Kirch's Lab:

Hawai'i is pretty much perfect--as a vacation destination and, according to Dr. Patrick Kirch, as a study system for examining how people and the environment interact. Kirch has a joint appointment with Integrative Biology and the Department of Anthropology, and his work combines the fields of ecology and archaeology. The archaeological aspect of his work allows him to take the long view, to understand the ecology of Hawai'i hundreds of years in the past. In Kirch's words, he is "contributing a long term perspective to human environmental interactions using islands as model systems."

As an undergraduate at the University of Pennsylvania, Kirch read MacArthur and Wilson's seminal book, The Theory of Island Biogeography. "It was really a mind opener for me," says Kirch. In their book, MacArthur and Wilson put forth the idea that islands can show us how biological processes work. These processes occur on islands, just like they occur on continents. But because islands are small and relatively isolated, they don't have all of the complexity and "noise" of larger landmasses. MacArthur and Wilson used islands as model systems to look at migration, speciation and extinction, among other things. Kirch uses islands as model systems to look at human culture and how humans and the environment interact.

Kirch is collaborating with Stanford ecologist Peter Vitousek. The two have a lot in common; both use clues found in the soil to examine the ecological processes of the past, and both were born and raised in Hawai'i. And so, says Kirch, "it's kind of ironic that we both ended up in the Bay Area, at these two rival schools." Despite this rivalry, they are working together to understand how gradients in soil nutrient composition and climate interacted with human agricultural practices on the Big Island of Hawai'i, over 200 years ago.

The islands of Hawai'i were formed one at a time as the Pacific Plate slid over a volcanic hot spot. The island Ni'ihau was formed first, about 5 million years ago. Next came Kaua'i as the Pacific plate slid to the northwest, and then O'ahu and Maui. The youngest island, at roughly one million years old, is Hawai'i, the Big Island.

Island age is important: when the islands are first formed, their soil is rich in phosphorus, a nutrient crucial for plant growth. Over time, rainfall leeches the phosphorus from the soil; older islands have less phosphorus. Because Hawai'i is young, its soil is rich in phosphorus. But the amount of phosphorus is not uniform across the island. Low elevation areas receive very little rainfall, so the soil still contains lots of phosphorus. High elevation areas get a lot of rain, and the phosphorus has been leeched from the soil. This sets up two biogeochemical gradients: rainfall increases with elevation, but phosphorus availability decreases with elevation. After Vitousek described these gradients, he and Kirch wanted to see how early farmers had dealt with them.

Kirch's archaeological work showed that the Hawai'ians were accomplished farmers. Their fields of taro and sweet potato blanketed between 50 and 60 square kilometers of the hillsides of Hawai'i. "We found that the system had basically expanded to fill what we call a sweet spot across the face of the mountain." The edge of the fields at the low elevation marked where the climate became too arid for crops to grow. And at the high elevation, the fields end at what Kirch describes as an invisible "cliff," where phosphorous levels decline steeply as a result of rain-induced leeching.

Kirch, Vitousek and their team are now discovering how the Hawai'ians drew down the phosphorous levels in the soil. The fields were separated by a grid of low walls. One of Vitousek's students compared the phosphorous levels in the soil in the fields and in the uncultivated soil beneath the walls. The soil that was farmed has much lower phosphorous content than the soil under the walls. The Hawai'ians were having a significant impact on their environment and, says Kirch, "this is really one of the first times in agricultural archaeology that anyone has been able to see a long term effect of agricultural production."

When Captain Cook came across Hawai'i in 1778, the Hawai'ian agricultural system was fueling a successful society. They had enough surplus food to feed pigs, which were sacrificed at cultural ceremonies. And the surplus food fed a complex society of soldiers and bureaucrats, hungry mouths who didn't pull their weight in the fields. Just how many Hawai'ians were there? This question is debated, and comprises another aspect of Kirch's research.

Says Kirch, Captain Cook "was killed there, so he didn't live to estimate population. But his second in command, Lieutenant King, did write in his journal a couple of pages on the question of population." He based his estimate on the number of houses per area of shoreline, the number of people per house, and the length of the shoreline (Captain William Bligh, of Bounty fame, made the maps). "It was a very scientific approach," says Kirch. "He came up with 400,000 or 420,000 people on the islands." Leaders of subsequent expeditions to Hawai'i were not interested in estimating population, and then it was too late: with the explorers came disease, rapidly decimating Hawai'i's population and putting an end to its agricultural empire.

Kirch is now estimating Hawai'i's historic population using methods quite similar to Lieutenant King's. He is digging at house sites, finding out how many people likely lived in each house, and how many houses once stood on the shoreline. At the house sites, Kirch is looking for evidence of another ingredient in the Hawai'ian diet: fish.

Archeologists sift the dirt and debris from their dig sites using screens, says Kirch, "to get the bones and shells and so on." On one particular trip, Kirch had sent a post doc and a grad student to Hawai'i a few days ahead of him, to start excavating house sites. When Kirch arrived, they were busily sifting using screens with holes of 1/8th inch, about the size of two letters in 12 point font.

"They were getting lots of shellfish," says Kirch, "and I said what about fish bone? They said ‘We're not getting any fishbone.' And I said my god, that can't be right, there have got to be fish here." But if people had been fishing intensively for a long period of time, always pulling out the biggest fish, the fish population could consist of very small fish--their bones could slide right through the 1/8th inch holes. Kirch's solution? "Let's get some window screen."

After a quick trip to the hardware store, Kirch and his team returned to the dig site. The window screen's holes were 1/16th of an inch--a single letter of type. They sifted the dirt with the fine screen, and "there was the fishbone… Things like Parrotfish, little dentaries and little pharyngogrinders. They were--" Kirch holds up his hand, his thumb and forefinger almost touching "--like this." The fish were tiny.

Kirch wants to quantitatively and statistically compare the size of the fish bones from the dig sites to the size of the fish today. People are now fishing recreationally, but "they're not doing the same intensive pounding on the resource" that they did in the 1700s. Kirch wants to know what effect the Hawai'ians had on fish populations, and what affect the fish had on the Hawai'ian's overall diet.

This will involve further collaborations with scientists from other disciplines, including people who study population modeling of both humans and fish. Says Kirch, "To me this is where the excitement is, in the contemporary sciences, trying to bring together these different perspectives for a larger understanding."


In 2008, Kirch will teach a new class, called Human Biogeography of the Pacific (IB 187). This course, says Kirch, will "look at humans from a biogeographic and evolutionary perspective," tracing human colonization and dispersal in the Pacific Islands. Kirch also teaches a course in Pacific Islands archaeology and pre-history (Anthro 124).

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