Approximating persistence in a general class of population processes

We provide a general framework for estimating persistence in populations which may be affected by catastrophic events, and which are either unbounded or have very large ceilings. We model the population using a birth-death process modified to allow for downward jumps of arbitrary size. For such processes, it is typically necessary to truncate the process in order to make the evaluation of expected extinction times (and higher-order moments) computationally feasible. Hence, we give particular attention to the selection of a cut-off point at which to truncate the process, and we present a simple method for obtaining quantitative indicators of the suitability of a chosen cut-off. (c) 2005 Elsevier Inc. All rights reserved.

Geogenic sources and sinks of trace metals in the Larsemann Hills, East Antarctica: Natural processes and human impact

To evaluate the extent of human impact on a pristine Antarctic environment, natural baseline levels of trace metals have been established in the basement rocks of the Larsemann Hills, East Antarctica. From a mineralogical and geochemical point of view the Larsemann Hills basement is relatively homogeneous, and contains high levels of Pb, Th and U. These may become soluble during the relatively mild Antarctic summer and be transported to lake waters by surface and subsurface melt water. Melt waters may also be locally enriched in V, Cr, Co, Ni, Zn and Sri derived from weathering of metabasite pods. With a few notable exceptions, the trace metal concentrations measured in the Larsemann Hills lake waters can be entirely accounted for by natural processes such as sea spray and surface melt water input. Thus, the amount of trace metals released by weathering of basement lithologies and dispersed into the Larsemann Hills environment, and presumably in similar Antarctic environments, is, in general, not negligible, and may locally be substantial. The Larsemann Hills sediments are coarse-grained and contain minute amounts of clay-size particles, although human activities have contributed to the generation of fine-grained material at the most impacted sites. Irrespective of their origin, these small amounts of fine-grained clastic sediments have a relatively small surface area and charge, and are not as effective metal sinks as the abundant, thick cyanobacterial algal mats that cover the lake floors. Thus, the concentration of trace metals in the Larsemann Hills lake waters is regulated by biological activity and thawing-freezing cycles, rather than by the type and amount of clastic sediment supply. (c) 2005 Elsevier Ltd. All rights reserved.

In vivo demonstration of load-induced fluid flow in the rat tibia and its potential implications for processes associated with functional adaptation

Load-induced extravascular fluid flow has been postulated to play a role in mechanotransduction of physiological loads at the cellular level. Furthermore, the displaced fluid serves as a carrier for metabolites, nutrients, mineral precursors and osteotropic agents important for cellular activity. We hypothesise that load-induced fluid flow enhances the transport of these key substances, thus helping to regulate cellular activity associated with processes of functional adaptation and remodelling. To test this hypothesis, molecular tracer methods developed previously by our group were applied in vivo to observe and quantify the effects of load-induced fluid flow under four-point-bending loads. Preterminal tracer transport studies were carried out on 24 skeletally mature Sprague Dawley rats. Mechanical loading enhanced the transport of both small- and larger-molecular-mass tracers within the bony tissue of the tibial mid-diaphysis. Mechanical loading showed a highly significant effect on the number of periosteocytic spaces exhibiting tracer within the cross section of each bone. For all loading rates studied, the concentration of Procion Red tracer was consistently higher in the tibia subjected to pure bending loads than in the unloaded, contralateral tibia, Furthermore, the enhancement of transport was highly site-specific. In bones subjected to pure bending loads, a greater number of periosteocytic spaces exhibited the presence of tracer in the tension band of the cross section than in the compression band; this may reflect the higher strains induced in the tension band compared with the compression band within the mid-diaphysis of the rat tibia. Regardless of loading mode, the mean difference between the loaded side and the unloaded contralateral control side decreased with increasing loading frequency. Whether this reflects the length of exposure to the tracer or specific frequency effects cannot be determined by this set of experiments. These in vivo experimental results corroborate those of previous ex vivo and in vitro studies, Strain-related differences in tracer distribution provide support for the hypothesis that load-induced fluid flow plays a regulatory role in processes associated with functional adaptation.

Noise in detection of qubit states using a quantum point contact

We present a model for detection of the states of a coupled quantum dots (qubit) by a quantum point contact. Most proposals for measurements of states of quantum systems are idealized. However in a real laboratory the measurements cannot be perfect due to practical devices and circuits. The models using ideal devices are not sufficient for describing the detection information of the states of the quantum systems. Our model therefore includes the extension to a non-ideal measurement device case using an equivalent circuit. We derive a quantum trajectory that describes the stochastic evolution of the state of the system of the qubit and the measuring device. We calculate the noise power spectrum of tunnelling events in an ideal and a non-ideal quantum point contact measurement respectively. We found that, for the strong coupling case it is difficult to obtain information of the quantum processes in the qubit by measurements using a non-ideal quantum point contact. The noise spectra can also be used to estimate the limits of applicability of the ideal model.