Minimum Viable Population: 5,000 Adults

Pandas are one of many endangered species who don’t meet the 5,000 minimum population for long term viability.

Dog breeds are not separate species. Still, the majority of the dog fancy behaves and enforces breeding rules as if they were. Having what could be the most extant genetic diversity of any species doesn’t really matter if all the diversity is never allowed to interact. It’s a theoretical advantage, not a realized one.

One of the most powerful forms of speciation is when two once related populations are no longer able to breed. Dogs, wolves, coyotes, dingoes, and golden jackals (members of the genus Canis) can all freely interbreed and produce fertile offspring. We can assign different species names to them but they can also be considered the same species depending on how you’d like to define that term. All of them have 78 chromosomes that come in 39 pairs and the uncompromised fertility of offspring of hybrids between these populations mean that any and all of them can be used for “genetic rescue” of each other. In fact, scientific investigation of DNA shows that such admixture is profound within various sub-groups of canids.

But dog breeders still treat mere breeds as permanent barriers and therefore many breeds are suffering effects similar to endangered species due to depleted genetic diversity, unavoidable inbreeding, and insufficient populations to maintain long-term viability.

So how many adults is the bare minimum to maintain a species? One research group from Australia claims that 5,000 adults is the minimum.

To ensure both long-term persistence and evolutionary potential, the required number of individuals in a population often greatly exceeds the targets proposed by conservation management. We critically review minimum population size requirements for species based on empirical and theoretical estimates made over the past few decades. This literature collectively shows that thousands (not hundreds) of individuals are required for a population to have an acceptable probability of riding-out environmental fluctuation and catastrophic events, and ensuring the continuation of evolutionary processes.

Here’s a decent article from American Scientist which outlines the concept:

An Australian team says it has figured out the minimum viable population for mammals, reptiles, birds, plants and the rest.

Imagine how useful it would be if someone calculated the minimum population needed to preserve each threatened organism on Earth, especially in this age of accelerated extinctions.

A group of Australian researchers say they have nailed the best figure achievable with the available data: 5,000 adults. That’s right, that many, for mammals, amphibians, insects, plants and the rest.

Their goal wasn’t a target for temporary survival. Instead they set the bar much higher, aiming for a census that would allow a species to pursue a standard evolutionary lifespan, which can vary from one to 10 million years.

That sort of longevity requires abundance sufficient for a species to thrive despite significant obstacles, including random variation in sex ratios or birth and death rates, natural catastrophes and habitat decline. It also requires enough genetic variation to allow adequate amounts of beneficial mutations to emerge and spread within a populace.

“We have suggested that a major rethink is required on how we assign relative risk to a species,” says conservation biologist Lochran Traill of the University of Adelaide, lead author of a Biological Conservation paper describing the projection.

…

In an effort to help guide rescue efforts, Traill and colleagues, who include conservation biologists and a geneticist, have been exploring minimum viable population size over the past few years. Previously they completed a meta-analysis of hundreds of studies considering such estimates and concluded that a minimum head count of more than a few thousand individuals would be needed to achieve a viable population.

“We don’t have the time and resources to attend to finding thresholds for all threatened species, thus the need for a generalization that can be implemented across taxa to prevent extinction,” Traill says.

In their most recent research they used computer models to simulate what population numbers would be required to achieve long-term persistence for 1,198 different species. A minimum population of 500 could guard against inbreeding, they conclude. But for a shot at truly long-term, evolutionary success, 5,000 is the most parsimonious number, with some species likely to hit the sweet spot with slightly less or slightly more.

Some criticisms are mentioned in the full article, and of course it should be noted that this is stochastic modeling which has its own set of assumptions and limitations.  The paper even cites climate change as similar issue, and it’s clear that there are many problems with modeling and convincing arguments in that arena as well.

So models are not guarantees, but they are in many ways the best thing we can do in the face of uncertainty and complicated systems that do not bend themselves to simplification.

I’ll also note that the application of this not-really-magic (despite the name of the linked article) number to dog breeds would also assume that dog breeds are bred in the manner assumed by the model.  There are several factors which would limit this assumption:

– There is not random mating or even animals selecting their own partners in dogs, thus in each generation the vast majority of the offspring are produced by a very few of the dogs in the last generation.  This isn’t particularly like most wild species.

It would be interesting to see the same sort of models carried out with data that more closely matches the number of founders and population structure of dog breeds.

– Dogs are not profoundly selected by natural forces.  We don’t allow the most fecund or hearty animals to breed the most, breeders select them based on other criteria, often having nothing to do with fitness.  For example, the mere fact that Bulldogs still exist speaks against the notion of survival of the fittest, as so few of them are actually able to whelp naturally.

Even with this caveats, it’s interesting to have a number to work with to identify dog breeds that don’t meet the minimum requirements.  And the speculate if other breeds that have more than that number but are built from only a few founders will likewise fail the model.

Recall that Border Collies, who are being bred on the order of 30,000 puppies a year between the 4 US and UK registries alone, have an effective number of founder genomes of only 8 dogs.  So some clarification is in order if it’s simply a matter of 5,000 adults or if these other factors would change that number.

Still food for thought.
Further Reading:

Pragmatic population viability targets in a rapidly changing world
Not Magic But Necessary
Minimum viable population size: A meta-analysis of 30 years of published estimates
Minimum viable populations: is there a ‘magic number’ for conservation practitioners?

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