30/05/2026
One third of all species. The dark is the habitat. The dark is disappearing.
Artificial Light at Night Now Affects an Estimated One Third of All Animal Species. Sea Turtle Hatchlings Navigate Away From the Ocean. Migratory Birds Collide With Buildings. Coral Spawning Is Suppressed. The Dark Is Disappearing.
Most animals evolved in a world where darkness was guaranteed. We have removed that guarantee across a third of all species.
Artificial light at night (ALAN) has grown approximately 2% per year globally since the 1990s, and is projected to continue growing. Its biological impacts extend far beyond the firefly decline already documented (post 301). A 2023 global synthesis paper in Nature estimated that ALAN meaningfully affects the behaviour, physiology, or ecology of over 30% of all animal species.
THREE CASE STUDIES — beyond fireflies:
SEA TURTLE HATCHLINGS: Newly hatched sea turtles navigate toward the sea using the brightness differential between the dark silhouette of land vegetation and the lighter open horizon over the ocean (which reflects starlight and moonlight). Artificial coastal lighting reverses this: hatchlings navigate toward lights, moving inland. Documented disorientation and mortality from ALAN affects all seven sea turtle species, all of which are threatened or endangered.
MIGRATORY BIRDS: Glass and building lights kill an estimated 600 million to 1 billion birds per year in the US alone — birds that navigate by stars at night, are attracted to lit windows during migration, and collide with glass they cannot perceive. The problem is highest during autumn and spring migration. Chicago, Houston, and New York have the highest documented bird strike mortality. Simple interventions (turning off lights during peak migration nights) reduce mortality by >80%.
CORAL SPAWNING: Many coral species spawn synchronously in response to moonlight cues. Artificial light — even at very low intensities — suppresses spawning synchrony. In areas with coastal ALAN: spawning success reduced, genetic mixing reduced, reef recovery from bleaching events impaired.
One third of all species. The dark is the habitat. The dark is disappearing.
When the simple act of turning off lights during bird migration nights reduces bird collisions by over 80% — and most cities haven't done it — what is the calculation that leads to that outcome?
