Even if you were the last rhino on Earth… why populations can’t be saved by a single breeding pair

Corey Bradshaw, Flinders University

Two days ago, the last male northern white rhino (Ceratotherium simum cottoni) died. His passing leaves two surviving members of his subspecies: both females who are unable to bear calves.

Even though it might not be quite the end of the northern white rhino because of the possibility of implanting frozen embryos in their southern cousins (C. simum simum), in practical terms, it nevertheless represents the end of a long decline for the subspecies. It also raises the question: how many individuals does a species need to persist?

Fiction writers have enthusiastically embraced this question, most often in the post-apocalypse genre. It’s a notion with a long past; the Adam and Eve myth is of course based on a single breeding pair populating the entire world, as is the case described in the Ragnarok, the final battle of the gods in Norse mythology.

This idea dovetails neatly with the image of Noah’s animals marching “two by two” into the Ark. But the science of “minimum viable populations” tells us a different story.

No inbreeding, please

The global gold standard used to assess the extinction risk of any species is the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species.

The Red List’s assessment criteria are based on the so-called “50/500 rule”. This states that to avoid inbreeding depression (the loss of “fitness” due to genetic problems), an effective population size of at least 50 individuals in a population is required.

To avoid eroding evolutionary potential (the ability of a population to evolve to cope with future environmental changes), an effective population of at least 500 is required.

The key here is that little qualifier “effective”. This refers to individuals who can breed with each other without causing inbreeding or loss of genetic diversity. A family unit, for example, might have only one or two reproductively effective members. But they would also need another, unrelated, family unit nearby for their offspring to reproduce with.

That means that the number of effective individuals is lower than the total population. On average, the ratio is about 0.1 to 0.2; that is, one effective individual (genetically speaking) for every five to ten members of the population.

This also assumes that the breeding pairs are matching up based on an optimal genetic basis – what geneticists call an “idealised population”.

In a perfect world, a breeding pair of animals would be completely unrelated and would have no chance of producing babies with any genetic defects caused by inbreeding. However, real populations rarely behave like this, so some pairs have a certain amount of relatedness. As the population gets smaller, the chance of breeding with a relative increases, which leads to more frequent and severe inbreeding.

Repopulating the world after the apocalypse

So let’s do the maths. Fifty effective individuals – the ICUN standard for avoiding inbreeding – equals a total population of 250 to 500. This means that, in a hypothetical apocalypse, humanity would need a lot more than a handful of survivors to repopulate effectively.

However, to retain evolutionary potential – to remain genetically flexible and diverse – the IUCN criteria suggest we would need at least 500 effective individuals. That requires a population of 2,500 to 5,000.

Some preliminary results emerging from ongoing research at the Centre of Excellence for Australian Biodiversity and Heritage appear to confirm this. Using both ancient DNA techniques and palaeo-demographic models, we have estimates of a minimum effective population size for Aboriginal Australians when they first appeared of about 250. This means at least several thousand had to arrive around the same time to manage to colonise the entire continent successfully.

Of course, not every species has the same ratio of effective to total population size, and not all populations necessarily need 5,000 individuals to survive. But without being able to measure the true ratio for a specific population, it helps to default to the average situation.

The idea that 50 individuals is enough to avoid inbreeding depression comes largely from laboratory populations that probably do not describe the situation for populations living in wild environments.

In species as varied as houseflies and pinkfairies, populations substantially greater than 50 individuals still succumb to inbreeding depression. So, in many cases, 50 effective individuals is in fact too low to ensure no inbreeding depression occurs. It may be that 100 effective individuals is closer to the true minimum, without even considering how populations respond to evolutionary challenges.

So, sensational analogies about the apocalypse aside, do human beings follow the same rule? We aren’t entirely sure, but evidence suggests that most species in vastly different groups roughly follow the same trend.

An emerging rule of thumb is that when a population starts to dip below several thousand individuals, it has a high likelihood of going extinct.

Corey Bradshaw, Matthew Flinders Fellow in Global Ecology, Flinders University

This article was originally published on The Conversation. Read the original article.

South Africa and the Rhino Horn Trade

Good news for the only place on Earth where tigers, rhinos, orangutans and elephants live together

Bill Laurance, James Cook University

Conservationists and environmental scientists are used to bad news. So when there’s some really good news, it’s important to hear that as well.

While the battle is far from over, there has been a series of breakthroughs in the long-running battle to protect the imperilled Leuser ecosystem in northern Sumatra, Indonesia – the last place on Earth where tigers, orangutans, rhinoceros and elephants still live alongside one another.

The government of Aceh Province – which controls most of the Leuser ecosystem and has been subjected to withering criticism for its schemes to destroy much of the region’s forests for oil palm, rice and mining expansion while opening it up with a vast road network through the forest – has agreed to a moratorium on new land clearing and mining.

The Leuser ecosystem.
Global Forest Watch, Author provided

This is huge news, and it’s clear that both the international community and Indonesia’s federal government have played big roles in making this happen. Indonesian President Joko Widodo deserves a great deal of credit for this accomplishment, which he has been pushing for many months, not just in Aceh but elsewhere in Indonesia too.

It is the culmination of an almost three-year battle by the Alliance of Leading Environmental Researchers and Thinkers (a scientific group I founded and lead) as well as many other dedicated researchers and conservationists.

Sumatran orangutans have lost huge areas of forest habitat.
Richard Whitcombe

Set in stone?

Moratoria can always be cancelled or weakened, but the chances of that happening seem increasingly remote. In a speech at last month’s signing of the Paris climate agreement in New York, Indonesia’s environment and forestry minister, Siti Nurbaya, underscored her commitment to the Leuser moratorium.

It seems unlikely that she would make this statement at such a high-profile event if there were any significant possibility that the moratorium will collapse.

And the news gets even better. Last week, Aceh’s deputy governor, Muzakir Manaf, declared that he will provide full support for ground-level measures needed to enforce the moratorium.

That is critical, for two reasons. First, it shows that the Aceh government is strongly behind the moratorium. Second, a moratorium is just a piece of paper unless there is real on-the-ground enforcement to ensure that illegal land-clearing, poaching, mining and other activities don’t continue unabated.

Limiting palm oil

A final piece of good news is that Nurbaya has confirmed her intention to halt completely the granting of new permits for oil palm plantations in state-owned forests right across the country.

To be clear, this doesn’t mean that oil palm plantations won’t keep expanding in Indonesia. There are thousands of existing permits encompassing many millions of hectares of native forest. Indeed, Indonesia has previously announced plans to clear a further 14 million hectares of native forest by 2020, mostly for oil palm and wood-pulp production.

But at least it means that the avalanche of new oil palm permits is coming to an end, for which both Widodo and Nurbaya deserve credit.

Rainforests being felled for oil palm in central Sumatra.
William Laurance

Not over yet

The fight to conserve Indonesia’s mega-diverse forests is far from over. The nation’s plans for massive road, dam and mining projects – many in forested areas where they can open a Pandora’s box of problems such as illegal poaching, logging and forest burning – is enough to frighten even the most sober of observers.

Fewer than 100 Sumatran rhinos survive in the wild, making it one of the world’s rarest species.
Lynsey Allen

But for today, at least, we can celebrate a very significant victory for conservation, and give credit to the many people who have worked to raise the profile of Leuser, including the actor Leonardo DiCaprio, who visited recently.

Few have had more impact than Ian Singleton, director of the Sumatran Orangutan Conservation Program. In a recent interview, Singleton laid out a remarkably compelling and detailed argument for saving Leuser, and for the surprisingly limited economic benefits its exploitation would generate for the local Sumatran citizens.

The economic and environmental think-tank Greenomics Indonesia also deserves a big round of applause for its efforts to facilitate this groundbreaking achievement.

But while we’re congratulating ourselves and others, we shouldn’t forget to keep a close eye on Leuser to ensure the promised moratorium really does take effect, and that one of the most important wild places in the world still survives.

This is an edited version of a blog post that originally appeared here.

Bill Laurance, Distinguished Research Professor and Australian Laureate, James Cook University

This article was originally published on The Conversation. Read the original article.