Prenatal screening for Down syndrome in twin pregnancies: Estimates of screening performance based on 61 affected and 7302 unaffected twin pregnancies

The aims of this study were to determine whether assumptions used in prenatal screening for Down syndrome in twin pregnancies are valid and derive estimates of risk and screening performance in twin pregnancies using observed data. Data were collected on nuchal translucency, chorionicity, pregnancy associated plasma protein-A (PAPP-A), and free ß human chorionic gonadotrophin (free ß-hCG) from 61 twin pregnancies with Down syndrome and 7302 unaffected twin pregnancies. Distribution parameters were determined and used to estimate screening performance. The assumption that proportional differences in serum marker levels in affected and unaffected singleton pregnancies apply to twin pregnancies was not confirmed. Median free β-hCG value in monochorionic affected twin pregnancies (2.63 multiples of the median [MoM]; 95% CI, 1.79-3.22 MoM) was lower than that assuming proportionality (3.76 MoM), and the median PAPP-A value in dichorionic affected twin pregnancies (1.88 MoM; 95% CI, 1.60-2.17 MoM) was higher than that based on proportionality (1.33 MoM). The detection rate was 87% for a 3% false-positive rate in monochorionic twin pregnancies and 74% in dichorionic twin pregnancies compared with 86% in singleton pregnancies. Estimates of screening performance in Down syndrome twin pregnancies do not need to rely on assumptions and can take account of chorionicity and gestational age.     

Modeling organic carbon dynamics and cadmium fate in long-term sludge-amended soil

A model is described that may help to resolve uncertainty and controversy over the long-term consequences of sludge applications to arable land, especially with regard to the effects of sludge adsorption characteristics on trace metal solubility and bioavailability (e.g., the sludge "time bomb" or sludge "protection" hypotheses). Mass balances of organic and inorganic material derived from sludge and crop residues are simulated. Each pool has a potentially different adsorption affinity for trace metals, and this leads to changes in the adsorption capacity of sludge-amended soil that influence leaching and crop uptake. Model simulations were compared with measured changes in organic carbon and ethylenediaminetetraacetic acid (EDTA)-extractable cadmium contents in a clay loam soil following 41 years of sludge applications. The model adequately reproduced the data, although discrepancies in the vertical distribution of Cd were attributed to the effects of macropore transport and root-uptake driven recirculation. A Monte Carlo sensitivity analysis demonstrated that the most important parameters affecting leaching and crop uptake were the Cd loading and parameters controlling adsorption, especially the partition coefficient for sludge-derived inorganic material and the exponent regulating the effect of pH on sorption. Scenario simulations show that no general conclusions can be drawn with respect to the validity of the sludge "time bomb" and sludge "protection" hypotheses. Either may occur, or neither, depending on three key system parameters: the ratio of sludge adsorption capacity to the initial adsorption capacity of the soil, the proportion of the sludge adsorption capacity contributed by the inorganic fraction, and the sludge Cd loading.     

DEVELOPMENT AND APPLICATION OF A SPATIAL HYDROLOGY MODEL OF OKEFENOKEE SWAMP,GEORGIA1

ABSTRACT: :The model described herein was used to assess effects of the Suwannee River sill (a low earthen dam constructed to impound the Suwannee River within the Okefenokee National Wildlife Refuge to eliminate wildfires) on the hydrologic environment of Okefenokee Swamp, Georgia. Developed with Arc/Info Macro Language routines in the GRID environment, the model distributes water in the swamp landscape using precipitation, inflow, evapotranspiration, outflow, and standing water. Water movement direction and rate are determined by the neighborhood topographic gradient, determined using survey grade Global Positioning Systems technology. Model data include flow rates from USGS monitored gauges, precipitation volumes and water levels measured within the swamp, and estimated evapotranspiration volumes spatially modified by vegetation type. Model output in semi‐monthly time steps includes water depth, water surface elevation above mean sea level, and movement direction and volume. Model simulations indicate the sill impoundment affects 18 percent of the swamp during high water conditions when wildfires are scarce and has minimal spatial effect (increasing hydroperiods in less than 5 percent of the swamp) during low water and drought conditions when fire occurrence is high but precipitation and inflow volumes are limited.     

Birds as predators in tropical agroforestry systems

Insectivorous birds reduce arthropod abundances and their damage to plants in some, but not all, studies where predation by birds has been assessed. The variation in bird effects may be due to characteristics such as plant productivity or quality, habitat complexity, and/or species diversity of predator and prey assemblages. Since agroforestry systems vary in such characteristics, these systems provide a good starting point for understanding when and where we can expect predation by birds to be important. We analyze data from bird exclosure studies in forests and agroforestry systems to ask whether birds consistently reduce their arthropod prey base and whether bird predation differs between forests and agroforestry systems. Further, we focus on agroforestry systems to ask whether the magnitude of bird predation (1) differs between canopy trees and understory plants, (2) differs when migratory birds are present or absent, and (3) correlates with bird abundance and diversity. We found that, across all studies, birds reduce all arthropods, herbivores, carnivores, and plant damage. We observed no difference in the magnitude of bird effects between agroforestry systems and forests despite simplified habitat structure and plant diversity in agroforests. Within agroforestry systems, bird reduction of arthropods was greater in the canopy than the crop layer. Top-down effects of bird predation were especially strong during censuses when migratory birds were present in agroforestry systems. Importantly, the diversity of the predator assemblage correlated with the magnitude of predator effects; where the diversity of birds, especially migratory birds, was greater, birds reduced arthropod densities to a greater extent. We outline potential mechanisms for relationships between bird predator, insect prey, and habitat characteristics, and we suggest future studies using tropical agroforests as a model system to further test these areas of ecological theory.     

Null model analysis of species nestedness patterns

Nestedness is a common biogeographic pattern in which small communities form proper subsets of large communities. However, the detection of nestedness in binary presence-absence matrices will be affected by both the metric used to quantify nestedness and the reference null distribution. In this study, we assessed the statistical performance of eight nestedness metrics and six null model algorithms. The metrics and algorithms were tested against a benchmark set of 200 random matrices and 200 nested matrices that were created by passive sampling. Many algorithms that have been used in nestedness studies are vulnerable to type I errors (falsely rejecting a true null hypothesis). The best-performing algorithm maintains fixed row and fixed column totals, but it is conservative and may not always detect nestedness when it is present. Among the eight indices, the popular matrix temperature metric did not have good statistical properties. Instead, the Brualdi and Sanderson discrepancy index and Cutler's index of unexpected presences performed best. When used with the fixed-fixed algorithm, these indices provide a conservative test for nestedness. Although previous studies have revealed a high frequency of nestedness, a reanalysis of 288 empirical matrices suggests that the true frequency of nested matrices is between 10% and 40%.