So how much inbreeding is too much?
Now that's the million dollar question. If inbreeding was only good, animal breeders would be merrily inbreeding and the animals would be thriving. But most animal (and plant) breeders go to some pains to manage inbreeding, and for reasons like the ones above. So how do you evaluate the costs against the benefits?
The rule of thumb for sustainable breeding of both wild and domestic animals is to keep inbreeding below 5%, and to consider 10% an upper limit for a population. There might be occasional animals that are much higher than this (for example, in maintaining multiple herds to use for outcrossing within a line), but when the population average goes higher than about 10% things begin to go downhill. As fertility goes down and litters get smaller, there are fewer animals born each generation, and those that are have an increased risk of genetic disorders, shorter lifespans, and general lack of vigor. This becomes a negative feedback loop that is called "the extinction vortex", and once a population heads down this path it can be very difficult to stop. Note again, that threshold for this vortex phenomenon is about COI = 10%. For most livestock breeds being bred sustainably, inbreeding is kept well below this. Organization breeding guide and service dogs keep inbreeding below 10%. But among purebred dogs otherwise, a COI (calculated back to founders) could be very difficult to find. And evidence of inbreeding depression and the other consequences of inbreeding are very evident - singleton litters, high rates of cancer in young dogs, allergies, and many other issues that seem to be "normal" in the breeding of dogs these days.
9/19/2014 \\ Inbreeding
3/6/2017 \\ Inbreeding
Most populations of wild animals don't survive with inbreeding levels above 25-30%. They are overtaken by small litter size, high puppy mortality, genetic disease, and physical defects. Modern veterinary care and our willingness to pay for it is sustaining many breeds with inbreeding levels even higher than this, but how much longer can they last?
If these breeds are going to survive, they need much larger population sizes, and the already breathtaking levels of inbreeding must be reduced to improve health. Otherwise, they will eventually go extinct.
Will this happen? Time will tell.