Food Webs, Climate Change and One Equation to Rule Them All

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Duke faculty member Jean-Philippe Gibert is the Joanne W. Markman and A. Morris Williams Jr. Associate Professor of Biology and a Simons Foundation Early Career Fellow in Microbial Ecology and Evolution. He and his lab study how climate change affects living things and the way organisms depend on others to survive and reproduce.

We caught up with Gibert to ask a few questions about his research. Below are excerpts from the conversation.

Things Eating Things

We study how climate change affects living things, and in particular these networks of species and their feeding interactions that we call food webs. Every living thing is a part of a food web. The fate of the biosphere under climate change is the fate of these food webs.

Every food web is different, but they’re also not different in the sense that there are properties of these food webs that are general across space and time.

A food web in the Cretaceous was different because it might have included dinosaurs, but it wasn’t fundamentally different than the way food webs are structured today. A food web in the middle of the Amazon is different because its species are different than ones you can find in the middle of the Arctic, but you still have the same processes in action: things eating things.

Less Energy for Big Things

When something chomps on something else, they get a chunk of organic matter they then process and convert into available energy to survive and reproduce.

It also means that there are things at the base of the food web that are somehow able to entrap energy into organic matter — plants, algae. When they get eaten by other things, they transfer their energy, fueling the rest of the food web — life, if you will.

But it’s physically impossible to get more energy from chomping on something than was originally contained in it. Worse, living things can’t even get the same amount of energy: they can only get less. The laws of physics don’t bend to those of biology — mostly. As more and more energy is lost through sequential chomping, there is less and less energy for big things at the top of food webs to survive and reproduce, so there are fewer of them. It means it’s hard being a polar bear!

These general rules make it such that we can use math to understand some generalities about the response of these food webs to things like climate change.

Pragmatic Biology

I describe what we do in the lab as working on pragmatic biology. By training I’m what’s called a theoretician, meaning that I have a math background in addition to a biology background. We use mathematical models to understand the natural world and its response to environmental change.

Mighty Microbes

A major focus of our work is on microbial food webs — the component of food webs that’s made of microbes — and their feeding interactions. They’re incredibly complex. There’s a mind-bogglingly large number of microbial species out there that do all the things you can possibly imagine, from acting like a plant or eating someone else like a lion, to swimming around the oceans like a whale or generating a lichen-type structure called a biofilm.

If you were to grab the entirety of the biosphere, put it on a scale and weigh it, microbes — despite being tiny — would be roughly a third of that by biomass. They also collectively send more carbon into the atmosphere than all humans and human activity combined, through the process of converting and using the energy they get from eating something else: respiration.

Cascading Effects

Our driving question is, how are these (microbial) food webs responding to climate change? By how I mean specifically who’s there, how many of each of the whos there are, and how that will change as the environment changes.

But why should they change? Simply put, we all like it neither too cold nor too hot. There is no organism on the planet that escapes that rule. If temperatures change, that alters their ability to survive or reproduce, which in turn leads to there being more, or fewer, of them.

As the abundance of the different species changes in response to climate change, they will be influencing each other. Imagine ten lions eating wildebeests: in the same way that none of us can eat an infinite amount of pizza even if we tried, lions will be able to eat only so many wildebeests. If I add 100 more wildebeests, maybe there could be a few more lions. The opposite is also true. If there are fewer wildebeests … something’s got to give, so fewer lions. But fewer lions also means more wildebeests, which means less grass for other things like zebras, and so on.

We say that changes in the abundance of the different species of the food web “cascade.” These ripple effects can propagate through the network and bounce around affecting everyone. Now, every species responds to temperature, but all do so differently. Add cascading effects, and that’s what makes studying food webs — and their environmental response — so complicated.

A Simple Equation for a Complex Process

A recent finding that we had, from January, is a mathematical equation that allows us to understand how changing temperature influences the abundance of all the species in the food web, just by taking into account two things: the response to temperature of each species; and information about who’s eating whom.

What that equation is telling us is precisely how those cascading effects are going to happen.

It’s an incredibly complex process that our finding breaks down into an incredibly simple mathematical equation. In fact, we did not believe that we could just with one mathematical equation understand, let alone predict, the response of these food webs to climate change. It didn’t seem possible. We have just shown that we can.

Summer Plans

We have grand plans about this “one equation to rule them all.”

We can do math all day long — it’s a lot of fun, I love math — but the question is, does it work? So, goal number one is to do an experiment with our microbes to test whether we can actually use this equation to predict how food webs will respond to temperature.

Goal number two is to use the equation on some of the data that we have for different food webs and compare the results with our predictions. If it works, we might be able to predict the fate of food webs across the planet.