Kōkiri Learn

Background reading · Science

Life between the tides

A rocky shore is sorted into bands by how long each part stays dry, and a missing predator can change a whole reef.

Twice a day the tide goes out and the rocky shore becomes one of the toughest places to live on Earth. Creatures that were underwater are suddenly baked by the sun, dried by the wind, or soaked by rain. Then the sea comes back and pounds them with waves.

Life in bands

Walk from the top of a rocky shore to the water and you pass through zones, or bands:

  • High zone: out of the water most of the time. Only tough animals such as tiny periwinkles and barnacles survive, sealed tight in their shells.
  • Middle zone: covered and uncovered every tide. Mussels, limpets, pūpū (cat's eye snails), chitons and seaweeds crowd together.
  • Low zone: only uncovered at the lowest tides. Kelp, sea anemones, starfish and kina live here.

    • The bands happen because each creature has features that let it cope with a certain amount of drying and heat. Grazers such as pūpū and limpets scrape algae off the rocks. Filter feeders such as mussels and barnacles strain food from the water. Predators such as whelks drill through shells.

    The case of the missing snapper

    Kina (sea urchins) graze seaweed. In north-eastern Aotearoa, big snapper and kōura (crayfish) eat kina and keep their numbers down. Where people have caught most of the big snapper and crayfish, kina have multiplied and eaten the kelp forests down to bare rock, called kina barrens. In the Leigh (Goat Island) marine reserve, set up in 1975 as NZ's first, fishing stopped, the predators returned, kina numbers fell, and the kelp grew back. Scientists compared the reserve with fished coast nearby to test the idea.

    What this means for you

    Removing one link in a food web can change everything below it. When you survey a shore, look for what is missing as well as what is there, and treat every rock and creature with care: put rocks back the right way up, and take nothing.

    Sources and further reading

    Written for Kōkiri Learn students in our own words. Check facts against the sources.

    Used in: Ngā Tohu o te Taiao: Signs in Our Environment