Why populations can’t be saved by a single breeding pair

3 04 2018


© Reuters/Thomas Mukoya

I published this last week on The Conversation, and now reproducing it here for CB.com readers.


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. Read the rest of this entry »

An appeal to extinction chronologists

2 06 2015

u7Pi3Extinction is forever, right? Yes, it’s true that once the last individual of a species dies (apart from insane notions that de-extinction will do anything to resurrect a species in perpetuity), the species is extinct. However, the answer can also be ‘no’ when you are limited by poor sampling. In other words, when you think something went extinct when in reality you just missed it.

Most of you are familiar with the concept of Lazarus1 species – when we’ve thought of something long extinct that suddenly gets re-discovered by a wandering naturalist or a wayward fisher. In paleontological (and modern conservation biological) terms, the problem is formally described as the ‘Signor-Lipps’ effect, named2 after two American palaeontologists, Phil Signor3 and Jere Lipps. It’s a fairly simple concept, but it’s unfortunately ignored in most palaeontological, and to a lesser extent, conservation studies.

The Signor-Lipps effect arises because the last (or first) evidence (fossil or sighting) of a species presence has a nearly zero chance of heralding its actual timing of extinction (or appearance). In paleontological terms, it’s easy to see why. Fossilisation is in fact a nearly impossible phenomenon – all the right conditions have to be in place for a once-living biological organism to be fossilised: it either has to be buried quickly, in a place where nothing can decompose it (usually, an anoxic environment), and then turned to rock by the process of mineral replacement. It then has to resist transformation by not undergoing metamorphosis (e.g., vulcanism, extensive crushing, etc.). For more recent specimens, preservation can occur without the mineralisation process itself (e.g., bones or flesh in an anoxic bog). Then the bloody things have to be found by a diligent geologist or palaeontologist! In other words, the chances that any one organism is preserved as a fossil after it dies are extremely small. In more modern terms, individuals can go undetected if they are extremely rare or remote, such that sighting records alone are usually insufficient to establish the true timing of extinction. The dodo is a great example of this problem. Remember too that all this works in reverse – the first fossil or observation is very much unlikely to be the first time that the species was there. Read the rest of this entry »