Impacts of earthworms on soil nutrients and plant growth in soybean and maize agroecosystems

The objective of this experiment was to determine the effects of earthworms on soil N pools and plant growth in soybean and maize agroecosystems. The species and number of individuals in earthworm communities were manipulated in plot-scale field enclosures (2.4 m × 1.2 m) by first reducing earthworm populations within enclosures with carbaryl pesticide, and then adding earthworm treatments to the enclosures. Soybean was grown in the enclosures in the first year and stover maize in the second year.The success of earthworm manipulations in field enclosures was affected by climate conditions and available food resources. The endogeic earthworm species Aporrectodea caliginosa was dominant in all enclosures, while introduced anecic Lumbricus terrestris earthworms had poor survival. In the first season, when climate conditions were favourable for earthworm survival and growth, there was a significant (P < 0.05) linear increase in soil mineral-N and microbial biomass N concentrations in the 0–15 cm depth of enclosures with more earthworms. Similarly, soybean grain and grain-N yield was significantly (P < 0.05) greater in enclosures with the largest earthworm populations than the control which had no earthworms added. In the second season, when climate conditions were less favourable, there was no effect of earthworms on soil N pools or maize plants, probably due to poor survival of introduced earthworms.     

Temporal genetic structure of a drone congregation area of the giant Asian honeybee (Apis dorsata)

The giant Asian honeybee (Apis dorsata), like all other members of the genus Apis, has a complex mating system in which the queens and males (drones) mate at spatially defined drone congregation areas (DCAs). Here, we studied the temporal genetic structure of a DCA of A. dorsata over an 8-day time window by the genotyping of sampled drones with microsatellite markers. Analysis of the genotypic data revealed a significant genetic differentiation between 3 sampling days and indicated that the DCA was used by at least two subpopulations at all days in varying proportions. The estimation of the number of colonies which used the DCA ranged between 20 and 40 colonies per subpopulation, depending on the estimation procedure and population. The overall effective population size was estimated as high as N _e=140. The DCA seems to counteract known tendencies of A. dorsata for inbreeding within colony aggregations by facilitating gene flow among subpopulations and increasing the effective population size.     

“双轨控制法”浅议

“浓度控制法”与“总量控制法”简称“双轨控制法”。“双轨控制法”的提出与实施,成为工业防治指导思想上一次大的进步。现就大气污染防治问题进行探讨。     

Parasite transmission in a migratory multiple host system

A transmission model was devised for trichostrongyloid nematodes of saiga antelopes and domestic sheep in Kazakhstan. The framework extends previous models by including seasonal migration of saigas, contact with separate populations of sheep, and climate-driven stochasticity in herbage biomass and in the development, survival and migration onto herbage of free-living larvae. The model was parameterised for the contrasting life histories of Marshallagia, Haemonchus and Nematodirus, three important parasites of saigas and sheep in the region, and was successful at predicting broad qualitative patterns of infection dynamics in sheep and saigas. Parasite transmission between saigas and sheep was predicted to be most important for Marshallagia (from sheep to saigas in the south in winter, and onward transmission to sheep in the north in summer) and Haemonchus (from sheep in the north in summer via saigas to sheep further south in autumn). Model predictions for winter transmission of Marshallagia infection in saigas were consistent with field data, which showed that saigas culled before they have grazed the winter range carry lower burdens of this parasite than older animals. The model provides a mechanistic explanation for its predictions, which will assist hypothesis formation, and further the epidemiological basis of efforts to control parasite transmission between wildlife and livestock in both directions. A similar modelling approach could prove useful in other situations where detailed mechanistic models of parasite transmission are inappropriate in the face of parameter uncertainty and spatio-temporal variation in climate and host density. This is likely to include the majority of wildlife-parasite systems.     

重庆市江北县两路镇环境噪声观测评价及预测

本文对两路镇环境噪声污染现状作了观测、分析和评价,应用重庆市人口密度和环境噪声模式及交通噪声污染模式,对未来10年环境噪声和交通噪声作了科学预测。     

Sex ratio dynamics and fluctuating selection pressures in natural populations of the Trinidadian guppy,<Emphasis Type="Italic"> Poecilia reticulata</Emphasis>

In many species, population sex ratios have far-reaching consequences for a wide variety of population-level and behavioural processes and can directly influence sexual selection through differential effects on male and female mating behaviour. Although sex ratios are often treated as more or less stable population characteristics, recent theoretical evidence suggests that sex ratios fluctuate under many conditions, and that the amplitude of these fluctuations can be considerable. Few studies have attempted to quantify this variation in systems with prominent, sex ratio-dependent sexual conflict. One of the species with the greatest potential to integrate these factors in the wild is the Trinidadian guppy, Poecilia reticulata. In this study, we quantified natural sex ratio variation both as detailed longitudinal studies of focal guppy populations and as snapshot estimates across a range of freshwater habitats. In line with theoretical predictions, we expected to detect significant sex ratio variation over time. We also investigated the association between juvenile and adult sex ratios to quantify a possible compensatory feedback implied in standard models of sex ratio evolution. Our results confirm that population-level sex ratios in wild guppy populations have a range of dynamic features, with all four focal populations showing significant variation in sex ratio over time. The survey showed that juveniles were generally close to equal (50:50) sex ratios whereas 7 out of 11 adult sex ratios differed significantly from equality. We found no evidence that a surplus of juveniles of the locally rarer sex had been produced. The results indicate that sex ratios and hence the balance between sexual selection and sexual coercion is normally fluctuating in nature, despite juvenile ratios being close to equality.Communicated by J. Krause     

A model of nematode dynamics in the Westerschelde Estuary

We developed a time dynamic model to investigate the temporal dynamics of nematode community in the brackish zone of the Westerschelde Estuary. The biomass of four nematode feeding groups observed from March 1991 to February 1992 is used to calibrate the model. Using environmental data as the input, the model predicts the temporal modification and interrelation of four nematode feeding groups. Nematodes achieve a dominant position in the community because of their lower loss rate (in respiration, excretion and natural death). Predators which are deposit-feeding macrobenthos control the variations of dominant nematodes, such as omnivores and non-selective deposit feeders. Food availability causes modification only for rare nematodes such as epigrowth feeders and selective deposit feeders. Temperature is a factor affecting both predation death and a loss including respiration, excretion and natural death. Overall, the modification of nematode community by food availability is much lower than by predator. The macrobenthos in the Westerschelde Estuary decrease from upstream to the estuarine mouth. The stability and standing stock of nematode population follow the opposite gradient of their predators. They increase from upstream to the estuarine mouth.     

Site fidelity and territorial movements of males in a rapidly declining population of yellow-headed blackbirds

We tested several hypotheses to explain low between-year territory fidelity in a breeding population of yellow-headed blackbirds (Xanthocephalus xanthocephalus). During a 5-year study the population of territorial males declined by two-thirds and some of the marshes that supported territories significantly deteriorated. Individual males held territories and bred for an average of 1.9 years. Of males that bred for at least 2 years, 30% skipped owning a territory in the study area during at least 1 year of their breeding lifetimes. Our information suggests that they may have bred outside of the area in those years. Of males with territories in two or more breeding seasons, 60% changed breeding marshes at least once. Males changed territories during 42.9% of between-year opportunities to do so. We found no support for the hypotheses that male yellow-headed blackbirds: (1) are more likely to move when territory density is low; (2) are likely to abandon territories that are deteriorating; or (3) change territories to improve their reproductive success. We suggest three non-mutually exclusive explanations for the yellow-headed blackbird's weak site fidelity: (1) it is a response to habitat deterioration and to other factors that may be causing the population's decline; (2) the males, being migratory, make fresh settlement decisions each year after they arrive on the breeding grounds in the general vicinity of their previous year's breeding; (3) yellow-headed blackbirds may have evolved in, and be adapted to, highly unstable habitats, moving frequently in response to changes in local breeding site conditions. Correspondence to: L.D. Beletsky     

Kin and population recognition in sympatric Lake Constance perch (Perca fluviatilis L.): can assortative shoaling drive population divergence?

Prior studies have shown that perch (Perca fluviatilis L.) of Lake Constance belong to two genetically different but sympatric populations and that local aggregations of juveniles and adults contain closely related kin. In this study, we analysed the genetic structure of pelagic perch larvae to investigate if kin-structured shoals already exist during early ontogenetic development or might be the result of homing to natal sites. Analysis of the gene frequencies at five microsatellite loci revealed that three out of five pelagic aggregations of larvae showed significant accumulation of kin. To investigate possible mechanisms of shoal formation, we tested if perch use olfactory cues to recognize their kin. Choice tests in a fluviarium showed preference for odours of unfamiliar kin vs unfamiliar non-kin. Additionally, we showed that perch could differentiate between the odours of the two sympatric populations and significantly preferred unfamiliar and unrelated conspecifics of their own over the foreign population. Our results present a behavioural mechanism that can lead to the observed formation of kin-structured shoals in perch. We further discuss if the ability to discriminate between their own and a foreign population can result in assortative mating within populations and thus form the basis of “socially mediated speciation” in perch.