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Science and Innovations

Study: Earth's Biggest Plants, Animals Key To Planet's Health


A new study released this month models what happen to the health of the planet when giant plants and animal vanish. It’s happened before—at the end of the last Ice Age when mammoths and enormous sloths died out. Humans likely played a part in that extinction and the researchers say Earth’s big animals today are headed for the same fate because of overhunting, habitat loss, climate change and other factors. KNAU’s Melissa Sevigny spoke with Northern Arizona University’s Andrew Abraham who was one of the authors of the study.

You invented a new word for this study, is that right?

“Megabiota.” So the largest animals, megafauna, and the largest plants, megaflora. Some of the megabiota living today—from the plant side, you can look at the sequoia or the giant redwoods. For the animal side, there’s lots of them especially if you look to places like Africa where we still hold most of our megafauna these days, so elephants, rhinos, giraffes. In Arizona in the past, there used to be an American lion, an American cheetah, there used to be mammoths which were similar to the elephant in the African savannah, and also strange creatures like the glyptodont and giant ground sloth.

Glyptodont, what is that?

It’s kind of like this big armadillo type creature, but significantly larger, maybe the size of a small car… The whole world over used to look something similar to an African savannah, with a whole myriad of different sorts of animals. What we’ve seen through time is that as human populations have gone into new landscapes these large animals are particularly susceptible to disappearing.

So one of the things you discovered in this study is that large animals are more threatened by extinction. Why is that?  

Exactly. During times of environmental change, large animals in particular are put under a lot of stress. There are three reasons for this. The first is that large animals and large plants tend to be at their physiological limits. They’re already pushing the boundaries of how large they can be. The second is they tend to have quite low reproduction capacity. The final reason is they require a lot of space. Especially large roaming animals like elephants, if they’re in a small fragmented landscape it’s very difficult for them to be able to survive.

So another thing you discovered with the computer modeling you did is that when large animals and plants disappear, that actually has a huge effect on Earth’s ecology. Can you talk about that?

We used a number of computer models to try to understand how the downfall of these large plants and animals will impact the world. One of these models in particular is the first model that is able to incorporate all the plants and all the animals in the entire world. One of the theories we tested in this paper was: does the removal of large organisms, large plants and animals, get compensated by smaller organisms? And what we found was that if you do remove the large organisms, you don’t really get a replacement service, an ecosystem service, from the smaller organisms. So if you have forests with very large plants, say the giant sequoia, they tend to hold much more carbon than a forest that didn’t have those large plants…. The largest animals tend to walk furthest and they move seeds and nutrients and pathogens across the landscapes, which means the Earth is more connected environment when these animals are present. Together they generated a much more habitable planet across the world.

There was an interesting result that if we didn’t have large animals there would be a 92 percent reduction in soil fertility. How does that work?

Exactly. This is related to the dispersal process. Large animals tend to eat a lot. Elephants eat 300 kg of vegetation a day and it takes a while for that vegetation to move through their body and come out as feces on the other end. During that time animals can move quite a large distance. We tend to find the large animals are disproportionally important to moving these nutrients across landscapes.

Did anything surprise you about the results?

One of the things that surprised us was just how vulnerable this plants and animals are to extinction. As a rule of thumb we found that for a 10 times increase in body size, the extinction possibility increases by about 300 times, which was vastly more than we’d thought or understood before…. In general these large animals are very, very vulnerable to changes in the environment, which may be caused by natural fluctuations, but as we’re seeing at the moment, humans also have a large role to play in that.

What does this all mean for in the real world, for conservation?

There is a debate in conservation whether we try and conserve lots of small areas of land or whether we aim to just conserve fewer large area. What we found in this paper is if you conserve larger areas with their megabiota, the whole system is more productive and the benefits gained are significantly more.

I found it fascinating in the paper that you modeled ahead to a world where there were no large animals left.

It was a pessimistic outlook to the future but it is the trend we’re seeing at the moment, so we shouldn’t shy away from that. There may be a time in the next 100 years or so where the largest animals we have on the planet is something like a cow. And we’re trying to understand what  are the implications are that, because we understand how important these large organisms are for our human health, perhaps that will give us a foundation from which to protect them into the future.

Andrew Abraham, thank you for speaking with me today.

Thank you, it’s been a pleasure. 


Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.
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