Disappearing Marine Iguanas by Conrad Toepfer

Part III—Coming to a Conclusion

It is not easy to explain why marine iguanas die during ENSO years. The connections were only recognized after the 1982–1983 event. Since the die-off was unexpected, relevant data from that time period are incomplete. There were three additional smaller ENSO events between the 1982–1983 and 1997–1998 events. Although scientists became better at recognizing the signals of an impending event, they were still surprised by the severity of the 1997–1998 ENSO. This means that what little data there are still come from the earlier events.

Liz did a lot of digging and decided that she could build a relatively strong argument for the cause of the marine iguana mortalities. Her argument depended on the following sources of information.

• Relationship of algal growth to temperature (Figure 4; will also need to consult Figure 3 for interpretation).

  • Figure 4—Algal growth 1991–1993

    

  • Figure 3—Temperature anomaly

    

  Note on interpreting Figures 4–7:The splash zone is located above the high tide line and while it may be moist at high tide it is not submerged. The upper or high intertidal zone is submerged at high tide but may be partially or completely exposed at low tide; the low intertidal zone is always submerged. In Figures 5–7 below, the location of the bars indicates where each species grows within the intertidal zone while the thickness of the bar gives an idea of relative abundance.

• Distribution of algae from 1981–1984 (Figures 5–7).

  • Figure 5—Distribution 1981

    

  • Figure 6—Distribution 1983

    

  • Figure 7—Distribution 1984

    

• Mass of stomach contents of marine iguanas from 1982–1984 (Figure 8).

  • Figure 8—Stomach contents

    

• Marine iguana condition on Isla Santa Fe, 1981–1987 (Figure 9).

  • Figure 9—Iguana condition

    

• Digestibility and cellulose content of algae (Table 1).

  • Table 1—Algae digestibility

    

Liz took a deep breath; that paper wasn’t going to write itself. With only a few days left until the deadline it was time for her to get going. “So now, what do these studies tell me about ENSO and marine iguanas?”

Questions

1. What conclusions do you think Liz made about the cause of population declines in marine iguanas?
2. Is it reasonable to assume that the data from the 1982–1983 and 1991–1992 ENSO events are representative of what happened in the later 1997–1998 event? Why or why not?
3. Do we have what we need or do we need additional information? Is there a potential problem with one or more of the studies?

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