Small is beautiful

Saving small populations is a big genetic challenge
14 November 2017

Interview with 

John Ewen, ZSL London Zoo


Tiger in a zoo


All of the species Kat Arney met in ZSL London Zoo - the blue-crowned laughing thrushes, the Sumatran tigers and the Partula snails - are small groups on the brink of extinction. That poses some significant genetic problems for conservationists, as she discovered when she called up John Ewen, ZSL’s expert on saving small populations.

John - Understanding the genetics of the population of the species is critical when you’re planning for recovery. However, I think what I voiced and what I always encourage others to do if I'm meeting with different groups on conservation problems is take a step back and try and understand first what it is they're trying to achieve.

In most cases when we’re looking at small populations of threatened species, and we’re worried they're facing an extinction event so we’re trying to recover them. So we’ve got this core ‘biological objective’ which is driving our management choices. Then we can start thinking about how genetics plays a role in their recovery.

So, it depends what you want and for me, it’s about recovering populations and recovering a species, and then I start considering how genetics plays a role in that.

Kat - When you do get to the genetic part of it, what sort of things are you thinking about and how do you sort of measure the genetics of a small population?

John - So, I immediately get worried when I'm faced with the situation where we’ve got a small and isolated population because that’s exactly the situation which can get you into trouble from a genetic perspective.

And those types of scenarios, the genetics of those populations will primarily be driven by two processes – one is genetic drifts, so loss of genetic diversity, then a second problem around inbreeding, so breeding between relatives. And both of those things will change the genetic constitution of their population.

When that happens, you can run into problems. So the two main problems that happen will be the risk of inbreeding depression and also, the lack of ability for these populations to adapt in the future.

Kat - What do you mean by inbreeding depression? Presumably, they're not just sitting around going, “I'm so depressed. All I've got are my cousins left.”

John - No. I mean, we start talking about inbreeding depression we’re starting to link the genetic effects on fitness. By fitness, I’m starting to look at those demographic parameters which are important for a population to persist. So, we have things like impacts on reproduction, impacts on survival and susceptibility to diseases, and other stresses from the environment on that species.

Kat - Is there any point when you're looking at a population that genetically speaking, they are just gone, they're too far gone, there's no way you can get them back?

John - No. I don’t believe that at all. I think that adds a separate challenge that we then have to deal with. So then we start talking in my world about recovery and how we might want to try and manage recovery, linking in with genetics then we want to try and recover some of the genetic diversity.

So there are options in how you might go about doing that and it really depends again on what your overarching objectives are to the management of that population. At the most basic level, we know that diversity arises because of mutation and that this process requires large population sizes.

So, in any case, I would be trying to grow my threatened population to as big as the population size as possible and as quickly as possible so I can remove any further loss of diversity through this random process of drift and I can create a situation where more diversity can arise through mutation.

Kat - So you're basically trying to grow some weird mutants to boost your population?

John - Well, not trying to purposely grow something like a weird mutant. This is what natural populations do. This is how genetic diversity arises. So, we’re just trying to put that population back in a place where nature can do its work.

Kat - What are your hopes for conservation in the future? What really needs to happen to make a difference and to save these species that do seem to be going extinct at an incredible rate?

John - Well for me, I'm actually much more of an optimist than a pessimist and I think the tools that we are using currently to recover threatened species, they can work and we need much more investment in them, and genetics certainly is an important component of that an informs them.

And so, with this increasing technology that we have and with dedicated people, we can bring species back from the brink of extinction. It works and there's evidence out there that works. So, we just need to do more of it.

Kat - John Ewen, from ZSL. 


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