An ecologist investigates how shell phenotype affects survival against song thrush predation by placing artificial snail shells on a deciduous forest floor.
She places 450 dark-banded shells, which match the dark leaf litter, and 350 yellow-unbanded shells, which are highly conspicuous.
After 48 hours, she checks the shells for signs of thrush predation (such as being cracked or moved to anvil stones).
The bar graph shows her results:
Calculate the difference between the number of predated yellow-unbanded shells and the number of predated dark-banded shells.
