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A guild-level food web for the Karoo Basin ecosystem.

A guild-level food web for the Karoo Basin ecosystem.

At the heart of our paper lies a model framework which we devised for analyzing fossil food webs. I stated in the previous post that our main question was “How would those food webs (important parts of the paleoecosystems) have responded to everyday types of disturbances, on the short-term, as the planet was busily falling apart?” We could approach this question in several different ways if we were working with modern food webs. We could conduct manipulative experiments with simple mock-ups of the food web, using for example some of what we believe to be the key species to represent the community. Or, we could conduct large scale manipulations, such as removing a species entirely; but that is very difficult to do, or to obtain permission! Or, we could measure variables such as species population sizes, how species interact with each other, and so forth, to then conduct numerical analyses and simulations. None of those approaches are available to us when dealing with ancient, extinct ecosystems. Therefore, what we did instead was to use the most accurate information that we have for each paleoecosystem, which consisted of categorizing species into “guilds”. Here, a guild is a group of species who shared the same habitat, and  potentially shared the same predators and prey. The “potentially” is based on our best interpretations of the ecologies of those extinct species, because without actually being there to witness their interactions (back to that Tardis again), we cannot be sure. The result is usually something like the box figure above. Even with species lumped, you can see how complex and busy the system would have been! And from this guild-level model, we can then construct many many different food webs, tweaking the specific links between species. An example is shown in the second figure.

Now, the number of different food webs that you could generate based on even a modest number of species, say 20, and a few guilds, is astronomically large (in fact beyond astronomical). The important thing, however, is that all of them would be consistent with the guild scheme. Let me give an example. Say we did a guild scheme for the modern African savannah. We would be justified to some extent to place lions and hyaenas in the same guild. We might not know exactly which antelope species (for example) each predator species was preying on, but we would never draw a food web where antelope were preying on lions, hyaenas or each other! So, what we have done for our food webs is constructed a mathematical space that contains all the food webs which could possibly have existed in our paleoecosystem. In other words, we have taken the full set of food webs that could be constructed for a certain number of species, and constrained ourselves to consider only those that are consistent with our accurate knowledge of the guild structure.

Detail of one possible food web just prior to the mass extinction.

Detail of one possible food web just prior to the mass extinction.

This still doesn’t solve the problem of how those food webs would have responded to various types of disturbances in the distant past. And in fact, we really cannot solve that problem, so we did what we think is the next best thing. We asked if there was anything special about those food webs, compared to any others that were not consistent with our guild structure. In other words, what if the ecosystem had evolved a bit differently, and comprised species a bit different from what we actually observe in the fossil record? We considered a number of such alternative models, differing from the real ecosystem in ways such as moving species around in the guilds, or moving guilds and the interactions between them, or removing guilds altogether. And each time we did that, and generated a food web from the new guild scheme, we examined the stability of the food web. Exactly what we mean by stability, and how we measured it, will be the subject of the next post.

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