Mean Population Density Research Articles

Energetics of a population of the isopod Asellus aquaticus: Life history and production

SUMMARY. A population of Asellus aquaticus in Wistow Lake, England, was sampled at monthly intervals from September 1973 to September 1974. The mean population density was 586 m−2 in autumn 1973 but it fell to 67 m−2 by autumn 1974. Evidence suggests that the lower density in autumn 1974 was due to premature mortality of gravid females. The cause of this is not known.Gravid females were present from February to June and July to September, giving rise to spring and autumn cohorts. The larger members of the spring cohort reproduced in the autumn, the remainder in the following spring. The autumn cohort reproduced mainly in the following spring but later than the spring cohort. The bimodal length‐frequency curves were analysed to separate the two cohorts and to estimate growth and production of each cohort separately. The overwintering spring cohort had a faster relative growth rate than the smaller individuals of the autumn cohort. Thus the spring cohort showed a P/B ratio of 2.62 for the period September–April compared with a ratio of 1.81 for the autumn cohort. The highest ratio of 6.42 was shown by the spring cohort in the summer (May–September).Annual population production (P) was 3004.9 mg dry wt m−2 from a mean biomass (B) of 757.2 mg dry wt m−2. The P/B ratio of 3.96 is double the value reported for A. aquaticus in Sweden.

The Density-Dependence of Spatial Behaviour and the Rarity of Randomness

1. Spatial distribution was analysed in 156 sets of field data comprising more than 200,000 sample units in 3840 samples from 102 species. 2. Taylor's model, proposing that spatial variance is proportional to a fractional power of mean population density, provided an appropriate description of 147 sets of data. 3. Iwao's mean crowding model provided a rather less adequate alternative but with marked limitations. 4. The negative binomial with a common k was not satisfactory. 5. Two data sets only were judged random at all population densities. 6. In all other sets of samples, only at or near the lowest densities was disposition indistinguishable from random and the range of spacing between individuals progressively and disproportionately changed as mean density increased. 7. Spatial disposition is thus density-dependent and we deduce that spatial behaviour is also density-dependent as required by Taylor and Taylor's delta-model for intrinsic population contril by movement. 8. The sepcies examined range from Protista through annelids, molluscs, crustaceans, arachnids, insects, echinoderms, fish, birds and quadrupeds to men and orchids. 9. The sampling scales range from ciliates on the surface of a flat-worm to the human population of the United States of America and include temporally stable and unstable populations. 10. The sampling methods range from quadrat counts, through trap, net and grab samples to volumetric samples and complete censuses.

Population distribution and structure of the free-living nematodes of Long Island Sound

The distribution and structure of nematode populations in 4 sedimentary environments (muds, muddy sands, fine sands and medium-coarse sands) in Long Island Sound were studied. Mean population densities were highest in muds and muddy sands. Cluster analysis suggested the presence of two basic faunistic units; a mud unit characterized by high species dominance, low species diversity and low species endemism, and a sand unit characterized by low species dominance, high species diversity and high species endemism. Species diversity in all habitats was a direct function of both species richness and equitability. Limited niche separation among deposit feeders, usually the dominant nematode trophic type in muddy sediments, is proposed as the cause for the high species dominance so often characteristic of shallow marine muds. The study afforded the opportunity to examine the quantitative and qualitative aspects of population structure in relation to environmental impact (as indicated by large differences in the heavy metal and organic carbon concentrations within each of the 4 sedimentary regimes). Within each sediment type no differences in population densities, species composition or species diversity of nematodes existed between heavily impacted and apparently non-impacted sediments, or between Long Island Sound and similar coastal regions. These findings cast doubt on (1) the use of heavy metal and organic carbon concentrations as indicators of environmental stress for marine nematodes; (2) the use of diversity indices alone as indicators of environmental deterioration; and (3) the usefulness of field monitoring studies alone for the assessment of pollution impact on marine nematodes.

Numbers and biomass of centipedes (Lithobiomorpha: Chilopoda) in a Betula-Alnus woodland in N.E. England.

Over a period of 1 year the mean monthly population densities of L. forficatus varied between 7.58 m-2 (February) and 24.07 m-2 (October). The equivalent values for L. crassipes were 17.05 m-2 (February) and 64.17 m-2 (October). The annual mean population density of the two species was estimated at 51 m-2 with a biomass of 0.585 g live wt m-2. Less than 4% of the total lithobiid population was found in decaying logs and tree stumps, most of the population inhabited the soil/litter layers.Seasonal fluctuation in numbers was shown to be unimodal in L. forficatus and bimodal in L. crassipes. A similar pattern was noted in the status of the ovaries, L. forficatus appears to have a single peak breeding period (September) while L. crassipes has two (June and September).In terms of biomass, the relative importance of centipedes among other woodland invertebrate predators is high.

Population dynamics and production of a brine shrimp, Parartemia zietziana Sayce (Crustacea : Anostraca), in two salt lakes in westren Victoria, Australia

Quantitative samples of P. zietziana were taken monthly for two years from Pink Lake and Lake Cundare. Shrimps were usually contagiously distributed. To reduce error, samples were stratified resulting in confidence limits of 40-50% for the mean population density. Despite this variability, stable trends emerged, and variation was not so great as to mask significant differences. Length-frequency analyses distinguished cohorts; a regression was established between length and dry weight, enabling growth to be estimated from samples. By combining growth with population densities in Allen curves, production was computed. In Pink Lake and Lake Cundare mean pro- duction was 11.3 and 1.0 g dry weight m-2 year-1 respectively. Generally there were two or three generations per year, but time and extent of recruitment were not predictable. Each generation suffered continuous mortality, the death of young shrimps accounting for most of the production. This mortality remains unexplained; there are no significant predators and salinity and temperature stress would occur only during summer.

Field Ecology of the Exotic Mexican Red-Bellied Squirrel in Florida

An introduced population of the Neotropical red-bellied squirrel ( Sciurus aureogaster ) from Mexico was studied in the Florida Keys. Live-trapping on Elliott Key, Florida, revealed the mean population density of squirrels in a mature subtropical hammock over a 12 month period in 1972 was 2.47 squirrels per hectare. Two pelage color phases present on the island (totally black, and gray with a red belly) occur in the population in a ratio of nearly 50 percent (based on 99 squirrels sampled by live-trapping). Visual censusing of the two color phases in the dense habitat resulted in an incorrect estimate favoring the more highly visible melanistic color phase. The average home range size calculated for 18 adult males was 2.3 hectares, and for nine adult females was 0.9 hectare. Reproduction occurred intermittently throughout the year with no tendency for a synchronized breeding season. Based on limited data from four nestling litters, one or two young may be the normal litter size for the species. The squirrels exhibited a high degree of nest box utilization in the tropical habitat characterized by few natural tree hollows. The species is largely frugivorous and was observed utilizing the fruit or seeds of sixteen different species of tropical trees and shrubs including wild mastic, papaya, coconut, sea grape, and thatch palm. There is little predation pressure on the population, but competition for food and denning sites by black rats, raccoons, frugivorous birds, owls, and other species seems primary limiting factors on the island's squirrel density.

The Determination of Normalizing Transformations for Nematode Count Data From Soil Samples and of Efficient Sampling Schemes

Soil sampling experiments were conducted on 0.01 hectare (ha) plots of Fox loamy sand in areas of low, medium, or high population densities of Pratylenchus penetrans (Cobb). Nematode count data from 1, 20, and 40 core samples were statistically analysed to find normalizing transformations and to compare the efficiency of different sampling schemes. Although negative binomial distributions fitted the observed frequency distributions, normalizing transformations of the form log (x + c), that stabilized variance for all population densities, were obtained empirically. It was estimated that at least seven hours of field sampling and laboratory analysis are required to obtain an estimate of mean population density of a plot within 20 % of the true mean with 95 % confidence. An estimate of nematode population density in a 0.01 ha plot based on a subsample from one 20 or 40 core sample is shown to have very low precision.

Significance of Spider Predation in the Energy Dynamics of Forest‐Floor Arthropod Communities

Spiders were the most important entomophagous predators of a 137Cs—tagged Liriodendron tulipifera decomposer community. Radioisotope techniques were used to quantify the bioenergetic impact of spider predation on the forest—floor detritus food web. Temperature dependent 137Cs elimination by spiders and 137Cs concentrations in field animals were used in intake—loss equations to estimate ingestion by the spider population. Radiocesium body burdens fluctuated seasonally in response to litterfall and seasonal differences in temperature and precipitation. Ingestion rates, assimilation efficiencies, and respiratory energy losses were used to estimate gross and net production of the forest spider population. Annual mean population density of spiders was 126 individuals/m2. Population—density peaks occurred in fall, early winter, and summer. Mean biomass for the year was 43 mg dry wt/m2; maxima occurred in early winter and spring. Spiders 1—10 mg in size accounted for 59% of total biomass; spiders <1 mg or > 10 mg comprised the remaining 24% and 17% of the population, respectively. Predicted values for ingestion, assimilation, and respiration indicate that forest—floor spiders were active throughout a temperature range of 5°—20°C, with 13.3° the optimum for net production by spiders of all size classes. Spiders killed an equivalent of 2.08 kcal prey tissue/m2 per year and ingested 1.68 kcal/m2 per year. Respiratory energy loss equaled 1.23 kcal/m2 per year. Trophic level efficiencies calculated from seasonal energy—budget data were: (1) intake efficiency (prey killed/ingestion), 73—86%; (2) assimilation efficiency (assimilation/ingestion), 90—99%; (3) respiration efficiency (respiration/ingestion), 58—96%; (4) respiration/assimilation, 63—106%; (5) ecological growth efficiency (net production/ingestion), —5—34%; and (6) tissue—growth efficiency (net production/assimilation), —4—37%.

Culture of rotifer <i>Brachionus angularis</i> Hauer feeding with dried <i>Chlorella</i>

A study was carried out to know the performance of powdered dried Chlorella for culture of rotifer Brachionus angularis. B. angularis was fed with powdered dried Chlorella in treatment 1, live or fresh cultured Chlorella in treatment 2 and baker’s yeast in treatment 3. The rage of physicochemical parameters during culture of rotifer such as water temperature, air temperature, pH and dissolved oxygen were within the suitable ranges for B. angularis culture and more or less similar in all the treatments. The highest population densities of B. angularis recorded in treatment 1, treatment 2 and treatment 3 were 50, 60 and 30 ind/ml, respectively. The mean population densities were 30.1±12.2, 37.4±14.6 and 21.1±6.1 ind./ml in treatment 1, treatment 2 and treatment 3, respectively. The results revealed that the powdered dried Chlorella had significant effect on the population density of B. angularis and was better than that of baker’s yeast. So, when live Chlorella will not be available, powdered dried Chlorella can be successfully used as feed for B. angularis culture. 
 
 Keywords: Rotifer, Chlorella, yeast, culture medium 
 
 Univ. j. zool. Rajshahi Univ. Vol. 26, 2007. pp. 73-76

Pythium and Mortierella in soils of Ontario forest nurseries.

Populations of Pythium and Mortierella species were estimated by plating surface soils in agar media containing pentachloronitrobenzene, pimaricin, and bacterial antibiotics. Pythium spp. were found in 16 of 17 Ontario soils, mainly seedbed soils in forest nurseries. There was significant variation in the mean population density of Pythium propagules between two seedbeds sampled in each nursery and between the nurseries. The population was high in 10 soils (7 to 57 propagules per cm3) with P. irregulare as the dominant species. The following species were fairly abundant (in decreasing order): P. rostratum, P. ultimum, and P. oligandrum. Pythium was not significantly correlated with properties of the nurseries or with populations of Mortierella, bacteria, Streptomyces, and heat-tolerant fungi. The Pythium population was high only in old nurseries. Mortierella spp. occurred in all soils varying from 1 to 500 propagules per cm3.

Production Rate, Population Density, and Drift of a Stream Invertebrate

Production rates of the stream mayfly, Baetis vagans McDunnough (Ephemeroptera: Baetidae), were estimated by two different methods, compared, and related to population density and drift rate. In the year of study, this mayfly had two summer generations and one winter generation. The two production rate methods yielded estimates of 9.1 and 12.6 g/m2 year as the sum of the three generations, although both estimates were minimal. Production rates for the summer generations were higher than for the winter generation, although winter production was still at a significant level. The summer generations drifted in large quantities, up to about 22 g/day, but the winter generation appeared to drift little, if any, until near the end of the generation in early spring. The relation between annual production rate and mean population density, or turnover ratio, was 9.7, or 3.2 times the number of generations.