-poor sediment conditions...

Constancy of population parameters for life history and floral traits in Raphanus sativus L. I. Norms of reaction and the nature of genotype by environment interactions

We observed norms of reaction for life history and floral traits in Raphanus sativus L. (wild radish: Brassicaceae) among genotypes raised in three planting densities. In the greenhouse, we used a nested breeding design to produce F1 seed representing 60 maternal plants and 15 pollen donors grown from field-collected seed. Eighteen hundred seeds were grown in three planting densities in an experimental garden. For each individual, we recorded survivorship, germination date, flowering date, petal area, ovule number, pollen production, and the mode of individual pollen grain volume/ flower. Planting density had a strong effect on survivorship, but differential mortality among genotypes was not density-specific. Two-way ANOVAS (block and density as class variables) were conducted on each paternal sibship to detect significant differences among densities with respect to mean phenotype. Among the 15 paternal genotypes, 12 exhibited significantly faster germination in the high-density plots. Three paternal families exhibited significantly delayed flowering at high-density. Two paternal families exhibited significant effects of density on petal area, but in opposite directions. Two paternal families had significantly lower ovule production at high-density. No paternal families exhibited significant effects of density on pollen production or pollen-grain volume. Strong differences among genotypes with respect to the effects of density on phenotype indicate genetic variation in the plastic response to density for these traits. Three-way ANOVAS of each density treatment measured the effects of block, paternal family and maternal family on phenotype; significant paternal effects indicated the presence of significant additive genetic variance ( VA) in the measured trait. The ability to detect VA and maternal effects nested within paternal genotypes in most of these traits was density-specific.

Growth and grain-yield of contrasting barley cultivars under different plant densities

The effect of planting density on dry-matter growth and grain-yield of contrasting cultivars of barley was examined over three years. The particular objective was to examine whether high-density planting or the use of vigorous cultivars would be disadvantageous in areas where frequent rainfall during early growth is followed by prolonged drought up to maturity.Pre-heading growth varied greatly among cultivars and densities; this variation was partly related to the number of tillers, with two-row cultivars and high densities having more tillers per unit area than six-rows and low densities respectively. The relationship between tiller number and canopy light interception, however, varied among cultivars. A detailed study of two contrasting cultivars showed that the cultivar which had the highest light interception per unit leaf area also had the highest dry-matter production. The result indicates the importance of radiation interception in pre-heading stages with incomplete ground cover.High densities promoted phenological development, with flag-leaf exsertion and heading occurring 6–7 days earlier in the high density (120 plants m−2) than in the low (36 plants m−2). No significant effect of density on grain-yield was observed in one season where water stress developed in the early-grain-filling period. Cultivars which headed earlier tended to escape water stress and produced higher grain-yield. Post-heading growth of late-heading cultivars was low, but this was partially compensated for by the apparent use of pre-heading assimilates.When there was frequent rain during the early stages of growth, water use was not affected by density and cultivar despite a large variation in total dry-matter production among treatments. In another season where favourable weather conditions were maintained throughout growth, yield response to plant density varied among cultivars. In one six-row cultivar with a limited ability to produce tillers, grain-yield increased with increase in density up to the highest of 150 plants m−2. In other cultivars, there was no consistent effect of plant density on grain-yield.It is concluded that grain-yield will not be reduced by the use of high plant density or vigorous cultivars which promote biomass production during early stages of growth, if rainfall is frequent at that time.

Phenological studies on the wing dimorphism of a semi‐aquatic bug, Microvelia douglasi (Heteroptera: Veliidae)

SummarySeasonal changes in the percentage of macropterous individuals in natural population of Microvelia douglasi in permanent habitats were investigated in Shimane, western part of Japan. Macropterous forms of 5th instar nymphs occurred most frequently from late June to early July. However, the seasonal appearance of macropterous adults differed between habitats and years. There were significant effects of density, temperature, photoperiod and food availability on the percentage of macropters under laboratory conditions. The percentage of macropterous forms generally increased with density, but the mode of the response was modified by temperature, photoperiod and food availability. When insects were reared under long day photperiod (16L8D) with abundant food (0.5–1.0 g/container/day), the percentage of macroptery varied most among the different density treatments at the intermediate temperature (24°C) and the difference was reduced at higher temperature (28° and 32°C). Effects of temperature were small under the short day photoperiod (12L12D). Under 12L12D photoperiod sensitivity of density responses were higher at 32°C and lower at 24°C than those under 15L8D photoperiod. When the insects were supplied with food every other day, the density response became obscure. Ecological significance of environmental responses in the wing morph determination was discussed in relation to the heterogeneous environment of water surface.

Pollen-Limited Fruit Set in Isolated Patches of Maianthemum canadense Desf. in New Jersey

WORTHEN, W. B. AND E. W. STILES (Department of Biological Sciences and Bureau of Biological Research, Rutgers-The State University of New Jersey, Piscataway, NJ 08855-1059). Pollen-limited fruit set in isolated patches of Maianthemum canadense Desf. in New Jersey. Bull. Torrey Bot. Club 115: 299-305. 1988.-We examined the effects of pollen addition and raceme density on fruit set in seven isolated patches of Maianthemum canadense Desf. (Liliaceae; False Lily-of-the-Valley) in central New Jersey. We removed racemes from two of four quadrants in each patch to create distinct high and low density sectors. We hand pollinated all the racemes in four 1-m quadrats (1/quadrant) within each patch, and compared fruit set in these quadrats with fruit set in eight naturally pollinated control quadrats (2/quadrant). We recorded the percentage of racemes that bore fruit and the mean number of fruits/infructescence in each quadrat, and analyzed these data with three-factor analyses of variance (patch, density, and pollination-treatment effects). There was a significant effect of pollen addition on fruit set. In four patches, hand pollination significantly increased both the percent of racemes that bore fruit and the number of fruits/infructescence. We conclude that fruit set in these patches of M. canadense was pollen limited. A previous study reported that fruit set in M. canadense was resource limited, but our results suggest that fruit set in different patches may be limited by different factors. The clonal architecture of a patch and the relatedness and distance between mating plants may influence these patterns. There were no significant density effects, in any patch, on fruit set in hand pollinated or naturally pollinated quadrats. Although this suggests that plant density (and inferentially, competition for resources) did not limit fruit set, clonal growth patterns may confound this conclusion.

Differential distribution between sub-species of the mangrove ceriops tagal: Competitive interactions along a salinity gradient

Field observations indicate that the two subspecies of the mangrove Ceriops tagal (Perr.) C.B. Robb (var. australis C.T. White and var. tagal) are often segregated in their distributions in mangrove forests. Measurements along gradients of interstitial soil water salinity in two forests indicated that C. t. australis was associated with higher salinity regions and C. t. tagal with lower salinity portions of the gradient. Seedlings were grown for 5 months at salinities from 0 to 60‰ in a reciprocal density series (100, 67 and 33% combinations). Increasing salinity significantly decreased the leaf emergence for both subspecies. This effect was significantly greater for C. t. tagal than C. t. australis. All C. t. australis propagules eventually developed their first pair of leaves. C. t. tagal had 100% leaf emergence only in the 0 and 15‰ treatments. At 60‰, less than 50% of the propagules developed leaves. Seedling mortality was greatest for both subspecies in the 60‰ treatment. C. t. tagal suffered no mortality in the 0 and 15‰ treatments, whereas C. t. australis had no mortality at 15 and 30‰. C. t. tagal had higher mortality than C. t. australis in the 30, 45 and 60‰ treatments. At 0‰, however, C. t. australis had significantly higher mortality than C. t. tagal. Both subspecies showed significant differences in growth depending on salinity, but had their highest growth at 15‰. C. t. tagal outgrew C. t. australis at 0 and 15‰. At 30, 45 and 60‰, C. t. australis grew more. Furthermore, there were significant density effects on growth which were opposite at different ends of the salinity gradient. C. t. tagal outgrew C. t. australis at all densities at lower salinities, and C. t. australis had better growth at all densities at higher salinities. Although both subspecies showed maximum growth at the same location (e.g. 15‰) on the salinity gradient, neither was abundant at that salinity in the field. Both were displaced towards higher salinities. The results indicate that competitive interactions occur between the subspecies, with C. t. tagal the superior competitor at lower salinities, and C. t. australis superior at higher salinities. This differential success of the two competitors at opposite ends of the salinity gradient seems to be an important factor in maintaining their distribution in the field.

Paedomorphosis in Ambystoma Talpoideum: Effects of Density, Food, and Pond Drying

The expression of metamorphosis and paedomorphosis in the salamander Ambystoma talpoideum was examined in a three—factor experiment using artificial ponds. This experiment was designed to measure simultaneously the effects of initial larval density, food level, pond drying regime, and their interactions on the percentage of individuals metamorphosing or becoming paedomorphic. A higher percentage of individuals metamorphosed from drying low—density, and high—food ponds than from constant water level, high—density, and low—food ponds. A higher percentage of individuals became paedomorphic in constant water—level and low—density ponds than in drying and high—density ponds. Food level had no effect on the expression of paedomorphosis. Initial density, food level, and drying regime all had significant effects on the mean snout—vent length of larvae in ponds. The percentage of paedomorphic individuals in a pond was positively correlated with mean snout—vent length in the population. Analysis of covariance using snout—vent length as the covariate removed density and food level effects on the percentage of metamorphic individuals but did not remove the effects of drying regime. Use of snout—vent length as covariate in an analysis of covariance for the percentage of paedomorphic individuals failed to remove significant effects of density or drying regime. Only initial density affected the survival of larvae. The factors manipulated in this experiment affected the expression of paedomorphosis both directly through the physical environmental conditions that they created and indirectly through the growth and body size of larvae. These results suggest that differences in environmental factors among populations can affect the life—history pathways of individuals and lead to a predominance of paedomorphosis in permanent aquatic habitats with favorable growth environments.

An initial description of the quantitative genetics of growth and reproduction in the mosquitofish, Gambusia affinis

Three stocks of mosquitofish were reared under identical conditions through one complete life cycle to quantify phenotypic and genetic variability for traits of growth and reproduction. At 60 days of age females and males averaged 473 mg and 183 mg in weight and 26.2 mm and 20.9 mm in length, respectively. Significant differences were found among stocks for both sexes in both traits. Rearing density strongly affected female size; its effection male size was much less pronounced. Stage of maturity and male size were closely related, with maturing males being significantly larger than either mature or very immature fish. Heritability estimates for males were 0.25 for both length and weight, for females 0.72 for length and 0.77 for weight. Males averaged 55 days of age at sexual maturity. There were no significant differences among stocks, nor was there a significant effect of density on this trait. Heritability of age at maturity was estimated to be 0.41. Maturation of the first male to mature in a tank appeared to be under much weaker genetic control than maturation of subsequent males, suggesting a genotype-environment interaction. The size of first broods of virgin females averaged 23 fish. Significant differences between stocks for size of first brood were apparent only when female weight was considered. Heritability of brood size was estimated to be 0.16 without correction for female weight, and was impossible to estimate but presumably very low when corrections were made for female weight. Although female weight significantly affected brood size, it accounted for only 13% of the variation in the trait.

THE BIOLOGY OF AMBROSIA TRIFIDA L.

Summary Ambrosia trifida plants were grown in monocultures in the field at five densities ranging from 4 to 500 plants m−2. Plant growth, productivity, biomass allocation, plant architecture, and seed predation were studied in repeated harvests. Plant growth was very plastic and the weight of individual plants was greatly influenced by density. Mean dry weight per plant varied from 11 g in the highest density to 320 g in the lowest density. At the final harvest in September the plants had lost up to 34% of their biomass through senescence of lower leaves. Above ground biomass production was extremely high and averaged from 1590 g m−2 in the lowest density to 3058 g m−2 in the highest density. Leaf area index was very high especially at high densities.The relationship between plant height and density was complex and some individuals in the intermediate density were more than 3 m tall. Reproductive ratios, the weight of reproductive materials/shoot biomass, generally declined with increasing density and were significantly higher when the weight of the male flowers was added to the weight of seed. With time a pronounced decline in leaf area ratio occurred in all densities. There was no significant effect of density on unit leaf rate. The plants were able to discard their lower unproductive leaves in dense plots and effectively raised their canopy further up the stem. But this left much unproductive stem structure below and leaf area ratio and relative growth rate declined. Plants in high densities were single stemmed while those in the lowest density were much branched.The presence in dense stands of tall and short plants created a bimodal distribution of leaf area and seed weight. Most seed biomass was located in the upper canopy of low density plants. The percentage of plants producing male flowers decreased with increasing density and, in the highest density, with decreasing height in the canopy. The shorter plants, without male flowers, had a higher reproductive ratio. They invested their reproductive energy into seed. Insects damaged many seeds and the percentage of damaged seeds per plant declined with increasing density. Furthermore in the population of highest density seed predation was higher in the ‘apparent’ tall plants than in less apparent short plants.

The effect of density of planting on the growth of two Phaseolus vulgaris varieties in England

SummaryThe growth and development of two varieties of Phaseolus vulgaris (Purley King and Limelight) were compared in two experiments in 1973 and 1974 at a range of planting density from 20 to 100 seeds/m2. Within this range, the relationship between seed yield and density in Purley King was asymptotic, although there was a suggestion that if even higher densities had been tested, a downward trend in yield might have occurred. The optimum density of planting for Purley King in Expt 1 was 50 seeds/m2 while that for Limelight was 40 seeds/m2. The corresponding densities in Expt 2 were 75 and 50 seeds/m2 respectively. Although number of branches per plant generally decreased with increasing density, there was no significant density effect on the number of nodes per plant. Thus stabilization of seed yield occurred even at quite low densities. Although in the low-density treatments, less vegetative tissue was produced, the peak of dry-matter yield occurred later after flowering and the slower subsequent senescence ensured the presence of active photosynthetic tissue throughout the pod-fill stage. Less pod retention occurred at high density which, combined with the ability of widely spaced plants to produce pods over a longer period, resulted in a similar number of pods per unit area over a wide range of density.Although the variety Purley King produced more than double the number of mature pods from its extra nodes and branches, it was outyielded by Limelight by 35% from the combined effect of more seeds per pod and a higher mean seed weight. Limelight also produced this high yield with less vegetative tissue. In both varieties it appeared that pod photosynthesis could take place, in Purley King because the pods were borne on higher nodes above the canopy and in Limelight due to the earlier senescence of its smaller leaf area. However, in spite of the apparent physiological advantages of Limelight, the pods are not borne high enough on this plant to enable satisfactory mechanical harvesting.

Total emittance of lunar fines

Description of the technique and discussion of the results of measurements of the total emittance of Apollo 11, 12, and 14 samples of lunar fines. The measurements were performed as a function of temperature upon samples of a density large enough to eliminate any significant density effect on the total emittance.

The Effect of Crowding on Human Task Performance1

In a series of three experiments, groups of people performed tasks under varying conditions of density. The tasks ranged from very simple to complex, from rote memory to a test of creativity. Subjects worked on the tasks for 4 hours at a time for two or three successive days. There were no significant effects of density on performance, nor any consistent trends.