Showing papers in "Philosophical Transactions of the Royal Society B in 1990"
TL;DR: Reconstruction of past lake-water pH from diatom data involves two steps; regression, where responses of modern diatom abundances to pH are modelled and calibration, where the modelled responses are used to infer pH fromdiatom assemblages preserved in lake sediments.
Abstract: Palaeolimnological diatom data comprise counts of many species expressed as percentages for each sample. Reconstruction of past lake-water pH from such data involves two steps; (i) regression, where responses of modern diatom abundances to pH are modelled and (ii) calibration where the modelled responses are used to infer pH from diatom assemblages preserved in lake sediments. In view of the highly multivariate nature of diatom data, the strongly nonlinear response of diatoms to pH, and the abundance of zero values in the data, a compromise between ecological realism and computational feasability is essential. The two numerical approaches used are (i) the computationally demanding but formal statistical approach of maximum likelihood (ML) Gaussian logit regression and calibration and (ii) the computationally straightforward but heuristic approach of weighted averaging (WA) regression and calibration. When the Surface Water Acidification Project (SWAP) modern training set of 178 lakes is reduced by data-screening to 167 lakes, WA gives superior results in terms of lowest root mean squared errors of prediction in cross-validation. Bootstrapping is also used to derive prediction errors, not only for the training set as a whole but also for individual pH reconstructions by WA for stratigraphic samples from Round Loch of Glenhead, southwest Scotland covering the last 10 000 years. These reconstructions are evaluated in terms of lack-of-fit to pH and analogue measures and are interpreted in terms of rate of change by using bootstrapping of the reconstructed pH time-series.
1,046 citations
TL;DR: In this article, a two-compartment model was proposed to represent the first few years of the decay of 14 C-labelled plant material in soil and showed to give a useful representation of data from the Rothamsted long-term field experiments.
Abstract: Although the decomposition of plant material in soil is an extremely complex process, relatively simple models can give good fits to the decay process. Thus a two-compartment model gives a close representation, over the first few years, of the decay of 14 C-labelled plant material in soil. A model containing a single homogeneous humus compartment decomposing by a first-order process is surprisingly useful for soil organic nitrogen over periods measured in decades. More sophisticated multicompartmental models are now widely used to represent turnover in soil. One of these, the Rothamsted turnover model, is described in detail and shown to give a useful representation of data from the Rothamsted long-term field experiments.
878 citations
TL;DR: There are several different ways of representing dispersal in spatially structured environments, as well as several possibilities for allowing environmental variation to come into play as mentioned in this paper, and the theoretical possibilities are thus contingent on spatial scale and dispersal rates.
Abstract: Population dynamics and species interactions are spread out in space. This might seem like a trivial observation, but it has potentially important consequences. In particular, mathematical models show that the dynamics of populations can be altered fundamentally simply because organisms interact and disperse rather than being confined to one position for their entire lives. Models that deal with dispersal and spatially distributed populations are extraordinarily varied, partly because they employ three distinct characterizations of space: as \`islands' (or \`metapopulations'), as `stepping-stones', or as a continuum. Moreover, there are several different ways of representing dispersal in spatially structured environments, as well as several possibilities for allowing environmental variation to come into play. In spite of this variety, a few common themes emerge from spatial models. First, island and stepping-stone models emphasize that little can be concluded from simply recording patterns of occupancy, instead a metapopulation's fate will be determined by the balance between local extinction and recolonization and how that balance interacts with random catastrophes. Island and stepping-stone models also make it clear that the spatial dimension, in particular spatial subdivision, can alter the stability of species interactions and opportunities for coexistence in both predator-prey and competitive systems. Continuum models, which usually take the form of reaction-diffusion equations, address slightly different questions. Reaction-diffusion theory suggests that in uniform environments, certain combinations of local dynamics and dispersal can produce persistent irregularities in the dispersion of species. These striking spatial patterns, which are called diffusive instabilities, can arise from predator-prey interactions, Lotka-Volterra competitive interactions, and from density-dependent population growth in an age-structured population. Moreover, although they differ fundamentally in their structure, the three major classes of spatial models share the common generalization that spatial effects should be expected only for: (i) selected spatial scales; (ii) specific dispersal rates, and (iii) particular patterns of environmental variation relative to the frequency and range of dispersal. The theoretical possibilities are thus contingent on spatial scale and dispersal rates. Although explicit experimental tests of spatial models are non-existent, a handful of studies report general changes in species interactions associated with manipulations of habitat subdivision. Observational studies with adequate data concerning dispersal and scale are also scarce; but those few observational studies with the appropriate supporting information consistently show profound spatial effects, especially effects due to habitat subdivision. The challenge for empiricists is to investigate more rigorously the roles of spatial subdivision and dispersal in natural communities. The challenge for theoreticians is to make the empiricist's job easier; this can best be done by delineating when spatial effects are most likely to be influential, and by offering guidance on how to design appropriate experiments. Simply saying that the spatial environment is important is to mouth a platitude: what we need to know is whether this presumed importance amounts to much in natural systems.
773 citations
TL;DR: This paper distinguishes between active and latent human failures and proposes a general framework for understanding the dynamics of accident causation, suggests ways in which current methods of protection may be enhanced, and concludes by discussing the unusual structural features of 'high-reliability' organizations.
Abstract: Several recent accidents in complex high-risk technologies had their primary origins in a variety of delayed-action human failures committed long before an emergency state could be recognized. These disasters were due to the adverse conjunction of a large number of causal factors, each one necessary but singly insufficient to achieve the catastrophic outcome. Although the errors and violations of those at the immediate human-system interface often feature large in the post-accident investigations, it is evident that these 'front-line' operators are rarely the principal instigators of system breakdown. Their part is often to provide just those local triggering conditions necessary to manifest systemic weaknesses created by fallible decisions made earlier in the organizational and managerial spheres. The challenge facing the human reliability community is to find ways of identifying and neutralizing these latent failures before they combine with local triggering events to breach the system's defences. New methods of risk assessment and risk management are needed if we are to achieve any significant improvements in the safety of complex, well-defended, socio-technical systems. This paper distinguishes between active and latent human failures and proposes a general framework for understanding the dynamics of accident causation. It also suggests ways in which current methods of protection may be enhanced, and concludes by discussing the unusual structural features of 'high-reliability' organizations.
631 citations
TL;DR: The problem is that the automation is at an intermediate level of intelligence, powerfulenough to take over control that used to be done by people, but not powerful enough to handle all abnormalities.
Abstract: As automation increasingly takes its place in industry, especially high risk industry, it is often blamed for causing harm and increasing the chance of human error when failures do occur. I propose that the problem is not the presence of automation, but rather its inappropriate design. The problem is that the operations under normal operating conditions are performed appropriately, but there is inadequate feedback and interaction with the humans who must control the overall conduct of the task. When the situations exceed the capabilities of the automatic equipment, then the inadequate feedback leads to difficulties for the human controllers. The problem, I suggest, is that the automation is at an intermediate level of intelligence, powerful enough to take over control that used to be done by people, but not powerful enough to handle all abnormalities. Moreover, its level of intelligence is insufficient to provide the continual, appropriate feedback that occurs naturally among human operators. This is the source of the current difficulties. To solve this problem, the automation should either be made less intelligent or more so, but the current level is quite inappropriate. The overall message is that it is possible to reduce error through appropriate design considerations. Appropriate design should assume the existence of error, it should continually provide feedback, it should continually interact with operators in an effective manner, and it should allow for the worst situations possible. What is needed is a soft, compliant technology, not a rigid, formal one.
552 citations
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TL;DR: In numerous experiments, computer simulated scenarios representing, for example, a small town, ecological or economic systems or political systems such as a Third World country, were used and subjects had to look for information, plan actions, form hypotheses, etc.
Abstract: Unlike other living creatures, humans can adapt to uncertainty. They can form hypotheses about situations marked by uncertainty and can anticipate their actions by planning. They can expect the unexpected and take precautions against it. In numerous experiments, we have investigated the manner in which humans deal with these demands. In these experiments, we used computer simulated scenarios representing, for example, a small town, ecological or economic systems or political systems such as a Third World country. Within these computer-simulated scenarios, the subjects had to look for information, plan actions, form hypotheses, etc.
460 citations
TL;DR: Two important shortcomings of traditional plant demography are emphasized; the dearth of simple manipulative experiments on such issues as seed limitation, and the tendency to locate study plots around existing mature individuals (the omission of ‘empty quadrats’ may introduce serious bias into the estimation of plant recruitment rates).
Abstract: Long-term studies of plant populations are reviewed, and their dynamics summarized in three categories. Many short-lived plants have ephemeral, pulsed dynamics lasting only a single generation, with recruitment determined almost entirely by germination biology and by the frequency and intensity of disturbance. Such populations are not amenable to traditional population models. At the other extreme, some long-lived plants have such protracted tenancy of their microsites that it is impossible to establish what pattern of dynamics (if any) their populations exhibit. A relatively small number of species show what we would traditionally regard as population dynamics at a given point in space (i.e. reasonably predictable trajectories that can be modelled by N t+1 = f ( N t )). A major difficulty in generalizing about plant dynamics is that the majority of species are successional; their recruitment depends upon the death, through senescence or disturbance, of the dominant plants. Where we do have data spanning several generations, it is clear that: (i) the populations are regulated by density dependent processes; (ii) in contrast to some animal populations, numbers appear to vary less from year to year in places where mean density is higher, and less from place to place in years when mean density is high than when density is low; (iii) few, if any, plant populations show persistent cyclic or chaotic dynamics, but (iv) there are several robust generalizations that stem from the immobility and phenotypic plasticity of plants (the law of constant yield; self-thinning rules, etc.). These generalizations are analysed in the context of simple theoretical models of plant dynamics, and the patterns observed in long-term studies are compared with similar data from animal populations. Two important shortcomings of traditional plant demography are emphasized; (i) the dearth of simple manipulative experiments on such issues as seed limitation, and (ii) the tendency to locate study plots around existing mature individuals (the omission of ‘empty quadrats’ may introduce serious bias into the estimation of plant recruitment rates).
378 citations
TL;DR: Beginning in 1981, a mathematical model called the erosion-productivity impact calculator model (epic) was developed to determine the relation between soil erosion and soil productivity throughout the U.S.A.
Abstract: Beginning in 1981, a mathematical model called the erosion-productivity impact calculator model (epic) was developed to determine the relation between soil erosion and soil productivity throughout the U.S.A. By 1985 the model was ready for use in the RCA (1977 Soil and Water Resources Conservation Act) analysis. Between 15000 and 20000 epic simulations of 100 years each were performed as part of the 1985 RCA analysis. After the RCA analysis, model refinement and development continued and epic has been applied to a number of agricultural management problems. For example, epic is capable of dealing with decisions involving drainage, irrigation, water yield, erosion (wind and water), weather, fertilizer and lime application, pest control, planting dates, tillage, and crop residue management. Example applications include: (i) 1988 drought assessment; (ii) soil loss tolerance tool; (iii) Australian sugarcane model ( AUSCANE ) ; (iv) pine tree growth simulator; (v) global climate change analysis, and (vi) farm level planning.
375 citations
TL;DR: The molecular basis of psychrophily is reviewed in terms of biochemical mechanisms and the ability to adapt their solute uptake systems through membrane lipid modulation may distinguish psychrophiles from psychrotrophs.
Abstract: Psychrophilic and psychrotrophic microorganisms are important in global ecology as a large proportion of our planet is cold (below 5 $^\circ$ C); they are responsible for the spoilage of chilled food and they also have potential uses in low-temperature biotechnological processes Psychrophiles and psychrotrophs are both capable of growing at or close to zero, but the optimum and upper temperature limits for growth are lower for psychrophiles compared with psychrotrophs Psychrophiles are more often isolated from permanently cold habitats, whereas psychrotrophs tend to dominate those environments that undergo thermal fluctuations The molecular basis of psychrophily is reviewed in terms of biochemical mechanisms The lower growth temperature limit is fixed by the freezing properties of dilute aqueous solutions inside and outside the cell In contrast, the ability of psychrophiles and psychrotrophs to grow at low, but not moderate, temperatures depends on adaptive changes in cellular proteins and lipids Changes in proteins are genotypic, and are related to the properties of enzymes and translation systems, whereas changes in lipids are genotypic or phenotypic and are important in regulating membrane fluidity and permeability The ability to adapt their solute uptake systems through membrane lipid modulation may distinguish psychrophiles from psychrotrophs The upper growth temperature limit can result from the inactivation of a single enzyme type or system, including protein synthesis or energy generation
368 citations
TL;DR: Evidence to date supports the view that mossy fibre convey conditioned stimulus information and that climbing fibres conveys the critical 'reinforcement' information to the cerebellum and that 'memory traces' appear to be formed in cerebellar cortex and interpositus nucleus.
Abstract: Evidence supports the view that 'memory traces' are formed in the hippocampus and in the cerebellum in classical conditioning of discrete behavioural responses. In the hippocampus learning results in long-lasting increases in excitability of pyramidal neurons that resemble the phenomenon of long-term potentiation. Although it plays a role in certain aspects of conditioning, the hippocampus is not necessary for learning and memory of the basic conditioned responses. The cerebellum and its associated brain-stem circuitry, on the other hand, does appear to be essential (necessary and sufficient) for learning and memory of the conditioned response. Evidence to date supports the view that mossy fibre convey conditioned stimulus information and that climbing fibres conveys the critical 'reinforcement' information to the cerebellum and that 'memory traces' appear to be formed in cerebellar cortex and interpositus nucleus.
317 citations
TL;DR: Two new studies are described whose results show that AP5 has no effect upon the retrieval of previously established memories, and that the dose-response profile of the impairment of spatial learning occurs across a range of extracellular concentrations in hippocampus for which receptor selectivity exists.
Abstract: There has recently been renewed interest in the idea that alterations in synaptic efficacy may be the neural basis of information storage. Particular attention has been focused upon long-term potentiation (LTP), a long-lasting, but experimentally induced synaptic change whose physiological properties point to it being a candidate memory mechanism. However, considerations of storage capacity and the possibility of concomitant activity-dependent synaptic depression make it unlikely that individual learning experiences will give rise to gross changes in field potentials similar to those that occur in LTP, even if learning and LTP utilize common neural mechanisms. One way of investigating the functional significance of LTP is to use selective antagonists of those excitatory amino acid receptors whose activation is essential for its induction. This paper discusses various design requirements for such experiments and reviews work indicating that the N-methyl-D-aspartate receptor antagonist AP5 causes a behaviourally selective learning impairment having certain common features to the behavioural profile seen after hippocampal lesions. Two new studies are described whose results show that AP5 has no effect upon the retrieval of previously established memories, and that the dose-response profile of the impairment of spatial learning occurs across a range of extracellular concentrations in hippocampus for which receptor selectivity exists. These experiments show that activation of NMDA receptors is essential for certain kinds of learning.
TL;DR: A broader approach than hitherto is advocated, which integrates ecological information with physiological data, in freezing tolerant individuals and in freezing intolerant species.
Abstract: Arthropods, as poikilotherms, adapt to cold environments in a variety of ways that include extension of locomotory activity to low temperatures, enhancement of metabolic rate and maintenance of a positive energy balance whenever possible. The ecological implications for many such animals are extension of the life cycle and a requirement for an individual to overwinter several times. Prolonged sub-zero temperatures increase the risk of tissue freezing, and two main strategies have been evolved, first avoidance of freezing by supercooling, and secondly, tolerance of extracellular ice. In the first strategy, freezing is invariably lethal and extensive supercooling (to -30 $^\circ$ C and below) occurs through elimination or masking of potential ice nucleators in the body and accumulation of cryoprotective substances such as polyhydric alcohols and sugars. Such species are termed freezing intolerant. The second strategy, freezing tolerance, is uncommon in arthropods and other invertebrates, and usually occurs in a single life stage of a species. Freezing of liquid in the extracellular compartment is promoted by proteinaceous ice nucleators. Freezing is therefore protective, and the lethal temperature is well below the supercooling point in freezing tolerant individuals, whereas in most freezing intolerant species it is close to or at the supercooling point. Proteins also act as antifreezes in insects of both strategies, producing a thermal hysteresis by lowering the freezing point of haemolymph in a non-colligative fashion while not affecting the melting point temperature. Recent studies and developments in arthropod cold tolerance are discussed against this background, and a broader approach than hitherto is advocated, which integrates ecological information with physiological data.
TL;DR: It is concluded that identification of errors as a separate class of behaviour is becoming increasingly difficult in modern work environments.
Abstract: Present technology is characterized by complexity, rapid change and growing size of technical systems. This has caused increasing concern with the human involvement in system safety. Analyses of the major accidents during recent decades have concluded that human errors on part of operators, designers or managers have played a major role. There are, however, several basic problems in analysis of accidents and identification of human error. This paper addresses the nature of causal explanations and the ambiguity of the rules applied for identification of the events to include in analysis and for termination of the search for ‘causes’. In addition, the concept of human error is analysed and its intimate relation with human adaptation and learning is discussed. It is concluded that identification of errors as a separate class of behaviour is becoming increasingly difficult in modern work environments. The influence of this change on the control of safety of large-scale industrial systems is discussed.
TL;DR: In this paper, a survey of the patterns in discovering and recording species of animals and plants, from Linnaeus' time to the present, is presented, along with various approaches to estimating what the total number of species on Earth might be: these approaches include extrapolation of past trends; direct assessments based on the overall fraction previously recorded among newly studied groups of tropical insects; indirect assessment derived from recent studies of arthropods in the canopies of tropical trees.
Abstract: This paper begins with a survey of the patterns in discovering and recording species of animals and plants, from Linnaeus' time to the present. It then outlines various approaches to estimating what the total number of species on Earth might be: these approaches include extrapolation of past trends; direct assessments based on the overall fraction previously recorded among newly studied groups of tropical insects; indirect assessment derived from recent studies of arthropods in the canopies of tropical trees (giving special attention to the question of what fraction of the species found on a given host-tree are likely to be `effectively specialized' on it); and estimates inferred from theoretical and empirical patterns in speciessizes relations or in food web structure. I conclude with some remarks on the broader implications of our ignorance about how many species there are.
TL;DR: It is suggested that the DNA-methylating system of mammals was derived from that of bacteria by way of a hypothetical intermediate that carried out selective de novo methylation of exogenous DNA and propagated the methylated DNA in the repressed state within its own genome.
Abstract: The amino acid sequence of mammalian DNA methyltransferase has been deduced from the nucleotide sequence of a cloned cDNA It appears that the mammalian enzyme arose during evolution via fusion of a prokaryotic restriction methyltransferase gene and a second gene of unknown function Mammalian DNA methyltransferase currently comprises an N-terminal domain of about 1000 amino acids that may have a regulatory role and a C-terminal 570 amino acid domain that retains similarities to bacterial restriction methyltransferases The sequence similarities among mammalian and bacterial DNA cytosine methyltransferases suggest a common evolutionary origin DNA methylation is uncommon among those eukaryotes having genomes of less than 10 $^8$ base pairs, but nearly universal among large-genome eukaryotes This and other considerations make it likely that sequence inactivation by DNA methylation has evolved to compensate for the expansion of the genome that has accompanied the development of higher plants and animals As methylated sequences are usually propagated in the repressed, nuclease-insensitive state, it is likely that DNA methylation compartmentalizes the genome to facilitate gene regulation by reducing the total amount of DNA sequence that must be scanned by DNA-binding regulatory proteins DNA methylation is involved in immune recognition in bacteria but appears to regulate the structure and expression of the genome in complex higher eukaryotes I suggest that the DNA-methylating system of mammals was derived from that of bacteria by way of a hypothetical intermediate that carried out selective de novo methylation of exogenous DNA and propagated the methylated DNA in the repressed state within its own genome During the evolution of complex plants and animals the inactivating effects of DNA methylation spread to extraneous cellular sequences, such as highly repetitive DNA and transposable elements, and later to tissue-specific genes Modern large-genome eukaryotes have adapted a primitive prokaryotic immune system to enable them to manage a genome that has expanded more than 1000-fold as a result of accumulation of extraneous sequences and tissue-specific genes
TL;DR: The functions of binding proteins are discussed in the light of numerous crystallographic and ligand-binding studies and a mechanism for the binding protein-dependent, high-affinity active transport is proposed.
Abstract: We have determined and refined the X-ray crystal structures of six periplasmic binding proteins that serve as initial receptors for the osmotic-shock sensitive, active transport of L-arabinose, D-galactose/D-glucose, maltose, sulphate, leucine/isoleucine/valine and leucine. The tertiary structures and atomic interactions between proteins and ligands show common features that are important for understanding the function of the binding proteins. All six structures are ellipsoidal, consisting of two similar, globular domains. The ligand-binding site is located deep in the cleft between the two domains. Irrespective of the nature of the ligand (e.g. saccharide, sulphate dianion or leucine zwitterion), the specificities and affinities of the binding sites are achieved mainly through hydrogen-bonding interactions. Binding of ligands induces a large protein conformational change. Three different structures have been observed among the binding proteins: unliganded 'open cleft', liganded 'open cleft', and liganded 'closed cleft'. Here we discuss the functions of binding proteins in the light of numerous crystallographic and ligand-binding studies and propose a mechanism for the binding protein-dependent, high-affinity active transport.
TL;DR: Its interactions with the major extracellular effector molecules that influence cell behaviour, its role in maintaining cell shape and its spatial and temporal changes in expression during development indicate that syndecan is involved in morphogenesis.
Abstract: Cellular behaviour during development is dictated, in part, by the insoluble extracellular matrix and the soluble growth factor peptides, the major molecules responsible for integrating cells into morphologically and functionally defined groups. These extracellular molecules influence cellular behaviour by binding at the cell surface to specific receptors that transduce intracellular signals in various ways not yet fully clear. Syndecan, a cell surface proteoglycan found predominantly on epithelia in mature tissues binds both extracellular matrix components (fibronectin, collagens I, III, V, and thrombospondin) and basic fibroblast growth factor (bFGF). Syndecan consists of chondroitin sulfate and heparan sulphate chains linked to a 31 kilodalton (kDa) integral membrane protein. Syndecan represents a family of integral membrane proteoglycans that differ in extracellular domains, but share cytoplasmic domains. Syndecan behaves as a matrix receptor: it binds selectively to components of the extracellular matrix, associates intracellularly with the actin cytoskeleton when cross-linked at the cell surface, its extracellular domain is shed upon cell rounding and it localizes solely to basolateral surfaces of simple epithelia. Mammary epithelial cells made syndecan-deficient become fibroblastic in morphology and cell behaviour, showing that syndecan maintains epithelial cell morphology. Syndecan changes in quantity, location and structure during development: it appears initially on four-cell embryos (prior to its known matrix ligands), becomes restricted in the pre-implementation embryo to the cells that will form the embryo proper, changes its expression due to epithelial-mesenchymal interactions (for example, induced in kidney mesenchyme by the ureteric bud), and with association of cells with extracellular matrix (for example, during B-cell differentiation), and ultimately, in mature tissues becomes restricted to epithelial tissues. The number and size of its glycosaminoglycan chains vary with changes in cell shape and organization yielding tissue type-specific polymorphic forms of syndecan. Its interactions with the major extracellular effector molecules that influence cell behaviour, its role in maintaining cell shape and its spatial and temporal changes in expression during development indicate that syndecan is involved in morphogenesis.
TL;DR: It is shown that completely deterministic regulatory factors can lead to apparently random fluctuations in population density, and a new method is developed to make practical distinctions between apparently noisy dynamics produced by low-dimensional chaos and population variation that in fact derives from random (high-dimensional) noise.
Abstract: Over the years, there has been much discussion about the relative importance of environmental and biological factors in regulating natural populations. Often it is thought that environmental factors are associated with stochastic fluctuations in population density, and biological ones with deterministic regulation. We revisit these ideas in the light of recent work on chaos and nonlinear systems. We show that completely deterministic regulatory factors can lead to apparently random fluctuations in population density, and we then develop a new method (that can be applied to limited data sets) to make practical distinctions between apparently noisy dynamics produced by low-dimensional chaos and population variation that in fact derives from random (high-dimensional)noise, such as environmental stochasticity or sampling error. To show its practical use, the method is first applied to models where the dynamics are known. We then apply the method to several sets of real data, including newly analysed data on the incidence of measles in the United Kingdom. Here the additional problems of secular trends and spatial effects are explored. In particular, we find that on a city-by-city scale measles exhibits low-dimensional chaos (as has previously been found for measles in New York City), whereas on a larger, country-wide scale the dynamics appear as a noisy two-year cycle. In addition to shedding light on the basic dynamics of some nonlinear biological systems, this work dramatizes how the scale on which data is collected and analysed can affect the conclusions drawn.
TL;DR: This paper reviews a series of approaches to the study of density dependence, regulation and variability in terrestrial animals, by using single-species, multispecies and life table time series data.
Abstract: This paper reviews a series of approaches to the study of density dependence, regulation and variability in terrestrial animals, by using single-species, multispecies and life table time series data. Special emphasis is given to the degree of density dependence in the level of variability, which is seldom discussed in this context, but which is conceptually related to population regulation. Broad patterns in density dependence, regulation and variability in vertebrates and arthropods are described, with some more specific results for moths and aphids. Vertebrates have generally less variable populations than arthropods, which is the only well documented, consistent pattern in population variability. The degree of density dependence of variability is negatively correlated with the average level of variability, suggesting that generally the more regulated populations are less variable. Most population studies, especially on insects, have involved outbreak species with complex dynamics, which may explain the common failures to detect density dependence in natural populations. In British moths, density dependence is less obvious in the more abundant species. The study of uncommon and rare species remains a major challenge for population ecology.
TL;DR: Links between population dynamics, the relative abundance of species and the richness of animal communities are reviewed within the framework of a simple conceptual model, based on body size and abundance.
Abstract: Links between population dynamics, the relative abundance of species and the richness of animal communities are reviewed within the framework ol a simple conceptual model, based on body size and abundance. Populations of individual species occupy positions in this body size: abundance space. Problems of relative abundance and absolute species-richness revolve around a number ol simple questions, including: what determines the upper and lower bounds (maximum and minimum population densities) of species in the assemblage; what determines the overall density of points (number of species) within these bounds; and how are the vertical and horizontal partitioning rules between species decided? The answers to these, and related questions, are briefly reviewed.
TL;DR: In this article, the authors used a first-order, finite-state, discrete-time Markov process to extract small, single channel ion currents from background noise, which can be used to detect signals that do not conform to a firstorder Markov model, but the method is less accurate when the background noise is not white.
Abstract: Techniques for extracting small, single channel ion currents from background noise are described and tested. It is assumed that single channel currents are generated by a first-order, finite-state, discrete-time, Markov process to which is added `white' background noise from the recording apparatus (electrode, amplifiers, etc.). Given the observations and the statistics of the background noise, the techniques described here yield a posteriori estimates of the most likely signal statistics, including the Markov model state transition probabilities, duration (open- and closed-time) probabilities, histograms, signal levels, and the most likely state sequence. Using variations of several algorithms previously developed for solving digital estimation problems, we have demonstrated that: (1) artificial, small, first-order, finite-state, Markov model signals embedded in simulated noise can be extracted with a high degree of accuracy, (2) processing can detect signals that do not conform to a first-order Markov model but the method is less accurate when the background noise is not white, and (3) the techniques can be used to extract from the baseline noise single channel currents in neuronal membranes. Some studies have been included to test the validity of assuming a first-order Markov model for biological signals. This method can be used to obtain directly from digitized data, channel characteristics such as amplitude distributions, transition matrices and open- and closed-time durations.
TL;DR: Mechanistically, TGF-beta s act to control gene expression of their target cells, many of their actions converging on a complex, multifaceted scheme of control of matrix proteins and their interactions with cells; these effects on matrix are thought to mediate many of the effects of TGF -beta on development.
Abstract: Transforming growth factors-beta (TGF-beta) are 25 kilodalton (kDa) homodimeric peptides with multifunctional actions controlling the growth, differentiation and function of a broad range of target cells of both epithelial and mesenchymal derivation. They are expressed early in embryogenesis and their tissue-specific and developmentally dependent expression is strongly suggestive of an essential role in particular morphogenetic and histogenetic events. Five distinct TGF-beta s have been characterized so far, with 65-80% homology to each other. By using both molecular biological and immunohistochemical techniques, we are currently attempting to define specific sites of expression of the different TGF-beta s and to determine whether TGF-beta s 1-5 might have unique functions in development and in the mature organism. Comparative study of the promoter regions for the different TGF-beta s and for any particular TGF-beta in different species is also underway. Mechanistically, TGF-beta s act to control gene expression of their target cells, many of their actions converging on a complex, multifaceted scheme of control of matrix proteins and their interactions with cells; these effects on matrix are thought to mediate many of the effects of TGF-beta on development.
TL;DR: It is concluded that the most important steps in crocodylomorph evolution, particularly in the skull, had taken place in sphenosuchians, hence they should be included in the same taxon as protosuchian and more advanced crocodilians, rather than with thecodontians.
Abstract: A detailed description is given of the osteology of the holotype of Sphenosuchus. The skull, particularly the braincase, is excellently preserved and shows a wealth of anatomical detail. Sphenosuchus was one of the largest of the early crocodylomorphs, with a skull length of 192 mm and an estimated total length of 1.4 m. The primary head of the quadrate meets the prootic and squamosal but not the opisthotic (or laterosphenoid); quadrate and pterygoid are not fused to the braincase and the basipterygoid articulation is free. The braincase and some other skull bones are pneumatized. The otic capsule is crocodilian but the subcapsular buttress (ossified subcapsular process) does not enclose the vagus nerve or the internal carotid artery. The scapula blade is triangular; the coracoid has a long posteroventral extension which is thought to have articulated firmly with a large interclavicle. Clavicles were absent. Metatarsal I is reduced; metatarsals II and IV are symmetrical about III, which is longest. A paired series of dorsal scutes was present. Sphenosuchus is considered to have been cursorial and carnivorous. Comparison is made between the pneumatic spaces in the Sphenosuchus skull and those of modern crocodiles and birds, and homologies are discussed. Representatives of the main cavities found in the crocodilian skull are present in the skull of Sphenosuchus, in some cases in a less clearly defined state. On the other hand, certain pneumatic spaces in the Sphenosuchus skull are not found in the modern crocodile but resemble cavities in the bird skull. The courses of the internal carotid and stapedial arteries are reconstructed; the latter is considered to have passed through the postquadrate foramen, temporal canal and anterior temporal foramen as it does in modern forms. The problem of the position of the stapedial artery in the crocodile is discussed. It is believed that enclosure of the artery took place as a result of the forward migration of the quadrate head, leading to the formation of a temporal canal. Detailed comparisons are made between the otic capsule of Sphenosuchus and those of modern crocodiles and birds, which it closely resembles. Changes in otic capsule structure in archosaurs to give the crocodilian or bird condition, starting from a primitive form like Euparkeria, are outlined. The skull is believed to have been kinetic, and the quadrate streptostylic, in the juvenile Sphenosuchus. The parts of the proximal end of the crocodilian quadrate are differentiated; in particular, the \`true' head is distinguished from the anterodorsal process. Although very reduced in the modern crocodile, the \`true' head is in the same morphological position as in Sphenosuchus; contact with the laterosphenoid has been brought about, not by further forward movement of the head, but by geniculation of the upper portion of the bone. The anterodorsal process is considered to have arisen as a result of the dorsal migration of an anterolateral projection somewhat similar to that of the thecodontian Stagonolepis. This change was also responsible for the elongation of the quadratojugal in crocodylomorphs. The validity of the order Crocodylomorpha is discussed. It is concluded that the most important steps in crocodylomorph evolution, particularly in the skull, had taken place in sphenosuchians, hence they should be included in the same taxon as protosuchians and more advanced crocodilians, rather than with thecodontians.
TL;DR: In collaboration with several companies and government departments, the MRC Applied Psychology Unit performed a series of auditory warning studies, showing that the number of immediate-action warning sounds should not exceed about six, and that each sound should have a distinct melody and temporal pattern.
Abstract: One of the most common auditory warnings is the ambulance 'siren'. It cuts through traffic noise and commands one's attention, but it does so by sheer brute force. This 'better safe than sorry' approach to auditory warnings occurs in most environments where sounds are used to signal danger or potential danger. Flooding the environment with sound is certain to attract attention; however it also causes startled reactions and prevents communications at a crucial point in time. In collaboration with several companies and government departments, the MRC Applied Psychology Unit performed a series of auditory warning studies. The main conclusions of the research were that the number of immediate-action warning sounds should not exceed about six, and that each sound should have a distinct melody and temporal pattern. The experiments also showed that it is possible to predict the optimum sound level for a warning sound in most noise environments. Subsequently, a set of guidelines for the production of ergonomic auditory warnings was developed. The guidelines have been used to analyse the environments in both fixed-wing and rotary-wing aircraft, and to design prototype warning systems for environments as diverse as helicopters, operating theatres and the railways.
TL;DR: In this paper, proteins for proton-linked transport of D-xylose, L-arabinose, D-galactose, Rhamnose and L-fucose into Escherichia coli are studied.
Abstract: Separate proteins for proton-linked transport of D-xylose, L-arabinose, D-galactose, L-rhamnose and L-fucose into Escherichia coli are being studied. By cloning and sequencing the appropriate genes, the amino acid sequences of proteins for D-xylose/H+ symport (XylE), L-arabinose/H+ symport (AraE), and part of the protein for D-galactose/H+ symport (GalP) have been determined. These are homologous, with at least 28% identical amino acid residues conserved in the aligned sequences, although their primary sequences are not similar to those of other E. coli transport proteins for lactose, melibiose, or D-glucose. However, they are equally homologous to the passive D-glucose transport proteins from yeast, rat brain, rat adipocytes, human erythrocytes, human liver, and a human hepatoma cell line. The substrate specificity of GalP from E. coli is similar to that of the mammalian glucose transporters. Furthermore, the activities of GalP, AraE and the mammalian glucose transporters are all inhibited by cytochalasin B and N-ethylmaleimide. Conserved residues in the aligned sequences of the bacterial and mammalian transporters are identified, and the possible roles of some in sugar binding, cation binding, cytochalasin binding, and reaction with N-ethylmaleimide are discussed. Each protein is independently predicted to form 12 hydrophobic, membrane-spanning alpha-helices with a central hydrophilic segment, also comprised of alpha-helix. This unifying structural model of the sugar transporters shares features with other ion-linked transport proteins for citrate or tetracycline.
TL;DR: It is shown that an `assured fitness returns' model provides a selective pressure for the evolution of worker behaviour which is at least about as strong as that of haplodiploidy, but free from such requirements of the latter as high levels of worker-brood genetic relatedness and ability of workers to manipulate brood sex ratios.
Abstract: Delineation of the selective pressures responsible for the evolution of sterile worker castes found in social insect colonies remains a major unsolved problem in evolutionary biology. There has therefore been a great deal of interest in suggesting ways by which the inclusive fitness of sterile workers can potentially be larger than those of solitary nest-builders. Queller's (1989) head-start hypothesis (Proc. natn. Acad. Sci. U.S.A. 86, 3224) suggests that workers may gain relatively more inclusive fitness because they have access to young of various ages which can be quickly brought to the age of independence, whereas a solitary foundress has to survive for the entire duration of the development of her brood. I argue here that Queller's quantitative analysis is incorrect because it gives an unfair advantage to workers, either by giving full credit of rearing an offspring to a worker who only cared for it for a short while or, by assuming that a worker can do much more work per unit time than a solitary foundress. I show, however, that workers do indeed have an advantage over solitary foundresses because they have assured fitness returns, even if in small amounts, for short periods of work. This results from a different reckoning from that used by Queller and gives a more moderate advantage, arising essentially from saving the wasted effort that occurs when lone foundress nests fail. Using field and laboratory data on the primitively eusocial wasp Ropalidia marginata, and re-analysing data on the four species of polistine wasps used by Queller, I show that such an `assured fitness returns' model provides a selective pressure for the evolution of worker behaviour which is at least about as strong as that of haplodiploidy, but free from such requirements of the latter as high levels of worker-brood genetic relatedness and ability of workers to manipulate brood sex ratios.
TL;DR: A simple model, which takes into account the mechanical properties of muscle, predicts optimum take-off techniques that agree well with those used by athletes and are remarkably insensitive to the numerical values assigned to the physiological parameters.
Abstract: High jumpers run at moderate speeds and set down the foot, from which they take off, well in front of the body. Long jumpers run up much faster and place the foot less far forward, with the leg at a steeper angle. A simple model, which takes into account the mechanical properties of muscle, predicts optimum take-off techniques that agree well with those used by athletes. These predictions are remarkably insensitive to the numerical values assigned to the physiological parameters.
TL;DR: It is suggested that shiftworkers merely 'stay up late' on the night shift, rather than adjust to it, and that this is responsible for the reduced safety at night.
Abstract: Safety and productivity are low at night and this would appear to be because we are a diurnal species. This is reflected not only in our habitual sleep time, but also in our endogenous body clocks that, together with exogenous influences, such as the patterning of meals and activity, result in predictable circadian (24 h) rhythms in our physiological processes. Our performance capabilities also vary over the course of our waking period, with task demands affecting both the precise trend over the day, and the rate at which it adjusts to the changes in sleep timing occasioned by shift work. Studies designed to examine the reasons for this have shown that memory loaded performance may have a quite separate endogenous component to that responsible for more simple performance, suggesting that these two types of performance cannot be causally related. Furthermore, it would appear that the exogenous component of circadian rhythms may also differ across measures, and our attempts to model these endogenous and exogenous components have led us to re-examine the evidence on adjustment to night work. Our findings suggest that shiftworkers merely ‘stay up late’ on the night shift, rather than adjust to it, and that this is responsible for the reduced safety at night. It would seem that in situations where safety is paramount, the only solution to these problems is the creation of a nocturnal sub-society that not only always works at night but also remains on a nocturnal routine on rest days.
TL;DR: The distribution limits of three species, in the British Isles, are discussed and low temperatures are shown to influence distribution, by limiting the capacity either to flower or to fertilize ovules, respectively.
Abstract: The distribution limits of three species, in the British Isles are discussed. For Verbena officinalis and Tilia cordata low temperatures are shown to influence distribution, by limiting the capacity either to flower or to fertilize ovules, respectively. In the case of Umbilicus rupestris , a long-term transplant population beyond the natural geographical limit of the species has evolved new low-temperature responses of seed germination and winter survival. The effect is a marked change of phenology, compared with populations of the species within its natural range, which enhances the capacity of the population to survive in a colder environment.
TL;DR: The vocal control system of oscine songbirds has some perplexing properties that are not predicted by common views of how vocal learning takes place and the relation between the direct pathway for the control of learned song and the recursive pathway necessary for song learning is not understood.
Abstract: The vocal control system of oscine songbirds has some perplexing properties e.g. laterality, adult neurogenesis, neuronal replacement that are not predicted by common views of how vocal learning takes place. Similarly, we do not understand the relation between the direct pathway for the control of learned song and the recursive pathway necessary for song learning. Some of the paradoxes of the vocal system of birds may disappear once the relation between the perception and production of learned vocalizations is better understood. To some extent, perception and production may be two closely related states of a same system.