The living dead

This might disappoint a few but this is not an article about how Andrew Lincoln tackles to save our human species from a zombie invasion.

In biology, the living dead is a concept referring to species whose only chance of survival is through rapid human intervention. These species cannot recover if habitat loss alone is stopped. Also referred to as “zombies”, they are left lingering on the planet until they die off into oblivion. Apologies for the dreary tone but this is unfortunately a fact that we must face. I promise to finish on a good note so do not get discouraged by the dark tale ahead.

As humans continue to bulldoze their way across our planet, they are tearing apart habitats of thousands of animals. As a result, animal homes are becoming fragmented at alarming rates and large populations of species previously able to roam for thousands of kilometres are broken up and confined into “virtual islands”. This is of course not breaking news and although we are being constantly warned about the dramatic effect we are having on the planet, not a lot seems to be happening to stop it.

We are mostly faced with news about species at the risk of extinction if we do not act soon, however this might be feeding our sense of hope that, like a late essay crisis, under pressure we’ll get our act together and help them pull through. What we don’t realise enough is that many species around us are already extinct but are still among us due to long generation times. In other words when the remaining ones die off then that’s it. These are the living dead.

They are part of what biologists like to call an “extinction debt”. The earth is keeping a tab of what we’ve been spending over the last couple of years and before you know it we’re in overdraft.

Now enough with the analogies and here’s the actual science behind this.

A term at the centre of this concept is the minimum viable population (MVP).

As populations decrease, a number of factors are at play that decrease their chances of reproduction. General variations in birth and death rates have an enhanced effect on small population than they would larger ones. This is known as demographic stochasticity. Take a population of three breeding individuals and let them have a one-year lifespan. If for some reason neither reproduce one year then their population goes extinct. Take a population of 100 on the other hand; even if a few don’t fancy getting it on one year, you’re still left with a very large population of reproducing animals. Variations in sex ratios also fall under the umbrella of demographic stochasticity. For example, the last six surviving individuals of the dusky sparrow were all male.

There must therefore be a threshold of how small the population can be to allow it to persist. There you have it, what is known as the minimum viable population. Shaffer first introduced the term in 1981 and described it as “the smallest isolated population (of a given species in a given habitat) having a 99% chance of remaining in existence for 1,000 years”.

So who are these living dead species? Unfortunately we don’t need to look very far to find them. The Florida panther is one amongst many to be heading down this dark path. These majestic animals have been reduced to a population of about thirty individuals in the area adjoining the everglades known as the Big Cypress Swamps. Reasons for their demise include as always habitat destruction.  Due to such a poor gene pool they are also prone to many genetic diseases such as cryptorchidism. In simpler terms this means one or both of their testicles remain undescended and as consequence, to put it grossly, this “cooks” the sperm that are not used to such high temperatures.

Rainforests are another example of “walk in graveyards”. The amazon is a host to a museum of species who are past their time. These include the tree ocelot, white-cheeked spider monkey, Rio Branco antbird, Brazilian tapir and Yellow-headed poison frog.


As promised, I will finish on a positive note so that you go away from this with the drive to do more. Raising the living dead is a 2010 article by Carlos Magdalena on the successful resuscitation of a seemingly doomed species of flower known as Café marron (for you biologists out there it is a member of the Rubiaceae family, the fourth largest of the angiosperms).

The Mascarene Islands on which it was found are known as the ‘Islands of the Living Dead’ due to the disastrous effects habitat destruction has had on their ecosystem. Following its initial discovery in the early 1700s, it then disappeared from our records until by luck 100 years later a schoolboy discovered a single specimen.

The plant was rushed to Kew gardens for ex situ conservation. Unfortunately attempts for it to produce seeds were in vain despite relentless efforts. Three years later, Kew garden scientists were finally rewarded for their efforts as seven seeds braved their way into the world.  This is very encouraging and good news for the future of the species. This success story should be used as a framework for future resuscitations and should help us remember that unlike Andrew Lincoln’s zombies, the living dead can be brought back.