First principles of physics predict predator–prey size ratios

In our latest Insight, Sébastien M. J. Portalier talks about his recent paper on The mechanics of predator–prey interactions: First principles of physics predict predator–prey size ratios,  his research and the best (and worst!) things about being an ecologist.

About the research

What is the background behind your paper?

Great white shark at Isla Guadalupe, Mexico, August 2006. Shot with Nikon D70s in Ikelite housing, in natural light. Animal estimated at 11-12 feet (3.3 to 3.6 m) in length, age unknown (source Wikipedia Commons).
Great white shark at Isla Guadalupe, Mexico, August 2006.  Source Wikipedia Commons.

The idea is that any organism is constrained by the medium in which it lives. We all live on Earth, we do not float in deep space; thus, we are submitted to physical factors such as gravity. When an animal moves, it has to deal with the density and the viscosity of the medium: for example, it is well known that buoyancy is higher in water than air. Starting from this idea, we thought that these constraints should apply for predators. Hence, a barracuda looking for a prey, or a leopard chasing a gazelle, have to counteract the effects of gravity, density, and viscosity of the medium (water or air), which constrain their respective performances (e.g., whether or not they will capture their prey), and the profit they will obtain from hunting that prey (how much energy they will get).

What’s your paper about?

We present a model that predicts the profitability of a given predator-prey interaction based on predator and prey sizes. We predict the range of prey sizes that a predator can consume. We apply the model to any sizes ranging from plankton to whales, in aquatic and terrestrial systems. But we have to say that we mostly describe swimming or flying predators, which represents a large amount of species already.

How is your paper different from other work in this area?

There are papers that use physics to describe animal motion, but most of them are very detailed. Hence, they are restricted to the species they describe, which make them difficult to generalize. On the other hand, many studies done on food webs use statistical regressions and do not provide a clear mechanism that explains the patterns shown. The novelty is that we built a model that starts from processes acting at the individual level to predict species interactions, and we based these processes on general principles of mechanics and well-known biological laws.

Where you surprised by anything when working on it?

Actually, we were not sure at all that the model would be able to predict a large number of existing interactions. I was personally surprised by the results. It seems that physical factors are strong ecological drivers of species interactions.

What are the key messages of your article?

The main idea is that it is possible to predict predator-prey interactions using only predator and prey sizes and general laws of mechanics and biology. The other key point is that the physical factors constrain species interactions.

Now that this paper has been published, what is the next step going to be?

The model presented in the paper is static: we describe a given interaction at a given moment in time. We are now working on a dynamic version of the model: a predator-prey model based on the present model to define a functional response. We expect to publish it soon.


About The Author

sebastien_portalierHow did you get involved in ecology?

I wanted to work in ecology, and I got the chance to be involved in an interesting project during my Master, then I stayed in the field with a PhD.

What’s your current position?

I am currently looking for a postdoctoral position.

What is the best thing about being an ecologist?

I always liked biology, and I chose ecology because I liked the integrative aspect of this field: we need to take into account processes acting at the individual level (sometimes at the gene level) to understand patterns at the ecosystem level. I think it is challenging but also fascinating.

What is the worst thing about being an ecologist?

The word “ecology” is often misused in the media, and many people do not really understand what we actually do. I find this quite frustrating.


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