1,721,012 research outputs found
Reliability of isotopic fractionation (Delta N-15, Delta C-13) for the delimitation of trophic levels of oribatid mites: Diet strongly affects Delta C-13 but not Delta N-15
No full textStable isotope analysis has become an analytical tool of central importance in trophic ecology. The actual degree of isotopic fractionation, however, remains a black-box in most studies. Hence, mean values for trophic enrichment from other taxa are commonly used to delimitate trophic levels. On the other hand, resource composition as well as consumer physiology both influence fractionation patterns. Especially in soil food webs, high variability of isotopic fractionation has been found for both N-15 and C-13. Here, we investigate effects of diet on trophic enrichment in a no-choice feeding, experiment with the oribatid mite model species Archegozetes longisetosus and a set of fungal, animal, algal and plant resources (seeds and vegetative tissues). We found consistent trophic enrichment for nitrogen irrespective of diet (Delta N-15 = 3.9 parts per thousand), but no reliable fractionation for carbon being negative for fungi (Delta C-13 = 1.8 parts per thousand), neutral for algae, and positive for the remaining plant and animal resources (Delta C-13 = 2.3 parts per thousand). The results suggest that delta N-15 is a reliable marker for delimiting the trophic level of oribatid mites and presumably other soil detritivores, while understanding delta C-13 signals needs a priori knowledge about isotopic fractionation with respect to consumed resources. (C) 2016 Published by Elsevier Ltd
Reducible defence: chemical protection alters the dynamics of predator-prey interactions
No full textMorphological and chemical defences are widespread anti-predator mechanisms in most domains of life, and play an important role in understanding predator-prey interactions. Classical concepts of dynamical protection ('inducible defence') include the morphological changes in certain crustaceans or the production of chemicals in many plants. Permanently stored defensive secretions are, to our knowledge, ignored in food web ecology. We show that this kind of chemical defence is highly dynamic and may loose its effect over time ('reducible defence'). Combining experimental and theoretical approaches, we show that chemical defence also changes the time budget of predators and decreases the strength of the functional response. However, this may be counteracted by increasing predator density-an effect we call 'apparent facilitation'. The interplay of 'reducible defence' and 'apparent facilitation' may substantially contribute to stability in terrestrial ecosystems
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
Track the snack – olfactory cues shape foraging behaviour of decomposing soil mites (Oribatida)
No full textAppropriate 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
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
Development and function of the genital organs in the parthenogenetic oribatid mite Archegozetes longisetosus Aoki 1965
Full text linkArchegozetes longisetosus Aoki 1965 is a parthenogenetic oribatid mite, that has a
pantropical-disjunct distribution. It is a member of the family of Trhypochthoniidae.
Its ease of rearing, rapid succession of generations and high fecundity fulfill
central requirements of a suitable model organism. Roy A. Norton in 1993 founded
a laboratory strain from a single puertorican female, Archegozetes longisetosus ran,
which is since kept in laboratories worldwide. This laboratory strain lead to A.
longisetosus being the most thoroughly studied oribatid mite. The species is of
special interest in studies on evolutionary biology, as it is a member of a cluster
of obligatory parthenogenetic species, for which molecular studies have indicated
a very old age of probably more than a hundred million years. Detailed insight
in the reproduction of A. longisetosus therefore does not only broaden our knowledge
of a model organism, but may also contribute to a better understanding of
the ancillary conditions of uni- vs. bisexual propagation, as well as the differential
likelihood of long term stability in different forms of unisexual propagation. One
step on the way towards this goal, and a necessary condition for future studies, is
the acquisition of anatomical data regarding structure, function and development
of organs and tissues. This cognitive interest lay at the starting point of the study
at hand. With the aid of high-resolution synchrotron X-ray micro computer tomographies
(SR-μCT), three-dimensional models of the genital organs and their precursors
were obtained from all freeliving instars (larva to adult). As several models
per instar at intervals of several days were obtained, important insight in developmental
processes was gained. Already in the larva, germinative and somatic
portions of the genital anlage can be distinguished. Further development proceeds
continously, and largely independent from cuticular moulting. Precursors
of the oviducts start to develop in the protonymphal stage, whereas proliferation
of the germcells takes place during the deutonymphal stage. Ectodermal portions
of the genital systems start development in the tritonymphal stage. The oviducts
apparently do not form as evaginations of a coelomic sac containing germline cells,
but either as lateral folds of the coelomic cavity or as lateral evaginations, which
retrogradally form secondary contact sites to the germinative portion. Further investigations
are needed to establish the mode of oviduct formation unequivocally.
Three-dimensional models also facilitated the planning of semi- and ultrathin serial
sectionings, which yielded histological and functional information. Observations
made using electron- and lightmicroscopy included the onset of meiosis I in
the tritonymphal stage, uptake of yolk precursors from the fat body into the oocyte
via microvilli and coated pits, accumulation of egg shell material simultaneous to
yolk accumulation, and the solidification of the egg shell upon passing into the
oviduct. These observations justified a nomenclature of the genital system and
the classification of the ovary as panoistic. Additionally, the oviducts could be described
as a sheltered space for embryonal development. In summary, the results
indicate that the loss of fertilization in the reproduction of A. longisetosus permits
both anatomical and temporal compaction of the related processes, enabling
relatively high average reproductive rate, even in the face of short-term unstable
environmental conditions. The selection of methods permitted to demonstrate the
mechanism of thelytoky by terminal fusion automixis with inverted (postreductional)
meiosis in a functional context, and to present A. longisetosus as a promising
model system also for questions beyond the borders of the taxon chelicerata.Archegozetes longisetosus Aoki 1965 ist eine parthenogenetische Hornmilbe mit
pantropisch-disjunkter Verbreitung aus der Familie der Trhypochthoniidae. Durch
ihre einfache Haltung, schnelle Generationenfolge und hohe Fruchtbarkeit erfüllt
sie wesentliche Merkmale eines geeigneten Modellorganismus. 1993 wurde von
Roy A. Norton aus einem einzelnen puertoricanischen Weibchen ein Laborstamm
etabliert, A. longisetosus ran, der inzwischen weltweit in Laboren vertreten ist, was
mit dazu führte, dass A. longisetosus mittlerweile die insgesamt am besten untersuchte
Hornmilbe darstellt. Von besonderem Interesse ist die Art für evolutionsbiologische
Studien, Da sie zu einem Schwarm obligat parthenogenetischer Linien
gehört, für die molekulare Studien ein sehr hohes Alter von evtl. über hundert Millionen
Jahren nahelegen. Ein genaues Verständnis der Fortpflanzungsvorgänge
von A. longisetosus dient also nicht nur der Erweiterung unserer allgemeinen Kenntnis
eines Modellorganismus, sondern darüber hinaus dem besseren Verständnis
der Randbedingungen ein- und zweigeschlechtlicher Fortpflanzung, sowie der
unterschiedlich langen Überlebenswahrscheinlichkeit der verschiedenen Formen
eingeschlechtlicher Fortpflanzung. Ein Schritt auf dem Weg zu diesem Verständnis,
und notwendige Bedingung für weiterführende Studien, ist der Aufbau einer
anatomischen Datengrundlage zu Aufbau, Funktion und Entwicklung der an der
Fortpflanzung beteiligten Organe und Gewebe. Dieses Erkenntnisinteresse lag der
vorliegenden Arbeit zugrunde. Anhand hochauflösender Synchrotron- Röntgen-
Mikro- Computer- Tomographien (SR-μCT) wurden dreidimensionale Modelle der
Genitalorgane und ihrer Anlagen in allen freilebenden Stadien von der Larve bis
zum Adultus erstellt. Da pro Entwicklungsstadium mehrere Modelle im Abstand
weniger Tage erstellt werden konnten, konnten so wichtige Erkenntnisse über
die Entwicklung der Genitalorgane gewonnen werden. So zeigte sich, dass schon
in Larvenstadien ein somatischer und ein germinativer Anteil der Genitalanlage
unterschieden werden kann. Die weitere Entwicklung dieser Anlage verläuft kontinuierlich,
und weitgehend unabhängig von den Häutungen der Körperhülle. Vorläufer
der Ovidukte beginnen sich ab der Protonymphe zu entwickeln, während die
Vermehrung der Eizellen in der Deutonymphe stattfindet. Ab der Tritonymphe
bilden sich auch die ektodermalen Anteile des Genitalsystems aus. Die Ovidukte
bilden sich dabei wohl nicht als Evaginationen eines die Eizellen enthaltenden
Coelomsackes, sondern entweder als randliche Abfaltungen des Genitalcoeloms,
oder als laterale Evaginationen, die retrograd sekundären Kontakt zum germinativen
Teil aufnehmen, eine Beobachtung, die noch weiterer Klärung bedarf.
Sehr erleichtert durch die Modellserie wurde auch die Planung von Semi- und Ultradünnschnittserien
ausgewählter Strukturen, die histologische und funktionelle
Einsichten lieferten. So konnte das Einsetzen der Meiose in der Tritonymphe, die
Aufnahme von Dottervorstufen aus dem Fettkörper in die Eizelle durch Mikrovilli
und ‘coated pitsˊ, die Ablagerung von Eihüllenmaterial und dessen Verfestigung
beim Übergang ins Ovidukt beobachtet werden. Durch diese Erkenntnisse konnte
eine Nomenklatur des Genitalsystems begründet werden, das Ovar als funktionell
panoistisch klassifiziert und die Ovidukte als geschützter Raum der Embryonalentwicklung
beschrieben. Insgesamt ergeben sich Hinweise darauf, dass der Reproduktionsmodus
durch Wegfall der Befruchtung die anatomische Verdichtung
und Verschränkung der Prozesse erlaubt, was eine konstant hohe Reproduktionsrate
auch unter kurzfristig wechselnden Lebensbedingungen ermöglicht. Mit der
vorgestellten Methodenauswahl konnte der Reproduktionsmodus der Thelytokie
mit terminaler Fusion und invertierter (postreduktionaler) Meiose im funktionalen
Zusammenhang dargestellt, und A. longisetosus als aussichtsreiches Modellsystem
für auch über den Bereich der Cheliceraten hinausweisenden Fragestellungen
vorgestellt werden
- …
