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400 563 566 Daily, G.C., P.R. Ehrlich and N.M. Haddad. 1993. Double keystone bird in a keystone species complex. Proceedings of the National

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Ecol. Monogr. 65 21 74 Mills, L. S., M. L. Soulé and D. F. Doak. 1993. The keystone-species concept in

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References Bond , W.J. 1994 . Keystone species . In: Schulze , E.D. and Mooney , H.A. (eds), Biodiversity and Ecosystem Function . Springer

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Macroscopic ecosystem studies often complete our knowledge based on population-level experiments and models. In this paper, the changed control of ecosystem functioning is reported by analyzing the structure of the energy flow network of a tidal marsh community (Crystal River, Florida). The positional importance of trophic components is characterized by a graph theoretical approach. Then, positional importance of points is compared to the magnitude of fitting carbon flows (i.e., the importance of links) and the congruency is expressed in percents. These results are presented for both an unperturbed (control) and a thermally stressed creek ecosystem of the river. The comparison of average congruency values for the two communities suggests that, first, trophic control may be stronger in the stressed community and, second, the reliability of carbon flows is also higher in the stressed ecosystem.

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The study of co-occurrence patterns has been extensively applied to propose assembly rules for community organization. Recently, a new interest has grown in the effect of gradients on these patterns and to analyze them through new approximations such as co-occurrence networks, through which keystone species can be identified. Neotropical floodplains represent interesting systems to study such patterns, because of their spatial heterogeneity, temporal variability and their high fish species richness. With this in mind, our goal was to study the co-occurrence patterns of fish in a segment of the Arauca River’s floodplain and the influence of the spatial and temporal variability on them. One stream and one floodplain lake were sampled with gill nets during 2014 – 2015 across a hydrological cycle and 5 matrices for each 5 sampled months in each water body were prepared to explore the co-occurrence patterns in each water body across months and 2 for the entire period, through a probabilistic pair-wise analysis of species co-occurrence that identified aggregated and segregated species pairs. With the observed cooccurrences × water body × month, the species weighted degrees and betweenness were calculated, and co-occurrence networks were constructed. The networks structures, in terms of the degrees of every species, were compared spatially and temporally through a generalized linear model. The stream showed the highest numbers of aggregated species pairs, and in general showed the most complex networks in terms of nodes, edges and degrees. The habitat type and the hydrological phases significantly influenced the structure of the fish co-occurrence networks. Two species, Loricariichthys brunneus and Pygocentrus cariba were identified as the core of the fish communities of the floodplain and as keystone species because they contribute to the connection of the networks by having a series of links with less frequent species.

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Facilitation by nurse plants is one of the most commonly reported interactions between plants and is regarded as an important factor in structuring plant communities. We used a meta-analysis to examine the generality of these effects, focusing on cushion plants, a common life-form occurring in high-Andean ecosystems. We targeted the following questions: (1) is there a generalized positive effect of cushions on other vascular plant species along the Andes? (2) do different species groups (i.e., annuals and perennials, natives and exotics) display different association responses to cushions? (3) does the nurse effect of cushions increase with environmental severity? Results indicated that the overall effect of cushions is positive, however these positive effects were more significant amongst exotic plants than in native plants; effects were only positive for perennial plants, and were notably negative for annuals. The positive effects of cushions also increased with physical stress, but only for perennial plants. These results allow us to suggest that as a whole cushions may be acting as keystone species that maintain the structure and diversity of high-Andean plant communities. Nevertheless, since cushions also positively affect the performance of exotic plants, we should be aware of their potential role in promoting biological invasions.

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302 Estrada, E. 2007. Characterization of topological keystone species: Local, global and “meso-scale” centralities in food webs. Ecol. Complex. 4: 48

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. Libralato , S. , Christensen , V. and Pauly , D . 2006 . A method for identifying keystone species in food web models . Ecol. Model. 195 : 153 – 171 . MacPherson , E. and Roel

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Community Ecology
Authors:
V. H. Cruz-Escalona
,
A. F. Navia
,
P. A. Mejia-Falla
,
M. V. Morales-Zárate
, and
C. A. Salinas-Zavala

. Jordán , F. , Liu , W. and Davis , A.J. 2006 . Topological keystone species: measures of positional importance in food webs . Oikos. 112 : 535 – 546 . Jordán , F

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Arena’s conceptualization is based on human ecology; Davis correlates the adoption of digital tools and the expansion of educational systems as one of coevolution. The keystone species in this theoretical framework is the teacher, as the most important

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