Over the past few weeks I have read articles about some species on the brink of extinction, their numbers dwindling in the low thousands or even hundreds. At what point does a species have no chance of recovery? Does it vary depending on whether the species is a mammal, bird, plant or otherwise? I thought I’d look into those questions and discuss what I’ll call the “extinction threshold” – the point of no return for a species.
Small populations of any species are generally at a much greater risk of extinction than large populations for several reasons. With fewer individuals there is a greater chance of inbreeding and the issues that creates. Small populations feel random variations in birth and death rates much more significantly than a large population would. And environmental fluctuations like predation, disease, and natural disasters take a much greater toll on small populations than large ones. Imagine if one wolf died out of a population of 100, that’s 10 percent. In a population of 1,000 wolves, however, that’s only 0.1 percent. Quite a difference! It’s like a slippery slope… once a species starts to decline those factors weigh more and more heavily on the population, leading to even greater declines.
So is there hope for a species that is sliding quickly down the slope towards extinction? Shaffer (1981) coined the term “minimum viable population” (MVP) to refer to the “smallest isolated population (of a given species in a given habitat) having a 99 percent chance of remaining in existence for 1,000 years. Populations smaller than the MVP are at significant risk of sliding down the slope to extinction. It can be difficult to determine where that MVP line is… and that is far too technical to get into here… but just know that it exists.
What really matters in terms of the long-term viability of small populations is their ability to disburse. If individuals from a small population can leave their habitat and find a new population of the same species to breed with, then there may be a chance for survival. This immigration brings more genetic diversity to both populations. However, given how habitats are fragmented by roads, transmission lines, housing developments and other man-made structures, many small populations don’t have the luxury of moving to find new mates.
Back to my question about “does it depend on whether a species is a mammal, plant, etc.”, well it depends. It depends on where that species is located, more than anything. But a bird would more easily be able to fly to a new habitat than a small rodent could crawl to one, and a lizard can get up and go far more easily than a plant (obviously plants can’t move, but they can spread their pollen and seeds, thus spreading their genetics on the wind or water). The more easily a species can get around in general, the greater are its chances in beating the “extinction threshold.”
Then there is the species’ charisma factor. Think about a cute, fluffy pike (a small, bunny-like create of alpine habitats) versus a poisonous snake. If you could only protect one of them from going extinct, which one would you choose. A majority, I will guess, would choose the pika, even though they will probably never see one in real life. Why is that? Because it is cuddly and charismatic. So if a species is attractive people may be more likely to try and help it survive, which would keep it above the “extinction threshold,” at least for a little while longer.