How can a very small population size increase a species’ risk of extinction?

A very small population size significantly increases a species' risk of extinction by reducing its ability to adapt to environmental changes and making it more vulnerable to random events. This heightened risk is primarily driven by the following factors:12

• Loss of Genetic Diversity: Small populations are prone to genetic drift, a process where random chance causes the loss of alleles from the gene pool. Without sufficient genetic variation, a species may lack the necessary traits to survive threats such as diseases, predators, or shifts in the physical environment.312
• Inbreeding: Limited mating opportunities in small groups often lead to inbreeding, which can increase the transmission of harmful traits and reduce overall ecological fitness.2
• Stochasticity: Small populations are highly susceptible to both environmental and demographic stochasticity. Random events—such as severe weather, fires, or fluctuations in birth and death rates—can disproportionately impact a small group, potentially driving it to extinction more easily than a larger, more resilient population.2

Conservation biologists use tools like population viability analysis to estimate the minimum number of individuals required to maintain long-term survival and mitigate these risks.2