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Ecologists carry a well-stocked toolbox with a great variety of sampling methods, statistical analyses and modelling tools, and new methods are constantly appearing. Evaluation and optimisation of these methods is crucial to guide methodological choices. Simulating error-free data or taking high-quality data to qualify methods is common practice. Here, we emphasise the methodology of the 'virtual ecologist' (VE) approach where simulated data and observer models are used to mimic real species and how they are 'virtually' observed. This virtual data is then subjected to statistical analyses and modelling, and the results are evaluated against the 'true' simulated data. The VE approach is an intuitive and powerful evaluation framework that allows a quality assessment of sampling protocols, analyses and modelling tools. It works under controlled conditions as well as under consideration of confounding factors such as animal movement and biased observer behaviour. In this review, we promote the approach as a rigorous research tool, and demonstrate its capabilities and practical relevance. We explore past uses of VE in different ecological research fields, where it mainly has been used to test and improve sampling regimes as well as for testing and comparing models, for example species distribution models. We discuss its benefits as well as potential limitations, and provide some practical considerations for designing VE studies. Finally, research fields are identified for which the approach could be useful in the future. We conclude that VE could foster the integration of theoretical and empirical work and stimulate work that goes far beyond sampling methods, leading to new questions, theories, and better mechanistic understanding of ecological systems.
Three dimensional modelling of fractured and faulted reservoirs : framework and implementation
(2010)
Modelling of coupled physical processes in fractured and faulted media is a major challenge for the geoscience community. Due to the complexity related to the geometry of real fracture networks and fault systems, modelling studies have been mainly restricted either to two dimensional cases or to simplified orthogonal fracture systems consisting of vertical and horizontal fractures. An approach to generate three dimensional meshes for realistic fault geometries is presented. The method enables representation of faults in an arbitrary incline as two dimensional planes within a three dimensional, stratified porous matrix of a generic geometry. Based on a structural geological model, the method creates three dimensional unstructured tetrahedral meshes. These meshes can be used for finite element and finite volume numerical simulations. A simulation of a coupled fluid flow and heat transport problem for a two layered porous medium cut by two crossing faults is presented to test the reliability of the method.
Seismic wide-angle data were collected along a 40-km-long profile centered at the geothermal research well GrSk 3/90 in the Northeast German Basin. Tomographic inversion of travel time data provided a velocity and a vertical velocity gradient model, indicative of Cenozoic to Pre-Permian sediments. Wide-angle reflections are modeled and interpreted as top Zechstein and top Pre-Permian. Changes in velocity gradients are interpreted as the transition from mechanical to chemical compaction at 2-3 km depth, and localized salt structures are imaged, suggesting a previously unknown salt pillow in the southern part of the seismic profile. The Zechstein salt shows decreased velocities in the adjacent salt pillows compared to the salt lows, which is confirmed by sonic log data. This decrease in velocity could be explained by the mobilization of less dense salt, which moved and formed the salt pillows, whereas the denser salt remained in place at the salt lows. We interpret a narrow subvertical low-velocity zone under the salt pillow at GrSk 3/ 90 as a fault in the deep Permian to Pre-Permian. This WNW-ESE trending fault influenced the location of the salt tectonics and led to the formation of a fault-bounded graben in the Rotliegend sandstones with optimal mechanical conditions for geothermal production. Thermal modeling showed that salt pillows are related to chimney effects, a decrease in temperature, and increasing velocity. The assumed variations in salt lithology, density, and strain must thus be even higher to compensate for the temperature effect.
The apparent isotope enrichment factor epsilon(macrophyte) of submerged plants (epsilon(macrophyte-DIC) = delta C-13(macrophyte) - delta C-13(DIC)) is indicative of dissolved inorganic carbon (DIC) supply in neutral to alkaline waters and is related to variations in aquatic productivity (Papadimitriou et al. in Limnol Oceanogr 50:1084-1095, 2005). This paper aims to evaluate the usage of epsilon(macrophyte) inferred from isotopic analyses of submerged plant fossils in addition to analyses of lake carbonate as a palaeolimnological proxy for former HCO3 (-) concentrations. Stable carbon isotopic analysis of modern Potamogeton pectinatus leaves and its host water DIC from the Tibetan Plateau and Central Yakutia (Russia) yielded values between -23.3 and +0.4aEuro degrees and between +14.0 and +6.5aEuro degrees, respectively. Values of epsilon (Potamogeton-DIC) (range -15.4 to +1.1aEuro degrees) from these lakes are significantly correlated with host water HCO3 (-) concentration (range 78-2,200 mg/l) (r = -0.86; P < 0.001), thus allowing for the development of a transfer function. Palaeo-epsilon (Potamogeton-ostracods) values from Luanhaizi Lake on the NE Tibetan Plateau, as inferred from the stable carbon isotope measurement of fossil Potamogeton pectinatus seeds (range -24 to +2.8aEuro degrees) and ostracods (range -7.8 to +7.5%) range between -14.8 and 1.6aEuro degrees. Phases of assumed disequilibrium between delta C-13(DIC) and delta C-13(ostracods) known to occur in charophyte swards (as indicated by the deposition of charophyte fossils) were excluded from the analysis of palaeo-epsilon. The application of the epsilon (Potamogeton-DIC)-HCO3 (-) transfer function yielded a median palaeo-HCO3 (-) -concentration of 290 mg/l. Variations in the dissolved organic carbon supply compare well with aquatic plant productivity changes and lake level variability as inferred from a multiproxy study of the same record including analyses of plant macrofossils, ostracods, carbonate and organic content.
African savannas are primarily used as pastures and are subject to changes in climate and management strategies. For sustainable management of these landscapes ecological knowledge on seasonal and long-term variability in plant community composition and the availability of green biomass is essential. In this study, we assessed the effects of dry and wet season on species richness and beta diversity for three sites along a gradient of increasing vegetation cover and precipitation in northwest Namibia. A hexagonal systematic sampling design was used to record floristic data. The Simple Matching, Soerensen, and multi-plot similarity coefficient and distance decay analyses were applied for examining beta diversity. Analyses were repeated while separating the plots according to the presence of woody vegetation. Species richness nearly doubled from dry to wet season; compositional similarity increased from dry to wet season and with increasing aridity of the study sites: distance decay was more pronounced in the dry season without any link to the precipitation gradient. Woody elements in the landscape, which occur along drainage lines or as tree islands, govern spatial and seasonal plant diversity fluctuations. Monitoring them is important for conservation strategies and for establishing grazing rules that ensure a sustainable use of savanna ecosystems.