Islands of Wolves, Rats, Lions and Dogs

Isle Royale, Michigan.  An island of wolves.

In the real world, population bottlenecks are not always quite as big a problem as some people imagine, nor are they quite as easy to correct as some people hope. 

If that sounds like two statements in direct opposition, then you have grasped a core message of this post, which is that not all animal populations are the same, that few real-world cases line up squarely with simple theory, and that there are multiple facets to both genetic isolation and genetic rescue.

In order to understand a little more, let's look at four real-world animal populations:

Imagine a pregnant rat jumps ship on to a large island that is 200 square miles in size.  The population of rats multiplies very rapidly but without any apparent long term problems due to inbreeding.  How is that possible?  Answer:  With rats, massive population numbers within a short period of time are possible, and a mid-sized island of 200 square miles (over 259,000 acres) can easily hold a million rats.  In this situation, genetic drift and mutation will eventually result in a population with little or no obvious genetic dysfunction.  On an island with 1,000,000 rats, in which each rat lives for a year or so, a 100-year old population of theoretically "inbred" rats will have very low coefficients of inbreeding, no inbreeding depression, and no discernible early mortality due to genetic weakness.  To be clear, this is not a theory; rats have colonized almost every island in the world exactly this way.

Now imagine that a pregnant wolf lands on an island that is 200 square miles in size.  The wolf whelps five pups, and the pups interbreed and the population grows for a time until it hits some sort of food-availability threshold.  A boreal island that is 200 square miles in size and with a sizable moose population can, for a while, feed a population of 50 wolves, but eventually that population can be expected to collapse down to as few as dozen individuals before it rises again and falls again due to the vagaries of disease and weather which will impact prey species such as moose, deer and rabbit.  After 75 years, the population of wolves on this island will be very inbred, and infecundity and disease will be common.  The reason for this is simple:  wolves typically live between six or seven years in the wild, and the smaller number of wolves on the island, combined with the smaller number of generations, means that there will be very little room for genetic drift or mutation.  Is is possible to import a single individual wolf to achieve a "genetic rescue" of this heavily inbred wolf population?  A single wolf, sadly, is not likely to do it.  The reason for this is simple:  the small number of wolves on the island (a population of 24, on average) and their high rate of infecundity due to inbreeding, means that the genes of the new male wolf will quickly dominate.  Inbreeding will then continue as before.  Though there may be a short temporal  improvement in population health, that may not be observed if there is a counter-balancing downtick in food sources occurring at the same time.  To be clear, this scenario is not one I have made up; it appears this is exactly what happened with the wolves on Isle Royale, Michigan.

Now imagine a large population of lions that has been reduced by hunting to just 30 individuals living on an isolated isthmus 200 square miles in size.  The population is so isolated that the coefficients of inbreeding within the lion population begin to rise, and a rise of infecundity and an increase in genetic defect is feared.   The good news, however, is that this isthmus is not at carrying capacity for lion, and so eight completely unrelated female lions are imported from another country more than 1,000 miles away -- a 25% population boost representing a massive increase in genetic diversity.  What happens?   A rather significant improvement in species health and fecundity seems to occur, and the lions begin colonizing more space on the isthmus.  Why did this genetic rescue work?  Well, for one thing, the original 30-lion population was more diverse than it at first appeared.  Remember that these 30 lions did not actually spring from a closed registry of two individuals, but were the remainder left from a massive population that once numbered scores of thousands.  While inbreeding to failure appeared to threaten this lion population, the actual genetic diversity beating under the hood was likely to be quite a bit more expansive than a purely mathematical population model would suggest.  Factor in the relative size of the genetic infusion coming from the other side of the country, and the ground is set up for a successful genetic rescue.  To be clear, this scenario is not one I have made up; this is exactly what happened with the lions of Southern Florida (aka, the Eastern Puma, Florida Panther, or Eastern Mountain Lion).

Now imagine a pair of dogs that are mated and their pups are then inbred to each other for 25 years at which time another dog is added to the population and the result is further inbred for another 40 years.  The result is a population of about 180 potential breeding dogs (30 dogs are born a year, and not all dogs will be bred) which have a Coefficient of Inbreeding of 80 percent.  What would it take to have a successful genetic rescue here?  If we use the Florida Panther model as a guide, we would need to see an addition of 45 new dogs to the gene pool.  Would the addition of  one or two dogs be enough to turn the tide?   Probably not.   The addition of only a handful of dogs in a show-breed situation, where dominant sire selection will continue unabated, will not provide the genetic fix needed to turn things around.  Remember we are starting here with a very high COI, and the public demand for these dogs is apparently quite low; this is a breed that hits its consumer "carrying capacity" with just 30 to 40 puppies a year.  To be clear, this scenario is not one I have made up; the dog presented here is the Cesky terrier, a breed that has never been very popular.

So what's the point? 
 
The point is this:  the issues associated with population bottle necks are not quite as simple as some imagine.  Yes, the relative size of the founding population matters, but so too does the genetic health of the founding population, the total number of animals bred, the number of generations it took for the population to achieve a large size, and whether the animal in question was breeding randomly or within an organized scheme that encouraged (or discouraged) genetic diversity.

Consider, for a moment, a dog breed that was drawn into a closed registry 120 years ago with 150 dogs in its foundation registry.  The breed grew rather quickly so that after 40 years it had a worldwide population of 125,000 dogs; a population it has maintained for the last 80 years.  Coefficients of inbreeding in the modern population of this dog will, on the whole, be quite low, and though the apparent "effective population size" may have dropped from 150 dogs to 80 dogs due to dominant sire selection, effective population size is quite a meaningless number when you have bred well north of 1 million dogs over a 100-year span.  In a situation like this diversity is not destroyed over time, but created through the sheer beat of numbers over vast distances.
 
Of course, there is another factor in all this, and it is an important one, and one that is too often left out.

You see, in the world of Kennel Club dogs there has always been a little "pedigree leakage."  

Pedigree leakage may be small or even nonexistent in a very rare breed where everyone knows everyone else, and the dog in question is likely to look a bit odd or extreme and not have an obvious analog in another breed or cross-breed.

But what of the other more popular breeds, where there are a lot more dogs in the wind, and only a small number are likely to ever see the inside of a show ring, and analogs exist all over? 

In a situation like this, instead of a reduction in effective population size, there may actually be a little growth! 

After all, a Labrador Retriever with five-generation pedigree papers from the AKC may, in fact, have a Flat-coated sire, or even be whelped by a cross-bred dog of "pedigree unknown." 

A Miniature Poodle may have a little Maltese coursing through its veins only a generation or two back. 

And what does it matter?  Not a whit if the cross was a sound one and did not add a new or heavier genetic load to the mix.

The bottom line is that there is more to population genetics than any one number, and while it's important to be worried, and to take action to increase breed genetic diversity, it's equally important to recognize that one can fall in love with less than robust genetic theory and falsely specific numbers just as easily as one can fall in love with cocked up doggy histories and the romance of breed purity.   In the end, population science and genetics is about more than one number; it's about a small stack of numbers meeting a real gene pool coursing through the flesh and blood of a real population that is living in a real environment (whether wild or artificial, free-feeding or subsidized).

Yes, let us work to increase diversity, but let us also look towards Mother Nature as well. 

Remember that while Mother Nature abhors closed breed pools, she also manages to work out solutions for most of those that wash up as single or paired animals on tropical islands or are introduced to foreign lands.  Observe the birds in the park.  How many Starlings did we start off with here in America?  How many English Sparrows?  How many Red Fox?   How many wolves in Yellowstone?  They are clearly thriving.  How is that possible?  And, of course, the answer is that they colonized in vast numbers over a vast space of land, and in the slow beat of numbers and space they managed forge their own genetic health.
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