You are doing biology field work studying two species of lizards on a small volcanic island. Although quite hostile to each other when they accidentally meet, the two species usually remain in separate parts of the island. Species A seems to live and hunt insects only in the lower grassland part of the island. Species B seems to live and hunt only in the upper rocky areas. Species A has a short tongue but has longer legs and is a faster runner. Species B is short, squat, runs slower, but has a long tongue. You observe Species A chasing down some pretty large, fast-moving insects, while Species B meanders from rock to rock, using its long tongue to get insects hiding between rocks and in cracks. What principle of ecology do your two lizard species demonstrate?

Barnacles are filter-feeding molluscs that live by filtering plankton from seawater. This is a description of the barnacles':

In a typical ecosystem, a species whose removal would have a bigger impact than any other would be a/an: