Why can biodiversity decline even when no species has become extinct?

Biodiversity is a broad term that encompasses not only the number of species in an area but also genetic variability, the number of individual organisms, and the complexity of biological communities. Consequently, biodiversity can decline significantly even when no species has gone extinct.1

This decline occurs through several key processes:

• Population Declines: A sharp reduction in the number of individuals within a species can disrupt social structures, making it difficult for survivors to find mates and leading to further population loss.1
• Loss of Genetic Diversity: As populations shrink, they often lose genetic variation through genetic drift and increased inbreeding. This reduces the species' ability to adapt to environmental changes, diseases, or competitors.123
• Diminished Ecological Roles: Even if a species remains present, its niche—the specific role it plays in an ecosystem—diminishes as its numbers fall. If a species is critical for functions like nutrient transport or habitat regulation, its decline can destabilize the entire ecosystem.12
• Ecosystem Homogenization: As specialist species decline, they are often replaced by generalist species or those favored by humans. This reduces the complexity and distinctiveness of ecosystems, making them more uniform across regions.2