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Science 21 April 2006:
Vol. 312. no. 5772, pp. 431 - 433
DOI: 10.1126/science.1123412
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Reports

Asymmetric Coevolutionary Networks Facilitate Biodiversity Maintenance

Jordi Bascompte1*, Pedro Jordano1 and Jens M. Olesen2

1 Integrative Ecology Group, Estaciòn Biològica de Doñana, Consejo Superior de Investigaciones Cientìficas, Apartado 1056, E-41080 Sevilla, Spain.
2 Department of Ecology and Genetics, University of Aarhus, Ny Munkegade, Building 540, DK-8000 Aarhus, Denmark.


Figure 1 Fig. 1. A network approach to plant-animal mutualisms. (A) Example of a community of plants and their seed dispersers in Cazorla, SE Spain (see Database S1 for references and data sets). Green circles represent plant species and red squares represent animal species. A plant and an animal interact if there is a qualitative link between them. (B and C) Each of the above plant-animal interactions is described by two weighted links (arrows) depicting the relative dependence of the plant on the animal (green arrow) and the animal on the plant (red arrow). The asymmetry of the pairwise interaction is proportional to the difference between the thickness of both arrows. Here we show a symmetric (B) and an asymmetric (C) example. (D and E) A species degree is the number of interactions it has with the other set. Species strength is the quantitative extension of species degree, and can be defined as the sum of dependences of the animals on the plant (D) and the plants on the animal (E). Although the degree is four in both (D) and (E), the strength of the animal (E) is higher than that of the plant (D). [View Larger Version of this Image (58K GIF file)]
 

Figure 2 Fig. 2. Frequency distributions of dependence values within a mutualistic community. Green solid histograms (A to F) represent dependences of plants on pollinators, and red dashed histograms (G to I) represent dependences of seed dispersers on plants. See Database S1 for references and data sets. [View Larger Version of this Image (16K GIF file)]
 

Figure 3 Fig. 3. Frequency distributions of asymmetry values of mutual dependences within a mutualistic community. (A to F) Plant-pollinator communities. (G to I) Plant seed–disperser communities. See Database S1 for references and data sets. [View Larger Version of this Image (18K GIF file)]
 

Figure 4 Fig. 4. Relationship between the number of interactions per species (degree) and its quantitative extension, species strength. (A to C) Pollinator species in plant-pollinator communities. (D to F) Plant species in plant-pollinator communities. (G and H) Animal species in plant seed–disperser communities. (I) Plants in a plant seed–disperser community. A quadratic regression is represented when the quadratic term is significant; otherwise a linear regression is plotted (G). As noted, in all cases but (G), species strength increases faster than species degree. See Database S1 for references and data sets. [View Larger Version of this Image (28K GIF file)]
 

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