Indices, Graphs and Null Models: Analyzing Bipartite Ecological Networks

Carsten Dormann, Jochen Frund, Nico Bluthgen, Bernd Gruber

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    Abstract

    Many analyses of ecological networks in recent years have introduced new indices to describe network properties. As a consequence, tens of indices are available to address similar questions, differing in specific detail, sensitivity in detecting the property in question, and robustness with respect to network size and sampling intensity. Furthermore, some indices merely reflect the number of species participating in a network, but not their interrelationship, requiring a null model approach. Here we introduce a new, free software calculating a large spectrum of network indices, visualizing bipartite networks and generating null models. We use this tool to explore the sensitivity of 26 network indices to network dimensions, sampling intensity and singleton observations. Based on observed data, we investigate the interrelationship of these indices, and show that they are highly correlated, and heavily influenced by network dimensions and connectance. Finally, we re-evaluate five common hypotheses about network properties, comparing 19 pollination networks with three differently complex null models: 1. The number of links per species ("degree") follow (truncated) power law distributions. 2. Generalist pollinators interact with specialist plants, and vice versa (dependence asymmetry). 3. Ecological networks are nested. 4. Pollinators display complementarity, owing to specialization within the network. 5. Plant-pollinator networks are more robust to extinction than random networks. Our results indicate that while some hypotheses hold up against our null models, others are to a large extent understandable on the basis of network size, rather than ecological interrelationships. In particular, null model pattern of dependence asymmetry and robustness to extinction are opposite to what current network paradigms suggest. Our analysis, and the tools we provide, enables ecologists to readily contrast their findings with null model expectations for many different questions, thus separating statistical inevitability from ecological process.
    Original languageEnglish
    Pages (from-to)7-24
    Number of pages18
    JournalOpen Journal of Ecology
    Volume2
    DOIs
    Publication statusPublished - 2009

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    pollinator
    asymmetry
    extinction
    power law distribution
    complementarity
    sampling
    pollination
    generalist
    index
    software

    Cite this

    Dormann, Carsten ; Frund, Jochen ; Bluthgen, Nico ; Gruber, Bernd. / Indices, Graphs and Null Models: Analyzing Bipartite Ecological Networks. In: Open Journal of Ecology. 2009 ; Vol. 2. pp. 7-24.
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    Indices, Graphs and Null Models: Analyzing Bipartite Ecological Networks. / Dormann, Carsten; Frund, Jochen; Bluthgen, Nico; Gruber, Bernd.

    In: Open Journal of Ecology, Vol. 2, 2009, p. 7-24.

    Research output: Contribution to journalArticle

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    T1 - Indices, Graphs and Null Models: Analyzing Bipartite Ecological Networks

    AU - Dormann, Carsten

    AU - Frund, Jochen

    AU - Bluthgen, Nico

    AU - Gruber, Bernd

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    N2 - Many analyses of ecological networks in recent years have introduced new indices to describe network properties. As a consequence, tens of indices are available to address similar questions, differing in specific detail, sensitivity in detecting the property in question, and robustness with respect to network size and sampling intensity. Furthermore, some indices merely reflect the number of species participating in a network, but not their interrelationship, requiring a null model approach. Here we introduce a new, free software calculating a large spectrum of network indices, visualizing bipartite networks and generating null models. We use this tool to explore the sensitivity of 26 network indices to network dimensions, sampling intensity and singleton observations. Based on observed data, we investigate the interrelationship of these indices, and show that they are highly correlated, and heavily influenced by network dimensions and connectance. Finally, we re-evaluate five common hypotheses about network properties, comparing 19 pollination networks with three differently complex null models: 1. The number of links per species ("degree") follow (truncated) power law distributions. 2. Generalist pollinators interact with specialist plants, and vice versa (dependence asymmetry). 3. Ecological networks are nested. 4. Pollinators display complementarity, owing to specialization within the network. 5. Plant-pollinator networks are more robust to extinction than random networks. Our results indicate that while some hypotheses hold up against our null models, others are to a large extent understandable on the basis of network size, rather than ecological interrelationships. In particular, null model pattern of dependence asymmetry and robustness to extinction are opposite to what current network paradigms suggest. Our analysis, and the tools we provide, enables ecologists to readily contrast their findings with null model expectations for many different questions, thus separating statistical inevitability from ecological process.

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    KW - linkage density

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    JO - Open Journal of Ecology

    JF - Open Journal of Ecology

    SN - 2162-1993

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