Ah, finally back to work. The lungs seem to cooperating again, and while not 100%, boy, it felt good to be back to the Academy. Also, being back meant that I was able to finally begin testing some of these couch-bound speculations from the past couple of weeks!
Shown in this first figure is a metanetwork representation of a Late Miocene, shallow marine community from the Dominica Republic. These data were compiled by one of my graduate students, Rachel Hertog. Recall that each sphere represents a set of one or more species that potentially share the same predators and prey. The guilds are colour-coded, but we’ll ignore that for now. The links between guilds represent sets of trophic interactions. This paleocommunity has 29 guilds and 139 guild-level links. The guilds range from phytoplankton to epifaunal benthic carnivores to pelagic carnivorous fish. There are 130 species in the community.
I ran topological-only and fully dynamic simulations of bottom-up perturbation on species-level networks derived from this metanetwork. The perturbation is a progressive reduction of primary productivity, implemented as a progressive reduction in the size of all four primary production guilds. The second figure shows the results. The fully dynamic results (in yellow) exhibit the typical CEG result. The topological-only results are shown in aqua. As expected, topological extinction underestimates the scale of extinction possible, and follows a predicted “exponential-type” of increase. Notice, however, that the pattern takes a little “hop” at an approximate perturbation magnitude of 0.67. Note, also, that it is at precisely this point that the dynamic results show the typically rapid increase in secondary extinction level.
An examination of the topological results reveal that the hop is due to the complete extinction, at that point, of two guilds: epifaunal herbivores, and shallow infaunal herbivores. These two guilds consume the macroalgae/seagrass guild exclusively. That this is the predicted point of extinction can be checked analytically. Remarkably, it explains the discontinuous/catastrophic increase seen in the dynamic results. There are five guilds that include these herbivore guilds as prey, and they are all carnivorous or omnivorous macroinvertebrates and fish, shallow infaunal, epifaunal and pelagic. All these consumers have a wide array of prey guilds, but the loss of the two herbivore guilds represent a significant enough loss of resources that the compensation of the predators, represented in the model as increases of interaction strength with remaining prey, causes top-down cascades strong enough to in turn cause the rise in secondary extinctions. The dramatic increase is, of course, also a function of the fact that the other producer guilds and their primary consumers are being perturbed. One of the next steps will be to repeat these simulations, but to perturb the herbivores only. And the really next big and tedious step is to work out the analytical predictions of the topological scenario.