Why are species with very small populations especially vulnerable to extinction?

Species with very small populations are especially vulnerable to extinction because they lack the resilience to withstand environmental changes, diseases, and random fluctuations in their surroundings. Their vulnerability is driven by several key factors:12

• Loss of Genetic Diversity: Small populations are highly susceptible to genetic drift, a random process where genetic traits are lost by chance. Without sufficient genetic variation, a species cannot adapt to new threats or changing environments.312
• Inbreeding: Limited mating opportunities in small groups often lead to inbreeding, which can increase the transmission of harmful traits and decrease overall ecological fitness.2
• Stochastic Events: Small populations are more vulnerable to both environmental and demographic stochasticity. Random events—such as severe weather, fires, or fluctuations in birth and death rates—can easily push a small group toward total collapse.2
• Lack of Rescue: Species with low dispersal rates are at a significant disadvantage because they cannot be easily "rescued" by individuals migrating from other, larger populations.4

Conservation biologists use population viability analysis to estimate the minimum viable population required to ensure a species' long-term survival.2