Elisa Thébault and Colin Fontaine, with a research carried out at Imperial College London, Wageningen University, part of Wageningen UR, and the Museum National d’Histoire Naturelle, have shown that the network architectures which favour the stability of ecological communities differ between trophic webs (“who eats who”) and mutualistic webs (“who pollinates who”). Their theoretical results reveal that, in order to be stable, mutualistic interaction webs should present a nested architecture whereas trophic webs should adopt a compartmented architecture. This difference in architecture can be found in a large number of empirical pollination (mutualist) and herbivory (trophic) webs. This work is a major breakthrough for a better understanding of the functioning and stability of communities. These results are published in the Science issue of 13 august 2010.
Networks of ecological interactions describe the relations between species within a community: for example “who eats who” for a trophic web1 or “who pollinates who” for a plant-pollinator mutualistic web2. The architecture of these networks describes the way the interactions are distributed among species; an architecture is compartmented when a network is built of different groups of species which interact more within groups than between groups (figure 1), while a nested architecture corresponds to a network that is organized around a unique group of generalist species interacting between themselves and with more specialist species (figure 2). As for the stability of a community (i.e. all the species of the network), it characterizes the ability of the community to resist to perturbations.
To date research on the relations between the architecture of ecological networks and the stability of communities had focused on one type of interaction at a time (mainly trophic interactions), making difficult the comparison between different types of networks.
In this study, the authors have realized a comparison between trophic and mutualistic networks and have investigated if the type of interaction (mutualistic or trophic) affects the relation between network architecture and community stability. They compared the results of a theoretical approach (dynamical model) with the architecture of a large dataset of published empirical networks describing 34 pollination networks (mutualist) and 23 herbivory networks (trophic).
Results show that the network architectures which favour the stability of ecological communities differ between trophic and mutualistic networks. Indeed a highly connected and nested architecture, i.e. with many generalist species interacting both between themselves and with specialist species, stabilize mutualistic networks; whereas a weakly connected and highly compartmented architecture, i.e. with few generalist species and species that interact within delimited groups, stabilize trophic networks.
This research brings important perspectives for a better understanding of the functioning of ecosystems and their response to environmental disturbances:
- How to define relevant and functional indicators of ecosystem stability with the architecture of interaction networks?
- How networks of different interaction types with different architectures can be combined together to form the large network that link all the species in an ecosystem? And how does it interact with ecosystem functioning and stability?
About the authors
Colin Fontaine, after being a post-doctorate research associate at Imperial College London, has been recently granted a fellowship to work at the Muséum national d'Histoire naturelle of Paris. Since last January, his research focuses on the creation of indicators for pollination systems in the laboratory « Conservation des espèces, suivi et restauration des populations laboratoire » (UMR 7204 MNHN/CNRS). He will analyze the data of the program of citizen science SPIPOLL launched by the Muséum and the Opie last May: www.spipoll.fr
Elisa Thébault is a post-doctorate research associate at Wageningen University (Netherlands), after having worked at Imperial College London. Her current research focuses on soil food webs and their response to agricultural land use, as part of the European project Soilservice (www.kem.ekol.lu.se/soilservice), in the laboratory Biometris (Wageningen UR). She will join the laboratory « Biogéochimie et écologie des milieux continentaux » in Paris (UMR 7618 CNRS/UPMC/ENS Paris/IRD/Université Paris 12) at the end of this year.
Illustration:
1. A trophic network is a network of feeding relationships between species in a community by which energy and matter are passed. .
2. A mutualistic network is a network of mutually beneficial relationships between species in a community.