The Spruce tree drops seeds every fall with its leaves, 80% of the seeds survive the winter. For a seed that survives its first winter there is a 40% chance it sprouts and a 40% chance it remains dormant and a 20% chance it dies each winter. About 60% of the sprouts survive to become mature trees capable of producing their own seeds. This sprout to mature tree process takes 4 years. What is the critical number of seeds dropped per tree to ensure the survival of the spruce tree species?