Why So Many Predators? Blog From Adjunct Professor Dr. Eleanor Sterling

(cross-posted from Dr. Sterling’s “Scientist at Work” blog from the New York Times)


photo credit Kevin Frey

This morning I woke up to a huge splash just offshore. I don’t even look any more as I know it is a large predator, either a jack or a shark, feeding in the lagoon. You get used to them here. You have to. The six scientists from the American Museum of Natural History, at this field station for two weeks to study sea turtles, regularly see at least 10 sharks, and sometimes we see 40 to 50 in a day if we concentrate. I am particularly fond of the shark pups – the size of my forearm or so – but the often-curious larger sharks (black tips in the shallow flats as well as Galapagos sharks on the deeper dives) can be a bit disconcerting, especially if you are concentrating on something else and only notice them as they swim past for a closer look. Other scientists have had close encounters with tiger sharks, but I have not yet seen them. I just see evidence of them on the turtles. When we are checking turtles over in the health analyses, we see healed wounds that make me crazy trying to figure out how the turtles survived. In some, the shell looks like a cookie with a bite out of it.

The spectacular wildlife at Palmyra Atoll makes it a treat to work here, but so do the living conditions. Many scientists brag about the hardships of their field locations, but our team has little to complain about (other than the long days and challenging work) and a lot to enjoy. Given the remoteness of the atoll one would anticipate sleeping in tents and cooking over campfires. In contrast, Palmyra Atoll is home to the world’s most natural marine laboratory, with a full-fledged research station with dive boat, sea kayaks, and skiffs for traveling around the atoll, a dry lab where we can process samples and work on computers, comfortable cabins, a galley serving scrumptious meals, electricity, showers and even washing machines. The field station staff has stored equipment we shipped out in the spring via the once-a-year barge, and it is always an adventure to track down the various boxes, crosscheck packing lists and inventories from last year’s stored equipment, and sort through the gear.

The station is managed by and funded primarily through private donations to the Nature Conservancy, in collaboration with the Palmyra Atoll Research Consortium — a group of scientists from eight different institutions.
The galley.Felicity Arengo The galley.

Through systematic surveys, researchers here have turned on its head one of the most basic tenets of ecology. The conventional story goes that food chains are pyramid-shaped, with larger numbers of organisms such as plants and other primary producers at the bottom. Next up in the pyramid are a smaller number of organisms that eat plants, and so on moving up to the apex of the pyramid, where carnivores preside. The pyramid emphasizes that carnivore numbers are smaller than those levels below, limited by the number of prey available to them. Here on Palmyra and in other remote, healthy marine systems along this long chain of islands in the Pacific, the inverse is true. There is a far higher biomass (essentially poundage) of predators than there is prey, turning the pyramid on its head. How the system can support so many predators is still a mystery.

Fortunately, within our research consortium we have a diversity of institutions and scientists able to work to understand this, tackling the individual parts of the system but also fitting them into questions about how the whole system works — in this case the relations between predators and prey. In essence we have a mini-ecosystem of scientists interacting to understand the marine and terrestrial systems of the atoll. We help each other by sharing gear, instruments and information. For instance, collectively we have set out an array of receivers within Palmyra’s lagoon system that can help track animal movements.

The U.S. Fish and Wildlife Service is interested in potentially restoring function to the lagoon system that was heavily modified when the atoll served for a short time as a military base during World War II. It is not clear what impact this restoration activity might have on endangered species currently using the lagoon. Consequently, our team is tracking turtle movements with acoustic tags, whereas others are tracking sharks and other species such as bonefish.

During the last field season we put our first acoustic tags on turtles and added our first set of receivers to the array. The receivers store information on animal movements until researchers retrieve the data. No one has checked the receivers since we were all here last summer, so we have no idea if the receivers have any relevant data on turtles. We are almost done with our unpacking, and then we can finally get started.

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