1,720,959 research outputs found
Topology of additive pairwise effects in food webs
No full textContrasting reductionistic versus holistic views, it is a general question whether adding the parts equals the sum. In the time of multiple drivers of anthropogenic change, it is a crucial issue, and better understanding additivity is critical for strategy and management. More particular research questions ask what are the community effects of the extinction of a single species and whether multiple local effects on different species will generate additive community responses. Here we perform food web simulations and study the community response to perturbing each species, one by one, and perturbing each possible pairwise combination of species. By comparing the two, we quantify additivity and analyze how does it depend on the topological positions of perturbed species pairs. Results increase the predictability of food web research, supporting systems-based conservation management and, possibly, multi-species maximum sustainable yield assessment in fisheries
Aggregating a plankton food web: Mathematical versus biological approaches
No full textSpecies are embedded in a web of intricate trophic interactions. Understanding the functional role of species in food webs is of fundamental interests. This is related to food web position, so positional similarity may provide information about functional overlap. Defining and quantifying similar trophic functioning can be addressed in different ways. We consider two approaches. One is of mathematical nature involving network analysis where unique species can be defined as those whose topological position is very different to others in the same food web. A species is unique if it has very different connection pattern compared to others. The second approach is of biological nature, based on trait-based aggregations. Unique species are not easy to aggregate with others because their traits are not in common with the ones of most others. Our goal here is to illustrate how mathematics can provide an alternative perspective on species aggregation, and how this is related to its biological counterpart. We illustrate these approaches using a toy food web and a real food web and demonstrate the sensitive relationships between those approaches. The trait-based aggregation focusing on the trait values of size (sv) can be best predicted by the mathematical aggregation algorithms
Who Is Where in Marine Food Webs? A Trait-Based Analysis of Network Positions
Full text linkNetworks of trophic interactions provide a lot of information on the functioning of marine ecosystems. Beyond feeding habits, three additional traits (mobility, size, and habitat) of various organisms can complement this trophic view. The combination of traits and food web positions are studied here on a large food web database. The aim is a better description and understanding of ecological roles of organisms and the identification of the most important keystone species. This may contribute to develop better ecological indicators (e.g., keystoneness) and help in the interpretation of food web models. We use food web data from the Ecopath with Ecosim (EwE) database for 92 aquatic ecosystems. We quantify the network position of organisms by 18 topological indices (measuring centrality, hierarchy, and redundancy) and consider their three, categorical traits (e.g., for mobility: sessile, drifter, limited mobility, and mobile). Relationships are revealed by multivariate analysis. We found that topological indices belong to six different categories and some of them nicely separate various trait categories. For example, benthic organisms are richly connected and mobile organisms occupy higher food web positions
Additivity of pairwise perturbations in food webs: Topological effects
Full text linkFood webs dynamically react to perturbations and it is an open question how additive are the effects of single-species perturbations. Network structure may have topological constraints on additivity and this influences community response. Better understanding the relationships between single-species and multi-species perturbations can be useful for systems-based conservation management. Here we study a single model food web by (1) characterising the positional importance of its nodes, (2) building a dynamical network simulation model and performing sensitivity analysis on it, (3) determining community response to each possible single-species perturbation, (4) determining community response to each possible pairwise species perturbation and (5) quantifying the additivity of effects for particular types of species pairs. We found that perturbing pairs of species that are either competitors or have high net status values in the network is less additive: their combined effect is dampened
The vulnerability of plant-pollinator communities to honeybee decline: A comparative network analysis in different habitat types
No full textThe populations of most pollinators, including honeybees, are declining that heavily affects both crop and wild plant pollination. Wild bee diversity and habitat type may modulate these effects. We addressed the question how the structure of plant-pollinator networks in different habitat types may influence the vulnerability of pollinator communities to the hypothetical loss of honeybees. We performed network analysis based on plant-visitation data in a traditional agricultural landscape and quantified the structural vulnerability (i.e. the effect of the loss of honeybee) of the plant-pollinator networks by a topological index (distance-based fragmentation). We found that very different plant-pollinator communities inhabited the studied different agricultural habitat types. The early summer arable fields had the most, pastures in mid-summer had the less vulnerable structure and, in general, an intermediate plant/pollinator ratio was associated with high vulnerability in the absence of honeybees. We suggest that increased plant species richness can ensure higher wild bee diversity and more stable plant-pollinator networks without honeybee, where flower-visitation can rely more on wild bees. Decreased management intensity in agricultural landscapes can therefore contribute to the maintenance of diverse plant-pollinator communities in agricultural landscapes and to sustainable farming
Food web dynamics in trophic hierarchies
No full textUsing network analysis for making predictions about food web dynamics is one of the major challenges in systems ecology. Since there are several notoriously difficult methodological problems with food webs, only a comparative perspective can help. We study a standard database for trophic flow networks and analyse the correlation between structure and dynamics in strictly hierarchical food webs (directed acyclic graphs, DAGs). To characterize the structural information about trophic nodes in food webs (their positional importance), we use 8 topological indices that had been developed for quantifying DAGs (3 indices related to status, s, and 5 indices related to the keystone index, K). For dynamics, we use the KS (keystoneness) index that quantifies the importance of trophic nodes in the food web, considering also carbon flows and biomass. We statistically compare the structural and dynamical importance of each network node and find that the K indices are much better predictors of KS than the s indices. Based on these results, we suggest that functional studies have to consider both bottom-up and top-down effects as well as indirect effects that are dampening with distance. We suggest that this kind of study can be helpful to better understand the relevance and applicability of network analysis, an otherwise popular research methodology with continuously increasing predictive power
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
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
- …
