Ecology

An article in a recent issue of Science (Elisa Thebault & Colin Fontaine, "Stability of ecological communities and the architecture of mutualistic and trophic networks", Science 329, Aug 13 2010, p. 853-856; free summary here) studies 2 kinds of ecological networks: trophic (predator-prey) and mutualistic (in this case, pollinators and flowers).  They looked at the effects of 2 properties of networks: modularity (meaning the presence of small, highly-connected subsets that have few external connections) and nestedness (meaning the likelihood that species X has the same sort of interaction with multiple other species).  (It's unfortunate that they never define modularity or nestedness formally; but this informal definition is still useful.  I'm going to call nestedness "sharing", since they do not state that their definition implies nesting one network inside another.)  They looked at the impact of different degrees of modularity and nestedness, in trophic vs. mutualistic networks, on persistence (fraction of species still alive at equilibrium) and resilience (1/time to return to equilibrium after a perturbation).  They used both simulated networks, and data from real-world ecological networks.

What they found is that, in trophic networks, modularity is good (increases persistence and resilience) and sharing is bad; while in mutualistic networks, modularity is bad and sharing is good.  Also, in trophic networks, species go extinct so as to make the network more modular and less sharing; in mutualistic networks, the opposite occurs.

The commonsense explanation is that, if species X is exploiting species Y (trophic), the interaction decreases the health of species Y; and so having more exploiters of Y is bad for both X and Y.  OTOH, if species X benefits from species Y, X will get a secondhand benefit from any mutually-beneficial relationships that Y has; if Y also benefits from X (mutualistic), then neither X nor Y will adapt to prevent Z from also having a mutualistic relationship with Y.  (The theory does not address a mixture of trophic and mutualistic interactions in a single network.)