Nature's Confluence

Nature’s Confluence from the field: the isolated atoll of Palmyra

9/19/2016

By Dena Spatz

I am 1,000 miles southwest of Hawaii and over 3,000 miles to the closest continent. I fall asleep to the squeaks of Brown Noddy and White Tern chicks and I wake up to the blow of the trade winds from the northeast. Before our 7:30 am breakfast, my team and I prepare our lagoon boats with a can of gas, the gear for the day, and our deep freezer-treated clothing that help to prevent insects and soils from spreading among all the islands we visit each day. By 8:30, we motor towards one of the many islets within Palmyra Atoll, a U.S. territory occupied by a handful of researchers and staff from the Nature Conservancy and the U.S. Fish and Wildlife Service.

Palmyra Atoll

We are on Palmyra Atoll to measure the recovery of the island ecosystem in response to the removal of invasive rats, which was completed five years ago. We spend the day surveying the habitat for land crabs, rare native tree seedlings, seabird nests, and indications of forest growth. On off days, we walk the shorelines to map fiddler crab habitat or to count migratory shorebirds, like the Bristle-thighed Curlew or the Wandering Tattler, which recently arrived from their arctic breeding grounds. As I move between islands, either by boat or by wading across shallow waters with black-tipped reef sharks and eels at my ankles, I notice slight differences in each island’s biological community. Many islands are dense with coconut palms and sprouting coco seedlings, some reaching 3 meters tall (and very difficult to walk through), while others have mixed forest habitats including Pisonia grandis and Heliotropium foertheria, two native coastal trees whose branches commonly support nests of the Red-footed Booby, Great Frigatebird, White Tern, and both Brown and Black Noddy. Some islands have chest-high ferns that we wade through on our surveys while others have dense vines that hang like curtains. Exposed coral rubble and little to no soil is common on some islands, while others present deep muddy pits that are easy to overlook until you find yourself stuck in muck up to your waist.

A Red-footed Booby nesting in a heliotrope tree, a native tree common to tropical islands in the Pacific. Note, the islands in the background, many of which have coconut palms that were introduced to the Atoll over 100 years ago.

Crab species are unevenly distributed across the atoll as well. Coconut crabs, the biggest land crabs in the world, live in underground burrows the size of a beach-ball or in crevices in the base of large native trees. Meanwhile, Strawberry hermit crabs (which look just like their name describes) are common across all islands and we find them mostly along the coral rubble coastlines or inside the open-air trunks of a Pandanus trees.

Millions of years of evolution, geological processes, and tropical weather shaped and created islands like Palmyra, and those critters that live on the Atoll today must have had the ability to fly or raft for many miles to be able to arrive on this isolated Atoll. Indeed, island biogeography theory provides a nice explanation of why and how islands are special places that support simple, yet unique, biological communities. However, as we surveyed the Atoll, I was surprised to observe any differences in the forests or the species that we observed between each of the islands. Why? First, most of the islands here are connected along the same shallow reef or sandy shoreline that is often exposed at low tides, providing a nice bridge for plants or animals to transit between islands.

Strawberry hermit crab (Coenobita perlatus, top) and coconut crab (Birgus latro, bottom). Photos by Coral Wolf

One of the many shallow crossings between islands

Second, the U.S. military was stationed on Palmyra during WWII, and in that time, they dredged the lagoon to make a shipping channel, built a mile-long air-strip for planes to land and created a “causeway” that provided driving access to all the islands in the atoll. Most importantly, Palmyra used to be a broken up landmass with hundreds of tiny islets. In the 1940’s these islands were connected or created with the dredge spoil from the lagoon, the vegetation was cleared, and the land was bomb-tested or converted for human use. Today, the Atoll is made up of just 25 islands and each has remnants of bunkers, airplane parts, unexploded bombs, and garbage from the war era.

"Bunker Island". One of the many sites on Palmyra Atoll where you can see historic remnants of WWII

So, despite millions of years of evolution, geological processes, and tropical weather that shaped and created Palmyra, the Atoll was essentially re-built just under 70 years ago; the ecosystem we observe today descended from that time period. I expected any obvious biological differences among islands to be eliminated. Yet, Palmyra may just be an example of what nature looks like when large densities of people (and rats) leave town - plants grow, seabirds breed, crabs disperse, cryptic species emerge. These changes are happening across the Atoll, but each island appears to experience this in slightly different ways. This is likely driven by each island’s size, location in the atoll, or the dominant species that survived the disturbances (like coconut palms). Perhaps the underlying soil types on each island control what forest communities can be sustained above. Or, maybe nesting seabirds, which drop their nutrient-rich guano on the ground below, could be fertilizing plants on some islands and not others. There are many hypotheses, and lots of evidence, for what drives the changes we can observe between islands, especially as they recover from a massive disturbance. With continued protection and research, we can learn more about this and the historical connections between Palmyra’s islands, the flora and fauna living upon them, and the food webs that unite the ecosystem. For now, as I finish up my field work, I will let my observations run wild and will enjoy each coconut crab sighting while doing my best to avoid the mud pits.

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