When only a few family lines dominate overall genetic diversity in small, dying populations, these populations become highly inbred, making them vulnerable to disasters such as epidemics.
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When a family line produces no surviving descendants, its genetic diversity is lost, and when the population is very small, this loss greatly decreases the genetic diversity of the population. When some families produce many offspring that live to reproduce while others produce none at all, it’s called breeding bias, and it’s a serious conservation problem for rare species in programs. captive breeding and reintroduction, such as the black rhinoceros, Diceros bicornisand the Galapagos Giant Tortoise from Floreana Island, Chelonoidis niger (ref & ref). Breeding bias intensifies inbreeding (ref) in an already small population, thereby decreasing the likelihood that the entire population can survive in the long term (ref).
In some small wildlife populations, such as the cheetah, Acinonyx jubatus, most individuals alive today may be descended from just a few individuals (ref) – or in extreme cases, such as that of the Chatham Island blackbird, Petroica crossed, a single pair (ref). Thus, even when the total population may be large, all of its individuals could be close relatives, so the overall genetic diversity could be very low (ref). For this reason, it is important to monitor lineage loss in populations of conserved wild animals, despite the fact that collecting the genetic data needed to detect and measure it accurately can be quite difficult. For this reason, this reproduction bias is little studied, although we know that its impacts are not good.
the critically endangered orange-bellied parrots in Australia, Neophema chrysogaster, are a tiny and dwindling population of migrating parrots. Although these tiny parrots can live up to 11 years, few live long enough to breed more than once and furthermore, juveniles have an extremely high mortality rate (ref). But are these juvenile deaths random? For example, because siblings have a similar start in life, perhaps if a chick dies before reaching reproductive age, all of its siblings are much more likely to die young too?
Conservation biologist Dejan Stojanovic, a postdoctoral researcher at the Australian National University and senior postdoctoral fellow at the Difficult Birds Research Group, wondered what happens to small populations when high juvenile mortality is not random among brothers and sisters. He and his collaborators realized that orange-bellied parrots could help provide answers. They analyzed 22 years (from 1995 to 2017) of field data collected by staff and volunteers from the wild population of orange-bellied parrots, to:
- quantify the frequency with which individuals fail to produce live offspring due to non-random juvenile mortality of sibling parrots
- identify living maternal lines and quantify the number of surviving offspring produced by each parrot mother, and
- assess the demographic impacts of non-random juvenile mortality of sibling parrots using population viability analysis (PVA), an important statistical tool for modeling population size, genetic diversity, and population risk of extinction over time
First, Dr. Stojanovic and his collaborators documented that non-random juvenile mortality of sibling parrots occurred more frequently than expected by chance. They found that the ratio of observed (black bars, Figure 2) to expected (gray bars, Figure 2) non-random child mortality was 1.37, meaning that if a chick died in its first year of life, all of his siblings were also 1.37 times more likely to die in their first year of life.
In a large and thriving population, the loss of a family line is what the process of natural selection looks like and therefore serves to ensure that only the healthiest lines pass on their genes to future generations. But because wild female orange-bellied parrots usually only have one opportunity to breed in their lifetime, a failed breeding attempt means the individual produces no living offspring.
“I was shocked that in just three years, 9/10 of the remaining wild family lines died out, leaving a single mother representing the entire evolutionary history of this species in the wild,” the Dr. Stojanovic via email.
In tiny inbred populations, the loss of a family line can be a big problem because family lines form the genetic substructure of populations. Additionally, in tiny populations, such as orange-bellied parrots, some families may possess significant genetic diversity that, if lost, decreases the overall genetic diversity of the entire species.
This genetic diversity cannot be replaced once it is lost.
Dr. Stojanovic and his collaborators used PVA analyzes to determine potential outcomes for two mortality scenarios: random child mortality and non-random child mortality among parrot siblings. They created two scenarios, Pair One (top panel, Figure 3) is the “optimistic model” based on 1995 child mortality rates (49%). He showed that, if conservation efforts are successful and child mortality could be reduced to pre-1995 levels, then the population could possibly survive non-random child mortality among parrot siblings despite declining diversity. resulting genetics (pink line).
On the other hand, pair two (lower panel, Figure 3) was based on the current high juvenile mortality rates (80%) documented in orange-bellied parrots. Simulation of current non-random high juvenile mortality among parrot siblings resulted in a decrease in genetic diversity, rapidly followed by extinction within 20 years (pink ribbon). However, even when testing for a high (blue tape) random juvenile mortality of 80%, most simulations still showed rapid erosion of genetic diversity, with the population likely dying out in 20 years.
PVA analyzes showed that regardless of mortality rates, non-random mortality in sibling parrots always resulted in a decrease in the genetic diversity of the population.
“These parrots show us the value of collecting detailed monitoring data, but more importantly, they demonstrate the need to regularly assess demographic processes in small populations,” Dr. Stojanovic told me via email.
This grim news is a powerful warning that underscores the need for conservation biologists to be vigilant against unseen threats, such as the loss of family lineages within tiny populations, and to gather and analyze relevant data to that these scenarios can be addressed early to avoid irreversible events. genetic damage to populations.
“It showed me that conservation practitioners need to be constantly alert to these types of threats – which are surprisingly difficult to detect,” Dr Stojanovic said in an email.
“If we had this information 20 years ago, it’s likely OBP would be in a much better place because we might have been able to preserve some of the genetic diversity that has now been permanently lost.”
Dejan Stojanovic, Teresa Neeman, Robert Lacy, Katherine A. Farquharson, Carolyn J. Hogg and Robert Heinsohn (2022). Effects of non-random child mortality on small inbred populations, Biological conservation 268:109504 | doi:10.1016/j.biocon.2022.109504
NOTE: This study was funded, in part, by a crowdfunding campaign (“Operation OBP”) to which I contributed.
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