Analysis and Comparison of Modern and Cambrian Food Webs

I read another paper, “Compilation and Network Analyses of Cambrian Food Webs” , looking at food webs as networks. This one was comparing modern food webs to those derived from fossils during the Cambrian period (time period between about 542 to 488 million years ago). It analyzed S (taxa), C (connectance), L/S (links per species) and 17 other separate components of networks to try and see if there were any differences between ancient and more current food webs and what might be learned from these differences. Dunne et al. analyzed 17 features including the fraction of species that are herbivores, the fraction of species without consumers and the mean length of food chains.

Dunne et al. tried to make sure that the representation of the Cambrian food web was as realistic as possible by ranking the surety of the links between nodes from 3 (highest) to 1 (lowest). They then tested their results to make sure the lack of certainty about connections in the food web did not skew their results. They did this by removing the 10, 25, 50,75 and 100 percent of number 1 links from the food web and then removed the same number of links randomly from the original food web and found the 17 conditions, S, C, and L/S for both these networks (each reduced food web was taken 100 times except the 100 percent removal of # 1 links because it would be the same each time and conditions were averaged) . When the two networks conditions were compared Dunne at al. found very little difference between the two results suggesting that the Cambrian food web is not being skewed by lower certainty links.

Of all the 17 components found, only a few proved to vary between the Cambrian food webs and modern food webs. The Cambrian food webs were found to have higher variability in total species links than modern webs. It is possible that this is due to some species having a large amount of predators. Since it is thought that the Cambrian food webs were possibly transitioning into more modern-like food webs during this period Dunne at al. suggest that the large amount of predators could be because certain species had not adapted to predation and that given time they would either become extinct or evolve defenses against the predators. One of the two ancient webs also differed from the other food webs in having a longer mean shortest path and a higher amount of loops. The longer mean shortest path length suggests a higher amount of separation between species and therefore a lack of influence between species. A high amount of loops in a food web are often thought to make the web less stable and therefore not to last very long. Both these conditions suggest that this food web is unstable and may be transitioning into a more modern-like food web.

Some of the methods performed here were a bit above my knowledge. Dunne et al. seem to have tested the assumptions in connecting the Cambrian food web quite thoroughly and the value of this model lies not in being 100% accurate but more in showing general trends. It would be interesting to see what the same analysis done on an older food web, thought to be before the transitions stage of the Cambrian food network, would look like compared to modern networks. It may be that there is not a food web available for this comparison but if there was it might show a different structure than the transitioning food web of Cambrian and the modern food webs.