The “Evil Quartet” of biodiversity loss is a term used to summarise the four primary drivers that together cause the most severe declines in species and ecosystems worldwide.

1. Habitat loss and fragmentation
- When natural habitats are converted for agriculture, urban development, or infrastructure, the area available for species shrinks.
- Remaining patches become isolated, limiting movement, gene flow, and access to resources, which raises extinction risk.

2. Over‑exploitation
- Direct removal of organisms through hunting, fishing, logging, or trade reduces population sizes.
- Unsustainable harvest rates exceed the ability of populations to recover, leading to rapid declines.

3. Alien species invasions
- Non‑native organisms introduced by human activities can outcompete, prey on, or transmit diseases to native species.
- Invasive species often lack natural predators in the new environment, allowing them to dominate ecosystems.

4. Co‑extinctions
- Species are interconnected through ecological relationships (e.g., pollinators, parasites, hosts).
- The loss of one species can trigger the extinction of dependent species, creating cascading effects.

These four factors—habitat loss and fragmentation, over‑exploitation, alien species invasions, and co‑extinctions—collectively form the “Evil Quartet” that drives the current worldwide biodiversity crisis.