Small is fragmented, breeding individuals may not

Small populations have unique concerns when trying to survive.

Do you know what they are? This lesson explains them as well as key concepts like the extinction vortex and effective population.

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Small Population Approach

Let’s pretend you’re a biologist tasked with saving a very small population of a given species. And I don’t mean that it’s just locally small.

No, it’s the last surviving population of this species in the wild in the entire world!You’ve got a tough job ahead of you. Before you can save this species, you must figure out what critical factors play a role in saving or driving a small population into extinction.So, what factors do you need to consider in each case? Let’s find out together.

Genetic Variation

Let’s just say that you are out in the field studying a small group of little birds called grouse. There aren’t a lot of these grouse left; in fact, you can quite certainly say there are about 100 of them left in the wild, and they’re only found in this particular area.What problems does this very small population of grouse face? Meaning, what factors may contribute to its extinction because the population is so small?Well, firstly, we can agree that because the number of individuals in the population is small, there isn’t a lot of genetic variation going on.

This means the small population is susceptible to inbreeding. Inbreeding increases the chances that the offspring of this population will have harmful recessive traits that will put that individual and the entire population in peril. Perhaps the offspring will be able to consume food but not reproduce as a result of inbreeding, thus lowering everyone’s chances of survival.Can you think of another problem in this small population? How about the fact that they can be easily overharvested by humans or predated upon to extinction by natural predators? It’s one thing if 10 individuals die in a population of 10,000. It’s another thing when 10% are killed off in a small space of time, especially if they’re breeding individuals.

What’s another factor that threatens a small population? They’re particularly vulnerable to habitat loss, as well. They may already be in a small habitat, or if their habitat is fragmented, breeding individuals may not be able to reach one another.

Extinction Vortex

Overall, small populations are susceptible to something called the extinction vortex, a downward trend in population where things like inbreeding, as well as genetic drift, cause a small population to decrease in number, all the way into extinction, unless this process is stopped and reversed.Genetic drift is where the frequency of a gene variant (allele) in a population changes due to random events. Again, this, alongside inbreeding, can lead to the extinction vortex because both can lead to a loss of genetic variation.

As an example, if a population doesn’t have enough genetic variation to resist something like a new strain of a disease, then the entire population can be wiped away into oblivion.Think about it: you and a group of your friends can all tolerate heat a bit differently. Some love the heat and can adapt to it readily and others wilt at anything above 70F. You are all different and can adapt to changes differently. This was a very simplistic example, but you can understand that the larger the group of friends you have, the more likely someone in that group can tolerate extreme temperatures than if you had a very small group of friends.

In our grouse example, the smaller the population, the less likely any individual in the population can tolerate something extreme, like a new strain of pathogen or disease-causing agent.

Minimum Viable Population

Now that you understand the factors that put a small population at risk, your next task as a biologist is to figure out how small the population of grouse needs to be before it starts on its way into the extinction vortex.This is actually not that simple! You need to figure out the minimum viable population (MVP) first, the smallest number of individuals in a population that can maintain the population.Such a number is calculated using computer models.

And many factors will go into determining the MVP. Which factors do you think would be important for this?A. The number of individuals in the population that are capable of breedingB. The number of individuals who will be killed naturallyC. The total number of individuals in the populationThe answers are A and B.

The total size of the population might actually be misleading. One hundred grouse is one thing, but if only two members of the population can breed, then the effective population size, the size of the population based on the amount of individuals that breed successfully, is far smaller than the total population size!Why is the effective population size so important? Remember, it all goes back to what I’ve been talking about ad nauseam: genetic variation. Genetic variation is the most important issue in the small population approach, and genetic variation depends on successful mating.

This is why the minimum viable population is best estimated using the effective population size. Other factors, like an estimate of how the population will be affected by natural events such as storms, droughts, and so on, can also help estimate the MVP in a computer simulation.

Lesson Summary

Let’s summarize the most important factors to consider when considering the small population approach in population conservation.A small population is highly susceptible to extinction as a result of a loss of genetic variation. Of course, other factors, like overharvesting, habitat loss, and even bad weather, can play their role.A small population may enter an extinction vortex, a downward trend in population where things like inbreeding, as well as genetic drift, cause a small population to decrease in number, all the way into extinction, unless this process is stopped and reversed.If we can figure out the minimum viable population (MVP), the smallest number of individuals in a population that can maintain the population, using the effective population size, the size of the population based on the amount of individuals that breed successfully, then we can figure out what the smallest number of individuals we need to have in a population is in order to avoid the extinction vortex.

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