 When a species becomes threatened, sometimes they're described as having fallen below the threshold of population viability. So what exactly is population viability and why does it matter? In short, population viability is the prediction of whether a species will go extinct or not in a defined period of time. And we can model this statistically with maths, using things we know about the species like how fast are they born, how fast do they die, are they immigrating between populations and then we can predict if they'll survive. So sometimes these processes are direct, you have more births, the population goes up, you have less births, the population goes down and sometimes they're a bit subtler. When populations become small or they're in the process of declining, this often exacerbates what's causing that decline and effects that happen to small populations are called alley effects. So for example, African wild dogs, they hunt in packs and they need a big enough pack of like a dozen to twenty members in order to take down their prey items to feed themselves when they're young. So as their population gets smaller and the packs shrink, they become less efficient at catching their prey, therefore there's less food to go around for hungry mouths, therefore they can produce less. And then there's less dogs, less food, less births, less dog and it's a vortex of where they get smaller and smaller and so that's a classic alley effect. An Australian example of this is very recently, researchers found that the critically endangered regent honey eater need to learn their songs from other regent honey eaters in order to sing beautifully and attract a mate. So when regent honey eater populations become small and there's less adult beds around to learn from, the young regent honey eaters start learning weird and sometimes different species songs and then they can't attract a mate because the females don't want to mate with the males who have weird songs. And so then their offspring get weirder and weirder songs, there's less birds, even less birds to learn off and so this behavioural trait of song learning is directly related to their population viability. The other thing we have to take into account with population viability analysis is stochasticity. Stochasticity is the random elements of the system, so things can happen that are unpredictable. They play by the rules of the system but we can never know the future truly. So in the same way that tomorrow's weather might predict two mils of rain but we cannot be 100% sure because of the stochasticity. Things can happen to populations too that we won't predict like random mutation of their genes or a random fire coming through and wiping out part of the population. Stochastic elements are also important to modelling. So when we've calculated the population viability of a species that helps us define exactly how threatened it is. So the IUCN definition of a critically endangered species is one that's at risk of coming extinct by more than 80% certainty in the next 10 years or three generations, whichever is longest. And then there's different definitions for endangered, threatened, near threatened. And that helps us to prioritise which species are in most need because if we're 80% sure something's going to go extinct in the next 10 years, we need to act now. And that's why we have those rankings of endangered, critically endangered, so on. Population viability also helps us frame conservation because it's a lens for thinking about the processes and the maths of a population and not just their behaviours. So for example, the Kakapo is a flightless nocturnal big green parrot endemic to New Zealand. They are absolutely beautiful. And they got down to 51 individuals which is a pretty small population. So when the conservation effort really kick-started, they were keen to get them breeding up and having that birth rate part of population increase. But what they found was that the Kakapo mothers were giving birth to more, well, birth through eggs of course, to more sons than daughters. And this is in the long term a bad thing because if you don't have enough daughters, you can't have enough offspring. And in the long term, that birth rate will go down and that will decrease the population viability. So the researchers took a step back using that lens to figure out what the heck is going on. And they realised it's because the Kakapo have an evolutionary strategy where when resources are abundant, they naturally have more sons because that's more efficient for them when resources are good. And so the researchers were just feeding them too much. So they tweaked the diet and they brought that sex ratio back to 50-50. And not only was this really good for Kakapo recovery, there's a couple hundred of them now, but it also gave us some amazing insights into the evolution of sex allocation. So that was fascinating for both the theory at large and the species themselves.