Question

Why are small populations more susceptible to extinction from demographic stochasticity than are larger populations?

Short answer

Answer: Smaller populations are more susceptible to extinction due to demographic stochasticity because random fluctuations in their population size have a greater proportional impact than in larger populations, making them more vulnerable to significant declines. Additionally, smaller populations are more likely to experience reduced genetic diversity due to random genetic drift, which can compromise their ability to adapt to changing environmental conditions and further increase their risk of extinction. On the contrary, larger populations can better buffer against such random fluctuations and are less likely to go extinct due to demographic stochasticity.

Step-by-step solution

Understanding Demographic Stochasticity

Demographic stochasticity refers to the random fluctuations in population size due to birth and death rates as well as immigration and emigration. These fluctuations can cause a population to grow or shrink over time, and in some cases even lead to extinction. The effects of demographic stochasticity are more pronounced in smaller populations.

Effects of Demographic Stochasticity on Small Populations

In small populations, demographic stochasticity can have a significant impact because even a small change in the number of individuals can represent a large proportion of the entire population. For example, if a population of 10 individuals loses two members, it represents a 20% decline. These fluctuations can result in a higher likelihood of extinction. Furthermore, a small population is more likely to lose genetic diversity due to random genetic drift, which can reduce the population's ability to adapt to changing environmental conditions and increase the risk of extinction.

Effects of Demographic Stochasticity on Large Populations

In larger populations, demographic stochasticity is usually less pronounced because a small change in the number of individuals has a smaller proportional impact on the overall population size. For example, if a population of 1,000 individuals loses two members, it represents only a 0.2% decline. This means that larger populations are better able to buffer against random fluctuations in birth and death rates and are less likely to go extinct due to demographic stochasticity.

Conclusion

In summary, small populations are more susceptible to extinction from demographic stochasticity because random fluctuations in their population size have a greater proportional impact than in larger populations. Additionally, smaller populations are more likely to experience reduced genetic diversity due to random genetic drift, which can further increase their risk of extinction.

Key concepts

Population Size

Population size refers to the number of individuals within a population. It plays a crucial role in determining the vulnerability of a species to external factors such as demographic stochasticity.
Small populations are more dramatically affected by changes in population size due to the greater proportional impact of each individual's birth or death.
This means that even a minor loss or gain can significantly influence the population dynamics, making them more vulnerable to potential extinction.

• A population of 10 individuals represents a 10% contribution to its total size for each individual.
• In contrast, a large population such as one with 1,000 individuals sees only a 0.1% impact per individual change.

Larger populations generally have a greater buffer against these fluctuations, providing them a better chance to maintain equilibrium and survival.

Genetic Drift

Genetic drift is the process where allele frequencies within a population change due to random sampling effects. Over time, these random changes can lead to significant differences in the genetic composition of the population.
This phenomenon can have more severe implications for small populations due to the higher impact of random changes in genetic makeup.

• In small populations, each individual's genetic contribution is magnified, leading to faster genetic drift.
• This can result in reduced genetic diversity, which is critical for a population's ability to adapt to environmental changes.

It is this decreased ability to respond to evolutionary pressures that can make small populations more susceptible to extinction compared to larger populations which experience more genetic stability.

Extinction Risk

Extinction risk is the probability of a species or population becoming extinct within a specific period. The risk of extinction is notably higher for small populations due to several interacting factors.

• The amplified effects of demographic stochasticity mean fluctuations can lead to either explosive growth or disastrous decline.
• With fewer individuals, there is less chance for favorable mutations to arise and spread through the population.

Also, as genetic drift acts more powerfully, the loss of genetic diversity can make it harder for populations to adjust to new threats or changes in the environment, increasing their susceptibility to extinction.

Small Populations

Small populations are groups of relatively few individuals of a species. They present unique challenges in maintaining sustainability and avoiding extinction.

• Due to their size, they experience more intense effects from demographic stochasticity, resulting in more extreme swings in population size.
• There are fewer genetic variations to contribute to offspring, increasing susceptibility to diseases and changing environments.
• The risk of inbreeding also rises, potentially leading to a decrease in fitness and adaptability over generations.

These challenges underscore the importance of conservation efforts targeted at increasing genetic diversity and population numbers to ensure long-term survival.

Environmental Adaptation

Environmental adaptation refers to a population's ability to adjust to changes in its surroundings. Genetic diversity within a population supports this adaptability, allowing species to thrive even as conditions change.
However, small populations often lack the genetic variation needed for effective adaptation.

• Lacking diversity can inhibit a population's capacity to evolve quickly in response to new predators, climate shifts, or disease outbreaks.
• When genetic drift reduces the presence of advantageous genes, it limits options for the population to adapt.

Ultimately, the ability to adapt is crucial for survival, and without adequate variation, small populations face a significantly heightened risk of extinction in changing environments.