Reconstructing Cambrian Food Webs

If I was given three wishes, I have always said that one of them would be to watch the evolution of life at my leisure, being able speeding things up and slowing them down at will. Of all the time periods we've designated, the Ediacaran and the Cambrian periods would be a frame by frame analysis. Were these organisms really that much different from modern organisms, and if so, did their ecology reflect these differences?

PLoS One published a paper today that attempts to make my pipe dream a reality by taking the well known geological snapshots of Cambrian life, the Chengjiang and Burgess Shales (520 and 505 ma respectively), and trying to reassemble the interactions of the living world of that era.

The desire to reconstruct historic ecosystems has been strong in the field, but many have felt the fossil record was not robust enough to construct any sort of models. Obviously this notion is beginning to change.

The researchers took data on the organisms identified from the Shales and then categorized them by trophic level (feeding level or a link/thread in the chain/web of energy transfer in ecosystems) based on their apparent morphology. This information was then used to create food webs based on the trophic designations of each organism.

For ecologists it's a daunting task to describe extant, accessible ecosystems, and it would seem that constructing a dead ecosystem from frozen, separate organisms would be nearly impossible or highly speculative (perhaps the latter remains true), but the paper suggests the opposite. Ninety percent of the taxa in the Cambrian Shales have been identified to species level, which gives the reconstructed webs a higher resolution than many extant systems.

The Burgess and Chengjiang food webs were run through the same rigorous analyses as eight well-studied modern food webs, including Bridge Brook and the Chesapeake, for the sake of comparison.

All 10 food webs were analyzed using:

• Original-species webs. Take the actual number of individual taxa in each web and assign numeric values for species richness (S), linkages (L), connectance (C) (realized feeding links) and trophic level (TL).
• Trophic-species webs. Same as above, but removes redundant trophic species, streamlining things a bit to reduce variation. So instead of 85 "original" or individual species in the Chengjiang web and 142 in the Burgess web, you have 33 and 48 theoretical species filling certain trophic roles, respectively.
• Scale-dependence. A relatively simple hypothesis that predicts that food-web properties will scale with species richness (S). The value of linkages and connectance are dependent on and will scale with S (or S and L).
• Ecological Niche Models. This model fills in the blanks, so to speak, addressing properties of the food web that the other methods cannot. It still uses S and C as inputs, but can describe other important factors affecting the web, such as percentages of different types of taxa, like carnivores or basal species.

In all three analyses, the diversity and complexity in the Cambrian webs were remarkably similar to the modern webs, mostly falling within numerical range across the board. However, there were a few notable differences.

In general, the Cambrian webs exhibit a higher distribution of linkages than the modern webs, especially in the case of certain prey species. They have higher than usual consumption rates, which might be the result of the proliferation of life in this era:

The rapid expansion of taxa into novel trophic roles in the early Paleozoic may have resulted in a large number of vulnerable taxa that had yet to develop effective predator defenses. A subsequent reduction of very vulnerable taxa could result from their extinction or the development of better defenses in response to the strong selective pressure of having many predators. These ongoing pressures on highly vulnerable taxa could constrain the upper bound of vulnerability to what is observed in modern webs, thus reducing overall variability in both vulnerability and total links.

The Chengjiang web especially seems to be less hierarchical than our modern systems, which have set, strong organization in their natural state (if that exists anymore), and rarely see the "feeding loops" ("A eats B eats C eats A"). If the famed rapid diversification of life in the Cambrian was indeed as dramatic as we think (in varying degrees), it would certainly explain this seeming instability or disorganization compared to our regimented modern systems. Obviously biological networks were pushed towards this relative stability, which is interesting in itself.

If this study shows or suggests anything, however, it is that the foundations of these stable systems were laid very early on, when our metazoan ancestors began to vie for a niche.

Ecologist Jennifer Dunne of the Santa Fe Institute, who led the study, has some offers a few suggestions for further study in the press release, but this one, from the paper itself is particularly interesting:

...carefully selected datasets could indicate whether mass extinctions break patterns of incumbency in trophic complexity and force the construction of new community structures, and whether those new structures converge on the apparently conserved patterns of species interactions suggested by the current and related analyses.

Which would provide a nice intermediate study to compare to this one. It's a fascinating question. Are the results of this study ignorant of the potential of reconstruction after a mass extinction?