Liz checked the number on her cell phone before answering. “Hey. What’s up?”
“I think I might have a lead on the iguanas. In ecology today, we covered Galápagos finches …”
“So?” said Liz sarcastically. She was still a little steamed that Abby got to take ecology this year. “Although finches are derived reptiles, I don’t see what they have to do with marine iguanas.”
“Let me finish! We were covering global weather patterns and talked about the effects of El Niño on finch populations. The normal weather in the Galápagos is really dry so the finches are limited by low food availability. But during an El Niño year, the vegetation grows like crazy and the finch populations may double or triple in size in just a few months.”
“I still don’t get it. The iguana population didn’t grow rapidly, it crashed.”
“I know, I know, but didn’t you say that your instructor went there in 1999? There was a severe El Niño in 1997–1998.”
“Really!” Liz moved to the edge of her seat. “But why would the iguanas die off when the finch populations exploded?”
“No clue. You’re the one writing the paper.”
By this point, Liz had done some background research on the marine iguanas and knew a little more about their basic biology. The iguanas spend most of their time basking on boulders on rugged shorelines but enter the water for feeding. They have flattened tails for swimming, strong claws to hang on to rocks while feeding, and tricuspid teeth to scrape algae off the surface of underwater rocks. The smaller iguanas, juveniles and females, feed in shallower water while the larger males may dive down 5–8 meters. All iguanas show a strong preference for red and green algae and avoid brown algae if the other varieties are present. Since the water in the Galápagos is usually much colder than most tropical water (about 18–23°C), the ectothermic (“cold-blooded”) iguanas can only feed for a limited time before they have to crawl out onto the black rocks to warm up in the sun. The iguanas’ natural predator, the Galápagos hawk, can only capture small hatchling iguanas, but predation risk can be high on islands with feral introduced dogs and cats.
Liz began to explore the El Niño (more appropriately referred to as the El Niño/Southern Oscillation, or ENSO) idea and quickly found that the main effects in the Galápagos were elevated air and water temperatures and elevated rainfall. The total rainfall at the Charles Darwin Research Station in 1998 was an astonishing 3.4m (over 11 feet!), second only to the 1982–1983 ENSO, and much more rain than typically falls in dry years (0.4–1.0m).
Questions
Examine the data that Liz ran into while researching ENSO in Figures 1–3. (Click on the figures for larger versions and use the back button on your browser to return to this page.)
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Figure 1—Monthly air temperatures
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Figure 2—Total annual rainfall
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Figure 3—Temperature anomaly
- Given what you know at this point about marine iguanas and the abiotic effects of ENSO, develop two possible directions of research that Liz should pursue to understand exactly why the iguanas suffered such a high mortality. Keep in mind that you need to consider indirect effects. While environmental temperature does change metabolic rates of ectotherms, the iguanas are exposed to a wide range of temperatures as they feed and bask on the lava. Direct mortality in response to a temperature change of a couple of degrees is unlikely.
- Choose one of those directions of research and determine what data you would need to find to support your ideas.
- Design an experiment that would help you collect the data you have identified in Question 2. Identify your independent variable(s), dependent variable(s), treatments, and control.
Originally published at http://www.sciencecases.org/iguanas/iguanas2.asp
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