Non-random Dispersal Research Articles

SEED DISPERSAL, SEED PREDATION, AND SEEDLING RECRUITMENT OF A NEOTROPICAL MONTANE TREE

Postdispersal fate of seeds from Ocotea endresiana (Lauraceae), a bird-dispersed Neotropical montane tree, was studied in Costa Rica to determine the influence of seed dispersers, seed predators, and microhabitat characteristics on seedling recruitment. Particular emphasis was placed on finding naturally dispersed seeds in order to study the link between dispersal and postdispersal fate of seeds. Four species of birds (Emerald Toucanet, Aulacorhynchus prasinus; Resplendent Quetzal, Pharomachrus mocinno; Three-wattled Bellbird, Procnias tricarunculata; and Mountain Robin, Turdus plebejus) dispersed the seeds by regurgitation, and one species (Black Guan, Chamaepetes unicolor), by defecation. Most seeds (80%) were dispersed within 25 m of parent trees and under high (>92%) canopy cover. Bellbirds deposited 52% of the seeds they dispersed under habitual song perches in standing dead trees on the edges of treefall gaps >25 m from parent trees. In contrast, the other four species dispersed only 6% of the seeds they dispersed >25 m from parent trees, and <3% of the seeds were dispersed to gaps. Thus, male bellbirds provided predictable, nonrandom dispersal to a different microhabitat than the other four species. Seed predation, germination, and one-year seedling survival were assessed for naturally dispersed seeds and for seeds placed at randomly located sites. Seed predation during the first 12 mo after dispersal was extremely high (99.7%). At least 50% of seed removal was attributable to small rodents, such as Peromyscus mexicanus, with no evidence of scatterhoarding or secondary dispersal. Seeds dispersed >20 m from parent trees were removed more slowly than seeds directly under the crown, but over the course of the fruiting season seed predation approached 100%, throughout the study site. Seed predation did not differ between gaps and forest understory. Germination of seeds protected from mammals was high (70–90%), regardless of microhabitat or seed size. Seeds regurgitated by birds and seeds removed from fruits by hand germinated equally well (98%), but seeds in intact fruits did not germinate. Seeds defecated by guans and regurgitated by other species in the field had similar germination success. Thus, dispersal by birds is important for removal of pulp but does not otherwise affect germination. Seedlings were taller in gaps (<90% canopy cover) than in closed-canopy forest (>90% cover). Fungal pathogens killed fewer seedlings in gaps than in understory, while other causes of seedling mortality were similar among microhabitats. Consequently, bellbirds, the most likely species to disperse seeds to gaps, had a disproportionate influence on seedling recruitment. Nevertheless, the absolute number of seedlings was higher in closed canopy forest, because the great majority of seeds (85%) landed there. The overall pattern of recruitment, averaged among trees, had a large peak corresponding with bellbird song perches in gaps, as well as a smaller peak near the parent tree corresponding with the seeds dispersed by other species. The relative height of these peaks is opposite that of the original pattern of dispersal. Frugivorous birds that use habitual perches may have disproportionate effects on plant recruitment. To test this idea further, it is necessary to study each stage of the recruitment process, including seed dispersal, postdispersal seed survival, and seedling survival.

In Situ Conservation of Wild Chiles and Their Biotic Associates

Wild congeners of domesticated crops increasingly serve as sources of genes for improving crop cultivars. Although wild congeners have been included in seed collections for ex situ storage, there has been little work to protect populations of these wild species in their natural habitats for in situ conservation. We assessed the distribution of chile plants (Capsicum annuum L. var. aviculare[Dierbach] D’Arcy and Eshbaugh) relative to the dominant woody vegetation of one subpopulation in a single drainage in southern Arizona, U.S.A. Wild chiles were not found in direct sun, and the distribution of chiles under different nurse plants could be a function of random chance, microenvironmental differences under different nurse‐plant species, or nonrandom dispersal by chile consumers. To examine chile distribution, we measured the association of wild chiles with nurse‐plant species and compared these associations with the available cover provided by each nurse plant. We also measured the buffering capacity of each nurse‐plant species, conducted mammalian and avian food‐preference experiments to determine the taxa dispersing chiles, and conducted time‐budget studies of potential chile dispersal agents. Wild chiles were not randomly distributed: over 75% were under the canopies of fleshy‐fruited shrubs that collectively made up less than 25% of the cover. We found limited evidence that differences in buffering capacity affected chile distribution. Food‐preference experiments suggested that birds are the only effective dispersal agents, and the time budgets of three common bird species were strongly correlated with chile plant distribution. These results lend support for directed dispersal by avian consumers. The distribution of chiles appears to be a function of interactions between consumers, nurse plants, and the secondary chemicals in the chiles themselves. Only through studies of in situ populations can we understand the interactions that sustain both wild‐crop relatives and the genetic variability essential to future crop management.

The hotspot effect in heterogeneous vegetation canopies and performances of various hotspot models

In an earlier paper (Qin and Goel, 1995), we reviewed various analytical hotspot models and evaluated them for homogeneous vegetation canopies by comparing them against Diana—a computer graphics, radiosity based, model. In this paper, we complete their evaluation by studying the hotspot effect of heterogeneous canopies (trees and crops). For this purpose, we first describe a computer graphics based method with which the cross‐correlation (or overlap) function that drives the hotspot effect can be accurately evaluated. We find that all the analytical hotspot models, more or less, overestimate the cross‐correlation function for tree canopies. Except the hybrid model, they perform worse in the principal cone (PC) than in the principal plane (PP); for big leaves or at low LAI or small spacing than for small leaves or at high LAI or large spacing. We investigate in detail various elements of heterogeneity (spacing distance between trees/plants, mutual shadowing between leaves and between tree crowns, crown porosity (within‐crown gaps), row structure (for crops), fraction of nonleaf organs, and nonrandom dispersion of vegetation elements etc.), and identify their influences on the canopy hotspot effect. The result shows that mutual shadowing, which is quite significant at high LAI and/or small spacing or under high solar/view zenith in PP, reduces the canopy hotspot and causes cross‐correlation function to decrease with LAI (for a fixed leaf size) rather than increase with LAI when mutual shadowing is neglected. The effect of spacing on the cross‐correlation function is significant if crown is used as the unit and insignificant (except for low LAI at low solar zenith angle) if leaf is used as the unit and LAI is kept fixed when spacing varies. At low LAI and/or small spacing (for a fixed LAI), crown porosity strongly affects the overlap function that uses crown as the unit, and causes geometric‐optics based hotspot models to considerably overestimate the overlap function. The large proportion of nonleaf elements in a canopy with small spacing and low or medium LAI often results in an asymmetric distribution of the canopy hotspot in PP. The row structure in row‐planted crops causes the zenithal width of hotspot to depend on the angle between solar azimuth and row direction and some unusual fluctuations of the cross‐correlation function (even a secondary peak) in PC. We present one modified expression for cross‐correlation function to account for mutual shadowing which is neglected in existing analytical models using leaf as a unit, and another for overlap function to allow for within‐crown porosity which is neglected in hotspot models using crown as the unit. Both expressions improve the accuracy in representing hotspot phenomenon

Seasonal, Diel, and Spatial Dispersion Patterns of Golden Eagle Autumn Migration in Southwestern Montana

In this report we describe three behavioral phenomena of Golden Eagle (Aquila chrysaetos) fall migration: agespecific seasonal differential migration, age-specific diel patterns of migration, and the spatial dispersion pattern of migrating Golden Eagles. Immature individuals of many North American raptor species migrate earlier in the season than adults (see Appendix 2 in Kerlinger 1989). As resource levels decline in the fall, less efficient foragers (Hoffman 1985) or subordinate birds (Gauthreaux 1978) are likely to be the first to seek resources elsewhere. Hoffman and Potts (1985) reported that immature Golden Eagles migrate earlier in the season than adults although Kerlinger (1989) reported the reverse. We examined our data to address whether there was seasonal differential migration between age classes of Golden Eagles migrating along the Bridger Range. Because different travel requirements or behavioral differences between age classes may result in age-specific migration patterns within periods of the day, we also investigated whether immature and adult eagles had different diel patterns of migration. A migrating raptor searching for lift may locate an updraft by flying towards other soaring birds. Such visual interaction among migrants would result in aggregation. This is similar to Kerlinger's (1985) aggregation model for flock formation. While his model applies to species that typically migrate in flocks, other species that do not form discrete flocks may still exhibit nonrandom dispersion due to visual interaction. Alternatively, Golden Eagles may avoid other eagles during migration. Pairs of Golden Eagles space out the nests that they use in a given year more regularly than random (Watson and Rothery 1986) and their territorial behavior may carry over to the migration season. In that case we would observe even dispersion. For aggregation or repulsion due to visual interaction to occur, concentrations of migrants would have to be sufficiently dense that individuals could see each other. Furthermore, aggregations of aerodynamically dissimilar birds would be ephemeral because of different climb and glide characteristics so it is appropriate to focus on concentrations of aerodynamically similar birds, e.g., birds of the same species, when testing for nonrandom dispersion. Golden Eagles passed our lookouts in sufficient densities that if they respond to the presence of other soaring Golden Eagles we should have observed nonrandom dispersion.

Environmental and Demographic Correlates of Spatial and Seasonal Genetic Structure in the Eastern Cottontail (Sylvilagus floridanus)

Elucidation of the complex interactions of forces that affect genetic structure within and among populations is important in the assessment of short-term viability and evolution of natural populations. We examined spatial and temporal changes in genetic, demographic, and ecologic characteristics of the eastern cottontail (Sylvilagusfloridanus; n = 543) within two isolated populations inhabiting playa lake basins in the Texas Panhandle. Spatial autocorrelation and contiguous clustering analyses documented nonrandom dispersion of genotypes within both populations, during each of four consecutive seasons. Considerable seasonal variation in genotypic autocorrelative patterns and size, position, and genotypic composition of significant contiguous clusters also was observed. Genotypic structuring within playa basins was particularly evident during spring, summer, and autumn; greater randomization of genetic distributions occurred in the winter, prereproductive season. Presence of spatial associations of genotypes and changes in their position and composition over seasons were influenced by episodic exchange of individuals from genetically different populations, subsequent random mating and restoration of Hardy-Weinberg genotypic proportions in resulting offspring, and patchy distribution of habitat types within playa basins. Mantel matrix regressions, which were used to test relationships among demographic, habitat, and genotypic variables show that the relative importance of each factor changes seasonally. Differences in habitat use among individuals were significantly correlated with genotypic cluster affiliation during the prereproductive period, whereas age structure became increasingly important in explaining spatial arrangement of genotypes during later periods. Although populations from different playa basins differ in gene frequency, seasonal changes in genetic structure within each population were correlated.

Baboons of the amboseli basin: Demographic stability and change

A cross-sectional demographic analysis of the entire baboon population of the Amboseli basin of southern Kenya was undertaken to complement the lon-gitudinal, intensive studies of a subpopulation. The present survey documented the extent and persistence of the influx of anubis baboons into the predominantly cynocephalus community and provided another example of the nonrandom dispersal patterns of cercopithecine males. In addition, the survey confirmed continued demographic stability of the basinwide baboon population and even growth in groups that had access to better feeding conditions, despite decline of the baboons' preferred habitat and expansion of human activities into wildlife areas. Conflicts with activities of humans, however, indicate that the present well-being of the Amboseli baboon population may be short-lived.

Conspecific Attraction and the Determination of Metapopulation Colonization Rates

Natural habitats, hence animal populations, are becoming increasingly fragmented and vulnerable as the result of human activities. The metapopulation concept (Levins 1970; Gilpin 1987; Harrison et al. 1988), which is applied to interdependent patches of occupied and unoccupied habitat, can be used to describe the dynamics of these fragmented populations. The ultimate goal is to understand these systems well enough to predict their behavior and manage for their persistence (Gilpin 1987). One of the most important aspects of metapopulation dynamics is the flow of individuals between patches (Gilpin 1987), yet dispersal between patches and their subsequent colonization have generally been assumed to be stochastic processes. Here we propose that the actions of dispersers may be profoundly influenced by the presence of conspecifics on neighboring patches, leading to a colonization process that may be much more deterministic than was previously believed. We advance the idea that taking nonrandom dispersal and colonization patterns into account can enhance management of metapopulation systems. The persistence of metapopulations is a function of the extinction rate of individual patches and their probability of subsequent colonization (Levins 1970; Gilpin 1987). A rich literature has outlined the factors that may influence the balance between extinction and coloniza-

Breeding seasons of oceanic birds in a subarctic colony

We studied the breeding seasons of marine birds on the Semidi Islands, western Gulf of Alaska, from 1976 to 1983. Distributions of laying or hatching observed in 11 species during 1–7 years are presented; less detailed information is available on the breeding schedules of three species. The combined laying period of 14 species lasted 3 months from mid-April to mid-July; the first eggs of the earliest and latest species were laid about 9 weeks apart. Mean laying dates varied by 2–16 days in nine species observed in 2 or more years. Although the egg-laying sequence of species was largely preserved from year to year, we found little evidence of concordant annual variation in breeding seasons. Species that fed lower in the food chain tended to breed earlier than those that fed at higher trophic levels. Early laying was correlated with longer laying periods, both within and between species. With the exception of three puffin species, birds with similar food habits exhibited less overlap in hatching and fledging dates than laying dates. A test for nonrandom dispersion of breeding times failed to find evidence for competitive avoidance of breeding overlap in piscivores or planktivores. Species may not have timed their egg laying to provide for maximum food availability during chick rearing. Rather, the comparative analysis of breeding schedules suggests that breeding times were determined more by the food requirements of laying females. Young females laying relatively late probably account for the right-skewed distributions of egg laying observed in this and other studies of colonial seabirds.

Automated Quantification of Roots Using a Simple Image Analyzer

AbstractManual methods for estimating root lengths in washed samples are time consuming, tedious, and subjective. The objective of this research was to evaluate the use of an inexpensive commercial image analysis system for measuring the length of roots in samples washed from soil. The analyzer consists of a high‐resolution television camera and a comparator, which measures the intersections of the TV scan lines and roots in the view area. Mathematical corrections are derived for overlapping roots in the sample and for limitations in system resolution. Errors due to nonrandom orientation of roots were minimized by scanning in two directions. Methods for contrast and light control are discussed. On a 1000 cm simulated root sample, the uncorrected length measurement was &lt; 50% of the actual length. Correcting for overlaps increased the measured length to around 70% of actual length. Overlap and resolution correction resulted in errors less than 5%. Coefficients of variation (CV) for repeated measurements on a single, undisturbed wheat (Triticum aestivum L.) root sample were around 0.5%, while errors resulting from nonrandom dispersion of samples in the tray were 2‐3%. The image analysis system therefore appears to capable of providing reliable root measurements.

Oviposition behavior of Aedes triseriatus (Diptera: Culicidae): prevalence, intensity, and aggregation of eggs in oviposition traps.

The oviposition behavior of the treehole mosquito Aedes triseriatus (Say) was studied by counting the eggs laid in 300 oviposition traps weekly for one season (May-October) in Illinois. Eggs were aggregated spatially among traps each week, and temporally among weekly samples and for individual traps over weekly counts. Aggregation could be associated with a nonrandom dispersion pattern of oviposition events and with the oviposition of 20-60 eggs per event. Thus, females were laying clumps of eggs in an overdispersed manner. The greatest number of eggs was found in late July when prevalence (proportion of traps with eggs) and intensity (number of eggs per positive trap) peaked simultaneously. Intensity also peaked in May and prevalence peaked in mid-June and early September. When traps from which eggs were removed each week were compared with traps in which eggs were allowed to accumulate, prevalence was naturally higher in the latter group. However, the intensity of eggs was similar in both groups, indicating that more eggs were laid in traps from which eggs were removed. Thus, the presence of eggs on oviposition surfaces was associated with a decrease in further oviposition.

Hilltopping Checkerspot Butterflies Revisited

Previous articleNext article No AccessNotes and CommentsHilltopping Checkerspot Butterflies RevisitedPaul R. Ehrlich and Darryl WheyePaul R. Ehrlich Search for more articles by this author and Darryl Wheye Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The American Naturalist Volume 132, Number 3Sep., 1988 Published for The American Society of Naturalists Article DOIhttps://doi.org/10.1086/284864 Views: 2Total views on this site Citations: 8Citations are reported from Crossref Copyright 1988 The University of ChicagoPDF download Crossref reports the following articles citing this article:Patrick Grof-Tisza, Zack Steel, Esther M. Cole, Marcel Holyoak, Richard Karban Testing predictions of movement behaviour in a hilltopping moth, Animal Behaviour 133 (Nov 2017): 161–168.https://doi.org/10.1016/j.anbehav.2017.08.028Victoria J. Bennett, Winston P. Smith, Matthew G. Betts Evidence for Mate Guarding Behavior in the Taylor’s Checkerspot Butterfly, Journal of Insect Behavior 25, no.22 (Aug 2011): 183–196.https://doi.org/10.1007/s10905-011-9289-1Oded Levanoni, Noam Levin, Guy Pe'er, Anne Turbé, Salit Kark Can we predict butterfly diversity along an elevation gradient from space?, Ecography 34, no.33 (Nov 2010): 372–383.https://doi.org/10.1111/j.1600-0587.2010.06460.xGuy Pe’er, Simone K. Heinz, Karin Frank Connectivity in Heterogeneous Landscapes: Analyzing the Effect of Topography, Landscape Ecology 21, no.11 (Jan 2006): 47–61.https://doi.org/10.1007/s10980-005-1622-7Guy Pe'er, David Saltz, Hans-Hermann Thulke, Uzi Motro Response to topography in a hilltopping butterfly and implications for modelling nonrandom dispersal, Animal Behaviour 68, no.44 (Oct 2004): 825–839.https://doi.org/10.1016/j.anbehav.2004.02.006Gretchen C. Daily, Paul R. Ehrlich, Darryl Wheye Determinants of spatial distribution in a population of the subalpine butterfly Oeneis chryxus, Oecologia 88, no.44 (Dec 1991): 587–596.https://doi.org/10.1007/BF00317724David Yeates, Gary Dodson The mating system of a bee fly (Diptera: Bombyliidae). I. Non-resource-based hilltop territoriality and a resource-based alternative, Journal of Insect Behavior 3, no.55 (Sep 1990): 603–617.https://doi.org/10.1007/BF01052331Gary Dodson, David Yeates The mating system of a bee fly (Diptera: Bombyliidae). II. Factors affecting male territorial and mating success, Journal of Insect Behavior 3, no.55 (Sep 1990): 619–636.https://doi.org/10.1007/BF01052332

Spatial and temporal genetic differentiation among the seed in a stand of Fagus sylvatica L.

In a beech population composed of three parts (demes), the genetic structure of the seed (after dispersal) of each deme was recorded in two successive years and at five protein loci, four of which were polymorphic. Spatial and temporal differentiation, as evidenced in allele frequencies, genie diversity and heterozygosity, was studied. While no differentiation was found for diversity, allele frequencies and heterozygosity showed significant spatial differentiation at the genically most diverse loci in both years. Temporal differentiation measured as genetic differences between the seed production of the two years was distinctly lower. The total seed production over both years showed a tendency of homogenising spatial genetic differences present in each single year. This observation can be explained by local differences in flowering of the trees from one year to the next combined with non-random dispersal of the seed, where, however, the reproducing parts of the stand were genetically similar in both years. Comparisons with the results of related studies on ponderosa pine revealed remarkable similarities in the amounts of spatial subpopulation differentiation. Noticeable differences occur with respect to the individual deme levels of differentiation. Consequences for the interpretation of adaptive strategies of trees are briefly discussed. The data on diversities and conditional heterozygosities suggest that there is a tendency for rare alleles to occur preferentially in homozygous form among the seed. The significance of this phenomenon for the maintenance of genetic polymorphisms is demonstrated in the light of recent population genetic selection theories.

Nonrandom Dispersal in Free-Ranging Vervet Monkeys: Social and Genetic Consequences

Male vervet monkeys in Amboseli National Park, Kenya, disperse nonrandomly from their natal groups at sexual maturity, and migrate to specific neighboring groups with their brothers or peers. Nonrandom dispersal in the company of allies appears to benefit young males by minimizing the risk of predation and reducing the probability of attack by resident males and females. Nonrandom dispersal also decreases the likelihood of mating with close female kin. Persistent nonrandom transfer, however, increases the risk of inbreeding depression. We emphasize the importance of considering social, as well as genetic, factors when evaluating the costs and benefits of any pattern of dispersal. In certain cases, there may be social advantages to a particular dispersal pattern even though it appears initially to carry some genetic costs. In other cases, a dispersal pattern that at first appears to increase the risk of inbreeding may in fact permit individuals to avoid mating with close kin. Finally, the benefits of a giv...

Epibenthic megafauna of a bathyal basin off southern California: patterns of abundance, biomass, and dispersion

A total of 50 species was recorded in a survey of near-bottom megafauna at a depth of 1300 m in the Santa Catalina Basin using the submersibles Alvin and Sea Cliff, and bottom trawls. The nine most common epibenthic types were counted in Alvin photographic surveys, yielding a total density of 16.7 individuals per m 2. Biomass of the four most abundant species (an ophiuroid, gastropod, holothurian, and rockfish) totaled 67.9 g m −2. The ophiuroid Ophiophthalmus normani comprised 99% of the community in terms of abundance and standing crop. All four dominant epibenthic species showed nonrandom dispersion patterns on the studied scales of one-half to hundreds of meters. Ophiophthalmus normani showed weak tendencies toward evenness on the smallest scale (0.5 m) and also formed randomly located patches 1 to 4 m in diameter. Both the gastropod Bathybembix bairdii and the holothurian Scotoplanes globosa aggregated in clumps from 25 to 96 m in diameter, but they showed no significant between-species correlation in patch location. The rockfish Sebastolobus altivelis was in most cases evenly distributed on scales of 6 to 158 m. Potential causes of patterns included attraction to detrital kelp parcels, avoidance of disturbed areas, breeding aggregation, and other social interactions. We conclude that the megafauna comprises a major fraction of the bathyal benthic biomass and that it, particularly the ophiuroids, significantly impacts the energetics and ecology of bathyal communities.

Spatial and Temporal Genetic Variability of the Eastern Cottontail on West Texas Playa Basins

Spatial and temporal changes in allelic frequency, genotypic proportions, and total and single locus heterozygosity of the eastern cottontail ( Sylvilagus floridanus ) were studied on two playa lake basins in the Texas Panhandle. Playa basins in this region are separated by extensive areas of agricultural and pasture lands, and therefore constitute the only significant cottontail habitat. Blood plasma samples were obtained from 543 individuals captured on both playa basins, during seven live-trapping periods from February through November, 1981. Adult and seven juvenile age classes were identified. Prereproductive adult cottontail populations had observed genotypic proportions that deviated significantly from expected Hardy-Weinberg equilibrium, seemingly the result of heterozygote deficiencies related to differential overwinter selection and/or nonrandom genotypic dispersal. These findings are inconsistent with previous studies in which increases in heterozygosity were shown to correlate with superior fitness. Significant ( P > 0.05) changes in allelic frequencies, genotypic proportions, and heterozygosity were related to population demographic characteristics. Apparent random matings by individuals of parental genotypes and subsequent juvenile recruitment dampened or eliminated the disequilibrium in Hardy-Weinberg proportions. Changes in population number due to reproductive recruitment and dispersal were accompanied by changes in gene frequency and heterozygosity. Spatial differences in genetic attributes were maintained between the two playa basins throughout the study because of the large distances between breeding groups. However, parallel trends were observed in population genetic and demographic characteristics on both playa basins.

Dispersion Patterns of the Red Imported Fire Ant (Hymenoptera: Formicidae), Aphids, and Some Predaceous Insects in East Texas Cotton Fields1

Discrete frequency distributions, obtained from count data on Solenopsis invicta Buren, Aphis spp., Pseudatomoscelis seriatus (Reuter) adults and nymphs. Orius insidiosus (Say). Nabis spp., and Geocoris spp., in 1973 and 1974, were fitted to seven mathematical frequency distribution models. The negative binomial distribution best described the dispersion pattern of these insects in both years, indicating an aggregated, nonrandom dispersion pattern. The parameter k of the negative binomial distribution was used to quantify the aggregation of these insects. The maximum likelihood estimate and the moment estimate were two methods used to estimate the parameter k. A common k was calculated for each insect studied, to determine whether the estimated k values were common to all sampling dates over a season. As indicated by common k values, Aphis spp. and S. invicta were highly aggregated. P. seriatus nymphs were more aggregated than the adults. Among the predators, Nabis spp. were more aggregated than O. insidiosus; and Geocoris spp. were the least aggregated. Taylor's power law was fitted to the variance and mean density values for each insect. The power function well described the relation between the variance and mean density value for each insect

Use of the Dirichlet tessellation for characterizing and modeling nonregular dispersions of second-phase particles

For the purpose of modeling the behavior of materials containing nonregular dispersions of second-phase particles, it is necessary to identify the parameters that best characterize the spatial distributions within the dispersions. For nonrandom dispersions it is proposed that the division of array space into Dirichlet regions centered on each particle will not only uniquely define near neighbors, but will enable clustering and the anisotropy of spacing to be dealt with in a systematic fashion. Using the Dirichlet tessellation procedure, a random array of points is shown to remain random during deformation, and three different classes of nonrandom dispersions are examined quantitatively.

Partitioning of Space in Communities of Ants

SUMMARY (1) The dispersion patterns of nests, size of foraging areas and overlap in foraging areas were measured for all species of ants at ten study sites in the Mojave and Great Basin Deserts of western North America. (2) Overdispersed patterns of nest distribution were common amongst the colonies of conspecifics. Such patterns are believed to result directly from defence of foraging areas and indirectly from intraspecific competition for food. Little evidence of non-random dispersion patterns was found when the colony positions of two species were considered together. (3) The dispersion pattern of all the colonies of all species in a community is uniformly spaced at lower elevations, random at intermediate elevations and aggregated at higher elevations. The trend towards increased aggregations is believed to be due to the increase in vegetation density with elevation. This would tend to limit the areas where ant colonies can be successfully established to unshaded areas between perennial plants. (4) Size of foraging area was found to decrease with increases in both elevation and latitude. It is postulated that size of the foraging area for these ants is negatively correlated with density of food. Overlap in foraging areas decreases with increases in species diversity and is not related to either elevation or latitude.

Cleavage of ferrite strengthened by interphase precipitation of vanadium carbide

AbstractCleavage fracture has been studied in a series of steels strengthened by vanadium carbide dispersions. It has been shown that the impact transition temperature is strongly dependent on the precipitation hardening contribution to the yield strength, this being more important than the precipitate volume fraction. Ageing of non-random dispersions shows that there is no significant difference between the mechanical properties of random and non-random (‘interphase precipitation’) dispersions. Fractographic investigation reveals that the interphase precipitate dispersion can change the path of a cleavage crack during fracture at room temperature, although this does not have any significant effect on the mechanical properties.

A refutation of critical tidal levels as determinants of the structure of intertidal communities on British shores

Early hypotheses to account for the pattern of zonation and vertical distribution of species on rocky shores in Britain invoked the concept of critical tidal levels. These levels were at heights on the shore where the upper or lower boundaries of distribution of a number of species coincided. Critical levels were correlated with heights on the shore where there were changes in the rate of change, with height, of the annual proportion of the time spent emersed. This was calculated from predicted tide tables. One implications of this was that different assemblages of intertidal species were present between particular levels on the shore. Futhermore, this hypothesis depended on patterns of vertical distribution of several species being identically controlled by physical factors associated with the rise and fall of the tide. This conflicts with more recent hypotheses, based upon experimental evidence, which include the effects of biological interactions among species on their patterns of distribution. The methods of calculation of the annual emersion curve were approximate, with a high degree of extrapolation. The hypothesis that critical levels exist has not been quantitatively tested. New, more accurate calculations of the emersion curve, from predicted tidal heights, indicate a smooth, monotonic curve against height on the shore. This eliminates the possibility of correlation between the coincident boundaries of distribution of a group of species and a height at which the curve changes in slope. The upper and lower boundaries of species were recorded in transects on five shores in different parts of Britain. If critical level exist, the upper or lower boundaries of a set of species must depart from random dispersion up and down the shore, to be clumped or aggregated at the critical levels. A test for non-random dispersion of the boundaries indicated no significant departure from random on any of the shores sampled. There is thus no evidence that critical tidal levels exist, because there is no evidence that the upper, or lower, boundaries of vertical distribution of intertidal species are in any way aggregated. There is no possibility of correlation between changes of slope of the emersion curve at particular heights on the shore and the patterns of distribution of intertidal species on British shores. The hypothesis of critical tidal levels must be abandoned and the rôle of the effects of tidal rise and fall on the vertical distribution of intertidal species must be re-examined in any new hypotheses to account for observed patterns of zonation.