Biology subjects

Gjoni, V.

Publications and source records attributed to Gjoni, V..

3 recordsLinked to original sources

bioRxiv2023

Detecting differences in Size Spectra

O_LIThe distribution of body size in communities is remarkably consistent across habitats and taxa and can be represented by size spectra, which are described by a power law. The focus of size spectra analysis is to estimate the exponent ({lambda}) of the power law. C_LIO_LIMany methods have been proposed for estimating{lambda} most of which involve binning the data, summing abundance within bins, and then fitting a ordinary least squares (OLS) regression in log-log space. However, recent work has shown that binning procedures may return biased estimates of size spectra exponents compared to procedures that directly estimate{lambda} using maximum likelihood estimation (MLE). Despite this variability in estimates, it is unclear if the relative change across environmental gradients is consistent across methodologies. Here, we used simulation to compare the ability of two binning methods (NAS, ELBn) and MLE to 1) recapture known values of{lambda} , and 2) recapture parameters in a linear regression measuring the change in{lambda} across a hypothetical environmental gradient. We also compared the methods using two previously published body size datasets across a pollution gradient and a temperature gradient C_LIO_LIMaximum likelihood methods always performed better than common binning methods, which demonstrated consistent bias depending on the simulated values of{lambda} . This bias carried over to the regressions, which were more accurate when{lambda} was estimated using MLE compared to the binning procedures. Additionally, the variance in estimates using MLE methods is markedly reduced when compared to binning methods. C_LIO_LIThe uncertainty and variation in estimates when using binning methods is often greater than or equal to the variation previously published in experimental and observational studies, bringing into question the effect size of previously published results. However, while the methods produced different slope estimates from previously published datasets, they were in qualitative agreement on the sign of those slopes (i.e., all negative or all positive). Our results provide further support for the direct estimation of{lambda} using MLE (or similar procedures) over the more common methods of binning. C_LI

ecology↗

bioRxiv2023

Bayesian hierarchical modeling of size spectra

O_LIA fundamental pattern in ecology is that smaller organisms are more abundant than larger organisms. This pattern is known as the individual size distribution (ISD), which is the frequency of all individual body sizes in an ecosystem. C_LIO_LIThe ISD is described by a power law and a major goal of size spectra analyses is to estimate the exponent of the power law, {lambda}. However, while numerous methods have been developed to do this, they have focused almost exclusively on estimating {lambda} from single samples. C_LIO_LIHere, we develop an extension of the truncated Pareto distribution within the probabilistic modeling language Stan. We use it to estimate multiple {lambda}s simultaneously in a hierarchical modeling approach. C_LIO_LIThe most important result is the ability to examine hypotheses related to size spectra, including the assessment of fixed and random effects, within a single Bayesian generalized (non)-linear mixed model. While the example here uses size spectra, the technique can also be generalized to any data that follows a power law distribution. C_LI

ecology↗

bioRxiv2022

Temperature and resources interact to shape phytoplankton size-abundance relationships at a continental scale

Communities contain more individuals of small species and fewer individuals of large species. The observed -3/4 slope relating mean size and mean abundance across communities (the cross-community scaling relationship or CCSR) is thought to arise from a 3/4-power scaling of metabolic rate with body size. Assuming that 3/4-power metabolic scaling is universal, as claimed by the metabolic theory of ecology (MTE), size-abundance relationships should also be invariant with a scaling exponent of -3/4 or nearly so, across environmental conditions. However, we find that the CCSR slope is actually -1 overall (perhaps due to isometric metabolic scaling), and varies substantially across temperature and nutrient gradients in 1048 freshwater lake phytoplankton communities in the USA. The size-abundance relationship is most shallow at low temperatures and high nutrients, and steeper and relatively insensitive to nutrients at high temperatures. Phytoplankton communities have orders of magnitude more small or large cells depending on environmental conditions than would be predicted from the MTE. Although based on observational data, these results suggest that the environment alters either metabolic scaling or other poorly understood processes shaping community size distributions.

ecology↗