The United Nations has set Halloween as the day when homosapiens are estimated to reach seven billion, up from six billion in 1999 and five billion in 1987.

Two centuries ago there were one billion people.

By the end of this century the UN estimates that the planet will be host to 10 billion humans.

To mark the latest milestone, the United Nations Population Fund (NFPA) has released The State of World Population 2011. In an accompanying statement, the UN says of the seven billion: “Together we can change and improve the world.”

But the rise of humans has been accompanied by an era of mass extinction. Humans, the king of all predators, are wiping out uncounted species of animals.

The Conversation asked Dr John Alroy, a Macquarie University analytical palaeobiologist and Future Fellow, about the shadow-side of humanity’s rise.

Everywhere people go, mass extinctions follow.

Australia was home to giant kangaroos, giant wombats and all manner of megafauna until around 40,000 or 50,000 years ago.

Then people arrived.

Next you see very large extinctions in North and South America that happened over quite a short period of time at the end of the last Ice Age, and then after that point you see extinctions on islands of all sizes whenever people show up, and that happens over and over and over again.

Any time people show up on an island with any native species that are big enough to hunt and don’t have good defences they go extinct.

Hawaii, New Zealand, Madagascar, lots and lots of little islands in the Pacific, little islands in the Mediterranean, islands throughout the Caribbean; there were multiple extinctions in Hispaniola [now Haiti and the Dominican Republic], and Jamaica and Cuba, and so on and so on.

The reason these extinctions happen is simply that humans can eat almost anything, unlike other efficient hunters. Take sabre-toothed cats: if their food starts to run out, their population decreases because they’re starving, so before they can drive something to extinction their population has already crashed.

There’s a feedback that creates an equilibrium between predators and prey, making it very hard for predators to cause an extinction.

But people can get by on absolutely anything – they don’t need large animals; they can eat bush tucker. Because of that they can keep on knocking off the occasional diprotodon [giant wombat] or mammoth or what have you, and every once in a while knock off another one, and so long as the increased death rate is greater than the built-in birth rate – even just a little bit higher – then you can cause an extinction.

And humans don’t really care because they can go, ‘Oh, I’ll find some salmon or eat large insects,’ or pretty much anything.

So the question is when do you get enough people to get that death rate up high enough that it’s faster than the birth rate, and that takes a fair amount of people.

I’ve run mathematical-simulation modelling on North America showing that not much more than a few hundred thousand people scattered throughout the lower 48 states [of the now US] were needed to cause the mass extinction that happened.

It’s not even vaguely like the population of the US right now, which is around 300 million.

In the Australian situation it was probably similar and it didn’t need to take more than a few hundred thousand people at the most to cause the extinctions that were seen.

The future of fauna?

Bad. Really bad.

One thing about trying to understand the current mass extinction is that people tend to think on very short time scales.

A lot of conservationists think in terms of what’s going to be here in 50 years or 100 years, and the problem isn’t that.

The problem is that we’re going to be here with a huge population size for many thousands of years to come.

As you bring many species close to the edge, sure you only lose so many of them every ten years or hundred years, but as you keep sustaining that pressure on all the species and you’re keeping their populations really small, you’re going to lose a lot of them.

An example of this is the very rare species of rhino called the Javan rhino. It’s called the ‘Javan rhino’ because it wasn’t described by Europeans until it was already confined to the island of Java, which isn’t terribly big. But it was previously found throughout South East Asia.

In 1988 a tiny, tiny remnant population of the species was found in Vietnam. The last of that remnant population, which was probably genetically distinct, was killed by poachers recently.

As a mammologist I found that horrifying.

The only population we have left of this species of very interesting rhino is under 50 individuals.

Maybe you could fit them all in a single CityRail train.

This population’s been fairly stable for decades now because it’s been very carefully protected by the Indonesian Government, but that’s not going to last forever.

At some point there’s going to be a revolution or a huge natural disaster, a volcanic eruption or whatever – it’s going to be in 100 years or 1000 years but it’s going to happen – and that national park where the 50 individuals are is going to get disturbed to the point where that species is going to go extinct.

If you wait long enough, with a highly threatened species, it’s going to have some bad luck and be wiped out.

There epidemic diseases that are causing big problems for plenty of species because there’s been rapid introduction of organisms carrying diseases throughout the world because you have so much exchange of materials. And not just pest insects, but micro-organisms.

A lot of animals that are big enough and interesting enough for us to care about them – like rhinos – those same species tend to be hunted heavily. And for stupid reasons like believing that rhino horn, which is made out of the same protein as fingernails, is somehow medicinal. It’s ridiculous but lots of people believe it and with seven billion people, if one person in 1000 believes it they’ll put enough financial incentive on to preserve poaching.

Shark fin soup is a foolish thing to eat and it’s causing big problems for sharks. There are plenty of species of sharks that are being hunted for that.

So foolishness is causing us to put relatively heavy pressure on animals we care about the most.

The major threats for most organisms – plants and insects and so on – are related to global warming or pollution, deforestation, ocean acidification. These sort of large scale factors are not targeted at those species but when you look at the large, charismatic animals, they face hunting on top of all that stuff.

On top of that we have only the vaguest idea of how many species are out there, and there have been huge numbers of extinctions that have gone undocumented because the forests or other habitats where those species lived are gone.

They’re already destroyed.

And we don’t even know because even if we have a couple samples in a jar in some museum, we don’t have the resources to go back and look for them where they were found years ago.

Comments welcome below.

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