Mean Crowding Index Research Articles

Development of microbial communities associated with tobacco root systems

Comparisons were made between patterns of microbial community development associated with tobacco root systems in nonautoclaved and autoclaved soils. Patterns of root system colonization by fluorescent Pseudomonas spp, actinomycetes and other bacteria did not differ between the two soils. Fungal community diversity, measured in terms of number and densities of taxa present, was greater in association with rhizospheres and root surfaces of tobacco grown in nonautoclaved soil than in autoclaved soil. Diversity within the rhizosphere and at root surfaces increased more slowly over time in autoclaved soil than in nonautoclaved soil. At all sampling dates fungal colonization of surfaces of first-order roots was more extensive in nonautoclaved soil than in autoclaved soil. As measured by Lloyd's index of mean crowding, average densities of hyphae in colonized regions of root surfaces were greater in nonautoclaved soil than in autoclaved soil at 7 and 28 days of plant growth. Colonized regions along root surfaces were more patchy or aggregated within autoclaved soil than in nonautoclaved soil as determined from values of Lloyd's index of patchiness. Measurements describing root surface colonization remained fairly constant over time because samples at each date were taken from tissues of similar physiological age.

Insect Distribution and Insecticide-induced Mortality in Corn and Potatoes as Affected by Plot Size and Location

This study was designed to determine if results of insecticide tests were affected by size and location of field plots used. Population density of insects in plots treated with carbaryl varied with plot size. Feeding damage by the potato leafhopper, Empoasca fabae (Harris), to potato foliage dropped rapidly as the treated plot size increased from 4 to 99 m2 and then continued to decline gradually to 1,000-m2 plots. Thus, a plot of ca. 100 m2 may be the minimum size to accurately determine insecticidal effect on the potato leafhopper. Corn earworm, Heliothis zea (Boddie), egg numbers changed the most sharply as plot size increased from 3 to 31 m2. Larval numbers dropped most sharply when plot size was increased from 3 to 14 m2. A minimum plot size of ca. 30 m2 appears to be necessary for corn earworm infesting sweet corn for tests done under similar conditions. Taylor’s index of aggregation and Lloyd’s mean crowding index and patchiness index were calculated for potato leafhopper injury in potatoes, caterpillar feeding damage in sweet corn ears, and numbers in sweet corn plots of Orius insidiosus (Say), Coleomegilla maculata (De Geer), corn earworm eggs and larvae, and European cornborer larvae, Ostrinia nubilalis (Hubner), in treated and untreated areas of plots.

Sampling of Bemisia tabaci (Genn.) (Sternorrhyncha: Aleyrodidae) in Sudanese Cotton Fields

The distribution of the red-eye nymphs of the tobacco whitefly, Bemisia tabaci (Gennadius), within the cotton plant was analyzed during the growing season 1980–1981. Whiteflies distinctly aggregate on and around a main stem leaf, whose location varies according to plant growth and cotton variety. Sampling costs were reduced considerably by counting red-eye nymphs on a distal leaf sector only. The mean crowding index was used to define numerical and sequential sampling plans with a given precision level.

Sampling statistics for Panonychus ulmi (Koch) (Acarina, Tetranychidae) and Tetranychus urticae Koch (Acarina, Tetranychidae) feeding on apple trees

The distribution of Panonychus ulmi (Koch) and Tetranychus urticaeKoch as observed in Swiss apple orchards was analyzed in a multi-stage sampling procedure. Based on the variance-mean relationship (Taylor, 1961) a transformation of mite counts was made. Major sources of variation were found in between-tree and between-crown level differences. In usingBliss andOwen's (1958) method the negative binomial model was concluded to be appropriate for the description of the P. ulmi but not for the T. urticae distribution. HenceCroft's et al. (1976) sampling statistics was recalculated for P. ulmi only. The mean crowding index (Lloyd, 1967) was linearly related (Iwao, 1968) to the mean for P. ulmi and to a less extent for T. urticae that was found in much lower density range. Based on the mean crowding statistics a sampling program afterKuno, (1976) is proposed for both species. Some differences between the sampling plans and their applicability are discussed.

Confidence Estimation of Ecological Aggregation Indices Based on Counts-a Robust Procedure

SUMMARY The problem of confidence-interval estimation of indices of spatial aggregation based on counts, such as Lloyd's indices of patchiness and of mean crowding, and the variance-mean ratio, is considered. A distribution-free method based on a straightforward application of the jackknife is presented. The accuracy of the method in small samples is investigated by simulation. A transformation which should improve the small-sample performance is suggested. Application of the method to both one-sample and two-sample problems is illustrated with data previously analyzed by methods that require specific distributional assumptions. Indices of nonrandomness in the spatial distribution of organisms, based on the mean and variance of counts of individuals per sampling unit, are frequently used by ecologists. Examples are the 'variance-mean ratio', David and Moore's (1954) 'index of clumping', Morisita's (1962) 'index of dispersion' and Lloyd's (1967) 'index of mean crowding' and 'index of patchiness'. Certain theoretical objections to the use of any of these indices, when the sampling units are quadrats placed randomly over a continuum, have been pointed out. Paloheimo and Vukov (1976), for example, show that none of the indices is invariant under changes in quadrat size, for a variety of theoretical clustering processes. For this reason it has been suggested that, whenever possible, count-based indices should be eschewed in favour of distance-based measures. Although, in certain circumstances, distance-based methods are on surer theoretical ground than the count-based methods, there remain, nonetheless, situations in which count-based methods present the only alternative. This may be for theoretical or practical reasons. An example of the latter, given by Patil and Stiteler (1974), is in the study of soil arthropods. Counts of animals per given volume are relatively easy to obtain, but distance measurements are virtually impossible. A similar situation pertains when plankton are collected in a plankton net. In the first example, contiguous quadrat analysis, along the lines initially proposed by Greig-Smith (1952), would present an alternative to the use of a single count-based index or statistic, but in the second example this alternative would not be possible. Count-based methods are theoretically justified when organisms occupy discrete habitat areas (such as, for example, insects living on leaves of trees), for in this case the habitat areas form natural sampling units. Distance-based methods are not useful in such situations. However, even in situations where the use of count-based indices can be justified, there often remains a statistical problem associated with their use, namely how to assess the sampling variation associated with an index calculated from sample data. Without this, of

Extraction from Foliage and Within-Plant Distribution for Sampling of Brachycolus asparagi (Homoptera: Aphididae) on Asparagus1

Methods for extracting Brachycolus asparagi Mordvilko from asparagus, Asparagus officinalis L., foliage and the distribution of the aphid on the plant were studied to increase sampling efficiency. Aphids were efficiently extracted by methyl isobutyl ketone and Berlese-Tullgren funnels, but not by beating. Aphids had a clumped distribution and numbers decreased with increasing height on the plant. Sampling efficiency was optimized by sampling the lower secondary branches on the plant. The advantage of using Taylor’s power law over the mean crowding index to determine the optimum number of samples is discussed.

Sampling Variability and Life History Features: Basic Considerations in the Design of Aquatic Insect Studies

Sampling variability in benthic studies may result from sampling device operation, physical features of the environment, laboratory sorting procedures, and biological features of study populations. Selected factors and procedures that influence variability, samplers affected, and proposed remedies are presented. Consequences of not considering autecological components in sampling designs are illustrated by analysis of larval counts of Cheumatopsyche pettiti (Banks), a multiple cohort caddisfly with an aggregated population. The range of mean numbers of C. pettiti was great with low sample numbers. Aggregation is more reliably measured at low sample numbers with the Index of Dispersion and the Mean Crowding Index than with the dispersion parameter k, the calculation of which from the maximum-likelihood equation, is integrally related to sample size. Nonrandom patterns of C. pettiti observed from samples collected in an Indiana, USA, stream riffle, may result from a failure to consider hyporheic distributions, spatial influences (e.g. sampling both favored and nonfavored microhabitats), instar-specific differences, and behavioral features. Variability in secondary production estimates of an aggregated population of Ceraclea ancylus (Vorhies) from a Kentucky, USA, stream indicated similar relationships to sample size.The size of the mean, the degree of aggregation, and the desired precision of the mean estimate will influence the number of samples required to estimate densities of benthic populations. Sample size requirements calculated from data reported in previous studies were high to achieve accepted levels of precision. Habitat stratification may reduce the numbers of samples required. Dicosmoecus gilvipes (Hagen) exhibited nonaggregated patterns and required fewer samples to estimate density in uniform substrate areas of a California, USA, river pool than did aggregated populations in both mixed substrate areas and the entire pool. Ceraclea ancylus required fewer samples for density estimates in stratified (by habitat or substrate type) than unstratified habitats, the fewest samples being necessary when the individual stone was the sampling unit. Judicious choice of study populations may permit larger numbers of samples to be collected and processed with reduced cost, as an alternative to stratification. Larvae of C. ancylus and D. gilvipes could be separated in the field; density underestimation due to a hyporheic population component was eliminated because of surface dwelling behavior or by choice of study sites; and compounded spatial distributions due to co-occurring instar-specific patterns were absent because the populations have a single cohort.Larger numbers of samples may be necessary than are generally taken in benthic studies. Further research is needed to assess variability in secondary production estimates and community diversity analyses. Improved methods for substrate surface area estimation and increased use of experimental approaches and sequential sampling techniques should be considered in future benthic sampling designs. Key words: sampling, benthos, aquatic, macroinvertebrate, Trichoptera, insect, experimental design, autecology, life history, variability

A STUDY ON THE DISPERSION PATTERN AND FOLIAGE INJURY BY TETRANYCHUS MCDANIELI (ACARINA: TETRANYCHIDAE) IN SIMPLE APPLE ECOSYSTEMS

AbstractPopulation growth and dispersion of Tetranychus mcdanieli McGregor were studied in an experiment conducted on Red Delicious apple trees grown in a light and temperature controlled walk-in growth room. Lloyd’s index of mean crowding was calculated as a means of interpreting aggregation patterns; however, it was found that mean colony size was a more generally useful population parameter . The regression of on mean density (m) and the ratio of mean density to were used to determine and characterize the patterns of dispersion obtained under the experimental conditions of the study.It was shown that the ratio of mean density to is the proportion of sampling units (i.e. individual leaves) that would have a colony size of individuals per unit, all other units being unoccupied. Characterization by the concomitant parameters and provides a more sensitive and meaningful method of describing populations than does the arithmetic mean.The data indicated that adult female populations of T. mcdanieli tend to be distributed as clumped colonies on apple trees. A plant feeding mite damage scale was devised to evaluate the reliability and accuracy of gross observational estimates as indicators of spider mite levels.