Twenty years ago, things seemed great for the Tasmanian devil.
The species had rebounded from several decades of intense persecution. It was originally believed that Tasmanian devils were a major threat to sheep, and from about 1830 to 1936, they were relentlessly hunted, trapped, and poisoned. Bounty systems were implemented, and their number did drop during that time period.
In 1936, Tasmania offered full protection to the Thylacine, which had become quite scarce in the wild. Because the devil was also quite uncommon, protection was offered to that species in 1941.
From 1941 until the 1960’s, the devils were allowed to exist without any sort of hunting pressure. And the numbers began to increase again. Occasionally, poisoning permits were issues to control devils in sheep production zones, but the animals were still officially protected.
By the 1970’s, there were concerns that the devils were overpopulated. In 1975, the population crashed, but it then rebounded. It continued to grow until 1987, when another crash happened. These sorts of boom and bust growth cycles exist in virtually all wildlife species, and it was assumed that the devils were developing along healthy lines.
Tasmania allowed permits to cull devils until the early 90’s.
Then in 1996, all hell broke loose for the devils. A contagious form of cancer was discovered. Called devil facial tumour [tumor] disease, it was originally believed to be caused virus. It is now founded to the result of a clone of malignant cells. It is transmitted with the devils fight over carcasses, which they are so famous for doing.
The devils are highly susceptible to this cancer. Since 1996, the devil population has crashed by 80 percent. Almost all of the loss is attributed to this disease. It is estimated that if the disease continues as it is now, the Tasmanian devil will be extinct in 25 years.
Why would the devils be so susceptible to this unusual cancer?
Well. It was always noted that devils were unusually susceptible to other forms of cancer. Cancer has always been a major cause of death in the populations that have rebounded since the devil was protected in 1941.
Devils have very low genetic diversity. There are certain reasons for this low diversity. One of these is that the Tasmanian devil in Tasmania is relict population. The animals were once found throughout Australia, but it is now represented only by population that became isolated on an island.
And this population experienced an extreme genetic bottleneck 10,000 years ago. The founding population that survived that bottleneck could have been as few as 500 individuals. This bottleneck has been exacerbated through intense persecution since Europeans arrived on the island, and because the MHC diversity was always compromised, the devils were often experiencing epidemics. It is thought that two population crashes that occurred in the first half of the twentieth century were the result of epidemics that rapidly spread through the genetically depauperate devils.
We are no longer talking about devils being overpopulated as they possibly were in the 1970’s and 80’s. We are now talking about possible extinction.
And it’s all because of inbreeding– inbreeding caused by natural causes 10,000 years ago and more recent inbreeding that has occurred because of intense persecution and habitat fragmentation.
This story should be a cautionary tale.
Just because a species has appeared to recover in numbers does not mean that all is well.
We have to pay attention to diversity within the MHC.
Many endangered species are in exactly the same position as the Tasmanian devil. Cheetahs are famously inbred, but thus far, no major disease has popped up that will kill them all. That does not mean that it won’t. It just means that it could easily happen.
And even our success stories might not be so successful.
In the US, we like to congratulate ourselves about the successful recovery of the northern elephant seal. In the early twentieth century, there could have been as few as 100 northern elephant seals left. The Mexican government protected the only surviving colony, and the US soon followed suit. Eventually, their numbers reached over 100,000 individuals. These are harem breeders, which means that only a few males produce offspring every generation. They suffer a definite popular sire effect, which wouldn’t be so bad if they weren’t already so inbred.
No bad diseases have popped up in these animals yet. However, one easily could.
As the devils have taught us, just because a species is particularly numerous does not mean it is not vulnerable. We have to pay attention to genetic diversity, especially in the MHC genes.
Unfortunately, many success stories in conserving wildlife species are likely to be similar to that of the Tasmanian devil. Genetic bottlenecks create small founder populations that may be able to recover in very large numbers, but because these populations have low genetic diversity, they are unable to survive epidemics.
This is why conservationist are so concerned with genetic diversity in all sorts of wild populations and within the zoo breeding programs. Genetic diversity is essential for endangered species to fully recover. Otherwise, they will always be vulnerable.
And if they are that vulnerable, how can we say that they have fully recovered? If one contagious disease can do this to the Tasmanian devil, what can another one do to the California condor, the Florida panther, or the giant panda?