Introduction to ecosystems
Introduction to ecosystems

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Introduction to ecosystems

3.2 Darwin’s thoughts on the iguanas

The marine iguanas of the Galápagos are the only marine lizards and occur nowhere else. They occupy a fragile ecosystem that is very vulnerable to changes in sea temperature. There is also a species of land iguana on the islands.

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Darwin’s thoughts on the iguanas

Some of the most remarkable creatures encountered by Darwin on the Galápagos were its iguanas. He was particularly fascinated by the marine variety though he was far from flattering. Darwin wrote that they were hideous-looking creatures of a dirty black colour, stupid, and sluggish in their movements. He did concede, however, that they were strong, graceful swimmers. Bearing in mind how much time they spent in the water, Darwin was surprised to find that marine iguanas didn't eat fish. He dissected several creatures and found that their stomachs were full of seaweed. Darwin noticed that the water was too cold for marine iguanas to stay in for a long time. When they weren't feeding, he observed them clinging to the rocks above the shoreline, basking in the sun. After the cold sea, these reptiles craved warmth. Darwin was also interested in their terrestrial cousins, although again, he wasn't very complimentary about their appearance. He called them small, ugly animals with a singularly stupid appearance and lazy, half-torpid movements. Today they're still plentiful on some islands. But marine iguanas are particularly vulnerable to changes in sea temperature, which can have a dramatic effect on the availability of seaweed.
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Activity 2

Why, do you think, is the marine iguana so vulnerable to sea temperature changes?

What impacts of environmental change might make the land iguana vulnerable?

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The marine iguana feeds exclusively on a small number of species of seaweed. The seaweed needs relatively cool water and in some years, when the ocean currents reverse, the seas around the islands get too warm for the seaweed and it dies back. The marine iguanas then have no food and die in large numbers. Any prolonged warming of the seas would make the species highly vulnerable to extinction. Land iguanas get most of their water from prickly pear cacti and the rest from rain fall. Any prolonged period without rain due to changes in climate put land iguanas at risk.

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