It is generally accepted that plant-pollinator webs are nested and that nestedness promotes species richness in those networks. I've just read a study of Kondoh et al. ("Food webs are built up with nested subwebs", 2010, Ecology) stating that trophic interaction networks are nested as well, but with the inverse consequences, that is nestedness prevents species coexistence. I am studying host-parasitoid networks and I am wondering if in those networks nestedness would promote or impair species richness. I would be very interested to have your opinions.
Hi François, thank you very much for your concise and clear revue of the current debate. A problem I think is the definition of specialization, which is extremely difficult if even possible to assess with confidence. In host-parasitoid networks the issue is certainly the same. I sampled an host-parasitoid community and find a typical asymmetric pattern, but how far it is due to undersampling, random attachement, or true specialization will be difficult to disentangle!
Best
Dear Valéria,
I have a paper in Journal Animal Ecology about nestedness and modules in host-parasite networks. Maybe it can help you.
I tend to lean toward the view that many empirically-based answers to this so far, as well as the same question regarding food webs, suffer from gross undersampling. Also, in cases where sampling might be good or adequate because the community is of low richness, one wonders whether the scaling to richer and more interesting communities would be nonlinear and allometric.
Thank you. Yes it is not an easy issue. Regarding host-parasite, a further difficulty is the possibility for hyperparasitism...
Anyway all these studies are very interesting to read at least to have a good idea of the wide variety of processes that may be at play in shaping species communities.
After reading some papers on the topic, I tend to agree that species richness is not dependent on the level of nestedness of trophic web but more on the nature (frequency/force of interactions, interchangeability, directions/magnitude, sensitivity to external changes) of the interactions among and witthin each compartment of the web. For example, it has been shown in a study on a semi-arid peathland that the degree of palability of plants was the dominant characteristic driving species richness in a predator-prey food web of insects.
Yes. Maybe there has been a problem of formulation too. Some studies advocate that netedness promotes species coexistence. But it is certainly as you say other factors that govern the presence of species within networks and nested structure arises as a consequence, not as a cause. Maybe then there is also no sense to try relating nestedness to network robustness.
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