Elton’s footsteps – Thursday Review

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

eltonsFOOTPRINTSThe Eltonian footprint we are stepping into with this paper is the concept of predator/prey body size ratios. The authors describe the prevailing paradigm as

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.

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One Response to Elton’s footsteps – Thursday Review

  1. Jan Klecka says:

    Hi, it’s interesting to read your thoughts about this paper.

    I am not sure what to think about this result: “It is interesting to find that larger predators consume prey that are on average, closer to their own size.” Other analyses found the oposite overall trend (and also a lot of variation between taxonomical groups of predators and prey):
    Brose et al (2006) Consumer-resource body-size relationships in natural food webs. Ecology, 87, 2411–2417.
    Bersier & Kehrli (2008) The signature of phylogenetic constraints on food-web structure. Ecological Complexity, 5, 132–139.
    Naisbit et al. (2011) Phylogenetic signal in predator-prey body-size relationships. Ecology, 92, 2183–2189.

    My own experiments with a limited set of species also found that larger predators feed on relatively smaller prey. One important issue is that almost all food web level studies work with average body masses of species and this may seriously distort estimates of the relationship between predator and prey mass (adults of one species feeding on juveniles of a larger species etc.; ontogenetic niche shifts are ubiquitous). We need to collect individual-level data to really understand how predator-prey interactions work! Species-level data simply don’t have the right resolution to address these questions.

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