Neural Plants (artwork by Ricard Solé)
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Pablo Marquet: We need a change in the way we dwell in the world. And that, I think, is urgent.


Abha: From the Santa Fe Institute, this is Complexity

Chris: I’m Chris Kempes.

Abha: And I’m Abha Eli Phoboo.


Abha: You know, over the past two episodes, we’ve looked at a lot of underlying laws that apply to life. And they’re deeply interesting, but it’s still striking to me that there’s so much diversity in the plants and animals we see around us.

Chris: Yeah, and one of the reasons, I think, that there’s been this lag in physicists getting involved in the life sciences is that the biosphere is really, really complicated. And that’s not to say that physics can’t get complicated too - we try to do things like simulate the surface of the sun -  but when you think about something like the scaling laws, there are many organisms that deviate from what the laws would predict.

Abha: Sure, like, if I think of a rabbit and a turtle — both animals are about the same physical size. But their lifespans are so different. A rabbit will probably live less than ten years, but a turtle can live for decades. So then, what’s going on there? What part of the picture are we missing?

Chris: Well, in today’s episode, this is exactly what we’ll dive into. We’ll get into what causes biodiversity and what happens when biodiversity disappears.

Abha: And we’ll hear from two researchers who will show us why being able to make predictions about the biosphere is really, really urgent for us as humans.


Abha: Part One: Why is our biosphere so diverse?


Abha: So scaling laws, which we talked about in our first episode are basically this: there’s an underlying relationship between an organism’s physical size and a bunch of traits, like how quickly it burns through energy, its lifespan, and how much it sleeps. You can even plot it out on a graph, and it follows this logarithmic curve.

Chris: And if you were to imagine all the plants and animals plotted out as little dots on this graph, there’s an obvious slope and underlying relationship, but the dots don’t adhere to a perfect line — it’s more like a tight cloud that’s in the shape of a line. 

Abha: Right so like, spraying an aerosol can of paint to make a line, versus drawing it with a really, really fine pen.

Chris: Exactly, the droplets kind of fly in different directions, but the overall shape is still there. So the scaling laws are literally the laws of physics that every organism feels equally — scaling laws are based on things like gravity and surface area and fluid viscosity. 

So, let’s pretend for a second that life evolved in a kind of purgatory — just a big, blank white space of nothingness with just the laws of physics. 

Abha: Which wouldn’t actually be possible.

Chris: Right, obviously that couldn’t actually happen. But if it did, everything would adhere perfectly to the scaling laws. That graph would be a thin, solid line, not a cloud. Because organisms would just be evolving to optimize for the laws of physics, to use energy in the most efficient ways relative to the size of their bodies.

Abha: Okay. But we don’t live in a vast white expanse of nothingness.

Chris: No, we don’t. We have a planet with weather patterns, wildfires, different biomes and altitudes. And so the environment we live in then adds this additional layer of influence on our bodies. That might make the organism deviate a bit from this perfect optimization to the laws of physics, because deviating is what allows it realistically to survive in the ecosystem it’s in. And each organism is playing a kind of game with its environment, trying to figure out how to adapt best to its surroundings, while still, on a grand scale, being bound by the basic laws of physics and the shape of this cloud. 

Abha: So that’s why you could have two animals that are roughly the same size — like the rabbit and turtle I mentioned earlier — which have different lifespans and move in different ways, right. A turtle can live a much longer life than a rabbit, and it moves more slowly. They’ve evolved unique strategies to survive in nature. 

Chris: Exactly. And even though all organisms feel the laws of physics equally, they don’t all feel the influences of the environment equally. Some plants, like aspens, exist in places with wildfires and have adapted to thrive with periodic burns. Other trees would be devastated by wildfires. Some animals live in really cold climates, others in tropical rainforests, etc. And this brings me to some work that my colleague, Brian Enquist, has done.

Brian Enquist: So my name is Brian Enquist. I'm a professor in the Department of Ecology and Evolutionary Biology at the University of Arizona here in Tucson, Arizona.

Abha: Brian is one of the few people we’ve spoken to who actually started out in the life sciences first and then became interested in physics, instead of the other way around.

Brian: I really do feel as if I am a biologist, a classically trained biologist that has found physics, and I found it rather early on. I don't know, somewhere around the age of 10 that I discovered that I really liked being outside. And I remember climbing a tree for the first time, just on my own. I kind of wandered away from the house and was out in the forest. And I was just very comfortable. And I decided to climb a tree all by myself and I was very proud of myself that I kind of decided to do that on my own. I remember sitting in the tree thinking, you know, this is pretty cool. I think I wanna do this for the rest of my life. 

Chris: Brian’s done some really interesting work on biodiversity. So, this cloud of biodiversity might look pretty random. But what he and several co-authors have done is zoom in on that cloud and ask, are there underlying laws here that we can tease out? If the basic laws of physics shape natural selection in predictable ways, then certainly, elements of the environment will shape natural selection in some predictable ways too. And he and some co-authors have coined what’s called the trait driver theory.

Brian: So what trait then of a plant or an animal best predicts whether or not you occur in an Arctic environment or a tropical environment? And it turns out that there are several traits then that are very predictive in a way. That is, if we see these traits, we know something very concrete about that organism in terms of how it lives, how long it lives, its physiology, its metabolism, and where it tends to live on the planet.

Abha: So trait driver theory takes a characteristic, like how dense a tree’s wood is, or how tall it is, and it makes a prediction about what type of environment that tree has adapted to. 

Chris: Right, like, trees tend to get shorter the closer you get to the arctic. Or if you think about wood density, for example, balsa trees and mahogany trees are both tropical plants that have found two different ways to thrive in their warm environments. 

Brian: And so if you've ever played around with wood at all, if you’re a kid maybe you made these balsa airplanes made out of balsa wood, which is this very light wood, or if move a desk made out of mahogany, right, or some sort of like really heavy tropical wood, you notice that there's tremendous variation in wood density. 

Chris: So when it comes to a mahogany tree — 

Brian: If you invest in very high density tissue, okay, that is an investment then for the future. That indicates that you're going to be there for a long time. So instead of kind of taking that carbon, that hard-earned carbon from photosynthesis, right, and all that metabolism then that's spent to obtain all this carbon, right, you then allocate it into something kind of, you know, seemingly non-useful, wood, or your tissue itself. And so that investment is made so that you kind of hang around longer, so that you can then obtain then your resources over a much longer than timeframe. And something like balsa wood has very little investment in your structure that you're building in order to live. And so that structure then is not... of built to last a long time. But instead what we find is that balsa wood lives a very fast life, but a very short life. It has a very high metabolic rate, it has a high photosynthetic rate, and it basically takes all that carbon and puts it right into seeds and babies. Balsa trees can blow over really easily in the wind, can easily get knocked down by animals. But if it can grow up really quickly because of this very cheap infrastructure, then it can throw all of that carbon instead into reproduction and then basically die.

Chris: So, balsa trees have adapted by being able to reproduce quickly and easily, while mahogany trees have adapted by investing more in their own bodies and growing slower. And it turns out, having denser wood makes a tree hardier in the face of a changing climate. And Brian and his co-authors have actually begun to identify specific plant traits that are better for adapting to climate change. 

Abha: And this means they can also identify which plants don’t have those traits. Brian published a paper about this recently.

Brian: Can we develop a more predictive theory for linking in these traits to how an organism or a phenotype then responds then to a change in climate?

Abha: You had a recent paper that you were part of published in Nature Communications that said more than 17,000 tree species are at risk from rapid global change. Could you tell us a little bit about this paper?

Brian: What we actually found was that when we looked at several different climate change scenarios and different human land use scenarios, we kept coming up with similar numbers in terms of the number of species then that seemed to be increasingly more threatened of having their total area collapsing and their habitable area then not available in future climate change and human land use scenarios. So we wanted to bookend a number of what we were talking about. in terms of the total number of species. And so our calculations based on these future projections indicate that, yeah, about 17,000 species are at risk in the future. And so this is a pretty daunting number. And of course then that also opens the door to additional research and as well as identify those species and locations where immediate conservation action would be needed.


Chris: 17,000 species are at risk. Some people might find that number unsettling, others might think it’s downright frightening. 

Abha: But why? Why is it bad if only the hardiest of organisms are left? Survival of the fittest, right?

Chris: Well, in Part Two, we’ll get into why biodiversity is important, and why finding fundamental, underlying laws of the biosphere is not just interesting for us as scientists, but it’s also crucial for human existence in the face of a changing planet.


Chris: This is Part Two: Why is biodiversity important?


Chris: So let’s say climate change has wiped out a bunch of trees, but the mahogany tree is still left. You could look at a mahogany tree and say, this tree is healthy and strong. But if there aren’t many other types of trees around, then the ecosystem as a whole is weak.

Pablo: Biodiversity is important. Diversity allows  the system to actually explore and be more efficient at harvesting resources and generating  biomass. But at the same time, you are more protected from pests, and pathogens, you are also, you might generate a much fast kind of decomposition rate. And you generate a soil formation in an amplified way. So it's, everything is better.

Chris: This is Pablo Marquet. 


Pablo: So I'm Pablo Marquet. I'm a professor at the Catholic University in Chile. By training, I'm an ecologist. I’m, well, right now in Mazatlan, you know, spending holidays here in Mexico. And when I’m not traveling, I’m in Santiago, in Chile.

Chris: Pablo has done some research on metastatic cancer, which, at first glance, seems a little far removed from work on biodiversity and ecology. 

Pablo: But then when we analyzed the network of the primary organ and the metastatic organ where it sends propagules, we realized that it was an ecological network. I mean, it has all the properties that most ecological network has

Chris: Metastatic tumors start out in one organ in the body, and then they look around for other organs to spread to — fertile grounds for reproduction. You could think of tumors as extremely successful organisms in their ecosystem — maybe a little too successful. What Pablo and his team of co-authors discovered is that the element phosphorus is like food or fertilizer for tumors, so they spread throughout the body to find more of it. 

Pablo: Because you need phosphorus to build proteins, because you need to build RNA that has a lot of phosphorus on it and ribosomes. So growing means that you are really have a very active way of creating RNA and creating proteins so you can start, you know, growing. So we found out, there was very limited data on the phosphorus content of the different organs of humans. So, we found out that usually the metastasis goes to an organ that actually have higher phosphorus content than the organ where the primary tumor actually started. And the reason for that seems to be that the primary tumor cells,  since they have altered the metabolism and they are very active in terms of ATP, in generating energy, they have the scope to actually outgrow the cells, but to do that, they need more phosphorus. So they will proliferate better in a place where the phosphorus content is higher. It's interesting to see that a tumor cell wants to actually capitalize on the energy, change the metabolism, start growing faster, outgrow other cells, recruit some normal cells in the body to actually help them. So that was basically the idea.

Chris: Cancer cells consume a lot of energy, and they outgrow the spaces they’re in. Metastatic cancer throws the body’s ecosystem out of whack. And what happens when an ecosystem is out of balance, is that eventually, that ecosystem breaks down. If we take a step back and move outside the ecosystem of the body to the broader ecosystem of our planet — well, we humans are like these tumors, always looking around for more phosphorus to consume.

Pablo: To find an organism that might act as a tumor, it would be an organism that somehow, the same as a cell within a body, kind of break its social contract with the rest of the cells, and that would be an entity that somehow broke its social connection to the rest of the entities. And the obvious kind of entities us. I mean, we have been kind of outgrowing beyond what a normal mammal species of 75 kilos will achieve in terms of density and in terms of impact.

Abha: I mean, this is awful. We humans, even if we want to be good individuals and live good lives, as a whole we’re like cancer. At least, that’s my initial reaction to this. But, Pablo doesn’t describe it in moral terms — as good or bad.

Pablo: But we are not bad. And metastatic cells are not bad either. They are doing something that might not be right for their own persistence. So that's why we have to learn that there is danger. In terms of what we are doing in the world right now, there is danger for ourselves. That's a problem. And it might not be easy to navigate through a degraded biosphere, you know, and we might want to learn the lesson before  it's inevitable. So that's why I think we have to change. We need a change in the way we dwell in the world. And that's, I think, is urgent.

Abha: Aside from whether or not this is good or bad, it’s just about basic survival. And as much as we humans love to think of ourselves as exceptional, we’re completely embedded in the biodiversity around us, we’re a part of it, and we need it.

Pablo: I mean, when life originated and started changing and generate biodiversity, actually it's like a wave of biomass covering the earth, you know, and that is one single thing that has many different appearances. It's just life. And we are all part of that tide of biomass that is transforming and have many different appearances. But at the end of the day, it's something that originated. 3.8 billion years ago to say a date, but long past and it's still here. And we are part of that. We are that moment. It is us, we are just a transformation, so to speak.

Abha: That wave of biomass is like a giant, moving quilt with all different colors, textures, and shapes. Let’s go back to Brian.

Brian: So climate change is going to be dramatically rearranging how that quilt is put together and built. And so the quilt then that we see of biodiversity is the result of, you know, millions, hundreds of millions of years of, you know, evolution. 

Abha: In the history of our planet, there have been events that caused mass extinctions. Famously, many scientists believe that most dinosaurs became extinct because of an asteroid that hit the earth 66 million years ago. It knocked out around 80 percent of all species of animals that were on the planet at that time. And obviously, if you look around today, you can tell that biodiversity eventually recovered and bounced back. But —

Brian: Climate change will also rip out important components of that quilt on shorter evolutionary timescales. What isn't emphasized enough about the nature of climate change is the  time scales associated with climate change relative to the time scales at which biodiversity and ecological processes you know, kind of emerge, right? So we're talking about an enormous change in the earth's climate and reorganization of the earth's biomes that basically occur close to human time scales. And examples of the past of mass extinction events shown how this amazing grandeur of biodiversity is able to reorganize itself and basically come back. And so life is tremendously resilient. But the changes that we're seeing now and increasingly are going to be seeing are going to be operating at timescales that are going to not only rearrange this quilt of life, but unfortunately rip out major components of that quilt. And as an ecologist, the concern is that how much of that tearing, that tattering, that reorganization then of life's rich tapestry can can our important ecosystem services of clean air, clean water, we rely on biodiversity for human health, how much can it take?

Chris: Brian, Pablo, myself, and other scientists are hoping that if we can tease out more of these fundamental laws of life, we’ll get a better understanding of what’s going to happen to our biosphere. 

Brian: When I step back and I think of all of my wonderful colleagues in the earth sciences, I'm very envious of their ability to predict the future of our climate system. But it's clear that one of the big uncertainties in understanding the future of the climate system has to do with what's going to happen to the biosphere. And if we focus then on the science of the biosphere, we don't have the same degree of predictive ability in terms of predicting how then the biosphere is going to then look and behave and function under different climate change scenarios, under different human land use scenarios, under different extinction scenarios. What's going to happen to the biosphere?

Chris: Brian and his co-authors have started to make predictions in a kind of brute-force way. And these predictions are based on what we already know about the 400,000 species ecologists have cataloged so far. We already know that an arctic fox obviously prefers a cold climate, for instance. But that’s not the same as making predictions based on a unified theory. And dealing with this massive collection of data is hard.

Brian: I have to say that you know, this work is incredibly, you know frustrating because dealing with biodiversity data, you know is very difficult. There's a lot of vagaries and and dirtiness of the data. The data are not very nicely organized, very patchy, there's a lot of issues. And so we've been spending a lot of time dealing with the dirtiness of biodiversity data. But it's also a little unsettling because we have very little theory for how we do basically kind of forecasting into the future how these different species will respond. But I have to say, it’s very challenging. 

Abha: So why is it like this? I mean, the earth sciences have prioritized making weather forecasts for a while, so why is it that we’re only just starting to think about making forecasts in the biosphere?

Chris: Well Pablo has some context for this and the history of ecology.

Pablo: If you look at the history of ecology, we ecologists come from a tradition that started with the big or large naturalists that were traveling the world and describing the world and being completely dazzled by the huge variety and diversity of different forms and interaction among them. And also because those things are kind of a match our scale in terms of space, time. So that's really kind of  lingers there.  

Chris: So much of biology is rooted in naming and cataloging — historically, many naturalists were exploring whatever was right in front of them. And because naturalists only had small slices of the world to look at, they were literally limited in how well they could see the forest for the trees. In contrast, physics and mathematics have always been about pulling back and finding abstract rules to explain our world, but only for the stuff that’s nonliving. And we’re now just starting to combine these two approaches.

Brian: And so increasingly we've been looking to some of these biological scaling laws, focused on trait environment interactions, trying to figure out if there are some underlying  approaches to scaling up and  forecasting how biodiversity will respond. But I have to say it's very challenging.

Chris: Brian and I are actually working on a paper to identify and name these different approaches to scientific inquiry, because being able to think more critically about how to use each of these approaches together gets at existential issues, for example how to move science forward as quickly as possible. Which, as Pablo and Brian have noted, is urgent.

Abha: Okay, so what are each of these approaches?

Chris: I’ll let Brian explain a little more. 

Brian: Yeah, so I should actually kind of step back and say that this is a paper we're trying to publish. It's not published yet. And we actually hope to hear back on the second round of reviews here sometime soon. But  scientific transculturalism  is a new idea that we developed at the Santa Fe Institute and, just to step back a little bit, the idea of scientific transculturalism kind of starts with this notion that there are multiple ways to kind of gain scientific insight then about the world, right? And so those different ways of gaining scientific insight kind of have kind of different, I think, philosophical and kind of cultural roots, right? And so one way that's pretty prominent in biology is more kind of the natural history kind of perspective. And so natural history has given us a wonderful catalog of life, kind of the description of biodiversity. And of course, the incredible kind of explosion of cellular biology and all the details of basically how cells work, how information is passed along, heritability, genetics, then and so on. 

Chris: This naturalist approach is an example of what we’re calling exactitude culture, which looks at the variability of the world in finer and finer detail, getting really really precise about what’s being observed and mapping out every single thing. And then in the other direction, we have coarse graining, which is pulling back and trying to simplify everything. If you haven’t guessed by now, the physics of life is all about applying that coarse graining, simplify-everything approach to the life sciences. Much of what we’ve talked about in the first two episodes are examples of this: scaling laws, assembly theory, the way organisms reproduce, and now, trait driver theory, too.

Brian 24:05

But coarse-grained culture also is seen in many different areas of science, of course. And within the life sciences, maybe this would be something like population genetics or quantitative genetics. And so the idea of coarse-grained culture is the importance of abstraction and the importance of things like parsimony and simplification,  so that you can kind of gain traction in understanding.

Chris: And it’s important to note that none of these approaches is better than any other. We need all of them working together in order to move science forward.

Brian 26:29

So the idea of scientific transculturalism gets to this kind of notion of kind of what then determines the pace of science, how quickly science then kind of proceeds.  27:23 There is this urgency of increasing the speed of scientific progress in terms of understanding how not only climate change, but the biodiversity crisis, how all of these urgent challenges are going to lead to not only cascade then through the earth system, but then are going to be presenting these new problems and challenges for humanity that we urgently need to identify, before they're set upon us. And so the notion of scientific transculturalism is that it can speed scientific progress, so that we can address many of these different challenges associated with the biosphere and the Anthropocene. And so to do so, one of the answers is to improve the predictability then of our models, and so on.

Abha: Each step of this process, discovering the scaling laws, and then, understanding the traits that allow plants and animals to adapt to different environments — it feels like unlocking pieces of a huge, grand puzzle. It’s actually quite hopeful, because the more we can predict and understand about the biosphere, the more we can, at least attempt, to prepare ourselves for what’s coming.

Chris: That’s the goal with integrating different cultures of science. It’s really about expanding the way we think about how science can be done, so we can improve and progress and solve really important problems in a better way. It’s a very SFI kind of attitude. And so far in this season, we’ve seen how this outlook can help us understand the connections between all forms of life, from the smallest cells to entire ecosystems. But there’s one more, kind of big area we haven’t really talked about yet. 

Abha: And what’s that?

Chris: It’s society. I mean, we are social animals. And this planet has many, many other social animals too.

Abha: That’s right. In our next episode, we’ll take this scaling laws-coarse-graining-physics approach and ask, what are the laws underlying communities?

Melanie Moses: But humans are so much more complex and so much more complicated and they have so many conflicting motivations. 

Chris: That’s next time, on Complexity. And before we go, we have a favor to ask. If you’ve been enjoying this show, the best thing you can do to support us is to tell a friend about it. Or tell two friends. Or five! And please rate and review us on Apple Podcasts, Spotify, or wherever you listen — it’ll help new listeners find the show. And thank you.