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Clear Gradient (clear + gradient)
Selected AbstractsA comparison of bacteria and benthic invertebrates as indicators of ecological health in streamsFRESHWATER BIOLOGY, Issue 7 2009G. LEAR Summary 1. We set out to evaluate the reliability of bacterial communities as an indicator of freshwater ecological health. 2. Samples of epilithic biofilm were taken over a 1-year period from four streams, each impacted by varying degrees of human modification. The bacteria within each sample were characterised using a whole community DNA fingerprinting technique (automated ribosomal intergenic spacer analysis). Spatial and temporal differences in community structure between samples were visualised using multi-dimensional scaling and quantified using permutational multivariate anova. Macrobenthic invertebrates, which are commonly used as indicators of stream ecological health, were also sampled for comparison. 3. Multivariate analysis revealed a clear gradient in macroinvertebrate community structure between sites exposed to increased human impact. Bacterial communities, however, could only distinguish the most impacted site from the remainder. 4. Additional research is required to increase the sensitivity of bacterial community analyses before endorsing their use as an indicator of freshwater ecological health. [source] Trends in savanna structure and composition along an aridity gradient in the KalahariJOURNAL OF VEGETATION SCIENCE, Issue 3 2002R.J. Scholes Abstract. The Kalahari sand sheet occupies 2.5 million ha in southern Africa. It is an area with relatively similar deep aeolian soils, and a strong south to north gradient in rainfall, from 200 to 1000 mm mean annual precipitation (MAP) in the region studied. This provides an excellent basis for gradient studies at the subcontinental scale. This paper briefly reviews the literature on the vegetation of the Kalahari and describes the vegetation structure and composition at 11 new sites. There is a clear gradient in woody plant biomass (as indexed by basal area) from south to north. Above the minimum level of 200 mm MAP, the woody basal area increases at a rate of ca. 2.5 m2.ha -1 per 100 mm MAP. Mean maximum tree height also increases along the gradient, reaching 20 m at ca. 800 mm MAP. The number of species to contribute > 95% of the woody basal area increases from one at 200 mm to 16 at 1000 mm MAP. Members of the Mimosaceae (mainly Acacia) dominate the tree layer up to 400 mm MAP. They are replaced by either the Combretaceae (Combretum or Terminalia) or Colophospermum mopane of the Caesalpinaceae between 400 and 600 mm MAP, and by other representatives of the Caesalpinaceae above 600 mm MAP. The vegetation is largely deciduous up to 1000 mm MAP, except for species that apparently have access to groundwater, which may be locally dominant above about 600 mm MAP. [source] Social adversity predicts ADHD-medication in school children , a national cohort studyACTA PAEDIATRICA, Issue 6 2010A Hjern Abstract Aims:, To test the hypothesis that psychosocial adversity in the family predicts medicated ADHD in school children. Method:, ADHD-medication during 2006 was identified in the Swedish Prescribed Drug Register in national birth cohorts of 1.1 million 6,19 year olds. Logistic regression models adjusted for parental psychiatric disorders were used to test our hypothesis. Results:, There was a clear gradient for ADHD medication with level of maternal education, with an adjusted odds ratio of 2.20 (2.04,2.38) for the lowest compared with the highest level. Lone parenthood and reception of social welfare also implied higher risks of ADHD-medication with adjusted ORs of 1.45 (1.38,1.52) and 2.06 (1.92,2.21) respectively. Low maternal education predicted 33% of cases with medicated ADHD and single parenthood 14%. Conclusions:, Social adversity in the family predicts a considerable proportion of ADHD-medication in school children in Sweden. [source] Latitudinal gradients in diversity: real patterns and random modelsECOGRAPHY, Issue 3 2001Patricia Koleff Mid-domain models have been argued to provide a default explanation for the best known spatial pattern in biodiversity, namely the latitudinal gradient in species richness. These models assume no environmental gradients, but merely a random latitudinal association between the size and placement of the geographic ranges of species. A mid-domain peak in richness is generated because when the latitudinal extents of species in a given taxonomic group are bounded to north and south, perhaps by a physical constraint such as a continental edge or perhaps by a climatic constraint such as a critical temperature or precipitation threshold, then the number of ways in which ranges can be distributed changes systematically between the bounds. In addition, such models make predictions about latitudinal variation in the latitudinal extents of the distributions of species, and in beta diversity (the spatial turnover in species identities). Here we test how well five mid-domain models predict observed latitudinal patterns of species richness, latitudinal extent and beta diversity in two groups of birds, parrots and woodpeckers, across the New World. Whilst both groups exhibit clear gradients in richness and beta diversity and the general trend in species richness is acceptably predicted (but not accurately, unless substantial empirical information is assumed), the fit of these models is uniformly poor for beta diversity and latitudinal range extent. This suggests either that, at least for these data, as presently formulated mid-domain models are too simplistic, or that in practice the mid-domain effect is not significant in determining geographical variation in diversity. [source] | ||||||||||