Legend
Bat guano (imported energy)
Fungi & bacteria (decomposers)
Cave crickets / isopods
Blind cave fish & spiders
There is zero light this deep — no photosynthesis is possible here at all. Every calorie in this cave arrived on a bat's wings from the surface. Cut the guano and the whole web starves from the bottom up, with no lighting involved anywhere in the collapse.
Why this works
Bats roost in the cave by day and fly out at night to feed on the surface, then return to deposit
nutrient-rich guano. That guano — not sunlight — is the base resource: fungi and bacteria decompose it,
small invertebrates like cave crickets and isopods graze the decomposers, and cave-specialist predators
such as blind fish and cave spiders sit at the top. Stop the bat colony's guano input and the base of
this web collapses first, then invertebrates starve as their food source vanishes, then predators
starve last — entirely independent of light, because none of this ever depended on light.
Legend
Surface population (reference, unchanged)
Cave-isolated population (evolving)
Same species, two populations. The surface group keeps full eyes and pigment because both are useful up there. The cave group is permanently isolated in total darkness, where eyes and pigment cost energy but return nothing — so each generation, individuals that invest less in them edge ahead.
Why this works
In complete darkness, eyes and skin pigment provide zero survival benefit — there's nothing to see and
no UV to guard against — but both still cost real metabolic energy to grow and maintain. Selection
doesn't actively pick "blindness"; it simply favors whichever individuals free up that energy for
finding scarce food or for reproduction. Generation after generation, the cave-isolated population
drifts toward smaller eyes and paler skin, while its surface-dwelling relatives — where eyes and
pigment still pay for themselves — stay exactly as they were.