1,720,972 research outputs found
Legacy effects and memory loss: how contingencies moderate the response of rocky intertidal biofilms to present and past extreme events
No full textUnderstanding how historical processes modulate the response of ecosystems to perturbations is becoming increasingly important. In contrast to the growing interest in projecting biodiversity and ecosystem functioning under future climate scenarios, how legacy effects originating from historical conditions drive change in ecosystems remains largely unexplored. Using experiments in combination with stochastic antecedent modelling, we evaluated how extreme warming, sediment deposition and grazing events modulated the ecological memory of rocky intertidal epilithic microphytobenthos (EMPB). We found memory effects in the non-clustered scenario of disturbance (60 days apart), where EMPB biomass fluctuated in time, but not under clustered disturbances (15 days apart), where EMPB biomass was consistently low. A massive grazing event impacted on EMPB biomass in a second run of the experiment, also muting ecological memory. Our results provide empirical support to the theoretical expectation that stochastic fluctuations promote ecological memory, but also show that contingencies may lead to memory loss
The role of wave-exposure and human impacts in regulating the distribution of alternative habitats on NW Mediterranean rocky reefs
No full textThe global decline of canopy-forming macroalgae has stimulated research on the mechanism regulating shifts among alternative habitats on rocky reefs. The effects of sea urchin grazing and alterations of environmental conditions are now acknowledged as the main drivers of shifts between canopy-formers and encrusting coralline barrens and algal turfs, respectively. The conditions under which these mechanisms operate remains, however, somewhat elusive. This is mostly a consequence of the fact that our current understanding has been generated by envisioning habitat shifts as dichotomic, at odds with rocky reef landscapes being composed by mosaics of habitats and with evidence of strong interactions among the species that compose each of the alternative habitats. Using data from a long-term sampling program and path analysis, we investigated how wave-exposure and human-induced degradation of environmental conditions regulate the mechanisms maintaining algal canopies formed by Cystoseira crinita, barren habitats and algal turfs as alternative states on subtidal reefs in the NW Mediterranean. In the Tuscan Archipelago, wave-exposure had positive effects on sea urchins, which, likely due to their low mean density, had weak effects on each of the alternative habitats. Canopy-forming macroalgae resulted, instead, to exert strong negative effects on the abundance of algal turfs. Since data from the Tuscan Archipelago did not explain any of the variation in the abundance of C. crinita canopies, a further analysis was performed including data from the coast of Tuscany to assess the role of cumulative human impacts in regulating habitat shifts. This showed that degradation of environmental conditions is a direct cause of the decline of macroalgal canopies, indirectly favouring the dominance of algal turfs. Our study suggests that management of human impacts should be considered a priority for preserving subtidal canopies formed by Cystoseira in the NW Mediterranean and that conservation efforts based exclusively on the control of sea urchin populations might be doomed to failure in some areas
Effects of grazer diversity on marine microphytobenthic biofilm: a ‘tug of war’ between complementarity and competition
Full text linkSpecies loss is one of the most striking problems related to human-driven environmental changes. Nevertheless, biodiversity and ecosystem functioning experiments have mainly focused on primary producers, paying less attention to the consequences of changing diversity at higher trophic levels. We performed a field experiment using cage enclosures to test the effects of species richness, identity and density of gastropod grazers on the photosynthetic efficiency and biomass of intertidal biofilm on an exposed rocky shore in the northwest Mediterranean. The diversity and composition of intertidal grazers affected the photosynthetic efficiency of biofilm with only negligible effects on biomass. Individual species showed strong identity effects. In assemblages of 2 or more species, positive or negative complementarity effects occurred. The magnitude of the ecosystem response is expected to depend on the particular species assemblage and its density, which will determine whether niche partitioning or competition is the prevailing process. Grazer preference in specific components of biofilm, characterized by different photosynthetic efficiency and competitive abilities, might explain concomitant changes in photosynthetic efficiency and comparable levels in biomass among treatments. The effects of grazers declined following the natural trend of decreasing biomass of biofilm during the study period, highlighting the importance of considering temporal variability in the effects of biodiversity on ecosystem functioning. This work emphasizes the key role of species identity to predict effects on their resources and ecosystem functioning.This work was partially supported by the University of Pisa
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Influenza dell'eterogeneità ambientale nella valutazione dell'effetto riserva: il caso dei popolamenti ittici dell'Arcipelago Toscano.
No full textIn un pianeta in rapida crescita demografica ed economica la presa di coscienza del progressivo deterioramento degli ecosistemi marini, da un lato, e il crescente interesse per la biodiversità degli ambienti marini ed i processi ecologici ad essi associati, dall’altro, hanno messo in luce l’incapacità dell’uomo di gestire l’immenso patrimonio custodito negli oceani e la totale mancanza di un approccio lungimirante, teso a quantificare l’impatto provocato dall’uso indiscriminato delle risorse marine. Una corretta tutela e una gestione sostenibile dell’ambiente, in particolare quello marino, vengono oggi avvertite, sia dai governi, sia dall’opinione pubblica come una necessità per il presente e un dovere verso le generazioni future. Le Aree Marine Protette (AMP), ad oggi, sono globalmente riconosciute come uno dei migliori strumenti volti alla protezione della biodiversità, la conservazione degli ecosistemi marini e la gestione degli stock ittici. All’interno delle AMP si riscontra, infatti, un generale aumento di densità, taglia e biomassa corporea per specie target della pesca, nonché un notevole miglioramento dell’output riproduttivo di tali specie. Tutto ciò si traduce spesso in un incremento di biodiversità dell’area soggetta a protezione. Le specie maggiormente soggette a prelievo, infatti, occupano spesso i livelli più alti delle reti trofiche e il loro sovrasfruttamento può determinare gravi conseguenze sull’intera comunità attraverso le cosiddette cascate trofiche con una riduzione della diversità complessiva. Le AMP hanno dimostrato di essere in grado di ristabilire le interazioni predatorie perdute e di promuovere il recupero a livello delle comunità.
Nonostante i risultati ottimistici ottenuti in molti studi, le risposte dei popolamenti ittici alla protezione variano sia in direzione, che entità e le basi di questa eterogeneità negli effetti delle AMP sono tuttora in gran parte sconosciuti. Le principali sorgenti di eterogeneità nell’efficacia delle AMP sono: taglia ed età (cioè il tempo trascorso dall’istituzione) della riserva, possibile inclusione in una rete di riserve, ciclo vitale e caratteristiche ecologiche delle diverse specie soggette a protezione ed eterogeneità spaziale nella distribuzione dei popolamenti protetti. In particolare, la frammentazione degli habitat naturali rappresenta un importante fattore di confusione che dovrebbe essere sempre considerato se si vogliono esaminare gli effetti di una AMP; tuttavia, raramente figura negli studi presenti in letteratura.
Il presente lavoro di tesi si inserisce in questo contesto e si propone valutare gli effetti di confusione che processi spaziali legati all’eterogeneità ambientale (riassunti negli indici di eterogeneità del substrato e della costa e dalle coordinate geografiche) possono introdurre nella valutazione dell’effetto riserva, presso alcune isole del Parco Nazionale dell’Arcipelago Toscano (Capraia, Giannutri, Montecristo e Pianosa), attraverso un campionamento della fauna ittica (effettuato tramite tecniche non distruttive di visual census) appropriato per comparare la struttura dei popolamenti ittici, le densità e le distribuzioni di taglia di singole specie target all’interno ed all’esterno delle AMP.
L’analisi è stata condotta avvalendosi di GAMM (Generalized Additive Mixed Models), che permettono di modellizzare in maniera flessibile gli effetti delle covariate (in questo caso le variabili predittive relative ai processi spaziali e ambientali) utilizzando come predittore lineare una combinazione additiva di funzioni non parametriche delle covariate ed effetti di fattori casuali (qui il tempo).
I risultati relativi alle modalità di distribuzione delle abbondanze delle specie censite non hanno mostrato significative differenze tra le aree protette e le località esterne alle AMP, mentre è emersa l’importanza degli effetti relativi alle coordinate spaziali e all’indice di eterogeneità della costa.
Nonostante i fattori da considerare siano molteplici, dalle caratteristiche del ciclo vitale proprie di ciascuna specie all’efficacia delle misure di controllo e tutela dell’area, i risultati ottenuti col presente studio suggeriscono l’importanza di includere, negli studi sulla valutazione dell’effetto riserva, indici relativi a processi spaziali legati all’eterogeneità ambientale
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
The coincidence of environmental and climatic extremes and the response of primary producers in marine coastal habitats and open systems
No full textUnderstanding how global change affects primary productivity in natural systems is of crucial importance. The majority of ecological studies on climate change focused the attention on the impact of mean changes in environmental conditions, but there is increasing evidence indicating that ecological responses may depend as much upon environmental variation and extremes. Extreme weather events, such as severe droughts, heavy rainfalls, heat waves and hot spells, are increasing in severity and frequency and are likely to cause severe impacts at all levels of biological organization. So far, most manipulative field experiments have examined the effects of individual extreme events, with little attention to the possible synergistic effects of multiple extreme disturbances. My thesis addresses the need to examine the combined effects of compounded extreme events on natural population. I focused on phytoplankton and intertidal epilithic microphytobenthos (EMPB) biofilms as model systems. EMPB has been used to test the hypothesis that the concomitance of distinct environmental extreme events elicits larger effects compared to the expected cumulative effect of individual extreme events. The importance of stochastic and determinist environmental changes in driving extreme events has been evaluated through the environmental bootstrap method applied at the scale of the Mediterranean basin.
Research on biofilms started from basic descriptions of spatial organization, as understanding these patterns is necessary to interpret the effects of climate extremes. Results indicated that microalgae develop scale-invariant structures, reflecting the influence of multiple processes operating at different spatial scales and possibly self-organization. A manipulative experiment was set up in order to test the separate and combined effects of warming and runoff following heavy rainfalls. Although a general pattern of reduced EMPB biomass in the clustered than the non-clustered scenario emerged in the first trial of the experiment, the hypothesis that compounded extreme events would elicit larger impacts than extremities of individual disturbances was not supported. EMPB biomass was susceptible to both warming and runoff, but the effects of the combination of these stressors was complex and context-dependent. Thus, repeating this experiment at different times will be necessary before generalities about EMPB responses to environmental extremes can be drawn.
The environmental bootstrap method resampled short-term data of sea surface temperature and patterns of geostrophic currents to obtain an ensemble of hypothetical time series that, when combined with a predictive model of chlorophyll a concentration allowed me to make inferences on primary productivity response to environmental extremes. The output of this analysis is a map of the distribution of chlorophyll a concentration values with 100 years return time periods for the Mediterranean basin, which highlights the areas that are likely to harbour exceptional greening events
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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