Activity 2 – Intertidal Zonation
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Materials & Setup
• “Hāmākua Poko” acetate (master, p. 13)
• “Intertidal Subzones Table” acetate (master, p. 14)
• “Intertidal Images” acetates (master, pp. 15-16)
• One copy of “Subzone Conditions Cards” (master, pp. 17-21)
• One set of “On the Edge Species Cards” (master, pp. 22-37), divided into five subzone categories using the “Subzones Species Key” (p. 38)
• Overhead projector and screen
1) Show the photo of Hāmākua Poko (near Mama’s Fish House on Maui, between Kūʻau and Hoʻokipa) on the overhead. The photograph generally illustrates the concept of zonation. Ask students to look at the photo and see if they can figure out what “zonation” means. This is the lead-in question to a brief class overview of the intertidal zone. (NOTE: You can find more background for leading this discussion in Teacher Background “Intertidal Conditions,” pp. 9-12.)
2) Lead the discussion to this definition of zonation: the distribution of plants and animals according to environmental conditions. The pattern of vertical banding that is shown in the photograph is similar to that seen along rocky shorelines where there are bands of “microhabitats” or “subzones” within a relatively narrow shoreline area.
3) The part of the shore that is underwater at high tide and exposed when the tide is low is called the “intertidal zone.” What factors do students think would influence the width of this zone? (Use “Intertidal Images” acetates to show variations in slope and tidal range.)
4) Ask students what the physical conditions would be like for species in the intertidal zone. As they answer, make the following points:
• Conditions vary in the intertidal zone, changing hour to hour, day to day, season to season. This is because of the ebb and flow of the tide, different seasonal wave patterns, light and temperature changes through the course of a day or night.
• Conditions vary within the intertidal zone itself, depending upon the beach slope, relationship to the high and low tide lines, and the terrain.
• Much of the intertidal zone should be alternately wet and dry, exposing organisms to fluctuations in moisture, wind exposure, etc.
• In tidepools the salt water can be concentrated by evaporation or diluted by rain water, so organisms are exposed to fluctuations in salinity.
• Wave action can displace or destroy habitat, and can also crush, break, or tear organisms. Wave action has different effects depending upon the location in the intertidal zone.
• Saltwater splash from waves can expose organisms to dessication (drying out).
5) Ask students if the intertidal zone is such a difficult place to live, why would any organisms live there at all? What are the benefits to organisms that can survive here? As students answer, make the following points:
• One advantage is avoiding many of the predators common in the more stable environmental conditions of the deeper waters at the edge of the intertidal. So snails, for example, can graze with less risk of predation.
• Under certain conditions (e.g., high tides at night) larger predators such as octopuses and eels can gain access to tidepools, but not consistently. Other predators include crabs, birds, and humans. Even experienced marine life observers have little specific information about predation. The intertidal zone seems to be a pretty safe place to live for organisms that can tolerate the environmental extremes.
• Also, through much of the intertidal zone, regular inundations by ocean water bring nutrients for algae growth, new food sources, and an abundant supply of oxygen.
6) Because of the variations in conditions, there are “sub-zones” within the intertidal zone. Show the “Intertidal Subzones Table” acetate and review the zonation within the intertidal area. Leave the “Intertidal Subzones Table” up on the overhead. Divide students into five groups. Give each group the Hypothesis Card for one of the subzones, from the “Subzone Conditions Cards.”
7) Have groups hypothesize about the environmental conditions in the subzone it was assigned. Students will consider three variables: fluctuations in salinity, wave action, and exposure to air. Have them write their hypotheses in the corresponding columns on the Hypothesis Card. Then, on the back of the card, have them write at least two hypotheses about how organisms that live in this subzone are adapted to live in these conditions.
8) After groups are finished recording their hypotheses, hand out the Comparison Card (from the “Subzones Conditions Cards”) and “On the Edge Species Cards” that correspond to each group’s subzone (see the “Subzones Species Key”). Have groups compare their hypotheses with actual conditions listed on the Comparison Card and compare their adaptations hypotheses with information available on the species cards. Students should make notes about these comparisons on the card or on a separate piece of paper.
9) Have each group share its hypotheses with the rest of the class, explaining similarities and differences between the Comparison Card and Species Cards.
10) Wrap up the class with a discussion based on the following questions:
• Which one or two of these subzones do you think would be the harshest environment for marine organisms? Why?
Well-reasoned responses are acceptable. In general, the splash zone and upper intertidal zones are considered the harshest zones because the organisms in these zones are exposed to greater extremes than those in other zones.
• In which subzone or subzones would you expect to find the most organisms and greatest diversity of marine organisms? Explain your answer.
Again, well-reasoned responses are acceptable. The lower intertidal and subtidal zones, being the most reliably submerged, are home to more and a greater variety of organisms than the others.
• Many of the plant and animal species in the intertidal zone were used for food and medicine in traditional Hawaiian culture. Many are still used today. Why do you think so many of these species would be used for food—other than how ono they are?
Well-reasoned responses are acceptable. The primary reason is probably that these species are accessible and relatively easy to gather or hunt.
• Are there any species that seem better adapted to avoid “human predation” than others? Why?
Well-reasoned responses are acceptable. Some possible answers include:
• Animals that live in the subtidal zone may be more difficult for humans to gather.
• Animals that grip tightly and are difficult to get off the rocks may be less vulnerable.
• Animals that are spiny or have sharp shells may be more difficult for humans to gather.
• Plants or animals that taste bad would discourage humans collecting food.
• Animals that quickly hide (such as rock crabs or zebra blennies) may be more difficult for humans to trap.
• Have you ever collected ʻopihi, limu, or other plants or animals in the intertidal zone? Or do you know someone who does? What is your (or their) favorite part of collecting?
• Hawaiians were careful observers of their environment. What observations do (or would) you make as you enter the intertidal zone? How would these observations affect your actions?
• Describe what conditions would be like in various parts of the intertidal zone if the following traditional Hawaiian prayer for surf were successful. This prayer would be chanted while lashing at the ocean’s edge with a length of pohuehue vine or after building a mound of sand and wrapping the pohuehue vine around it.
• Hypothesis Cards
• Participation in group work and class discussion
• Journal entries