Predicting species abundance distributions by simultaneously using number and biomass as units of measurement

The universal observation that some species in an ecological community are common, but many more are rare, is neatly encapsulated in a species abundance distribution (SAD)1. However, the shape of the distribution can depend on the currency used to measure abundance 2. Here we show how the SADs for numerical abundance and biomass are related and how this relationship can be used to predict the form of the SAD. When plotted in log numerical abundance, log biomass space, species points lie within an approximately triangular area the limits of which are set by body size range, and the upper limit of abundance in both metrics. Under the simplifying, but reasonable, assumption that the observed scatter of species within this region is random, the shape of the SAD is immediately derived from simple geometrical considerations. For the SAD of numerical abundance this is a power curve. The biomass SAD can be either a power curve or, more frequently, a unimodal curve, which can approximate a log normal. This log triangular random placement model serves as a null hypothesis against which actual communities can be compared. Data from two intensively surveyed local communities indicate that it can give a good approximation, with species scattered within a triangle. Further, we can predict the consequences, for the SAD, of size-selective sampling protocols. We argue that mechanistic models of SADs must be able to account for the relative abundance of species in alternative currencies. Moreover, this approach will shed light on niche packing and may have application in environmental monitoring

The important challenge of quantifying tropical diversity

The tropics are the repository of much of the world’s biodiversity, yet are undersampled relative to temperate regions. To help fill this knowledge gap, a paper in BMC Biology explores diversity patterns in tropical African plants, as revealed by the RAINBIO database. The paper documents spatial variation in diversity and data coverage, but also highlights the challenges faced in quantifying diversity patterns using data collated from a range of sources including herbaria. See research article: http://bmcbiol.biomedcentral.com/articles/10.1186/s12915-017-0356-8.Publisher PDFPeer reviewe

Q&A: What is biodiversity?

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The Shape of Species Abundance Distributions Across Spatial Scales

Species abundance distributions (SADs) describe community structure and are a key component of biodiversity theory and research. Although different distributions have been proposed to represent SADs at different scales, a systematic empirical assessment of how SAD shape varies across wide scale gradients is lacking. Here, we examined 11 empirical large-scale datasets for a wide range of taxa and used maximum likelihood methods to compare the fit of the logseries, lognormal, and multimodal (i.e., with multiple modes of abundance) models to SADs across a scale gradient spanning several orders of magnitude. Overall, there was a higher prevalence of multimodality for larger spatial extents, whereas the logseries was exclusively selected as best fit for smaller areas. For many communities the shape of the SAD at the largest spatial extent (either lognormal or multimodal) was conserved across the scale gradient, despite steep declines in area and taxonomic diversity sampled. Additionally, SAD shape was affected by species richness, but we did not detect a systematic effect of the total number of individuals. Our results reveal clear departures from the predictions of two major macroecological theories of biodiversity for SAD shape. Specifically, neither the Neutral Theory of Biodiversity (NTB) nor the Maximum Entropy Theory of Ecology (METE) are able to accommodate the variability in SAD shape we encountered. This is highlighted by the inadequacy of the logseries distribution at larger scales, contrary to predictions of the NTB, and by departures from METE expectation across scales. Importantly, neither theory accounts for multiple modes in SADs. We suggest our results are underpinned by both inter- and intraspecific spatial aggregation patterns, highlighting the importance of spatial distributions as determinants of biodiversity patterns. Critical developments for macroecological biodiversity theories remain in incorporating the effect of spatial scale, ecological heterogeneity and spatial aggregation patterns in determining SAD shape.Peer reviewe

Predation pressure shapes brain anatomy in the wild

The predation pressure data were collected by AEM and AED as part of an ERC-funded project (BioTIME 250189), AEM was supported by the Royal Society, AK and NK were supported by the Knut and Alice Wallenberg Foundation (KAW2013.0072 to NK).There is remarkable diversity in brain anatomy among vertebrates and evidence is accumulating that predatory interactions are crucially important for this diversity. To test this hypothesis, we collected female guppies (Poecilia reticulata) from 16 wild populations and related their brain anatomy to several aspects of predation pressure in this ecosystem, such as the biomass of the four major predators of guppies (one prawn and three fish species), and predator diversity (number of predatory fish species in each site). We found that populations from localities with higher prawn biomass had relatively larger telencephalon size as well as larger brains. Optic tectum size was positively associated with one of the fish predator’s biomass and with overall predator diversity. However, both olfactory bulb and hypothalamus size were negatively associated with the biomass of another of the fish predators. Hence, while fish predator occurrence is associated with variation in brain anatomy, prawn occurrence is associated with variation in brain size. Our results suggest that cognitive challenges posed by local differences in predator communities may lead to changes in prey brain anatomy in the wild.Publisher PDFPeer reviewe

Change in the dominance structure of two marine-fish assemblages over three decades

Funding: FM and AEM are grateful to the European Research Council (ERCAdG BioTIME 250189 and ERCPoC BioCHANGE 727440).Marine fish are an irreplaceable resource but are currently under threat due to overfishing and climate change. To date, most of the emphasis has been on single stocks or populations of economic importance. However, commercially valuable species are embedded in assemblages of many species and there is only limited understanding of the extent to which the structure of whole communities has altered in recent years. Most assemblages are dominated by one or a few species, with these highly abundant species underpinning ecosystem services and harvesting decisions. This paper shows that there have been marked temporal changes in the dominance structure of Scottish marine assemblages over the last three decades, where dominance is measured as the proportional numerical abundance of the most dominant species. We report contrasting patterns in both the identity of the dominant species, and shifts in the relative abundance of the dominant in assemblages to the east and west of Scotland. This result highlights the importance of multi-species analyses of harvested stocks and has implications not only for fisheries management but also for consumer choices.PostprintPeer reviewe

Measuring temporal turnover in ecological communities

Range migrations in response to climate change, invasive species and the emergence of novel ecosystems highlight the importance of temporal turnover in community composition as a fundamental part of global change in the Anthropocene. Temporal turnover is usually quantified using a variety of metrics initially developed to capture spatial change. However, temporal turnover is the consequence of unidirectional community dynamics resulting from processes such as population growth, colonisation and local extinction. Here, we develop a framework based on community dynamics and propose a new temporal turnover measure. A simulation study and an analysis of an estuarine fish community both clearly demonstrate that our proposed turnover measure offers additional insights relative to spatial context-based metrics. Our approach reveals whether community turnover is due to shifts in community composition or in community abundance and identifies the species and/or environmental factors that are responsible for any change

Increases in local richness (α-diversity) following invasion are offset by biotic homogenization in a biodiversity hotspot

This work was funded by the Brazilian Ciência sem Fronteiras/Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) (grant number 1091/13-1), European Research Council (AdG BioTIME (grant number 250189) and PoC BioCHANGE (grant number 727440)) and the Royal Society.The world's ecosystems are experiencing unparalleled rates of biodiversity change, with invasive species implicated as one of the drivers that restructure local assemblages. Here we focus on the processes leading to biodiversity change in a biodiversity hotspot, the Brazilian Cerrado. The null expectation that invasion leads to increase in local species richness is supported by our investigation of the grass layer in two key habitats (campo sujo and campo úmido). Our analysis uncovered a linear relationship between total richness and invasive richness at the plot level. However, because the invasive species—even though few in number—are widespread, their contribution to local richness (α-diversity) is offset by their homogenizing influence on composition (β-diversity). We thus identify a mechanism that can help explain the paradox that species richness is not declining in many local assemblages, yet compositional change is exceeding the predictions of ecological theory. As such, our results emphasize the importance of quantifying both α-diversity and β-diversity in assessments of biodiversity change in the contemporary world.PostprintPostprintPeer reviewe

Sociability between invasive guppies and native topminnows

This investigation was funded by the European Research Council though a grant (project BioTIME 250189) awarded to AEM and the Concejo Estatal de Ciencia y Tecnología del Estado de Michoacán through a grant awarded to MCC.The role of interspecific social interactions during species invasions may be more decisive than previously thought. Research has revealed that invasive fish improve their foraging success by shoaling with native Mexican species, and potentially increase the chances of invasion success. However, do native individuals tend to associate with invaders as well? We tested the hypothesis that the twoline skiffia (Neotoca bilineata) and the Lerma livebearer (Poeciliopsis infans), both native endemic Mexican topminnows, will associate with guppies, a notorious invasive species present in Mexico. Our investigation shows that guppies, twoline skiffias and Lerma livebearers have a mutual tendency to associate with each other. Although there is a marked tendency to shoal with heterospecifics in this system, shoaling partners do not necessarily benefit equally from the association. Further research on invasive-native social interactions is needed to promote our understanding of potential facilitation by natives.Publisher PDFPeer reviewe