1,721,034 research outputs found
Diversity of vegetation patterns and desertification
No full textA new model for vegetation patterns is introduced. The model reproduces
a wide range of patterns observed in water-limited regions, including
drifting bands, spots, and labyrinths. It predicts transitions from bare
soil at low precipitation to homogeneous vegetation at high
precipitation, through intermediate states of spot, stripe, and hole
patterns. It also predicts wide precipitation ranges where different
stable states coexist. Using these predictions we propose a novel
explanation of desertification phenomena and a new approach to
classifying aridity
Bistability and regular spatial patterns in arid ecosystems.
No full textA variety of patterns observed in ecosystems can be explained by resource–concentration mechanisms. A resource–concentration mechanism occurs when organisms increase the lateral flow of a resource toward them, leading to a local concentration of this resource and to its depletion from areas farther away. In resource–concentration systems, it has been proposed that certain spatial patterns could indicate proximity to discontinuous transitions where an ecosystem abruptly shifts from one stable state to another. Here, we test this hypothesis using a model of vegetation dynamics in arid ecosystems. In this model, a resource– concentration mechanism drives a positive feedback between vegetation and soil water availability. We derived the conditions leading to bistability and pattern formation. Our analysis revealed that bistability and regular pattern formation are linked in our model. This means that, when regular vegetation patterns occur, they indicate that the system is along a discontinuous transition to desertification. Yet, in real systems, only observing regular vegetation patterns without identifying the pattern-driving mechanism might not be enough to conclude that an ecosystem is along a discontinuous transition because similar patterns can emerge from different ecological mechanisms
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
A mathematical model of plants as ecosystem engineers
No full textUnderstanding the structure and dynamics of plant communities in
water-limited systems often calls for the identification of ecosystem
engineers-key species that modify the landscape, redistribute resources
and facilitate the growth of other species. Shrubs are excellent
examples; they self-organize to form patterns of mesic patches which
provide habitats for herbaceous species. In this paper we present a
mathematical model for studying ecosystem engineering by woody plant
species in drylands. The model captures various feedbacks between
biomass and water including water uptake by plants' roots and increased
water infiltration at vegetation patches. Both the uptake and the
infiltration feedbacks act as mechanisms for vegetation pattern
formation, but have opposite effects on the water resource; the former
depletes the soil-water content under a vegetation patch, whereas the
latter acts to increase it. Varying the relative strength of the two
feedbacks we find a trade-off between the engineering capacity of a
plant species and its resilience to disturbances. We further identify
two basic soil-water distributions associated with engineering at the
single patch level, hump-shaped and ring-shaped, and discuss the niches
they form for herbaceous species. Finally, we study how pattern
transitions at the landscape level feedback to the single patch level by
affecting engineering strength. (c) 2006 Elsevier Ltd. All rights
reserved
Ecosystem engineers: From pattern formation to habitat creation
No full textHabitat and species richness in drylands are affected by the dynamics of
a few key species, termed ``ecosystem engineers.{''} These species
modulate the landscape and redistribute the water resources so as to
allow the introduction of other species. A mathematical model is
developed for a pair of ecosystem engineers commonly found in drylands:
plants forming vegetation patterns and cyanobacteria forming soil
crusts. The model highlights conditions for habitat creation and for
high habitat richness, and suggests a novel mechanism for species loss
events as a result of environmental changes
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
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
Vegetation patterns along a rainfall gradient
No full textA continuum model for vegetation patterns in water limited systems is
presented. The model involves two variables, the vegetation biomass
density and the soil water density, and takes into account positive
feedback relations between the two. The model predicts transitions from
bare-soil at low precipitation to homogeneous vegetation at high
precipitation through intermediate states of spot, stripe and gap
patterns. It also predicts the appearance of ring-like shapes as
transient forms toward asymptotic stripes. All these patterns have been
identified in observations made on two types of perennial grasses in the
Northern Negev. Another prediction of the model is the existence of wide
precipitation ranges where different stable states coexist, e.g. a bare
soil state and a spot pattern, a spot pattern and a stripe pattern, and
so on. This result suggests the interpretation of desertification
followed by recovery as an hysteresis loop and sheds light on the
irreversibility of desertification. (C) 2003 Elsevier Ltd. All rights
reserved
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