In this post, and others to come every Thursday I will choose a random paper from my stash of 1500 or so, read it and review it.
Today’s paper is: Stepping in Elton’s footprints: a general scaling model for body masses and trophic levels across ecosystems
1. predator and mean prey mass are positively correlated
2. predator masses increase with trophic level
3. predator-prey body mass ratios do not vary consistently across trophic levels
They immediately follow with a list of the issues surrounding this paradigm:
1. the scatter in the relationships among prey mass, predator mass, and trophic level can be substantial
2. a few studies have reported different relationships
3. [the third concept (above)] has not [to the author’s knowledge] been explicitly analysed, neither within nor across ecosystems
In order to reconcile the current Eltonian paradigm with these apparent issues the authors use a dataset of 35 food webs; from marine (5), stream (12), lake (10), and terrestrial (8) habitats. At this point I should note that I think it is a plus that while they do not include their data alongside their publication they do provide a very nice table that serves as a lovely treasure map telling the reader exactly where to find all the information.
So what they do is split their data into marine, stream, lake, and terrestrial habitats, and analyze the relationship between prey mass vs predator mass, predator mass vs trophic level, and predator-prey mass ratio vs trophic level. For each habitat type the consumers were split up into invertebrates, ectotherm vertebrates, and endotherm vertebrates. The authors note three general results that they have shown:
1. the log10 of average prey mass increases with the log10 of predator mass with a slope greater than unity
2. the log10 of predator mass increases with predator trophic level
3. the log10 of the predator-prey body-mass ratio […] decreases with predator trophic level
It is interesting to find that larger predators consume prey that are on average, closer to their own size. I suppose from an evolutionary standpoint it makes sense, you get more bang for your buck with a larger prey and should therefore be able to spend more of your time and energy growing and reproducing, rather than finding more small things to eat. I do wonder, however, whether social or pack hunting could be taken into account. Of those large carnivores that consume even larger prey, I would think a decent portion utilize pack hunting, e.g., lions or wolves. If that were the case is the appropriate body mass ratio the predator/prey or the pack/prey. Another concern I have with body mass ratios is that very rarely do predators consume their entire kill, especially when that kill is large (e.g., an elephant). Some portion of the prey goes towards feeding scavengers and insects. Should that be taken into account when discussing body mass ratios, or is what we are interested in merely the ability to kill a prey individual? And what about parasites? In this study they focused solely on predator-prey interactions, ignoring as well herbivores and detritivores. I wonder if parasites would follow the opposite trend as regular predator prey, or if they would be closer to whatever the equivalent plant-herbivore ratio scaling would be.
I really liked the last paragraph of their discussion, where they relate their findings of body mass ratios to studies of interaction strength and stability. According to their findings we should expect that interactions should be strongest amongst the intermediate trophic levels, and comparably weaker interactions at higher trophic levels. Compare this with what was found by Ulanowicz, Holt, and Barfield (2014), that in trophic chains that are unusually long (6-9 trophic levels) the interactions at the top of the chain are relatively weak. Precisely what is predicted by Riede et al.