What it means to be "depauperate"

Rapa is biotically amazing. The island is shaped like a C, with a bay (Baie Ha'urei) in the center. The terrain, which is highly eroded since the island is 5 million years old, is rugged, with many valleys, peaks, and cliffs. Being so far south (27° S), the island is well out of the tropics, too cold for breadfruit, and coconut palms grow only at sea level, and only produce dwarfed coconuts about half the length of normal ones. Some typical French Polynesian plants, like hibiscus (purau, Hibiscus tiliaceus), or giant nahe ferns (Angoipteris evecta, Marattiaceae) are much rarer, and grow only at very low elevations near water. Conversely, because of the cooler temperature, there are a number of plants found here that are shared with New Zealand, but are found nowhere else in French Polynesia, such as the shrubs Corokia and Hebe. Unlike almost everywhere else in French Polynesia, there are native forests of puarata (Metrosideros collina) and tree ferns (Cyathea) persisting at sea level.

The mountains and sea cliffs of Rapa are home to nesting noddies, petrels, shearwaters, tropicbirds, and storm-petrels; again some of these are species of the Southern Ocean that nest nowhere else in French Polynesia. We had a white-bellied storm petrel land in our camp on top of Mt. Perau the first night we were there, probably attracted by and confused by the blacklight we had to collect moths. The streams teem with freshwater shrimp and gobies. The secluded bays on the outer coast of Rapa harbor small cold-water coral reefs, with butterflyfishes and groupers and parrotfishes, and huge spiny lobsters. Rapa is too isolated for a commercial fishery to develop for any of these. Some of these fishes are found nowhere else but in the waters around Rapa. In the winter, seals occasionally come up from the Antarctic.

Rapa is probably the closest thing we have in French Polynesia to the Galápagos Islands. There is an amazing adaptive radiation of 67 species of tiny weevils in the genus Miocalles, each of which is specialized to a different species of host plant or different part of the same host plant. (Miocalles are found elsewhere in French Polynesia, but nowhere else are they as diverse as on Rapa.) The same is true of tortricid moths in the genus Dichelopa, which have 15 species endemic to Rapa; this is half the known species in the genus, which is found across the South Pacific. There was an amazing radiation of Partula tree snails, many of which are now extinct. We saw representatives of all of these during our fieldwork. Rapa has its own an endemic family (with one species) of pyraloid moths. Herbaceous plants have a tendency to evolve woodiness on islands; in other words, they evolve into shrubs and trees. This is particularly pronounced in the family Asteraceae, which is familiar to North Americans as the family of daisies, thistles, dandelions, and sunflowers. Rapa has five genera of woody Asteraceae, including the tree the Rapa call 'ānei and scientists call Fitchia (which grows up to 5 m in height, and has prop roots, and huge orange thistle-like flowers--see photo), and the shrub Oparanthus, with tiny yellow compound flowers and huge, stiff round leaves.

So going to Rapa is a bit like Christmas for the Pacific island biologist. The climax of our trip was hiking up to Mt. Perau, at 650 m the only cloud forest in the Austral Islands, and the highest point between Tahiti and Antarctica. We spent three days collecting insects; a big part of this was sifting soil and fogging (ie, spraying with commercially available pyrethrin) the mossy branches and tree trunks that were everywhere. A lot of what fell out were crustaceans (amphipods and isopods, or beach hoppers and sowbugs). Perau had the weirdest sowbugs I have ever seen, looking like minature ankylosaurs and trilobites. There were also rove beetles (staphylinids) and orobatid mites (which look like tiny spiders), and some other things, but after half a day of doing this, I realized that I wasn't getting anything new; there were no more than about 20 species of arthropod living in the moss and soil. And then it hit me: Rapa is depauperate. In fact, everywhere in French Polynesia is depauperate--the term means having lower biodiversity than an equivalent area somewhere else. In this case, the islands of French Polynesia, despite being tropical and subtropical, have fewer species than an equivalent area of land in the continental tropics (like in Southeast Asia) because they are so small, and so isolated in the Pacific. If I were to try fogging or sifting litter in New Guinea, or Southeast Asia, I would never run out of new stuff. The fact that French Polynesia was depauperate in everything was depressing to me when I first started coming here. It was the sheer out-of-controlness of tropical biodiversity in the Dominican Republic, or Costa Rica, or Malaysia--huge, crazy-looking insects, a dozen species of bird that all look the same, reptiles and frogs everywhere, the strangest flowers you've ever seen--that attracted me to doing tropical fieldwork, and wanting to study tropical biodiversity in the first place. It became pretty obvious once I got to Tahiti after starting grad school that despite being tropical, and despite having been colonized mostly by things from New Guinea and Southeast Asia, French Polynesia had none of this crazy stuff. But eventually I came to appreciate it for what it is. And ironically, my thesis project here will hopefully elucidate some of the mechanisms responsible for the crazy biodiversity that is in the continental tropics. But it took ending up in a truly tiny patch of habitat on a tiny island in the middle of the ocean--an island within an island--to realize again just how little biodiversity there actually is here in French Polynesia.