Utilization of autochthonous macroinvertebrate drift by a pool fish community in a woodland stream

Qualitative and quantitative drift data were collected simultaneously above and below a pool both before and after the pool was heavily electrofished. These data revealed no significant difference between drift organism densities (#/m³) above or below the pool before or after fish collection. Qualitative and quantitative fish gut analyses suggest that the sunfishes Lepomis megalotis, L. macrochirus and L. cyanellus may be feeding on drifting invertebrates as these organisms comprised 58%, 37% and 35% gut volume, respectively. A design for a long-term drift net apparatus is presented.     

Mesh-size effects on drift sample composition as determined with a triple net sampler

Nested nets of three different mesh apertures were used to study mesh-size effects on drift collected in a small mountain stream. The innermost, middle, and outermost nets had, respectively, 425 µm, 209 µm and 106 µm openings, a design that reduced clogging while partitioning collections into three size groups. The open area of mesh in each net, from largest to smallest mesh opening, was 3.7, 5.7 and 8.0 times the area of the net mouth. Volumes of filtered water were determined with a flowmeter. The results are expressed as (1) drift retained by each net, (2) drift that would have been collected by a single net of given mesh size, and (3) the percentage of total drift (the sum of the catches from all three nets) that passed through the 425 µm and 209 µm nets. During a two day period in August 1986, Chironomidae larvae were dominant numerically in all 209 µm and 106 µm samples and midday 425 µm samples. Large drifters (Ephemerellidae) occurred only in 425 µm or 209 µm nets, but the general pattern was an increase in abundance and number of taxa with decreasing mesh size. Relatively more individuals occurred in the larger mesh nets at night than during the day. The two larger mesh sizes retained 70% of the total sediment/detritus in the drift collections, and this decreased the rate of clogging of the 106 µm net. If an objective of a sampling program is to compare drift density or drift rate between areas or sampling dates, the same mesh size should be used for all sample collection and processing. The mesh aperture used for drift collection should retain all species and life stages of significance in a study. The nested net design enables an investigator to test the adequacy of drift samples.     

A model for accurate drift estimation in streams

1. This paper explores the experimental difficulties involved with the use of drift nets in small streams, and outlines a method whereby the estimation of drift density (number of specimens m⁻³ of water) can be improved.
2. Changes in the filtering efficiency of the net caused by trapping of organic debris ('clogging') has the effect of reducing net entrance velocities, causing errors in the calculation of sampled water volume, and thus drift density. A model of the reductions in net entrance velocity based on empirical measurements of trapped debris is developed.
3. Cross-sectional velocity calculations suggest that errors can also be introduced into drift density calculations by positioning sampling nets only on the bed. A method to allow this effect is demonstrated.
4. As adjustments to the calculation of sampled volume are required when sampling in rivers that undergo marked changes in discharge during the sampling period, a method whereby these effects can be accommodated to improve drift density estimations is also outlined.
5. The results of this study imply that theoretical links between flow hydraulics and short-term drift behaviour are poorly understood.     

A comparison of the relative contributions of temporal and spatial variation in the density of drifting invertebrates in a Dorset (U.K.) chalk stream

1. Invertebrate drift is commonly investigated in streams, with the majority of studies focussed on temporal (typically diel) variation. In comparison, few studies have investigated spatial variation in drift and there is little consensus among them. We tested the hypothesis that spatial variation in invertebrate drift is as important as temporal variation. 2. The density of drifting invertebrates in a chalk stream was sampled using an array of nets arranged to determine vertical, lateral and longitudinal variation. Samples were collected at dawn, during the day, at dusk and by night, on four separate monthly occasions. Insecta and Crustacea were analysed separately to identify the effect of differing life history strategies. The density of drifting debris was also recorded, to act as a null model. 3. Time of day and vertical position together explained the majority of the variance in invertebrate drift (79% for Insecta and 97% for Crustacea), with drift densities higher at dusk and night, and nearer the stream bed. Independently, time of day (38%, Insecta; 52%, Crustacea) and vertical position (41%, Insecta; 45%, Crustacea) explained a similar amount of the observed variance. Month explained some of the variance in insect drift (9%) but none for Crustacea. 4. Variation in the density of drifting debris showed little in common with invertebrate drift. There was little variation associated with time of day and only 27% of the observed variation in debris could be explained by the factors investigated here, with month explaining the largest proportion (20%). We suggest the difference in drifting debris and invertebrates provides further evidence for a strong behavioural component in invertebrate drift. 5. Spatial variation in invertebrate drift can be of the same order of magnitude as the much‐described diel temporal variation. The extent of this spatial variation poses problems when attempting to quantify invertebrate drift and we recommend that spatial replication should be incorporated into drift studies.     

大陆漂移与白蚁的系统发育

白蚁Termite是属于古老而又原始的一类昆虫。从许多生活习性和形态特征证明,等翅目Isoptera——白蚁与蜚蠊目Blattariae昆虫的亲缘关系十分接近,特别与隐尾蠊科Cryptocercidae的某些类群有着更为密切的关系。它们同以木材纤维为食,并且都是借助于体内共生的细菌和原生动物消化木纤维。当地球上高大乔木出现之后,就已经为这类蛀木昆虫的生存提供必要的生活条件。石炭纪Carboniferous是蜚蠊目昆虫最为繁盛的地质时期(林启彬,1978),为此,白蚁在地质历史上有可能与蜚蠊同时期,或接近     

Drift of stream invertebrates below a cave source

Stream invertebrate drift below the cave source of South Branch Creek, Minnesota, generally increased rapidly. Ephemeroptera and Trichoptera increased rapidly and then decreased at the lower stations (to 430 m) below the cave. Chironomidae drift, in high numbers but low biomass because of their small size, increased fairly rapidly and leveled out at the lowermost stations. Gastropods increased slowly below the cave, reached a maximum, then decreased somewhat at the lowermost station. Drifting oligochaetes, small in size but in very large numbers, increased more slowly below the cave and appeared not to have reached a maximum at the lowermost station. It was concluded that, in general, drift increases fairly rapidly below the stream origin and fluctuates in the upper reaches, probably reflecting benthic population abundance and local ecological conditions, before attaining equilibria downstream, rather than increasing linearly due to cumulative effects. Paper no. 11,172 Scientific Journal Series, Minnesota Agricultural Experiment Station, St. Paul, Minnesota 55108 Paper no. 11,172 Scientific Journal Series, Minnesota Agricultural Experiment Station, St. Paul, Minnesota 55108     

Drift patterns of particulate matter and organisms during artificial high flows in a large experimental channel

Using a large experimental channel, five artificial high flows with different flow regimes were launched to reveal the precise temporal sequences of drifting particulate matter and organisms during the high flows. Drifting fine particulate matter and organisms were collected by sampling bottles and a water pump, respectively. The peaks of drift abundance occurred before the peak discharges, and the abundance declined quickly within several minutes during the rising phase of high flows. The major determinant of drift abundance of particulate matter and organisms was periods of stable conditions before each high flow (tested by determination coefficients in a correlation analysis). The drift of macroscopic plant material also accelerated the drifts of epiphytic fauna and infauna, which drifted with plants and bed sediments. The magnitude (discharge and duration of peak flows) of high flows only slightly affected drift abundance. The fauna more resistant to high flows were invertebrates that fasten their cases or retreats by silk threads on the substrata. Invertebrate taxa having sucking apparatus or high swimming activity were also resistant to high flows.     

Genetic load,inbreeding depression,and hybrid vigor covary with population size: An empirical evaluation of theoretical predictions

Reduced population size is thought to have strong consequences for evolutionary processes as it enhances the strength of genetic drift. In its interaction with selection, this is predicted to increase the genetic load, reduce inbreeding depression, and increase hybrid vigor, and in turn affect phenotypic evolution. Several of these predictions have been tested, but comprehensive studies controlling for confounding factors are scarce. Here, we show that populations of Daphnia magna, which vary strongly in genetic diversity, also differ in genetic load, inbreeding depression, and hybrid vigor in a way that strongly supports theoretical predictions. Inbreeding depression is positively correlated with genetic diversity (a proxy for N_e), and genetic load and hybrid vigor are negatively correlated with genetic diversity. These patterns remain significant after accounting for potential confounding factors and indicate that, in small populations, a large proportion of the segregation load is converted into fixed load. Overall, the results suggest that the nature of genetic variation for fitness‐related traits differs strongly between large and small populations. This has large consequences for evolutionary processes in natural populations, such as selection on dispersal, breeding systems, ageing, and local adaptation.     

Drift of ostracod crustaceans in adjacent intermittent and permanent streams

Ostracods were studied in the drift of adjacent permanent and intermittent streams in Southern Ontario. More species were drifting in the permanent stream than in the intermittent one. The latter developed a pool fauna during early summer and stagnant water species were predominant. Both standing and running water species were common in the permanent stream. Eleven were recorded in the drift and all but one of them are strong swimmers. I. bradyi, the only poor swimmer usually lives among vegetation and it is suggested that truly herpobenthic ostracods are less subject to drift than the others. The pattern of ostracod drift over a period of 13 months in those streams was different. The numbers drifting at the upstream and downstream stations of the same stream in a given month was variable. Ostracods were the major component of crustacean drift in both streams. Stream drift is considered as a mechanism of ostracod dispersal.     

Parallel evolutionary forces influence the evolution of male and female songs in a tropical songbird

Given the important role that animal vocalizations play in mate attraction and resource defence, acoustic signals are expected to play a significant role in speciation. Most studies, however, have focused on the acoustic traits of male animals living in the temperate zone. In contrast to temperate environments, in the tropics, it is commonplace for both sexes to produce complex acoustic signals. Therefore, tropical birds offer the opportunity to compare the sexes and provide a more comprehensive understanding of the evolution of animal signals. In this study, we quantified patterns of acoustic variation in Rufous‐and‐white Wrens (Thryophilus rufalbus) from five populations in Central America. We quantified similarities and differences between male and female songs by comparing the role that acoustic adaptation, cultural isolation and neutral genetic divergence have played in shaping acoustic divergence. We found that males and females showed considerable acoustic variation across populations, although females exhibited greater population divergence than males. Redundancy analysis and partial‐redundancy analysis revealed significant relationships between acoustic variation and ecological variables, genetic distance, and geographic distance. Both ambient background noise and geographic distance explained a high proportion of variance for both males and females, suggesting that both acoustic adaptation and cultural isolation influence song. Overall, our results indicate that parallel evolutionary forces act on male and female acoustic signals and highlight the important role that cultural drift and selection play in the evolution of both male and female songs.     

The relation between invertebrate drift and two primary controls,discharge and benthic densities,in a large regulated river

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Stream drift,size-specific predation,and the evolution of ovum size in an amphibian

Summary A stream-breeding race of small-mouthed salamanders (Ambystoma texanum) in central Kentucky produces ova that are twice as large as those of a pond-breeding race found nearby. Embryos of stream-breeders also hatch at a more advanced developmental stage than those of pond-breeders. Morphological evidence indicates that stream-breeders were derived from pond-breeding stock. Assuming that differences between stream and pond-breeders reflect evolutionary change, and that the ancestral pond stock that invaded streams was similar to extant pond-breeders, we examined three hypotheses that might explain changes in ovum size and stage at hatching following the invasion of streams. (1) Larger ovum size evolved indirectly as a consequence of selection for rapid development which minimizes mortality risk from stream drying. (2) Increased ovum (hatchling) size and stage at hatching of stream-breeders are adaptations to resist stream current. (3) Increased ovum (hatchling) size and stage at hatching are adaptations to reduce predation on hatchlings from stream invertebrates. The results of field and laboratory studies only support hypotheses (2) and (3). Hatchlings that were relatively large or at a more advanced developmental stage had slower drift rates and were less vulnerable to predation by Phagocata gracilis, a flatworm abundant in streams in central Kentucky. Developmental and growth parameters were not correlated significantly with ovum size in populations of either geographic race. Differences in degree of parental care among races also cannot explain variation in ovum size since both races abandon their eggs immediately after oviposition.     

Behaviour of drifting insect larvae

The larval drift behaviour of 23 species representing Ephemeroptera, Plecoptera and Trichoptera was investigated in the laboratory using different current regimes. Mayfly nymphs often performed swimming, while caddis larvae were reluctant to do so. Stonefly nymphs were intermediate. In mayflies swimming seemed to be used to reach the substrate as soon as possible. In contrast most stonefly nymphs by swimming prolonged the time spent in the water column. Modes of swimming and sinking posture differed markedly between the orders. Living passively sinking animals often reached bottom faster than dead control specimens, so consequently behaviour did not always express itself in activity. Some caddis larvae spun adherent anchor lines. Differences among taxa seemed more important in explaining swimming activity compared to preferred habitats (as stream, river and lake) in each species. However, observed differences among closely related species indicated subtle differences related to microhabitat to be of profound importance in explaining the alternative behavioural strategies used.     

Three New Species of Lagenophryid Peritrichs (Ciliophora) Ectocommensal on Freshwater Decapod Crustaceans from Madagascar1

One species of Lagenophrys and two species of Operculigera are described for the first time. Lagenophrys machaerigera n. sp. was discovered on the freshwater crab Gecarcinautes goudoti and varies between two extreme forms in the structure of its lorica aperture. Operculigera carcini n. sp. was also found on G. goudoti and exhibits several characteristics that set it apart from other members of its genus. Some of these characteristics also suggest a phylogenetic link between O. carcini and the genus Lagenophrys. Operculigera madagascarensis n. sp. was discovered on the parastacid crayfish Astacoides granulimanus. The occurrence of O. madagascarensis on a Madagascan parastacid and other species of Operculigera on Chilean parastacids suggests that parastacids are the oldest hosts of the genus Operculigera. Continental drift is the most likely mechanism by which species of Operculigera and parastacids could have been dispersed to distant parts of the southern hemisphere. The absence of Operculigera on Australian parastacids may be due to its replacement by the genus Setonophrys on those hosts.     

Insect drift over the northern Arabian sea in early summer

Air borne insects, mostly carried by wind currents, were trapped over the northern Arabian sea (16‡ to 20‡ N; 68‡ to 72‡ E), in the course of cruise No. 111, ORVSAGAR KANYA (March 14 to April 7, 1996). A total of 2,301 insects belonging to 8 different orders, 47 families and 173 species were trapped. Of these, Hymenoptera was represented by the largest number (1082), which was followed by Hemiptera (586), Diptera (552), Coleoptera (51), Neuroptera (10), Trichoptera (03), Lepidoptera (03) and Orthoptera (01). The trapped insects were mostly between 0.6 mm to over 11 mm long. The data was examined for α-diversity as well as for possible correlations between various parameters like the diversity index, size and number of insects trapped on one hand and the distance of the nearest land mass in wind direction, on the other. Deceased.     

Founder effect and genetic disease in Sottunga, Finland

Pedigree data are analyzed in order to determine the factors responsible for the high frequencies of certain genetic disorders in an isolated Swedish-speaking population of Finland's A land archipelago. The founders of Sottunga are identified, and the genetic contributions of each founder to descending birth cohorts are estimated. Founders born before 1700 have far more descendants in the contemporary gene pool than do more recent founders. However, because of migration and depopulation since 1900, the expected genetic contributions of the early founders to the present-day population are similar to those of later founders. A descendant in the contemporary population has a 2% chance of having inherited a particular gene from the founder who makes the largest single contribution to the gene pool. This corresponds approximately to a 2% probability of inheriting an autosomal dominant disease gene from this founder. Given an average inbreeding coefficient of 0.0016, the probability of inheriting two recessive disease genes from this founder is 0.000032. The incidence of autosomal dominant von Willebrand disease in Sottunga is greater than 10% while that of autosomal recessive tapetoretinal disease is 1.5%. We conclude, therefore, that the high frequencies of these diseases are not due to the disproportionate genetic contribution of one or a few particular founders. It is more likely that these disease genes occurred in high frequency in the initial population or were introduced repeatedly through time.     

Heterosis in an isolated, effectively small, and self-fertilizing population of the flowering plant Leavenworthia alabamica

Mildly deleterious mutations are thought to play a major role in the extinction of natural populations, especially those that are small, isolated, or inbred. Self-fertilization should reduce the effective size of populations and simultaneously reduce migration between populations. A history of self-fertilization should therefore cause a population to harbor a substantial "local drift load" caused by the fixation of mildly deleterious mutations. This hypothesis was tested in Leavenworthia alabamica, which contains large, self-incompatible populations and smaller self-compatible populations with adaptations for self-fertilization. The fitness of offspring from within- and between-population crosses was compared to quantify heterosis caused by the masking of deleterious alleles in the heterozygous state. Little heterosis was observed in crosses between five large, self-incompatible populations and two of the three small, self-fertilizing populations of L. alabamica. However, the most geographically isolated and genetically divergent self-fertilizing population (Tuscumbia) exhibited a 110.2% increase in germination and a 73.6% increase in fitness, which is consistent with a sizeable local drift load. The finding of substantial heterosis for fitness supports the idea that small effective size, reproductive isolation, and self-fertilization can make populations particularly vulnerable to mutation accumulation.     

Population size and the nature of genetic load in Gentianella germanica

Abstract Theory predicts a significant relationship between the size of a population and the magnitude and composition of its genetic load, but few natural populations have been investigated. We examined the magnitude of genetic load due to recessive deleterious alleles (GL) both segregating and fixed within Gentianella germanica populations of varying size by selfing and reciprocally crossing plants within and between natural populations according to a partial diallel design and by comparing the performance of the experimental progeny in a common-garden experiment. The results show that GL for total fitness in small populations (fewer than 200 plants) was mainly due to fixed recessive deleterious alleles, whereas GL for total fitness in larger populations (more than 200 plants) appeared to be mainly due to segregating deleterious recessive alleles. The total fitness of selfed plants increased with decreasing population size, indicating some purging of deleterious alleles associated with declining population sizes. The magnitudes of GL due to fixed deleterious alleles in small populations and segregating deleterious alleles in large populations, however, were overall similar, suggesting that purging selection was an insignificant force when compared to genetic drift in determining the magnitude of GL in small natural populations in this species. The results of this study highlight the importance of population size in determining the dynamics of genetic loads of natural populations and are overall in line with a large body of theoretical work indicating that small populations may face higher extinction risks due to the fixation and accumulation of deleterious alleles of small effect.     

Spatio-temporal variation in the strength and mode of selection acting on major histocompatibility complex diversity in water vole (Arvicola terrestris) metapopulations

Patterns of spatio-temporal genetic variation at a class II major histocompatibility complex (MHC) locus and multiple microsatellite loci were analysed within and between three water vole metapopulations in Scotland, UK. Comparisons of MHC and microsatellite spatial genetic differentiation, based on standardised tests between two demographically asynchronous zones within a metapopulation, suggested that spatial MHC variation was affected by balancing selection, directional selection and random genetic drift, but that the relative effects of these microevolutionary forces vary temporally. At the metapopulation level, between-year differentiation for MHC loci was significantly correlated with that of microsatellites, signifying that neutral factors such as migration and drift were primarily responsible for overall temporal genetic change at the metapopulation scale. Between metapopulations, patterns of genetic differentiation implied that, at large spatial scales, MHC variation was primarily affected by directional selection and drift. Levels of MHC heterozygosity in excess of Hardy–Weinberg expectations were consistent with overdominant balancing selection operating on MHC variation within metapopulations. However, this effect was not constant among all samples, indicating temporal variation in the strength of selection relative to other factors. The results highlight the benefit of contrasting variation at MHC with neutral markers to separate the effects of stochastic and deterministic microevolutionary forces, and add to a growing body of evidence showing that the mode and relative strength of selection acting on MHC diversity varies both spatially and temporally.