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Unmeasured Factors (unmeasured + factor)
Selected AbstractsInheritance of mercury tolerance in the aquatic oligochaete Tubifex tubifexENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 9 2003Dora Elva Vidal Abstract Resistance to contaminants is an important yet unmeasured factor in sediment toxicity tests. The rate at which mercury resistance develops and its genetic persistence in the oligochaete worm Tubifex tubifex were studied under laboratory conditions. Worms were raised for four generations under two different sediment treatments, one reference clean sediment, the other contaminated with mercury. Worms raised in mercury-contaminated sediment developed mercury tolerance that persisted even when the worms were raised for three subsequent generations in clean sediment. Mercury tolerance was determined by comparative water-only toxicity tests with mercury as the only stressor. Control worms had a mean lethal concentration (LC50) of 0.18 mg/L,1. Worms exposed to high levels of mercury in sediment had high mercury tolerance with a mean LC50 of 1.40 mg/L,1. When mercury-tolerant and control mercury-intolerant worms were crossed, their descendants also demonstrated mercury tolerance during lethal toxicity tests. The LC50 for worm descendants resulting from this cross was 1.39 mg/L,1. Adaptation to mercury exposures occurred rapidly in this group of worms and appears to be due to both phenotypic and genotypic mechanisms. Development of contaminant resistance and adaptation may be common phenomena in aquatic benthic invertebrates, which should be considered during the design and interpretation of toxicity tests. [source] The importance of interspecific interactions for breeding-site selection: peregrine falcons seek proximity to raven nestsECOGRAPHY, Issue 6 2004Fabrizio Sergio The advent of GIS is initiating a rapid increase in the utilization of wildlife-habitat models as tools for species and habitat management. However, such models rarely include estimates of interspecific interactions among explanatory variables. We tested the importance of such variables by using the peregrine falcon Falco peregrinus, a medium-sized raptor frequently reported to be affected by heterospecifics, as a model species. In an Alpine population, compared to random locations, peregrines selected breeding sites farther from conspecifics, on taller cliffs, with higher availability of farmland and closer to raven Corvus corax nests. Within suitable habitat, peregrines selected sites near ravens and far from elevations associated with golden eagle Aquila chrysaetos nests. Productivity increased with cliff size, farmland availability (rich in the local main prey) and with proximity to ravens, suggesting that the observed choices were adaptive. Finally, at the regional level, peregrine density peaked at low elevation and was positively associated with raven density. The results suggested an active breeding association of peregrines with ravens, which may provide early-warning cues against predators and safe alternative nest-sites. They also confirmed the importance of including estimates of interspecific interactions among explanatory variables, which may: 1) make models more realistic; 2) increase their predictive power by lowering unexplained variance due to unmeasured factors; 3) provide unexpected results such as the cryptic, large-scale breeding association of our study; and 4) stimulate further hypothesis formulation and testing, ultimately leading to deeper ecological knowledge of the study system. [source] Brood conspicuousness and clutch viability in male-caring assassin bugs (Rhinocoris tristis)ECOLOGICAL ENTOMOLOGY, Issue 2 2009JAMES GILBERT Abstract 1.,Conspicuousness to mates can bring benefits to both males (increased mating success) and females (reduced search costs), but also brings costs (e.g. increased predation and parasitism). Assassin bugs, Rhinocoris tristis, lay egg clutches either on exposed stems or hidden under leaves. Males guard eggs against parasitoids. Guarding males are attractive to females who add subsequent clutches to the brood. This is an excellent opportunity to study the effects of conspicuousness on the fitness of males and females. 2.,Using viable eggs in a multi-clutch brood as a correlate of fitness, the present study examined whether laying eggs on stems affected (1) female fitness, through exposure to parasitism and cannibalism, and (2) male fitness, through attracting further females. 3.,Stem broods were more parasitised. However, males on stems accumulated more mates and more eggs, a net benefit even accounting for parasitism. The eggs gained from being on a stem were cannibalised. By contrast, higher mortality on stems suggests that females should gain by ovipositing on leaves. To the extent that egg viability represents fitness, male and female interests may therefore differ. This suggests a potential for sexual conflict that may affect other species with male care. 4.,Despite higher costs, females actually initiated more broods, and subsequently added bigger clutches to broods, on stems than under leaves. This suggests either that viable eggs do not reflect fitness, or that females laid in unfavourable locations. The key is now to address lifetime fitness, since unmeasured factors may affect offspring viability post-hatching, and to investigate who controls the location of oviposition in R. tristis. [source] Rejecting the mean: Estimating the response of fen plant species to environmental factors by non-linear quantile regressionJOURNAL OF VEGETATION SCIENCE, Issue 4 2005Henning K. Schröder Abstract Question: Is quantile regression an appropriate statistical approach to estimate the response of fen species to single environmental factors? Background: Data sets in vegetation field studies are often characterized by a large number of zeros and they are generally incomplete in respect to the factors which possibly influence plant species distribution. Thus, it is problematic to relate plant species abundance to single environmental factors by the ordinary least squares regression technique of the conditional mean. Location: Riparian herbaceous fen in central Jutland (Denmark). Methods: Semi-parametric quantile regression was used to estimate the response of 18 plant species to six environmental factors, 95% regression quantiles were chosen to reduce the impact of multiple unmeasured factors on the regression analyses. Results of 95% quantile regression and ordinary least squares regression were compared. Results: The standard regression of the conditional mean underestimated the rates of change of species cover due to the selected factor in comparison to 95% regression quantiles. The fitted response curves indicated a general broad tolerance of the studied fen species to different flooding durations but a narrower range concerning groundwater amplitude. The cover of all species was related to soil exchangeable phosphate and base-richness. A relationship between soil exchangeable potassium and species cover was only found for 11 species. Conclusion: Considering the characteristics of data sets in vegetation science, non-linear quantile regression is a useful method for gradient analyses. [source] | ||||||||||