Love Fruit? Thank (Dinosaur) Mass Extinction

Summary

Short Wave Podcast Summary

Episode: Love Fruit? Thank (Dinosaur) Mass Extinction
Release Date: March 14, 2025
Hosts: Emily Kwong and Regina Barber
Guest: Christopher Doughty, Associate Professor of Ecoinformatics at Northern Arizona University
Duration: Approximately 12 minutes of content

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Introduction to the Cretaceous Period

The episode opens with Emily Kwong setting the stage by transporting listeners back to the Cretaceous period, a time when sauropods roamed the Earth. She paints a vivid picture of a "very bright forest" dominated by open spaces and abundant pine trees (00:30).

Christopher Doughty and Sauropods as Ecosystem Engineers

Christopher Doughty introduces himself and his fascination with sauropods, the largest terrestrial animals ever to walk the Earth. He explains how these dinosaurs functioned as ecosystem engineers, performing vital roles such as knocking down trees, distributing nutrients, and dispersing seeds (00:35–00:58).

"They can knock down trees, they can distribute nutrients, they can move seeds. They do a lot of really important things, and big animals tend to do that differently than small animals."
— Christopher Doughty (00:58)

Sauropods managed nutrient and seed distribution primarily through their poop and the decomposition of their corpses, maintaining the health and diversity of their ecosystems (01:07–01:18).

The Mass Extinction Event and Its Aftermath

Emily Kwong delves into the catastrophic mass extinction event triggered by an asteroid impact, which wiped out approximately 75% of all life on Earth (01:51). This extinction removed large dinosaurs, leading to significant ecological shifts:

• Forest Structure Changes: Without large herbivores to knock down trees, forests became lush and tall, resulting in a 20% darker understory (01:51–02:09).
• Impact on Seed Evolution: Smaller seeds, adapted to the brighter understory, found themselves at a disadvantage in the darker conditions, prompting an evolutionary shift toward larger seeds (02:09–02:57).

Evolution of Seed Sizes and the Fruit Boom

The fossil record revealed a sudden increase in seed and fruit sizes following the dinosaur extinction. Doughty's research suggests that darker understories favored plants with larger seeds, which in turn led to the proliferation of fruits that could entice larger animals for seed dispersal (02:14–06:20).

"The verdict is, yeah, it worked. So, you know, basically what we saw was the darker understory basically created an evolutionary advantage for bigger seeds."
— Christopher Doughty (06:20)

Modeling the Ecosystem Changes

Doughty explains the methodology behind his research, which involved:

1. Fossil Analysis: Scraping fossils from leaves before and after the extinction to assess light availability in the understory (04:45).
2. Rainforest Studies: Observing modern plant growth in varied light conditions to correlate seed size with seedling development (05:56).
3. Comparative Analysis: Studying modern megafauna like forest elephants to understand their role in forest ecology (05:56–06:05).
4. Computer Simulations: Integrating data into a model to simulate ecological changes over millions of years (06:05–06:19).

Implications for Primate Evolution

The transformation in plant life had profound effects on the evolution of other species. Doughty highlights that the ancestors of primates benefited from the abundance of fruit, which provided both food and an incentive to climb trees—traits that were crucial for their development (07:01–07:14).

"Had there not been fruit, you wouldn't have this incentive to climb up in the tree and eat fruit."
— Christopher Doughty (07:14)

Reversing Trends and Subsequent Extinction Events

About 35 million years ago, a new wave of large mammals emerged, altering the previously established seed size trends. These mammals, similar to modern elephants and saber-toothed cats, began modifying the forest structure again, leading to a decrease in seed sizes (07:26–08:10).

The Sixth Extinction Overlap

Doughty connects these historical trends to the current sixth mass extinction, emphasizing that many tree species co-evolved with now-extinct large mammals. He cites examples like avocado and chocolate, which evolved alongside South American elephants, culminating in their reliance on these animals for seed dispersal (08:10–08:52).

Human Impact and the Current Mass Extinction

Addressing the present-day scenario, Doughty asserts that humans have become the dominant ecosystem engineers. Through activities such as logging, farming, and altering landscapes, humans fulfill roles once occupied by large mammals, albeit in ways that may not be beneficial for ecosystem health (09:07–10:32).

"We are metabolically King Kongs running around the planet... we're really changing this long term evolutionary trajectory of the planet in interesting ways."
— Christopher Doughty (10:32)

Reflecting on Ecosystem Coevolution and the Future

Doughty encourages listeners to consider how human actions are reshaping ecosystems on a global scale, mirroring past mass extinctions' impacts on seed and plant evolution. He emphasizes the importance of understanding these dynamics to predict and potentially mitigate future ecological shifts (11:30–12:12).

"I'd like people to reflect on our ecosystem co-evolved with these big animals. They play really critical roles and now they're missing."
— Christopher Doughty (11:47)

Conclusion

The episode concludes with a thoughtful reflection on the intricate relationships between species and their environments, highlighting the profound and lasting impacts of mass extinctions. Through insightful dialogue, Emily Kwong and Christopher Doughty illuminate how past events shape present ecosystems and offer a lens to understand humanity's role in the ongoing sixth extinction.

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Produced by: Burley McCoy
Edited by: Rebecca Ramirez
Fact-Checked by: Tyler Jones
Audio Engineer: Robert Rodriguez
Senior Director: Beth Donovan
Senior Vice President of Podcasting Strategy: Colin Campbell

Transcript

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[Emily Kwong]

You're listening to Short Wave from npr. Today we are going back in time all the way to the Cretaceous period. Here's what it looked like.

[Christopher Doughty]

It would have been a very bright forest. They would have been very open. You have a lot of pine trees.

[Emily Kwong]

A time that Christopher Doughty loves because of this one kind of dinosaur.

[Christopher Doughty]

The sauropods are the biggest terrestrial animal that's ever walked the Earth.

[Emily Kwong]

Chris uses big data sets to understand ecosystems. He's an associate professor of ecoinformatics at Northern Arizona University. And he told me that sauropods were so big, they acted like ecosystem engineers.

[Christopher Doughty]

They can knock down trees, they can distribute nutrients, they can move seeds. They do a lot of really important things, and big animals tend to do that differently than small animals.

[Emily Kwong]

And the way sauropods move nutrients and seeds around was through their poop.

[Christopher Doughty]

Yeah. And their bodies.

[Emily Kwong]

The decaying corpse of a sauropod.

[Christopher Doughty]

That's true. That's not quite as fun to think about, but, yeah.

[Emily Kwong]

Circle of life, maybe. There was, like, a Mufasa sauropod who is like my sun. You'll be the grass someday.

[Christopher Doughty]

That's right. Yeah, you are. Those same elements have cycled through your sun that were in a sauropod someday, probably.

[Emily Kwong]

Those sauropods were herbivores, eating plants. And the plant seeds from this time were actually kind of small, like a centimeter across. And that was fine for them because of all this light in the understory.

[Christopher Doughty]

And so there was no evolutionary reason for seeds to get big. But what happened was after the dinosaurs went extinct, the forest got really dark.

[Emily Kwong]

When the asteroid hit, it wiped out as much as 75% of all life on Earth. And suddenly, there were no longer these big dinosaurs to knock down the trees. So the trees grew lush and tall, and the understory between the forest floor and the tree canopy got roughly 20% darker.

[Christopher Doughty]

For a little seed, that's trouble. You know, these little seeds, they need light.

[Emily Kwong]

And paleontologists observed something weird in the fossil record around this time. Not long after the asteroid hit, seeds suddenly got bigger, much bigger, and there was this sudden boom in fruit. The edible plant ovaries we love today. But why? And what did the death of the dinosaurs have to do with it? Today on the show, how the mass extinction of the dinosaurs may have led to the fruit and vegetable isle you see today and how we're living in another time of mass extinction where the dinosaurs are us. I'm Emily Kwong, and you're listening to Shortwave, the science podcast from npr.

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[Emily Kwong]

So, Chris, let's talk about mass extinctions. The world has gone through five of them. How do you know something is a mass extinction?

[Christopher Doughty]

That's right. So species go extinct all the time. And it's not unusual, it's just part of nature. But certain things happen that drastically accelerate this process, that make it so, you know, 80, 90% of species go extinct. So what distinguishes a mass extinction from just a normal extinction is many more species are going extinct, and it's global.

[Emily Kwong]

All right, so you have this theory that the mass extinction got rid of these dinosaurs that were changing the forest and knocking down trees, and that meant that the forest understory was getting darker. How did you go about testing this idea that that dark understory then led to bigger seeds and fruits?

[Christopher Doughty]

Yeah.

[Emily Kwong]

So, you know, it's not like you can go back and check. You can't set up a wildlife cam.

[Christopher Doughty]

No, no, no. Yeah. It is difficult to try to piece these together. So the first starting point was we had colleagues that literally scraped off fossils from leaves right before the extinctions and pretty much right after the extinctions. And we could test the chemistry of those leaves, and we could actually say how much light these leaves got. And so for one of the first times, we were able to accurately quantify how much less light was in this understory after the dinosaurs went extinct. And then what we were able to do was go into rainforests and study how plants grow. And so from that perspective, we could see how tall would you get at a given seed size. And it's a very strong correlation. So if you're small, you develop into a tiny little seedling, and if you're a big seed, you develop into a big seedling. And then we can go even further and look at modern megafauna, like forest elephants and see how do they open up the understory. And so then what we do is we take all this information going from fossil records to how plants grow, to how seeds grow, to how forest elephants affect forests, and we put it into a model. You can think of it like a computer simulation.

[Emily Kwong]

It's like rollercoaster tycoon.

[Christopher Doughty]

I don't know that one, but yeah. So like, you know, like, you know, SimCity or something, Sim dinosaurs genre of.

[Emily Kwong]

You'Re taking all this real world data and you're putting it into a model and you're saying like, what if?

[Christopher Doughty]

Exactly, yeah. And then you just allow millions of years of ecology to happen. And so basically you just kind of input this into our model and then you see what happens.

[Emily Kwong]

So, yeah, what's the verdict?

[Christopher Doughty]

The verdict is, yeah, it worked. So, you know, basically what we saw was the darker understory basically created an evolutionary advantage for bigger seeds. And so those bigger seeds then created fruit that would entice bigger animals to eat them. And these bigger animals would distribute the fruits farther. And the trees that are moved farther away from their mother tree are very happy because you don't have to put up with the same diseases that your mother has. You don't have to be under her shade. And so that's an evolutionary incentive as well. So basically they're bribing these animals with fruit to move them around.

[Emily Kwong]

In the time immediately following the mass extinction of the dinosaurs, did that eventually give rise to the ancestors of primates?

[Christopher Doughty]

Yes, our primate ancestors developed because they were good at climbing trees and eating fruit. And so had, had there not been fruit, you wouldn't have this incentive to climb up in the tree and eat fruit.

[Emily Kwong]

So the death of the dinosaurs was our boon?

[Christopher Doughty]

It was, it was, yes.

[Emily Kwong]

Well, I feel bad for them. Okay. Did the model ever show this trend happening in the other direction?

[Christopher Doughty]

Yes, at one point, all the seed sizes were getting bigger. And then about 35 million years ago, these seed sizes started getting smaller. And it was a big mystery as to, you know, what's driving this, you know, and what our model explained was that it was actually the evolution of these really large mammals now like who.

[Emily Kwong]

Who were the new influencers?

[Christopher Doughty]

You know, you had all sorts of like, you know, weird looking mammal groups that, you know, grew enormous that are kind of the, some of the relatives of what we see now out on the African savannah. So they'd get really big and then they'd start opening up these understories as well. And so long those big seeds were no longer way better than the small seeds. And so the small, the seeds, the seed size started to decrease but then.

[Emily Kwong]

You know, boom, seed size goes up again. Several million years after that. Woolly mammoths, saber tooth tigers, giant sloths. What happened to those mammals and what happened to the plants?

[Christopher Doughty]

That's right. So that's another really interesting extinction event because it overlaps with us. So this is commonly referred to as the sixth extinction. There was a lot of tree species that co evolved with these big animals to have them move their seeds around. So think of the avocado, think of chocolate, two of our favorite plant species. Those co evolved with these now extinct South American elephants. So that's why you kind of see this increase in seed size over time more recently.

[Emily Kwong]

Yeah, I mean animals are clearly such a huge factor in shaping ecosystems as engineers, as influencers in a way. And we are currently in another mass extinction, the Holocene. Who are the influencers of today?

[Christopher Doughty]

That's right, yeah. So, you know, I'm really interested in trying to predict the future with some of these models, not just thinking about the past. And so, you know, one of the fun things we did in this paper was say, all right, we went through another mass extinction recently of these really large important animals. What do we predict for future seed size? And yeah, without these large mammals, like you know, the gomp, the fears, mammoths, giant ground sloths, these forest understories are a lot darker. And so our model would say, okay, well it's dark, the seeds want to grow bigger again. But the interesting thing was we couldn't actually say that because there's another enormous ecosystem engineer that is dominant on the planet now, which is us.

[Emily Kwong]

We're quite a variable to add to the.

[Christopher Doughty]

Exactly, yeah. So we've taken on the ecological roles of some of these now extinct animals.

[Emily Kwong]

Oh, how so?

[Christopher Doughty]

Through logging and creating farms, creating grasslands. It's not uncommon for larger animals to create grazing lawns. We just call it agriculture. They're excellent at distributing nutrients across the landscape. We're not so good at moving nutrients, but we dig up a lot of elements and spread it across the planet. And so A lot of these big animals used to consume a lot of this vegetation that when they went extinct, you got a big increase in fire. So there's a lot of interesting ecological changes that occurred once these, these animals went extinct.

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Yeah.

[Christopher Doughty]

And actually have a. I have an interesting little, little tidbit for, you know, we're, we are King Kong. And I don't mean that as a metaphor. I mean that we're metabolically King Kong.

[Emily Kwong]

What do you mean by that?

[Christopher Doughty]

Humans, you know, use other energy. We heat our houses, you know, we have fires. And so if you figure out how much energy we use, like by burning oil on top of like everything we eat, you could predict what size animal would be. And so we would be 8 billion King Kongs. We're metabolically 8 billion King Kongs running around the planet. And so we in the western world consume a lot more energy than typical global citizen. We might be the ultra source. I haven't done that actual calculation. We might even be bigger than King Kong. So we're really changing this long term evolutionary trajectory of the planet in interesting ways. But of course, you know, who knows where we're going to be in a thousand years versus a million years. That's something that our models cannot predict, unfortunately.

[Emily Kwong]

And considering the way the dinosaurs went, I just, I don't feel great about this. So with all this in mind, how do you hope people think about the kind of like wax and wane of the understory in relationship to mass extinctions? Because we're in one right now.

[Christopher Doughty]

Yeah, I mean, I'd like people to reflect on our ecosystem co evolved with these big animals. They're used to having these big animals both as nutrient distributors affecting forest structure. They play really critical roles and now they're missing. And so that's kind of like our motivating factor for creating these models is to say something about our future world on big spatial scales over long periods of time.

[Emily Kwong]

Thank you so much for coming on the show.

[Christopher Doughty]

Oh, thank you so much for having me.

[Emily Kwong]

This episode was Produced by Burley McCoy and edited by our showrunner, Rebecca Ramirez. It was fact checked by Tyler Jones. The audio engineer was Robert Rodriguez. Beth Donovan is our senior director and Colin Campbell is our senior vice president of podcasting strategy. I'm Emily Kwong. Thank you for listening to Short Wave, the science podcast from N.

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