Each year, the Hawai’i Conservation Alliance (formerly known as the Secretariat for Conservation Biology) brings together hundreds of researchers, land managers, agency officials, students, and others with an interest or stake in the preservation of Hawai’i’s natural resources. This year’s conference was held in Honolulu in early July. Last month, we published summaries of selected presentations; we conclude our coverage this month.
We also include write-ups of discussions that took place at the more informal Hawai’i Project Meeting, an annual summer event organized by Stanford University’s Peter Vitousek and held this year on the campus of Kamehameha Schools at Kea’au.
‘Eat ‘Em’
Misinformation takes on a life of its own; it almost becomes fact,” William Walsh warned his audience. “And in no way has [misinformation] been more detrimental for our management aims than with the introduction of two marine species, ta’ape and roi.”
Walsh, a biologist with the Department of Land and Natural Resources’ Division of Aquatic Resources, was instrumental in the establishment of marine reserves on the Kona coast a few years ago, set up to protect fish from over-exploitation by the aquarium trade. More recently, Walsh has looked into the widely held notion that these two non-native snappers are responsible for degrading near-shore marine ecosystems.
The two species were introduced in the mid-1950s, when fisheries managers for the territorial government noticed declining populations of native fish. To increase fish fauna, they brought in game and food fish that they felt would thrive in Hawai’i.
“The intention of the introductions was to improve the near-shore ecosystem,” Walsh said. Most fish didn’t survive, but ta`ape, also known as blue-lined snapper (Lutjanus kasmira) and roi (Cephalopholis argus) prospered. Fifty-five ta’ape arrived on O’ahu in 1958. Within 19 days, they had spread 50 miles, eventually spreading across the entire archipelago. “In one sense it was a wonderful natural experiment that obviously will never be done again,” he said.
Since then, ta’ape and roi have been blamed for out-competing native fish, degrading marine habitats, even eating rare aquarium fish. However, Walsh said, work by James Parrish of the Hawai’i Cooperative Fishery Research Unit shows that ta’ape don’t have a strong negative effect on adult native snappers in deep-water habitats.
Based on fish catches made by commercial methods and underwater observations, Parrish and others found very little habitat overlap with the native snappers; ta’ape populations occur in shallower waters. The introduced fish also show little aggression against native fish. What’s more, the ta’ape feed at different times than most natives. While ta’ape do share some habitat with opakapaka (pink snapper), these, too, feed at different depths and on different prey.
Since ta’ape live in shallow water, do they impact shallow-water fish populations? Shallow-water native species are declining in population, though ta’ape aren’t necessarily to blame. Many fish and invertebrate populations declined well before the introduction of ta’ape in the 1950s. And Walsh’s data show that if the fish are impacting anything, it’s Kona crab, whose population has declined since the ta’ape’s introduction and which has been found in ta’ape stomachs.
As for roi, it, too, was found to have little impact on native fish populations in Hawai’i. One trend Walsh reported was that the more roi at a site, the more piscovores (fish that eat other fish) are found there, too. Roi eat whatever fish are most readily available; contrary to a belief commonly held among fishermen, they do not target aquarium fish.
After dispelling lore and myth, Walsh’s advice regarding ta’ape and roi?
“Eat ’em!”
– Trevor Stokes
As natural resource managers consider re-introducing birds to areas where they once were prevalent but have since disappeared – due to disease or other factors – controlling mosquito populations looms as a huge issue. Determining the extent of mosquito habitat in native forest areas is thus a critical factor in assessing the suitability of areas for possible future bird release.
In 2001 and 2002, a team of researchers at the U.S. Geological Survey’s Kilauea Field Station surveyed a square kilometer of mesic-dry forest in Hawai’i Volcanoes National Park, collecting adult Culex quinquefaciatus mosquitoes on a monthly basis and dissecting them to determine whether they carried the parasite for avian malaria. The scientists – Matt Reiter, Julie Lease, Carter Atkinson, and Dennis LaPointe – also mist-netted ‘amakihi to determine rates of infection among the native birds.
Giving weight to the common wisdom that undisturbed native forests provide few opportunities for mosquitoes to breed, the researchers found that 95 percent of the larval habitats in the area surveyed were built, such as catchment systems and watering troughs. In only one case – a tree hole in an introduced species – were mosquitoes found in a habitat that was not manmade.
Generally, there was low abundance of mosquitoes in the surveyed area; the average capture rate was .052 mosquitoes per trap per night. And the fraction that carried the malaria parasite was small: 7 percent. “Despite apparently low mosquito densities,” Reiter and the others report, “malaria was moderately prevalent (18 percent) in the ‘amakihi population and there was active disease transmission.”
The discovery that in mesic-dry forests nearly all larval habitat is of human origin allows for easier identification and removal. Or, as the authors write, “the elimination or treatment of anthropogenic larval habitat should result in a break of malaria transmission in this mesic-dry forest, thus creating a disease-free area for the potential reintroduction of birds now extirpated from the park.”
– Patricia Tummons
Arthropods of ‘Ohi’a
Birds need bugs for food. But do native bugs need the birds? Daniel Gruner’s study of arthropods living in and around ‘ohi’a trees suggests they do.
The trees themselves play an important role in the web of an arthropod’s life: ‘Ohi’a (Metrosideros polymorpha) is the most widespread and abundant native tree in the islands. Living among ‘ohi’a are 7,000 species of arthropods from 145 families in 24 arthropod orders.
“Hundreds of endemic herbivorous and predatory species are found,” reported Gruner of the University of Hawai’i’s Zoology Department, including many specialized on ‘ohi’a. But most of the biomass of those species that eat detritus is made up of introduced species, he said.
In his three-year study of Big Island ‘ohi’a, Gruner tinkered with the relationship between bird, bug, and tree by draping ‘ohi’a plots with nets, fertilizing them, or doing both, and then compared these plots with control areas.
In plots where birds were excluded by the nets, Gruner found more spiders. He also found that detritus herbivores and native species were more abundant in fertilized plots.
“In rare evidence of biotic resistance, bird presence subdued the invasion of introduced species,” Gruner wrote. “Forest birds, even the introduced Japanese white-eye, may be important food web stabilizing factors. As ‘ohi’a lehua is the foundational resource in most Hawaiian natural areas, more attention to the integrity of the resident arthropod community is important to adequate management of Metrosideros forests.”
– Teresa Dawson
Most research on avian pox and malaria in native forest birds has taken place on Hawai’i, with little done to assess the disease rates on other islands. But last fall, Samuel Aruch and his team from the U.S. Geological Survey took a stab, casting their mist nets in the mountains and valleys of Maui and Moloka’i.
At six sites across five different elevations (2,200 to 4,700 feet) at Haleakala National Park’s Ka’eapahu and Kipahulu valleys, Aruch caught, banded, measured, and sampled the blood of dozens of native forest birds. Spinning the birds’ blood in a centrifuge, he searched for evidence of malaria in their red blood cells. He also looked for telltale disease-related lesions on their feet, bills and eyes. Aruch tested mosquitoes, as well, catching them in carbon dioxide and traps, and conducting larvae surveys of stagnant water.
Aruch’s results correspond to the general expectation that malaria is more common in the warmer climes. The higher – and cooler – one goes, the less prevalent the incidence of disease.
- At the lower elevations (2,200 and 2,500 feet), about a third of the mosquitoes were diseased.
- At 2,500 feet, about a fifth of 23 birds Aruch caught were diseased, the highest rate of all sites.
- At 3,000 and 3,200 ft, the birds’ infection rate dropped to 11 and 12 percent, respectively.
- No disease was detected at the 4,700-foot elevation.
Aruch’s work on Moloka’i was ongoing when he presented his data at the Hawai’i Conservation Conference. So far on Moloka’i, Aruch has found a 45 percent pox frequency in 31 natives birds caught, only one of which was an ‘amakihi (Moloka’i has only two native bird species left, ‘apapane and the ‘amakihi). Of 14 birds tested for malaria, 93 percent were infected. Despite this, he and his crew collected just one infected mosquito, suggesting that there is little correlation between the rate of infection among mosquitoes and that among birds.
– T.D.
Ancient stone fences run across the hills of the Kohala district of the Big Island. These, according to Hawai’i Project Meeting organizer Peter Vitousek, mark the boundaries of farmers’ fields and testify to “an amazingly intensive dryland agricultural system that raises for us all sorts of questions about how [native Hawaiians] chose their sites over time, and how their use of land interacted with the development of society.”
One site extensively studied by Vitousek is amazing indeed. Over a 9-mile transect from the top of a hill to the bottom, annual rain fall ranges from 12 feet to 3 inches.
How did ancient Hawaiian farmers maintain intensive agriculture for hundreds of years throughout this site? And how did the changes in the agricultural system affect the surrounding land?
To get at the answers, Vitousek’s group worked with Oliver Chadwick of the University of California at Santa Barbara to examine the soils in the region. They found two types of soil in Kohala, a younger, mineral-rich series about 150,000 years old, and an older, mineral-leached soil dating back about 400,000 years.
An important factor in the soils’ ability to support agriculture is precipitation, the scientists found. In another presentation, Lars Hedin of Princeton University described that in wetter soils, the water displaces air in the soil. Microorganisms and plants use up all the oxygen and produce an oxygen-poor environment. This increases the acidity of the soil, causing it to release minerals and metal. The drier the soil, the fewer the nutrients released.
Vitousek found that a “sweet spot” exists in soils, where precipitation balances the nutrient availability and soil type. This allowed the Hawaiian farmers to intensify agriculture in a narrow strip of land.
Hawaiian farming practices not only changed the soil chemistry in the region, but made it better for agriculture. Phosphorus, a nutrient necessary for life, dramatically illustrates how long-term land use can alter the chemistry of soils. In the regions intensively farmed, Vitousek found “very significant enrichment of phosphorus” that isn’t found anywhere else on the Kohala hill transect, giving evidence that plants actually increased the lands’ arability.
To better understand what practices ancient Hawaiians used to change the soil, Vitousek used niobium as a reference mineral to follow the fate of phosphorus. Vitousek assumed that Hawaiians mulched their cultivated lands, which would bring in extra phosphorus without extra niobium. This was consistent with their observations.
While the stone walls run along the contours of the hill, the trail system runs up and down it. This summer, Vitousek and his group planned to look at the soils under the walls, hoping they might provide an idea of the history of certain soils before they were influenced by intensive agriculture over time.
– T.S.
In another study about ancient farming methods, Patrick Kirch of the University of California at Berkeley discussed his research into Hawaiian life at Kahikinui, Maui. There, he and his colleagues have found a ritual garden and a surprising connection among soil quality, habitation sites, and farms.
The Kahikinui study area is 13 square kilometers, over which his group walked “every square meter.” They found about 2,100 Hawaiian archeological stone structures, most at 250 to 800 feet above sea level.
Agriculturally, “there is no formalized field system like Kohala,” Kirch said. There are, however, “mini field systems in some regions with parallel field walls called swales, and endless stone mounds; we actually found ritual gardens enclosed that have branched coral offerings. This is the first evidence of a ritual garden in Hawai’i.”
Throughout the region, the age of the soil ranges from 8,000 years old to 200,000 years old. Hawaiian settlers immigrated to the region by 700 A.D. if not earlier, and tilled the soil for at least 300 years there. Kirch has observed that the house sites tend to be found on the poorer soils, adjacent to the nutrient-rich kipuka. It is still unclear, Kirsch said, whether the ancient people selected the sites for growing plants or whether by growing plants in a concentrated area, the people further enhanced the mineral and nutrient content of the soil.
– T.S.
Koa, a valuable and beautiful native hardwood, has both economic and ecological importance. But when it comes to understanding koa growth and silviculture, “we don’t really have any data,” according to Patrick Baker, a postdoc with the U.S. Department of Agriculture’s Forest Service and The Nature Conservancy.
With the constant market demand for koa, Baker’s research aims to develop practical, sustainable silviculture alternatives for landowners who want to grow koa. At one research site at Honomalino, South Kona, the spindly but numerous koa trees that have sprouted in the wake of clearing by commercial loggers now form a dense “dog-hair” forest, worthless for all practical purposes but a great site for research.
By thinning plots in the dog hair and measuring the growth of the trees, Baker seeks to find the maximum allowable density of koa that still allows it to grow to its full potential. So far, Baker has found that the trees in thinned areas increase their girth within the first two months, a fast response for trees. In addition, there is two to three times more growth six months after the trees are thinned.
Baker is also finding that the species composition in the neighborhood of the koa tends to have an effect on the koa’s growth. The more ‘ohi’a, the more koa growth, he says, although no one is venturing to say why.
– T.S.
Once an exotic plant takes hold of a new place, it often grows to greater stature and abundance than it did in its home range. It can even expand its range of habitat.
Julie Denslow of the U.S. Forest Service looked into whether genetic or environmental factors (such as lowered pressure from herbivores) can account for the invasiveness of certain species. As a first step in discovering what factors promote invasiveness, Denslow and others looked at clidemia (Clidemia hirta), a native to American neotropics that has become invasive across the Pacific islands and in Asia and Africa.
By growing clidemia in controlled greenhouse conditions, the researchers found that Costa Rican and Hawaiian populations don’t really differ in their growth rates. However, when herbivory is taken into account, Denslow found that pests do make a difference in the plant’s invasiveness.
“In Costa Rica, there is a pest pressure that has affected the survivorship of these plants in the understory but not in the open,” she says. So, in the case of clidemia, it looks like both genetics and the environment play roles in determining the plant’s invasiveness.
— Trevor Stokes
Volume 14, Number 4 October 2003
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