Interaction strength, food web topology and the relative importance of species in food webs

Authors

  • Eoin J. O’Gorman,

    1. Environmental Research Institute, University College Cork, Lee Road, Cork, Ireland
    2. Department of Zoology, Ecology and Plant Sciences, University College Cork, Distillery Fields, North Mall, Cork, Ireland
    Search for more papers by this author
  • Ute Jacob,

    1. Environmental Research Institute, University College Cork, Lee Road, Cork, Ireland
    2. Department of Zoology, Ecology and Plant Sciences, University College Cork, Distillery Fields, North Mall, Cork, Ireland
    3. Alfred Wegener Institute for Polar and Marine Research, PO Box 120161, D - 27515 Bremerhaven, Germany
    Search for more papers by this author
  • Tomas Jonsson,

    1. Ecological Modelling Group, Systems Biology Research Centre, Skövde University, Box 408, SE-54128 Skövde, Sweden
    Search for more papers by this author
  • Mark C. Emmerson

    1. Environmental Research Institute, University College Cork, Lee Road, Cork, Ireland
    2. Department of Zoology, Ecology and Plant Sciences, University College Cork, Distillery Fields, North Mall, Cork, Ireland
    Search for more papers by this author

Correspondence author. E-mail: e.ogorman@mars.ucc.ie

Summary

1. We established complex marine communities, consisting of over 100 species, in large subtidal experimental mesocosms. We measured the strength of direct interactions and the net strength of direct and indirect interactions between the species in those communities, using a combination of theoretical and empirical approaches.

2. Theoretical predictions of interaction strength were derived from the interaction coefficient matrix, which was parameterised using allometric predator–prey relationships. Empirical estimates of interaction strength were quantified using the ln-ratio, which measures the change in biomass density of species A in the presence and absence of species B.

3. We observed that highly connected species tend to have weak direct effects and net effects in our experimental food webs, whether we calculate interaction strength theoretically or empirically.

4. We found a significant correlation between our theoretical predictions and empirical estimates of direct effects and net effects. The net effects correlation was much stronger, indicating that our experimental communities were dominated by a mixture of direct and indirect effects.

5. Re-calculation of the theoretical predictions of net effects after randomising predator and prey body masses did not affect the negative relationship with connectance.

6. These results suggest that food web topology, which in this system is constrained by body mass, is overwhelmingly important for the magnitude of direct and indirect interactions and hence species importance in the face of biodiversity declines.

Ancillary