A year after a bumper crop of the spike-tipped orbs littered streets and parks throughout the region, acorns seem to have vanished this autumn, a little-understood phenomenon that scientists say will ripple throughout the ecosystem and devastate the population of everything from chipmunks to owls. The cycles of acorn production are erratic. Some years there’s a bounty, and others there’s a smattering, but rarely do oaks produce next to none.