Islands and Alleles: How genetics can help protect endangered species

Reference: du Plessis, S.J. et al. (2006) Genetic diversity and cryptic population re-establishment: management implications for the Bojer’s skink (Gongylomorphus bojerii). Conserv Genet 20: 137.

Why Islands and Why Genetics?

There have been 5 mass extinctions throughout earth’s history, and humans are currently causing the sixth. A recent report predicted that over 1 million plant and animal species face extinction in the next few decades due to habitat destruction, invasive species, and climate change.

Island life is especially vulnerable to extinction. Even though islands make up 5% of the earth’s land area, over 60% of recorded extinctions have taken place on them since the 16th century. Many species live only on one or a few islands. If something goes wrong in such a limited area, the entire species could be lost.

Island species have evolved traits to survive in a very specific environment. When an invader enters, it’s like an ambush attack- native species often lose because they haven’t had time to evolve a defense strategy.

Isolated island populations often have low genetic diversity. A population of plants or animals can contain many versions (alleles) of genes. For example, all humans have genes that control for eye color, but there are many alleles that result in different versions of the trait: blue, green, hazel, brown. The number of versions of a gene that exist in a population determines its genetic diversity.

Genetic diversity can be a measure of the stability and health of species. Imagine a field of flowers in a drought-stricken area. All of those flowers will have a gene (or group of genes) that determines how much water they need to survive. Many of the flowers will die of thirst, but if there is high genetic diversity, the chances are better that some will have a version of the gene that allows them to survive with less water. These super-flowers could enable the population to survive.

Managers can maintain genetic diversity by moving a few individuals of a species from one location where the population is relatively strong to another location where that species has a smaller, less diverse population. This strategy is used by Mauritius Island to manage the critically endangered Bojer’s skink.

The Skink Study

Plessis et. al. studied the genetic diversity and structure of the Bojer’s skink in Mauritius.

The Bojer’s skink is Critically Endangered– facing an extremely high risk of extinction according to the International Union for the Conservation of Nature (IUCN). You can only find them on a few small offshore islands of Mauritius. They historically lived on the country’s mainland, but were wiped out by small invasive mammals called musk shrews that were introduced by humans.

A musk shrew- cute but deadly (to skinks)! (“Musk Shrew” by GubbiHakki is licensed by CC BY-NC-ND 2.0.)

The musk shrews took quite a toll on the skinks. Today, managers actively look out for skink populations to make sure they remain strong and healthy.

Bojer’s skinks live in the areas marked with a red box on the map. The north and southeast groups don’t mix. Each has a unique genetic makeup that managers work to preserve.

All southeastern skinks come from one island: Ilot Vacoas (IV). In the past, they spread from IV to Ile de la Passe (IDLP) and Ile aux Fouquets (IAF), then went extinct in the 1900s when they were wiped out by the shrews.

In the 2000s, managers reintroduced skinks to IAF by bringing a few from IV. They did not bring any to IDLP, so in 2009 they were surprised to find that the skinks had somehow bounced back there as well! They likely snuck in as stowaways with travelling tourists. Managers responded by bringing more skinks over from IV to strengthen the surprise population. Before this study, we didn’t yet know the effectiveness of this action.

The goal

Scientists captured 254 skinks from 5 islands, removed tiny pieces of their tails, and then released them. DNA was extracted from the tail pieces and analyzed.

The scientists aimed to use genetic analyses to

  • confirm that the northern and southeastern populations are indeed genetically distinct
  • find out where the surprise IDLP skinks came from
  • see if the individuals that showed up on IDLP in 2009 were mixing and mating with the skinks that we then introduced from IV, and
  • assess the current genetic diversity on IDLP.
What they found out

Analyses showed that the IDLP skinks were similar enough to the other southeast islands that they probably came from nearby, instead of up north. The scientists also confirmed previous assumptions that the northern and southeastern islands are genetically distinct and should be thought of as two separate groups to conserve.

Analyses also showed that the skinks discovered on IDLP in 2009 did mate with the introduced skinks, increasing genetic diversity to a degree. However, genetic diversity is still not high enough to ensure long term survival of IDLP’s population.This lingering low diversity is likely due to the founder effect. This phenomenon explains that when just a few individuals colonize a new habitat, normally rare traits may become common in the newly founded population.

A large population gives rise to varying founder populations. (This image by Qz10 is marked by Public Domain Mark 1.0.)
What should we do next?

Relocation efforts have helped, but the study’s findings show that the work is not yet finished. We need to continue relocating skinks from IV to IDLF. This will be a balancing act. We need to make sure we don’t take so many skinks from IV that its population loses genetic diversity. According to these genetic analyses, we are doing well with this so far- IV still had a lot of genetic variation. While maintaining diversity on IV, we need to also make sure we introduce enough genetic variation to IDLP to make the effort worthwhile. The study suggested that we could do a better job with this.

Why does this study matter?

We gained a better understanding of the genetic structure of skink populations, showing us how we are doing with conservation and how we can do better. We should continue to use studies like this one to monitor and evaluate our efforts. This study provides a useful framework that we can use to study the invisible diversity of other endangered species.

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Sarah Shainker

Sarah is a Phd student at the University of Alabama in Birmingham interested in evolutionary ecology, population genetics, citizen science, and macroalgae. Before beginning grad school, she worked as an outdoor educator in the north Georgia mountains and as a coastal resource management volunteer for Peace Corps Philippines. Twitter: @SarahShainker

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