Consequences of large animal extinctions

Across the last 50,000 years or so, the planet has lost the majority of its large animals, known as megafauna, from most of its landmasses. It’s tempting to blame the climate, or just evolution’s natural ebb and flow, but those familiar with the timeline will recognise that this disappearance coincides with our ascendency, putting us squarely in the frame. Presenting to Chris Smith the case against us, and the consequences, is Aarhus University’s Jens Christian Svenning…

Jens - If you're interested in prehistory, one thing that really stands out is that large animals used to be all over nature on all continents, whether dry, wet, cold, warm. The situation nowadays, where the biggest animals we have in nature are usually some kind of deer or something like that, it's really unusual. That's my starting point.

Chris - And what do you define as a large animal, then? Are we talking elephant size or, you highlight deer, just bigger than a deer?

Jens - Animals come in all kinds of body sizes. A classical definition is that they should have a body weight of at least 45 kilos or 100 pounds. That's still pretty arbitrary, but a sizeable animal like a fallow deer.

Chris - And if one looks back in the fossil record, is there evidence that it was the case that they were everywhere and now they're not? Or have they always been in this fairly restricted distribution?

Jens - No. Big animals and even the very big, so elephant sized, were all over the place until very recently in pre-history, let's say, 10,000 to 30,000 years ago. If you go into any good natural history museum in North America or in Europe or in Asia or in Africa, you'll find elephants from these timescales. You see that these kinds of animals were there. And in fact, we had 15 species of elephants from Patagonia to Alaska, from Alaska to Britain, and from Britain to the Cape, until 30,000 years ago.

Chris - And when did they start to decline?

Jens - They started to decline roughly 30,000 - 50,000 years ago. And then it kept going until 12 out of 15 species went completely extinct.

Chris - The timeline that you've highlighted does map onto when we've seen some climatic changes on Earth. Can we explain this loss in the same way as we've seen boom and bust for all kinds of species over Earth's evolutionary history and just say, well, this is down to some kind of changing environment and that's why these animals have gone. The niche that they occupied has been snatched away by climate change?

Jens - No, we actually can't explain this from climate. It's very clear, in fact. In the last million years we've had these very strong climatic cycles with ice ages and warm periods like the present coming and going with a timing of 100,000 years. We've had 10 of those cycles roughly. Elephants did very well through these cycles until the last 50,000 years. That makes good sense because elephants are super generalised animals that can eat all kinds of vegetation. We had elephants from super cold areas, like arctic areas, to the warmest areas, and from desert-like areas, through forest areas. They were all over the place and very generalised, so you can't explain this from climate.

Chris - So if it isn't climate, what do you think has led to the decline in not just elephants but these big animals in general and everywhere?

Jens - The explanation is clearly with ourselves, our species. We see this decline really starting with the emergence of really modern people and then the expansion across the planet. Then we see very consistently these declines in elephants and in all kinds of other big animals. If we look in detail at the archaeological evidence, we can also see that our ancestors were hunting mammoths and other elephants, made big traps and were able to kill really big individuals or even whole flocks. We can also see from their bones that they had a large amount of their protein often coming from elephants and rhinos and so on.

Chris - And is that why the loss is chiefly focused then on these big animals rather than the smaller ones? Because those were the ones that our ancestors were going after?

Jens - Yes. If you go out hunting and you could get a big animal like a mammoth or something like that, that would give food for your family and the people in your tribe for quite a long time and would be a very efficient use of your energy relative to trying to hunt smaller animals, which could be equally difficult to kill.

Chris - People often say that these big animals deliver a lot of ecosystem services. They're effectively farmers for us. And if they're not there, you don't have those services. What have been the consequences then of the loss of these big animals from nature over this time period?

Jens - The consequences are likely fundamental. It is something that we are still working on, but it's very clear from both first principles but also from empirical evidence that this has had big consequences. First of all, the larger animals interact very strongly with vegetation. They eat plants, elephants tumble over trees, they generate pathways and so on. We can see actually that European forests, for example, before the extinction of these large animals, was really a mix of, say, classical tall forests, really disturbed vegetation, and even open vegetation. And it's very hard to explain this other than from the effects of these animals. They likely had really strong impacts on the ecosystems; changing vegetation, controlling fire regimes, and also dispersing lots of other species around.

Chris - So what differences does the world display today? When we look at the world around us today, how different does it look because these animals are now missing, do you think?

Jens - Well, of course, the place to look is in our natural areas because the rest we have fundamentally changed in other ways. Of course that deviates completely from the natural state. But even if you go into our natural areas, they deviate strongly from what was normal if you look into prehistory. In Europe, I would say you very often find much denser forests than you otherwise would find. It's not that we wouldn't have lots of trees, but it would be a much more mixed landscape than we usually find today in those kinds of settings.

Chris - And hence, if we've changed the environment because we don't have these big animals, there must be knock on consequences for other animals that we don't go hunting because their environment has changed?

Jens - Yes, there are likely lots of consequences. Of course, many other animals directly depend on the large animals, like dung beetles or scavenging birds and so on, and we actually do see big extinctions in scavenging birds associated with megafauna, large animal declines. But also, lots of forest plants actually associated with quite high light levels, they really suffer in modern dense forests. That's another example that they're missing something to keep the system more varied, more open. We also see that lots of plants are very strongly dispersed and limited in our natural landscapes today, and they really miss the animals to move them around.

Chris - Us humans are not really good news for the planet, are we?

Jens - We haven't been very good news for the other species, no. But of course we have the ability to think and rethink and that's something we can still do. I would say there's a way forward. I don't think it's about going back, but we can definitely restore the large animals. Many of the species are not extinct and if we give them the chance, they can rebound, they can come back. Not necessarily the exact same species, but at least other large animals in our systems. That would be really good for the functioning of our ecosystems.