Hope is a Thing With Feathers: Genetic Diversity and a Little Bird that Recovered From the Brink of

Extinction is not a goal that any species strives towards. The ultimate goal of a species is to survive and one of the ways to ensure that is through genetic diversity. Genetic diversity is the variation in genes within a species. It is incredibly important for the survival of a species because it allows them to adapt to environmental changes or diseases, something more pronounced now in the face of climate change causing changes in the environment faster than ever seen in history. Low genetic diversity is linked to members of a species being closely related which can lead to inbreeding depression: the reduced state of fitness that can potentially result from inbreeding. One of the concerns with endangered species is not just their low populations but also the low genetic diversity that comes with it, affecting the fitness of future generations and their adaptability to survive. Typically, the mortality rates for the offspring of related parents are 30-40% higher than those of unrelated parents, and any surviving offspring have lower fitness in general, affecting future generations’ ability to survive.

Despite this statistic, there is an example of a species recovering despite all of them being essentially related: the Black Robin. The Black Robin (or the Chatham Island Robin) is found in New Zealand and in 1980 was critically endangered with a population of just 5 individuals, only one of which was a fertile female. This was due to them being vulnerable to introduced species such as cats and rats, making them extinct in most of the Chatam Islands. Now their population is over 300, thanks to conservation efforts by a New Zealand conservationist by the name of Don Merton. Miraculously, despite the entire population being genetically as closely related as siblings, the only issue that has come out was the perpetuation of the maladaptive gene where females would lay their eggs on the rims of their nests, leading to the deaths of their unborn children when the eggs would fall off. Because Merton’s team would push those eggs into the middle of their nests, it allowed individuals with this gene to survive and pass it on to their offspring. The gene spreading to 50% of the population at one point. After humans stopped intervening, however, this behaviour became less frequent and now only occurs in 9% of the population, a high proportion nevertheless but a number that will most likely continually decrease over time without human intervention.

(Mangere Island (left) and Little Mangere Island (right). The 5 remaining black robins were found in Little Mangere Island back in 1980)

Despite the entire population being so closely related, the only inbreeding problem displayed was the one caused by humans. This begs the question, what is the minimum number of individuals needed to prevent inbreeding depression? Are just 5 individuals enough like in the case of black robins or does this vary from species to species? A rule was developed known as the “50/500” rule, where 50 individuals are required to prevent the population from inbreeding and 500 individuals required to reduce genetic drift (the random change in the frequency of a gene within a population which may lead to decreased genetic variation). However, this is an inexact estimate and different species will have different minimum viable populations (MVPs). Generally, that number can be below 10 for small island species that rapidly reproduce, to over 100 for longer-lived species such as elephants or trees.

In the case of the black robins, it is speculated that the reason no inbreeding depression occurred is because this species may have encountered several of those population bottlenecks in their history and have lost the alleles that can cause debilitating effects, allowing their population to recover from just one female over 50 years ago. Specific MVPs can be calculated through the help of ecological modelling where different factors and characteristics of a species is considered to create an estimate. Because a lot of data is required to calculate this, the general 50/500 rule is often used instead.

MVPs can be useful for conservation. Often times it cannot be applied at all because some populations may already be below the MVP. On the other hand, MVPs can be used to enforce and create policies for the conservation of certain species. An example of this is in the conservation of the northern spotted owl, where conservationists managed to force the U.S. Forest Service to maintain sufficient old growth of forest to support an MVP of northern spotted owls. However, working at this minimum is also risky, because other unaccounted factors could also cause populations to decline. As in the case of the northern spotted owl, populations continued to decrease despite the new policy due to competition from barred owls. While MVPs provide the urgency required for governments to conserve the minimum, it is only the bare minimum.

The story of the black robin gives hope for the recovery of organisms with dwindling populations. But the black robin’s population only recovered thanks to efforts by conservationists who took the time and effort to care about a small island bird that essentially had no significance prior to their “miraculous” recovery, the same conservationists who also inadvertently harmed their populations by trying to help. Conservation is a necessity and we must protect all forms of biodiversity including those we cannot see like genetic diversity.