Homogeneous Environmental Conditions Research Articles

The influence of micro-habitat segregation on size assortative pairing in Gammarus pulex (L.) (Crustacea, Amphipoda)

Hypotheses to explain assortative pairing in gammarids (Amphipoda) include size-related habitat segregation and male-male competition. We investigated the influence of micro-habitat heterogeneity on size-assortative pairing in Gammarus pulex, using field-based studies and complementary laboratory-based studies. Firstly, we studied the distribution of males and females of different size and the intensity of size-assortative pairing in micro-habitats differing in particle size at three different study sites. In one site both males and females distributed among micro-habitats in relation to their size, with larger individuals in vegetation and smaller individuals in fine gravel. In a second site, there was no difference in the size of both males and females between two micro-habitats of different particle size, whereas in a third site with two micro-habitats of different particle size the difference was significant for males, but not for females. However, size-assortative pairing was significant within each micro-habitat in each study site. In the first site, pooling data from the three microhabitat subsamples resulted in a higher correlation coefficient between male size and female size, compared to the ones obtained within each micro-habitat, whereas no such difference was observed for the two other sites. Secondly, in experimental conditions, we compared the strength of size-assortative pairing under combinations of both heterogeneous and homogeneous environmental conditions and male-biased or female-biased sex ratios. Size assortative pairing was significant only when the sex-ratio was male-biased. Environmental heterogeneity had no effect on size-assortative pairing. We conclude that male: male competition is the major determinant of size-assortative pairing in G. pulex and that micro-habitat segregation plays only a marginal, if any, role.

Effects of environmental factors on child survival in Bangladesh: a case control study

The need for further studies on relationships between deaths and environmental variables has been reported in the literature. This case-control study was, therefore, carried out to find out the associations between several social and environmental variables and deaths of children due to infectious diseases such as those leading to diarrhoea, acute respiratory infection, measles and other diseases. Six hundred and twenty-five deaths (cases) and an equal number of matched living children (controls) aged 1–59 months, were studied in rural Matlab. An analysis of crude and adjusted odds ratio showed differential associations. Sources of drinking water, amount of stored water, conditions of latrines, number of persons sleeping with the child and the type of cooking site were statistically significantly associated with deaths due to infectious diseases after controlling for breast feeding, immunization, and the family size. Significant associations were also observed between: (i) the sources of drinking water and deaths due to ARI, and (ii) conditions of latrines and deaths due to diarrhoeal diseases, after controlling for the confounding variables. Several other environmental factors also showed associations with these various death groups, but they were not statistically significant. The size of the samples in death groups (small) and the prevalence of more or less homogeneous environmental health conditions probably diminished the magnitude of the effects. The results of the study reconfirm the importance of environmental health intervention in child survival, irrespective of breast-feeding, immunization, and selected social variables.

Effects of environmental factors on child survival in Bangladesh a case control study

The need for further studies on relationships between deaths and environmental variables has been reported in the literature. This case-control study was, therefore, carried out to find out the associations between several social and environmental variables and deaths of children due to infectious diseases such as those leading to diarrhoea, acute respiratory infection, measles and other diseases. Six hundred and twenty-five deaths (cases) and an equal number of matched living children (controls) aged 1-59 months, were studied in rural Matlab. An analysis of crude and adjusted odds ratio showed differential associations. Sources of drinking water, amount of stored water, conditions of latrines, number of persons sleeping with the child and the type of cooking site were statistically significantly associated with deaths due to infectious diseases after controlling for breast feeding, immunization, and the family size. Significant associations were also observed between: (i) the sources of drinking water and deaths due to ARI, and (ii) conditions of latrines and deaths due to diarrhoeal diseases, after controlling for the confounding variables. Several other environmental factors also showed associations with these various death groups, but they were not statistically significant. The size of the samples in death groups (small) and the prevalence of more or less homogeneous environmental health conditions probably diminished the magnitude of the effects. The results of the study reconfirm the importance of environmental health intervention in child survival, irrespective of breast-feeding, immunization, and selected social variables.

Operational yield forecast using AVHRR NDVI data: Reduction of environmental and inter-annual variability

Operational millet yield forecast has been conducted in Senegal using AVHRR data. This was possible by establishing a linear relation between yield collected in the field and NDVI integrated during the reproductive period of millet growth. A bio-physical frame was adapted in order to understand and reduce inter-annual and environmental variability. This was done by limiting potential variation of plant physiological parameters by accounting for vegetation complexity and differences in productivity: (1) Subtracting a pre-growing season NDVI reference level from the NDVI integral improved significantly the level of explained yield variance and was interpreted to reduce the influence of non-crop vegetation within the agricultural domain. (2) Soil and vegetation maps were used to identify areas of homogeneous environmental production conditions. Using this information along with the NDVI integral, accorded a millet yield regression model with a correlation coefficient of r2 0.72 and a standard error of estimate of 190kg ha 1. Yield data could be available one month before the harvest.

Outdoor Experimental Ponds (Mesocosms) Designed for Long-Term Ecotoxicological Studies in Aquatic Environment

Outdoor artificial ponds (mesocosms) of 12 m3were designed for long-term ecotoxicological studies. Sediment, macrophytes (Typha angustifoliaandElodea canadensis), and free and caged freshwater snails [Lymnaea palustris(Müller)] and wood lice (Asellus aquaticusL.) were collected in nearby natural ecosystems and introduced in the mesocosms. Sixty goldfish (Carassius auratusL.) were caged in each pond. Introduced species developed and reproduced in every mesocosm. Animal species (mainly insects and amphibians) spontaneously colonized the ponds, developed, and reproduced. The resulting communities qualitatively resemble those living in natural lentic systems in the surrounding area. Homogenity in physical and chemical conditions and in abundance of phytoplanktonic, periphytic, and macroinvertebrate communities between the different mesocosms was assessed during the stabilization period (8 months). Except for periphyton biomass, no divergent evolution was observed between the ponds. Mesocosm water was slightly eutrophic, alkaline (mean pH: 8.47 ± 0.09), and moderately hard and mineralized. The homogenous and realistic environmental conditions and high ecological representativity of the outdoor experimental ponds were suitable for extensive ecotoxicological studies. Considerations on the choice and origin of introduced species and on possible interactive effects of the transfer of organisms from natural environments, maintainance conditions, and pollutant exposure are discussed.

Effects of Natural Variation in Sex Ratio and Habitat Structure On Mate-Guarding Decisions in Amphipods (Crustacea)

AbstractWe employed field-based studies, with complementary laboratory-based studies, to investigate social and environmental influences on tactical mate-guarding decisions in amphipods (Crustacea). Firstly, we investigated variation in precopulatory mate-guarding duration in Cammarus duebeni celticus in relation to the social structure of natural populations. Variation in population density of up to two orders of magnitude had no effect on precopula duration, whereas guarding durations increased as the sex ratios of the populations became more male biased. That is, males have some ability to assess the probability of other males taking females into precopula and are prepared to guard for longer as this threat of male: male competition increases. A field demonstration of tactical shifts in reproductive behaviour in response to pertinent social conditions is thus provided. Secondly, the 'habitat segregation' hypothesis, which proposes that positive size-assortative pairing in amphipod populations arises due to variation in the use of micro-habitats, was tested in natural field populations and under laboratory conditions in Echinogammarus marinus. This was necessary in order to distinguish any purely environmental determinants of size-assortment from the role of active decisions by males concerning mate choice and male: male competition. The hypothesis was rejected on the grounds that size-assortative pairing arises under both heterogeneous and homogeneous environmental conditions. Further, in both study species, male and female body size were positively correlated with precopula duration. Thus, indirect competition for access to large, fecund females, based on the timing of male entry into precopula, together with direct aggression, provides the explanation for size-assortative pairing in amphipods.

EFFECTS OF INBREEDING ON CLOSED POPULATIONS OF PREDACEOUS MITES (ACARINA: PHYTOSEIIDAE)

AbstractPopulations of Neoseiulus fallacis (Garman) and Phytoseiulus persimilis Athias-Henriot, predators of Tetranychus urticae Koch, were reared under homogeneous environmental conditions by inbreeding without introducing new germ plasm.Mites from 1-year-old inbred cultures had high mortality in all stadia and generally lower viability. Oviposition was reduced and a high percentage of eggs failed to hatch. Further inbreeding resulted in the production of only sterile female mites. These data cast serious doubt on the possibility of genetically improving populations of predaceous mites with only limited germ plasm resources.