A discrete space model for continuous space dispersal processes

The simulation of dispersal processes in landscapes over large spatial extents is challenging because of the large difference in geographical scale between overwhelmingly dominant localised dispersal events, and rare long-distance dispersal events which typically drive overall rates of spread. While localised dispersal may point to high resolution individual level models, long-distance dispersal events are likely to involve much coarser grid-based models. In this paper we propose a discrete space (i.e., grid-based) model for dispersal processes in continuous space. We start by illustrating the behaviour of continuous space walks when their movement is discretised to a grid. The importance of short time period cell-to-cell moves which return a walk to its previous grid cell location is identified. A conceptual model which uses a Markov chain buffer phase between cells to replicate the observed behaviour of discretised continuous space walks is proposed. Analysis of the Markov chain shows that it can be parameterised using just two parameters in addition to the dispersal kernel. An algorithm for implementation of the proposed model is presented. Empirical results demonstrate that the proposed mechanism produces good matches to continuous space dispersal processes with both exponential and heavy-tailed dispersal kernels.     

Free space is valuable space: lessons from Chocolate Cities

In our reading of Chocolate Cities: The Black Map of American Life as a work of liberation sociology, we discuss contributions to Black critical thought and praxis this book uniquely makes. Hunter and Robinson craft a Black-owned analytical approach by rezoning the United States map into a charting of African American social and cultural milestones. To apply the re-centering methodological and theoretical innovations this book encourages, we extend its themes to the contested racialized terrains of literacy and education in the US. One contemporary response to systemic educational racism, Black homeschooling, is reconceptualized as a resistance movement. Inspired by the provocative work Hunter and Robinson have done, we situate the Black homeschooling within the tradition of critical Black thought, and its enduring emancipatory warrant.     

Visual space distortion

We are surrounded by surfaces that we perceive by visual means. Understanding the basic principles behind this perceptual process is a central theme in visual psychology, psychophysics, and computational vision. In many of the computational models employed in the past, it has been assumed that a metric representation of physical space can be derived by visual means. Psychophysical experiments, as well as computational considerations, can convince us that the perception of space and shape has a much more complicated nature, and that only a distorted version of actual, physical space can be computed. This paper develops a computational geometric model that explains why such distortion might take place. The basic idea is that, both in stereo and motion, we perceive the world from multiple views. Given the rigid transformation between the views and the properties of the image correspondence, the depth of the scene can be obtained. Even a slight error in the rigid transformation parameters causes distortion of the computed depth of the scene. The unified framework introduced here describes this distortion in computational terms. We characterize the space of distortions by its level sets, that is, we characterize the systematic distortion via a family of iso-distortion surfaces which describes the locus over which depths are distorted by some multiplicative factor. Given that humans' estimation of egomotion or estimation of the extrinsic parameters of the stereo apparatus is likely to be imprecise, the framework is used to explain a number of psychophysical experiments on the perception of depth from motion or stereo. Received: 9 January 1997 / Accepted in revised form: 8 July 1997     

Life in space

The physical conditions of Space are most inhospitable and the higher forms of life probably could exist extraterrestrially only on Venus, Jupiter, and Saturn in our Solar System, and the chances there are poor in light of present knowledge. Thus intelligent life probably exists only on the Earth.Although indigenous intelligent extraterrestrial life seems to be improbable it is by no means clear that man cannot learn to live reasonably comfortably on most of our planets and planetoids such as our moon, and it seems certain that he will be able to travel great distances in the solar system.Lower forms of life may well occur extraterrestrially.     

Exploring structure space

The genome projects are changing biology by providing the genetic blueprints of entire organisms. The blueprints are tantalizing but we cannot deduce everything we need to know from them, including the structures and detailed functions of proteins. In this paper we describe an approach for obtaining structural information about proteins on a genomic scale. We describe how structural and functional information might eventually be put together to form a basis for describing life at many levels. We then describe how structural information fits into this picture and classes of proteins for which structural information would be useful in a genomic context. We conclude with a proposal for an initiative to determine protein structures on a very large scale.This revised version was published online in October 2005 with corrections to the Cover Date.     

Cells in space

How does one treat in a seriously injured astronaut in outer space or even another planet? To answer such a question, the US National Aeronautical Space Administration (NASA) has embarked on a program of growing tissues--and possibly whole organs--in space. NASA has developed a unique rotating bioreactor that allow cells to be grown in a microgravity environment that eliminates almost all shear forces placed upon a cell culture system while entering space. Back on earth, this novel bioreactor has led to exciting discoveries and applications by scientists trying to get cells to differentiate and form their natural three-dimensional tissue matrices--the holy grail of tissue engineers. NASA's bioreactor has allowed various labs to culture cells and even viruses previously impossible to grow using traditional methods. These successes are attributed to the bioreactor's ability to provide an unique environment that closely resembles tissue differentiation during embryogenesis, and thus allowing cellular expression of surface epitopes similar to that of intact tissues. It also appears that cells grown in a microgravity, low-shear environment allows for greater chemical signaling, probably as a result of more surface contact between cells. Realizing the bioreactor's commercial potential, Santa Monica, California-based VivoRx licensed exclusive rights from NASA for both therapeutic and diagnostic commercial applications. VivoRx has, in the past, successfully transplanted encapsulated islet cells from cadavers and porcine pancreas into insulin-dependent diabetics, perhaps a major breakthrough in the treatment of diabetes. However, pancreas from cadavers are in very short supply. The bioreactor may be the answer; VivoRx hopes the bioreactor will allow them to propagate enough human islet cells to use their cell-based approach to treat a large diabetic population. The company has already successfully grown islet cells generated from the bioreactors, and is beginning FDA-approved Phase I/II clinical trials.     

Microbiology in space

Editorial

Microbiology in space     

Plants in space

Virtually all scenarios for the long-term habitation of spacecraft and other extraterrestrial structures involve plants as important parts of the contained environment that would support humans. Recent experiments have identified several effects of spaceflight on plants that will need to be more fully understood before plant-based life support can become a reality. The International Space Station (ISS) is the focus for the newest phase of space-based research, which should solve some of the mysteries of how spaceflight affects plant growth. Research carried out on the ISS and in the proposed terrestrial facility for Advanced Life Support testing will bring the requirements for establishing extraterrestrial plant-based life support systems into clearer focus.     

Chemical evolution in space

The bulk of complex molecules in the space between the stars is probably contained in small frozen interstellar dust grains. A typical grain is about as old as the earth and has, as a result of photochemical processing, converted a large fraction of its oxygen, carbon and nitrogen bearing mantle into large organic molecules whose maximum molecular weights are limited only by the grain size of about 0.1 m. Laboratory and theoretical methods provide the basis for explaining the evolution of interstellar grains from the time they are formed as seedlings in the atmospheres of cool evolved stars to the time they are destroyed by being incorporated into the material of new stars. The organic dust constitutes about one tenth of a percent of the total mass of the Milky Way and far outweighs any estimates of the total mass of all the planets. A planet like the earth is continually and directly accreting interstellar dust from space. Primitive carbonaceous meteorites show evidence of containing interstellar dust. Since comets are possibly almost pure aggregated interstellar dust they also provide a source of interstellar organic material on the earth.     

Radiation protection in space

The challenge for planning radiation protection in space is to estimate the risk of events of low probability after low levels of irradiation. This work has revealed many gaps in our knowledge that require further study. Despite investigations of several irradiated populations, the atomic-bomb survivors remain the primary basis for estimating the risk of ionizing radiation. Compared with previous estimates, two new independent evaluations of available information indicate a significantly greater risk of stochastic effects of radiation (cancer and genetic effects) by about a factor of three for radiation workers, including space travelers. This paper presents a brief historical perspective of the international effort to assure radiation protection in space.Invited paper presented at the International Symposium on Heavy Ion Research: Space, Radiation Protection and Therapy, Sophia-Antipolis, France, 21–24 March 1994