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Species Relative Abundance (species + relative_abundance)
Selected AbstractsMeasurement of body size and abundance in tests of macroecological and food web theoryJOURNAL OF ANIMAL ECOLOGY, Issue 1 2007SIMON JENNINGS Summary 1Mean body mass (W) and mean numerical (N) or biomass (B) abundance are frequently used as variables to describe populations and species in macroecological and food web studies. 2We investigate how the use of mean W and mean N or B, rather than other measures of W and/or accounting for the properties of all individuals, can affect the outcome of tests of macroecological and food web theory. 3Theoretical and empirical analyses demonstrate that mean W, W at maximum biomass (Wmb), W when energy requirements are greatest (Wme) and the W when a species uses the greatest proportion of the energy available to all species in a W class (Wmpe) are not consistently related. 4For a population at equilibrium, relationships between mean W and Wme depend on the slope b of the relationship between trophic level and W. For marine fishes, data show that b varies widely among species and thus mean W is an unreliable indicator of the role of a species in the food web. 5Two different approaches, ,cross-species' and ,all individuals' have been used to estimate slopes of abundance,body mass relationships and to test the energetic equivalence hypothesis and related theory. The approaches, based on relationships between (1) log10 mean W and log10 mean N or B, and (2) log10 W and log10 N or B of all individuals binned into log10 W classes (size spectra), give different slopes and confidence intervals with the same data. 6Our results show that the ,all individuals' approach has the potential to provide more powerful tests of the energetic equivalence hypothesis and role of energy availability in determining slopes, but new theory and empirical analysis are needed to explain distributions of species relative abundance at W. 7Biases introduced when working with mean W in macroecological and food web studies are greatest when species have indeterminate growth, when relationships between W and trophic level are strong and when the range of species'W is narrow. [source] On testing predictions of species relative abundance from maximum entropy optimisationOIKOS, Issue 4 2010Stephen H. Roxburgh A randomisation test is described for assessing relative abundance predictions from the maximum entropy approach to biodiversity. The null model underlying the test randomly allocates observed abundances to species, but retains key aspects of the structure of the observed communities; site richness, species composition, and trait covariance. Three test statistics are used to explore different characteristics of the predictions. Two are based on pairwise comparisons between observed and predicted species abundances (RMSE, RMSESqrt). The third statistic is novel and is based on community-level abundance patterns, using an index calculated from the observed and predicted community entropies (EDiff). Validation of the test to quantify type I and type II error rates showed no evidence of bias or circularity, confirming the dependencies quantified by Roxburgh and Mokany (2007) and Shipley (2007) have been fully accounted for within the null model. Application of the test to the vineyard data of Shipley et al. (2006) and to an Australian grassland dataset indicated significant departures from the null model, suggesting the integration of species trait information within the maximum entropy framework can successfully predict species abundance patterns. The paper concludes with some general comments on the use of maximum entropy in ecology, including a discussion of the mathematics underlying the Maxent optimisation algorithm and its implementation, the role of absent species in generating biased predictions, and some comments on determining the most appropriate level of data aggregation for Maxent analysis. [source] Long-term compositional responses of a South African mesic grassland to burning and mowingAPPLIED VEGETATION SCIENCE, Issue 1 2005Richard W.S. Fynn Abstract Question: What is the long-term compositional response of grass and forb species to various combinations of burning and mowing? Can these responses be predicted from simple plant traits? Location: Ukulinga research and training farm (24°24,E, 30°24,S), Pietermaritzburg, KwaZulu-Natal, South Africa. Methods: Grass species relative abundance in 1996 in various burning and mowing treatments of a long-term (> 50 a) experiment was calculated from data obtained using a point sampling method, whereas forb species abundance in 1999 was determined using the importance score method. The experiment consisted of different frequencies (annual, biennial and triennial) of burning and mowing in winter or spring in combination with different frequencies of summer mowing (none, early, late or both). Results: Grasses responded to the type of disturbance (burn or mow) and frequency of burning, whereas forbs responded primarily to the presence or absence of any form of disturbance and secondarily to the timing of burning (spring versus winter). Summer mowing and annual or biennial dormant-period burning maintained communities dominated by short grasses, whereas tall grasses dominated under annual dormant-period mowing, triennial burning or protection from disturbance. Grass tillering strategy (below- or above-ground) influenced response to burning frequency. Many erect herbaceous dicot species with aerial meristems were reduced in abundance by summer mowing whereas most small creeping herbaceous dicot species appeared to be dependent upon summer mowing. Conclusions: This long-term experiment demonstrated that: (1) grasses and forbs responded differently to burning and mowing; (2) simple plant traits, such as height, position of tiller initiation, and position of meristems have potential for predicting the response of species to the timing and frequency of burning and mowing. [source] Assemblage level variation in springtail lower lethal temperature: the role of invasive species on sub-Antarctic Marion IslandPHYSIOLOGICAL ENTOMOLOGY, Issue 3 2009CHARLENE JANION Abstract. It is widely held both in the physiological literature, and more generally, that the average characteristics of species within an assemblage differ among sites. Such generalizations should be based on investigations of whole assemblages at sites, but this is rarely done. Here, such a study is undertaken for virtually the full assemblage of springtails found at sub-Antarctic Marion Island, by investigating supercooling points (SCPs) of 12 of the 16 species that occur there. Assemblage level variation tends to be less than that documented for assemblages across northern hemisphere sites but similar to that found at some Antarctic locations. Across this set of species, the mean SCPs of the indigenous species (mean ± SE =,17.2 ± 0.4 °C) do not differ significantly from that of the invasive species (,16.3 ± 0.7 °C). Overall, the introduction of several species to the island does not appear to have led to functional homogenization (for this trait). By combining the assemblage-level SCP data with information on the abundances of the species in each of four major habitats, it is also shown that severe but uncommon low temperature events could substantially alter species relative abundances. By resetting assemblage trajectories, such events could play an important role in the terrestrial system at the island. [source] Bayesian Shrinkage Estimation of the Relative Abundance of mRNA Transcripts Using SAGEBIOMETRICS, Issue 3 2003Jeffrey S. Morris Summary. Serial analysis of gene expression (SAGE) is a technology for quantifying gene expression in biological tissue that yields count data that can be modeled by a multinomial distribution with two characteristics: skewness in the relative frequencies and small sample size relative to the dimension. As a result of these characteristics, a given SAGE sample may fail to capture a large number of expressed mRNA species present in the tissue. Empirical estimators of mRNA species' relative abundance effectively ignore these missing species, and as a result tend to overestimate the abundance of the scarce observed species comprising a vast majority of the total. We have developed a new Bayesian estimation procedure that quantifies our prior information about these characteristics, yielding a nonlinear shrinkage estimator with efficiency advantages over the MLE. Our prior is mixture of Dirichlets, whereby species are stochastically partitioned into abundant and scarce classes, each with its own multivariate prior. Simulation studies reveal our estimator has lower integrated mean squared error (IMSE) than the MLE for the SAGE scenarios simulated, and yields relative abundance profiles closer in Euclidean distance to the truth for all samples simulated. We apply our method to a SAGE library of normal colon tissue, and discuss its implications for assessing differential expression. [source] Trophic-dynamic considerations in relating species diversity to ecosystem resilienceBIOLOGICAL REVIEWS, Issue 3 2000KRIS H. JOHNSON ABSTRACT Complexity in the networks of interactions among and between the living and abiotic components forming ecosystems confounds the ability of ecologists to predict the economic consequences of perturbations such as species deletions in nature. Such uncertainty hampers prudent decision making about where and when to invest most intensively in species conservation programmes. Demystifying ecosystem responses to biodiversity alterations may be best achieved through the study of the interactions allowing biotic communities to compensate internally for population changes in terms of contributing to ecosystem function, or their intrinsic functional redundancy. Because individual organisms are the biologically discrete working components of ecosystems and because environmental changes are perceived at the scale of the individual, a mechanistic understanding of functional redundancy will hinge upon understanding how individuals' behaviours influence population dynamics in the complex community setting. Here, I use analytical and graphical modelling to construct a conceptual framework for predicting the conditions under which varying degrees of interspecific functional redundancy can be found in dynamic ecosystems. The framework is founded on principles related to food web successional theory, which provides some evolutionary insights for mechanistically linking functional roles of discrete, interacting organisms with the dynamics of ecosystems because energy is the currency both for ecological fitness and for food web commerce. Net productivity is considered the most contextually relevant ecosystem process variable because of its socioeconomic significance and because it ultimately subsumes all biological processes and interactions. Redundancy relative to productivity is suggested to manifest most directly as compensatory niche shifts among adaptive foragers in exploitation ecosystems, facilitating coexistence and enhancing ecosystem recovery after disturbances which alter species' relative abundances, such as extinctions. The framework further explicates how resource scarcity and environmental stochasticity may constitute ,ecosystem legacies' influencing the emergence of redundancy by shaping the background conditions for foraging behaviour evolution and, consequently, the prevalence of compensatory interactions. Because it generates experimentally testable predictions for a priori hypothesis testing about when and where varying degrees of functional redundancy are likely to be found in food webs, the framework may be useful for advancing toward the reliable knowledge of biodiversity and ecosystem function relations necessary for prudent prioritization of conservation programmes. The theory presented here introduces explanation of how increasing diversity can have a negative influence on ecosystem sustainability by altering the environment for biotic interactions - and there by changing functional compensability among biota - under particular conditions. [source] | ||||||||||