The key here is that few qualifiers are effective. Is the number of individuals that would result in the same loss of genetic diversity, inbreeding or genetic drift if they behaved like an idealized population. Great, you say? More like, “What does that mean?” […] C.J.A. 2009 We`re sorry, but 50/500 is still too little. Nature Conservation Bytes […] This means that the number of effective individuals is less than the total population. On average, the ratio is about 0.1 to 0.2; That is, an effective individual (genetically speaking) for the five to ten members of the population. So let`s do the math. Fifty effective individuals – the IUCN standard for avoiding inbreeding – correspond to a total population of 250 to 500 people. This means that in a hypothetical apocalypse, humanity would need much more than a handful of survivors to effectively repopulate.
Under the direction of the eminent conservation geneticist, Professor Richard Frankham, and including my longtime partner in scientific crime, Professor Barry Brook, we have just published a comprehensive review of the “50/500” rule that has existed since the work of Franklin and Soulé in 1980. Let`s get back to the details. As already mentioned, the so-called “50/500” rule has been in existence for more than 30 years and continues to be a general guideline for management in almost all circles of small population management. Basically, the rule states that to avoid inbreeding depression (i.e. loss of “fitness” due to genetic problems), an effective population size (Ne) of at least 50 individuals in a population is required. To avoid eroding evolutionary potential (the ability of a population to evolve to cope with environmental change), a Ne of at least 500 is required. So let`s do the math. Ne = 50 means on average Nc = 250 to 500, and Ne = 500 means Nc = 2500 – 5000. In fact, about 5000 is what our meta-analysis of the minimum viable size of the population (i.e., the census population) suggests. “Our research suggests that the 50/500 rule is at least an order of magnitude too small to effectively avoid extinction,” says Dr.
Traill. “This does not necessarily mean that populations of less than 5,000 people are condemned. But it underscores the challenge small populations face in adapting to a rapidly changing world. A long-standing idea in species recovery strategies is the so-called “50/500” rule. This states that at least 50 adults are required to avoid the harmful effects of inbreeding and 500 to avoid extinction due to the inability to develop to cope with environmental changes. An emerging rule of thumb is that if a population begins to fall below several thousand individuals, it has a high probability of disappearing. Some preliminary results of ongoing research at the Australian Centre of Excellence for Biodiversity and Heritage seem to confirm this. Using both ancient DNA techniques and paleodemographic models, we have estimates of a minimum effective population size for Australian Aborigines at their appearance, of about 250.
This means that at least several thousand people had to arrive at about the same time to successfully colonize the entire continent. But if you just look at the genetic arguments, the 50/500 rule is starting to collapse. As a basic assumption in many IUCN Red List criteria, it is extremely important to make the rule “correct”. […] You may remember Dick Frankham, Barry Brook and I recently wrote a review in Biological Conservation that challenges the status quo regarding the famous 50/500 “rule” in conservation management (effective population size [Ne] = 50 to avoid inbreeding depression in the short term, and Ne = […] […] The 50/500 Rule in Conservation Great blog post presenting the results of a new article (sub) on why the 50/500 rule in conservation is too weak. The argument is mainly based on the fact that 50 and 500 are actual population sizes and not census sizes, a fact overlooked in the application of the idea. […] The 50/500 rule has been used as a conservation guiding principle for the assessment of the minimum effective population size (N(e)). There is a lot of confusion in the recent literature about how the value of 500 should be applied to assess extinction risk and set priorities in conservation biology. Here, we argue that confusion arises when the genetic basis of a short-term N(e) of 50 is used to prevent inbreeding depression in order to justify a long-term N(e) of 500 to maintain evolutionary potential. This confusion can lead to misleading conclusions about how genetic arguments alone are sufficient to set minimum thresholds for viable populations (PVPs) to assess the risk of extinction of endangered species, particularly those that point out that MVPs would need to multiply by the thousands to maintain evolutionary potential.
[…] Recommendations for 50/500 rules, Red List criteria and population viability analyses. The one we missed. 100/1000 is the new 50/500. Multiply by 10 for the census population size at […] This means, of course, that for some species, the categories on the Red List should change – especially those classified according to criterion C. More importantly, this means that if you don`t shoot for population size in the 1000s (preferably the 1000s), you accidentally (or intentionally) cause extinction. So, sensational analogies about the apocalypse aside, do people follow the same rule? We`re not entirely sure, but the evidence suggests that most species from very different groups follow pretty much the same trend. “Negligible” is a subjective term in itself, just as the word “very” can mean different things to different people. For this reason, we have sought to standardize the “negligible” criteria for minimum viable population sizes, almost exactly what the almost universally accepted IUCN Red List, with its various (categorical) extinction risk categories, attempts to do. The Red List`s evaluation criteria are based on the “50/500 rule”. This indicates that to avoid inbreeding depression (the loss of “fitness” due to genetic problems), an effective population size of at least 50 individuals in a population is required.
The results were published online in the journal Biological Conservation. There is no clear definition of what constitutes a population sufficient for the survival of a species, as the survival of a species depends to some extent on random events. Therefore, any calculation of a minimum viable population (MVP) depends on the population projection model used.  A series of random (stochastic) projections could be used to estimate the initial size of the population needed (based on model assumptions) so that (say) there is a 95% or 99% probability of survival, say 1,000 years in the future.  Some models use generations as a unit of time rather than years to maintain consistency between taxa.  These projections (population viability analyses or PVA) use computer simulations to model populations using demographic and environmental information to project future population dynamics. The probability attributed to a PVA results after thousands of repetitions of the environmental simulation. This idea fits perfectly with the image of Noah`s animals walking “twice two” in the ark. But the science of “minimally viable populations” tells us a different story. Conservation biologists set their minimum population size targets too low to prevent extinction.
MVP does not take external intervention into account. Therefore, it is useful for conservation managers and environmentalists; A population can be increased beyond the MVP through a captive breeding program or by bringing in other members of the species from other reserves. Fiction writers have enthusiastically taken up this issue, mainly in the post-apocalypse genre. It is an idea that has a long history; The myth of Adam and Eve, of course, is based on a single breeding pair that populates the entire world, as described in Ragnarok, the last battle of the gods in Norse mythology. With species as diverse as houseflies and pink fairies, populations of more than 50 individuals still succumb to inbreeding depression. In many cases, 50 effective individuals are indeed too weak to ensure that inbreeding depression does not occur. It may be that 100 effective individuals are closer to the true minimum without even considering how populations respond to the challenges of evolution. “Often they aim to entertain dozens or hundreds of individuals when thousands are actually needed.
Our review found that populations below about 5,000 had unacceptably high extinction rates. This suggests that many conservation restoration goals are simply too small to do much good in the long run. » […] More than one million isolated small population fragments of endangered species are likely to suffer from inbreeding depression, loss of evolutionary potential and increased risks of extinction (genetic erosion).