Science Questions

How few individuals is too few to save a species from extinction?

Sun, 30th May 2010

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Andrea Lewis asked:

When are there too few animals to save a species? Pandas and other endangered animals for example. What is the cut-off when there won't be enough genetic diversity left in the numbers that are left in the wild for them to be saved?


Helen -  That’s a really good question and it opens up an awful number of issues to consider, but we can certainly just get into that to some extent.

I think my first question will be, what do we mean by saving the species?  Are we talking about them persisting in the wild?  Do you want to keep them in captivity?  Those will require very different numbers of individuals within a population.  But also, we’re talking about genetic diversity and one thing you can think of is that when a population is cut down, when it declines, you do get the problem of inbreeding and that’s what we’re really hinting at.

If there’s not enough genetic diversity, you might get inbreeding and you might actually have the appearance of deleterious genes coming together and causing problems in the animals that are being born.  But that said, there have also been situations where we’ve thought that maybe genetic diversity is the reason why some wild species are not doing so well, when in fact, it isn’t, one example is the cheetahs.  

We think that historically they went through a very big bottleneck, these lovely, very fast running cats that run around in Africa, and there were various studies that pointed towards genetic diversity as the problem.  Things like, they could take skin grafts from any cheetah and put it onto another one and it wouldn’t be rejected.  So conservationists began to think, “Aha!  It must be this lack of genetic diversity which is why we’re not seeing enough new baby cheetahs surviving in the wild.”

In fact, this wasn’t true at all.  It was a glitch in the studies of those skin grafts that meant it wasn’t anything to do with genetic diversity.  It was because lions were eating the baby cheetahs and I think hyenas as well.  They were being predated upon by other animals, and when they were in areas where they weren’t, they were doing fine. 

So, it’s a tricky thing to think about.  You can also think about the island of Madagascar, which was colonised by just a handful of different mammals.  So they had very low genetic diversity.  It doesn’t necessarily mean a species is going to go extinct.  It’s all about lots of different factors that affect whether or not it’s going to survive in the wild which I think is what really we’re talking about - are they going to survive and can we try and stop extinctions from happening?  There are lots of things we have to consider.

Chris -   And as we heard last week when we were talking about Tasmanian Devils, actually being a bit inbred can be dangerous for another reason.  You can get something happening to a population similar to the Devils.  They have a line of cells which had become cancerous, and they can then go from one animal to the next, almost like an organ graft.  This is accepted and not rejected by the immune system of the recipient because they're so inbred.  There are dangers of a dwindling population.


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