ISLANDS AND THEIR LESSONS IN BIODIVERSITYi -Jack R. Holt ISLAND OF THE DODO The extinction of the dodo is representative of modernity in several ways, not the least of which is that the event occurred on a small island. -David Quammen (1996) When I was in London five years ago I had one day to stay in the city before our plane left for New York. With all of London to choose from, what does one do in one day? Well, for me there was no question. I had to go to the British Museum of Natural History. The main attraction was its collection of dinosaurs. I walked throughout the museum, and then paused at a display of animals that had become extinct during historic times. There was a stuffed Passenger Pigeon, birds once so numerous in North America that their flocks blackened the sky as they flew over. Nearby, there was a Dodo, another pigeon. This animal was large. In fact everything about this bird was large except its wings. Its skeleton indicated that the Dodo was flightless. Not only were its wings short, but its breast bone was too small to support flight muscles. Clearly, it could not fly, and its reconstruction made it look even more comical with a bald face and a body covered with fluffy down-like feathers (see Figure 1). Portuguese sailors discovered the Dodo's Island in 1510. The island, a small point of land in the Indian Ocean east of Madagascar, became known as Mauritius. The earliest record of the Dodo was in 1598. By 1681, the last Dodo had been killed. Isolated island habitats seem to favor the evolution of flightless birds (as well as miniature mammals and giant reptiles). Birds probably become flightless because birds that lose their ability to fly are not selected against in habitats with plenty of food and no predators. Thus, on its little island in the Indian Ocean, the Dodo was a successful species. However, when Portuguese sailors began to kill them for FIGURE 1. Top: A reconstruction of the dodo food, they brought their associates like pigs and that emphasizes the stubby wings and stocky rats. Together with the wholesale slaughter, body of this large flightless bird. Bottom: An destruction of forests and nests, the last Dodo articulated skeleton of a dodo. died in 1681, only 83 years after it was first sighted. Mauritius and similar isolated islands are volcanic and therefore can be made very far from continental land masses (and their biota). The island also is surrounded by a large fringing coral reef. Although volcanic activity seems to have been quiet for the past million years, the island continues to grow through the accretion of limestone by the growth of coral. ARCHIPELAGOS AND BARRIERS Islands have had two distinct modes of origin – they have either been separated from continents of which they are but detached fragments, or they have originated in the ocean and have never formed part of a continent or any large mass of land. -Alfred R. Wallace (1880) Unlike Mauritius, Puerto Rico is part of a chain of islands (archipelago) called the Greater FIGURE 2. A relief map of Puerto Rico. Antilles and forms a boundary between the Atlantic and the Caribbean. Like Mauritius, Assateague Island is quite different from Puerto Rico was formed by volcanic activity and Puerto Rico. It is part of the North American is surrounded by a fringing coral reef. However, continent and formed by the outwash of its geology is older and much more complex. sediments from the eastern seaboard and their Puerto Rico seems to have been formed from the reallocation by ocean currents. Barrier islands floor of the Atlantic Ocean around 190 million are long and narrow bits of land with narrow years ago. Originally, It was an arc of volcanic strips of different environments. Most of the islands that coalesced. Around it in the shallow wave action occurs on the ocean side. The sea, reef builders made thick deposits of mainland side of the barrier island is marsh and limestone. bay (see Figure 3). This history can still be seen in the landforms of the island. The ancient island arc is now a mountain chain that runs east to west across Puerto Rico. On the north and south are areas of limestone from the reef-builders. This geological mosaic creates a large number of habitats in a small area. In fact, the island is only about 100 miles long (E-W) and 40-50 miles wide (N-S). In that relatively small area (3,515 mi2), Puerto Rico has rain forests, dry forests, mangrove swamps, coral reefs, etc. Consider the number of bird species in Puerto Rico. The island has about 280 species of living birds (both residents and migrants). All the rest of the United States has about 650 species of birds. Thus, the number of birds in Puerto Rico may be less than half that of the rest of the US, but the surface area of Puerto Rico is less than one-tenth that of Pennsylvania FIGURE 3. Top: Map of Assateague Island. (46,058mi2). Since the surface area of the 2 Note the long, thin aspect of the island. North is United States is about 3,787,425 mi , Puerto to the left. Bottom: Diagram of a barrier island Rico has 469 times more bird species per square that illustrates the open ocean, island, bay and mile than does the rest of the US! mainland. Barrier islands also serve to dampen Consider what that means in the context of wave action thereby changing the character of bidiversity. The numbers indicate that the the marsh than fringes the bay. destruction of a square mile in Puerto Rico has a much greater impact on bird species than the loss of a square mile in central Pennsylvania. By and large, barrier islands like Assateague are dynamic; that is, they are constantly changing. Consequently, permanent construction on them often is a lesson in futility. The dynamic nature of the island can be seen on the beach where black peat and blackened clam The beach and main dune. shells from an ancient marsh continually erode in the surf. That is because the island is slowly rolling over itself toward the mainland as it has done since its formation at the end of the last ice age. As you walk from the beach and dunes toward the bay, you encounter a forest dominated by long-leaf pine. The forest, in turn abruptly changes to marsh. Pine trees grow on higher ground with less chance of salt water intrusion. Scrub behind main dune. All of these different zones occur in an area that is sometimes less than a mile across. The diversity of habitat allows nearly 300 species of birds to occur there. Although the island has a high diversity of plants and animals, a barrier island like Assateague is too close to the mainland to have unique species (This includes the ponies and Sitka Deer, animals that are artificially maintained and controlled). Unique or endemic island species occur on isolated islands like Mauritius. Long-leaf pines with marsh in the foreground. AMONG THE FIRST TO STUDY ISLANDS Note how the height of the trees rises the farther Mr. Darwin appears to have been the first writer they are from the bay (to the right). who called attention to the number and importance, both from a geological and a biological point of view, of oceanic islands. -Alfred R. Wallace (1880) The Galapagos Islands (Figure 5) are much like Mauritius in that they are volcanic and rise from a very deep ocean bottom. Unlike Mauritius, though, the Galapagos are an archipelago and relatively close to South America. Although the islands lie on the equator, they are bathed by a cold water current Salt marsh, one of the most productive so that coral does not grow on them. This island ecosystems on earth. chain is relatively young and many of the FIGURE 4. Illustrations of the beach, dunes, organisms that inhabit it show clear affinities scrub, pine forest, and salt marsh in a straight with those of the South American continent. line across the barrier island. These characteristics helped to give Charles Darwin some of the important clues about the A walk across the island will take you operation of evolution. through several distinct environments (see The Galapagos Islands are true desert Figure 4). The leading edge of the island is a islands. The surrounding cold current causes the sandy beach that is delimited by a main dune. air to dry out so rains rarely fall. In this harsh Other smaller dunes occur in the area environment relatively few species live on the immediately behind the main dune. This area island surface, but the surrounding waters are looks almost like a desert with some cactus and teeming with life. Thus, a species of Iguana has scrubby plants. taken to the sea and feeds on algae. Another amphibious oddity is the Galapagos penguin, an that these areas must have been connected to animal whose ancestors must have traveled their respective continents when ocean water northward on the Antarctic current. levels were lower as they must have been during The Beagle circumnavigated the globe the great ice ages. during Darwin’s five-year voyage. After the ship left the Galapagos, it headed across the Pacific toward Australia. On the way, Darwin visited many other oceanic islands, many of which had coral reefs, most of which were atolls. One of the first books that Darwin wrote after his return to England was The Structure and Distribution of Coral Reefs in 1842.