Ecological studies of collembolan populations in a pine forest soil IV. Comparison of distribution patterns

The distribution pattern of ten species of Collembola was studied during the four years period from July 1971 to May 1975 in a pine forest soil. The distribution patterns were analysed for two scales of distribution, i. e., the distribution over the plot of 10×10 m² and the micro-distribution within a block sample consisting of 36 contigious units each 2×2 cm² in area, by applying the -m regression method. The fundamental pattern which appeared was quite similar for the species examined and individuals were aggregated in response to the heterogeneity of habitat conditions. The causes of aggregations were discussed with regard to some environmental factors. The relative abundances of 10 species within the collembolan community was examined in relation to the habitat utilization and the relative abundance was not related to the degree of aggregation but rather to the area occupied by individuals. This suggests that the more numerically abundant species tend to occupy broader micro-habitat. Biological meaning of aggregation was discussed in connection with the population biology and community organization of collembola.     

Evolutionary processes and evolutionary noise at the molecular level

Summary On account, notably, of a competition between different component functions for individual sites in polypeptide chains, each protein molecule represents a functional compromise, with some functions optimized, but the overall state of the molecule –suboptimal–. The proposal is made that the selection coefficient relating to a protein molecule under given conditions can in principle be broken down into partial selection coefficients relevant to the different functions that the molecule carries out. At generalfunction sites, each fixation improves some function, while others deteriorate, at first nonsignificantly, and the overall adaptive state of the molecule fluctuates around its maximum. A selective mechanism is described whereby kaleidoscopic changes in primary structure at variable sites are indefinitely promoted, independently of any environmental changes and with the molecule remaining close to a state of maximal overall adaptation. The paradoxical aspect of this proposal is analyzed. The implication of specific functions in substitutions at general-function sites is noted. Further, it is shown that a certain category of changes in the internal environment of the organism can be integrated into the constantenvironment model for selection. Genetic sufficiency is considered a notion more adequate than genetic optimality for describing biological fitness and for providing a basis for the present model. On this basis selection occurs without genetic load. Multipolymorphism is one of the consequences. Several lines of evidence, in particular observations on polymorphism in deep sea organisms, seem to support the model. It is pointed out that it provides a theoretical foundation for a molecular evolutionary clock. The theoretical constancy of the clock depends on the constancy of functional density. The question of the evolution of functional density is examined. Comparisons of observed substitution frequencies with values expected on a random basis are rejected as a measure of the contribution to evolution of nondetermination. They are considered to reflect a hierarchy in the resistance of the molecules to different amino acid residues as substituents. A limited component of –true– randomness, again accompanied by selection, is on the other hand provided by the model. Most amino acid substitutions are considered evolutionary noise, even though noise compatible with selection. It is proposed that evolutionarily significant substitutions may be identified by monitoring changes in functional density and weighted functional density.Directeur de Recherche at Centre National de la Recherche Scientifique, Paris.     

On the Information Matrix of Exponential Mixture Models with Long-term Survivors

The aim of this paper is to study the properties of the asymptotic variances of the maximum likelihood estimators of the parameters of the exponential mixture model with long-term survivors for randomly censored data. In addition, we study the asymptotic relative efficiency of these estimators versus those which would be obtained with complete follow-up. It is shown that fixed censoring at time T produces higher precision as well as higher asymptotic relative efficiency than those obtainable under uniform and uniform-exponential censoring distributions over (0, T). The results are useful in planning the size and duration of survival experiments with long-term survivors under random censoring schemes.     

Goodness of Fit of Discriminant Functions from the Vector Space of Fixed Variates

Alternative proofs of some of KSHIRSAGAR's (1971) results on testing discriminant functions or canonical variables in the vector space of fixed variates are given. These results are derived in terms of the original variates unlike KSHIRSAGAR (1971) who derives the results by using random orthogonal transformations and triangular decompositions of the original matrix variates.     

Some variables of the craniofacial complex in a Venezuelan population of negroid ancestry

A total of 226 individuals (101 males and 125 females), from La Sabana, a Venezuelan Negroid isolate, with ages between 8 and 60 years, were studied in order to characterize the population for its craniofacial variables and to study the behavior of these variables in relation to age and sex. The variables studied were grouped in three categories: direct cephalometric variables, which included 6 measurements taken directly on the individuals; indirect cephalometric variables, which included 18 measurements (9 angular and 9 linear), taken on lateral head films; and dental variables, which included 9 measurements taken from dental models. In general the direct variables showed the lowest coefficients of variation (CV), suggesting homogeneity within this sample. They were followed by the dental and the indirect variables, which had the highest CV values. In order to detect age and sex effects on the variables, sex and age group comparisons were performed with Student t tests. A greater proportion of significant differences were found among the direct variables, indicating that age and sex have more influence on this group of variables than upon the other two. Comparisons of our sample from La Sabana, with samples from African Negroid, Caucasoid, and Amerindian population show that La Sabana individuals have a craniofacial pattern basically Negroid, as we expected, although some contribution from Caucasoides and especially Amerindians is also suggested in our data.     

First capture success in two dimensions: The search for prey by a random walk predator in a comprehensive space of random walks

Random walk models are an important tool used for understanding how complex organisms redistribute themselves through space and time in search of targets such as food, shelter, or mates. These walks are easily studied with agent-based models, which can be used to ask which search strategy is best according to some efficiency metric. Current studies however, generally do not consider the full range of potential random walks, success metrics, and constraints on the walker, and implementation details vary widely. It is therefore difficult to compare results across studies. In this paper, we investigate predator search behaviour in a comprehensive space of key movement variables that allows the predator to select from a continuum of random walks ranging from Brownian walks (BWs) to correlated random walks (CorRWs) which include directional persistence, to composite random walks (ComRWs) which feature intensive and extensive search modes (ISMs and ESMs), and finally to more complex correlated composite random walks (CCRWs). We specifically focus on the search behaviour of a predator between the initiation of a search for a prey item and the first successful acquisition of a prey target: we call this interval the “search-to-capture” event. We measure the predator's success against three metrics of energetic cost: (1) the time elapsed, (2) the distance travelled, and (3) an equally weighted combination of time and distance. In addition, we explore the effect of three different constraints on the predator: (1) hunting success in the extensive search mode, (2) detection radius when in the extensive search mode, and (3) prey density. Our work confirms the broadly held notion that CCRW movement patterns should always outperform BWs, but find instructive cases where other walks are superior. We also show that, within the CCRW category, there is a wide range of possible walks and rank these according to measures of energetic cost. Our work also offers insights into the evolutionary pressures surrounding the “search-to-capture” event, and suggests that CCRW predators with low hunting success in one movement mode experience higher evolutionary pressures and are thus more likely to adopt a nearly optimal random walk. Our work highlights the need for comprehensive studies that examine several aspects of random walks simultaneously.     

Connecting species’ geographical distributions to environmental variables: range maps versus observed points of occurrence

Connecting the geographical occurrence of a species with underlying environmental variables is fundamental for many analyses of life history evolution and for modeling species distributions for both basic and practical ends. However, raw distributional information comes principally in two forms: points of occurrence (specific geographical coordinates where a species has been observed), and expert-prepared range maps. Each form has potential short-comings: range maps tend to overestimate the true occurrence of a species, whereas occurrence points (because of their frequent non-random spatial distribution) tend to underestimate it. Whereas previous comparisons of the two forms have focused on how they may differ when estimating species richness, less attention has been paid to the extent to which the two forms actually differ in their representation of a species’ environmental associations. We assess such differences using the globally distributed avian order Galliformes (294 species). For each species we overlaid range maps obtained from IUCN and point-of-occurrence data obtained from GBIF on global maps of four climate variables and elevation. Over all species, the median difference in distribution centroids was 234 km, and median values of all five environmental variables were highly correlated, although there were a few species outliers for each variable. We also acquired species’ elevational distribution mid-points (mid-point between minimum and maximum elevational extent) from the literature; median elevations from point occurrences and ranges were consistently lower (median −420 m) than mid-points. We concluded that in most cases occurrence points were likely to produce better estimates of underlying environmental variables than range maps, although differences were often slight. We also concluded that elevational range mid-points were biased high, and that elevation distributions based on either points or range maps provided better estimates.     

Ecological memory at millennial time-scales: the importance of data constraints,species longevity and niche features

Ecological memory describes how antecedent conditions drive the dynamics of an ecological system. Palaeoecological records are paramount to understand ecological memory at millennial time-scales, but the concept is widely neglected in the literature, and a formal approach is lacking. Here, we fill such a gap by introducing a quantitative framework for ecological memory in palaeoecology, and assessing how data constraints and taxa traits shape ecological memory patterns. We simulate the population dynamics and pollen abundance of 16 virtual taxa with different life and niche traits as a response to an environmental driver. The data is processed to mimic a realistic sediment deposition and sampled at increasing depth intervals. We quantify ecological memory with Random Forests, and assess how data properties and taxa traits shape ecological memory. We find that life-span and niche features modulate the relative importance of the antecedent values of the driver and the pollen abundance over periods of 240 yr and longer. Additionally, we find that accumulation rate and decreasing pollen-sampling resolution inflate the importance of antecedent pollen abundance. Our results suggest that: 1) ecological memory patterns are sensitive to varying accumulation rates. A better understanding on the numerical basis of this effect may enable the assimilation of ecological memory concepts and methods in palaeoecology; 2) incorporating niche theory and models is essential to better understand the nature of ecological memory patterns at millennial time-scales. 3) Long-lived generalist taxa are highly decoupled from the environmental signal. This finding has implications on how we interpret the abundance-environment relationship of real taxa with similar traits, and how we use such knowledge to forecast their distribution or reconstruct past climate.