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There are three responses to bottom-up perturbation that have been observed in both the models and simulations of real communities. First, there is the “typical” CEG response (described in an earlier post). The low levels of secondary extinction at low levels of perturbation are basically as expected and predicted theoretically by the topological collapse of the network as the perturbation propagates through the network.

The second response is also part of the typical CEG response, and is the nearly discontinuous and significant increase in secondary extinction, implicating a threshold response. It isn’t clear at this point exactly what precipitates the threshold change, that is, is it the loss of highly linked species? Or is it threshold responses in compensatory increases of link strength? Or something else? But, at the very least I know that it does not involve topological effects, because the threshold is neither predicted mathematically, nor is it present in the results of topological extinction-only simulations. It is therefore definitely a feature of the demographic (Lotka-Volterra-type) interspecific interactions.

Finally, there is the occasionally observed high level of secondary extinction at low levels of perturbation. Surprisingly, I think that this is not a demographic effect, as we’ve long hypothesized. It is not likely to be, since it occurs in the topological extinction simulations of the 3-guild model! Those results point to the presence of at least two distinctly different topological classes of species-level networks. One class responds according to the mathematical predictions, and the other exhibits the anomalous response. I believe that this response is essentially a result of constructing networks that have very low linkages in the critical places. The probability of this occurring must lie in the multinomial probabilities of species linkages based on guild trophic link distributions, and the uncertainty of these probabilities, or the entropy of the metanetwork, increases under certain combinations of guild species richnesses.

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