Posted by: Andrew Guerin | April 1, 2008

What killed the mammoths?

ResearchBlogging.orgAccording to the evidence we have, the woolly mammoth Mammuthus primigenius disappeared about 3600 years ago; pretty recently in the grand scheme of things.

For some time now, debate has raged over why, exactly, they disappeared.

They might have been hunted to extinction by expanding human populations. Indeed there is evidence that many disappearances of populations of large vertebrates coincide rather neatly with the arrival of humans on any particular continent. This is known as the overkill hypothesis.

There is a competing hypothesis: that the mammoths were driven extinct by a drastic loss of suitable habitat due to the changing climate, which was warming up from the last major ice age (the Last Glacial Maximum was 21,000 years ago).

As is often the case in biology, where there are two strong competing hypotheses like this, it is not a simple case of one hypothesis being wrong, and one being right. In fact it now seems most most likely to have been a combination of the two.

A recent analysis (Nogues-Bravo et al) has used a modelling approach to attempt to shed some light on the contributions of these factors (anthropogenic and climatic) to the extinction of this particular species.

A ‘climate envelope’ model (which is used to work out the geographical extent of suitable climate conditions for mammoths and how that changed over time) showed that from about 42,000 years ago the amount of suitable mammoth habitat was decreasing, reaching a very low level by about 6000 years ago (see figure below) – and this is expected to decrease the overall population size.


Maps of projected climatic suitability. Red: most suitable, orange to green indicating decreasing suitability. Black dots are records of mammoth presence, black lines are northern limit of human presence. kyr BP = 1000s of years Before Present

A dynamic population model was then used to calculate, given the shrunken populations of mammoths, what level of hunting intensity would lead to the extinction of the species. This figure was found to be very low for the period starting about 6000 years ago, compared to 30-42000 years ago.

So it seems that the two factors acted together; first the changing climate contracted the geographical range of the species, decreasing the population to a level where human hunting became a threat to the survival of the species. Humans, then, pushed the species over the edge. This view is supported by the fact that mammoths had persisted through earlier climatically induced shrinkages in their range – back when humans hadn’t been around to deliver the final blow.

I think we should take all this as a warning.

It shows that climate change can reduce a species’ populations down to the point where they become vulnerable to extinction due to other factors, such as exploitation at levels which they might otherwise be able to sustain. This means that human-induced climate change may make species even more vulnerable to our other environmental effects.

Coming from a slightly different direction to get a different (but similar) lesson, it shows that species are resilient to climate-driven extinction, when not exposed to other negative factors. However, by our many activities which negatively affect the environment and the biological populations within it (such as habitat destruction, over-exploitation, introduction of invasive species, etc, etc) we are removing the very resilience which might allow species to cope with the climatic changes that we ourselves are wreaking on the environment.

Nogues-Bravo, D., Rodri­guez, J., Hortal, J., Batra, P., Araujo, M.B. (2008). Climate Change, Humans, and the Extinction of the Woolly Mammoth. PLoS Biology, 6(4), e79. DOI: 10.1371/journal.pbio.0060079

PLoS journals are open-access and all articles are available online for everyone, for free!
www.plosbiology.org
They also have their own ‘editors summary’ of the work, which is probably better than mine.

Advertisements

Responses

  1. This sounds like an interesting paper

  2. Yep. Not really in my field, but it was interesting so I wrote a little about it!

  3. Many animals have died to extinction because of man. I heard of several wiped out just due to gladiator games, i.e. Roman conquest.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Categories

%d bloggers like this: