2002 Progress Report: Habitat Degradation and Introduced Diseases Stress the Endangered Hawaii AkepaEPA Grant Number: R829093
Title: Habitat Degradation and Introduced Diseases Stress the Endangered Hawaii Akepa
Investigators: Freed, Leonard A. , Cann, Rebecca L. , Goff, Lee M.
Institution: University of Hawaii at Honolulu , Chaminade University of Honolulu
EPA Project Officer: Packard, Benjamin H
Project Period: July 1, 2001 through June 30, 2004 (Extended to October 25, 2006)
Project Period Covered by this Report: July 1, 2001 through June 30, 2002
Project Amount: $510,375
RFA: Wildlife Risk Assessment (2001) RFA Text | Recipients Lists
Research Category: Biology/Life Sciences , Ecological Indicators/Assessment/Restoration , Aquatic Ecosystems
The objectives of our research project deals with two stressors on the Hawaii akepa (Loxops coccineus coccineus), an endangered Hawaiian Honeycreeper that nests obligately in cavities, which are only found in large trees. One stressor is infectious disease, which is assumed to limit the current distribution of this bird to upper elevation forests, in which the temperature is too cool for the introduced Culex quinquefasciatus mosquito or the Plasmodium relictum malarial parasite during its sexual stage inside the mosquito (asexual stage is inside the avian host). With climate warming, both the mosquito and malarial parasite will fare better at upper elevations. The increase in range of the mosquito also will increase transmission of avian pox virus at upper elevations. This disease causes inflammation of toes, feet, ankles, and bills, with frequent loss of these parts, and it is potentially deadly. Hawaiian birds evolved in the absence of mosquito-transmitted diseases for 5 million years; their immune systems may not deal effectively with diseases that have suddenly appeared.
The second stressor is habitat degradation that operates at upper elevations. Old-growth forests at upper elevations have been disturbed by cattle ranching for over 150 years. This has resulted in the replacement of much of the forest floor with introduced grasses that form a pasture. In addition, many trees were cleared to make room for expansive pasture. Strong winds during winter storms and hurricanes permeate the habitat with less interference by intermediate size trees. The result is that large trees with cavities are falling at a rate of approximately 2 percent per year. Tree growth is so slow that it is likely that cavity trees are falling faster than they can be replaced.
Therefore, the Hawaii akepa has a dual problem: (1) carrying capacity is reduced at upper elevations, where the birds are relatively safe from disease; and (2) climate warming is expanding the range of disease from lower elevations to upper elevations. A major issue is whether tolerance to disease will evolve fast enough to prevent extinction.
Our approach to these stressors is a study of the Hawaii akepa, the forest, the mosquito vector, and the pathogens along an elevational gradient at Hakalau Forest National Wildlife Refuge on the windward slope of Mauna Kea on the Island of Hawaii. One objective is to document forest structure at three study sites along the gradient so that carrying capacity based on numbers of large trees with cavities can be compared with the censused population size of the bird. This will identify study sites in which the population is below carrying capacity. The second objective is to compare the prevalence of disease and mosquitoes along the gradient. This will determine the extent to which mosquito-transmitted disease is maintaining host population size below carrying capacity. The third objective is to compare genetic population structure of the bird along the gradient. This will determine net direction of dispersal. With this information, we can determine if genotypes that might be tolerant or resistant to disease are likely to disperse to upper elevations or if managers must move appropriate genotypes throughout the population.
We conducted a census of endangered birds, while inspecting potential study sites at lower elevations. This was the first census at this elevations since the Hawaii Forest Bird Survey during the late 1970s. We performed the census during the early breeding season when birds are most conspicuous. We were surprised to find a very rapid decline in numbers over a distance of 400 m in continuous forest. Between elevations of 5,000 ft and 4,830 ft, the birds became extremely rare. Large trees also became very rare over this distance. Therefore, it is possible that the virtual end of the range at this point was based in part on carrying capacity. However, there were many downed tree ferns with pockets that serve as oviposition sites for Culex mosquitoes. The forest bird survey documented birds down to an elevation of 4,500 ft. Our finding may indicate that the mosquito line has risen several hundred feet in the last 25 years. Support for this interpretation comes from the rarity at 4,830 ft of the endangered Hawaii creeper, which does not depend on big trees such as the Hawaii akepa.
At the upper elevation site, where the birds exist at highest density, we documented an epizootic (equivalent to an epidemic for human diseases) of avian malaria and pox virus. We captured an adult male Hawaii akepa that was missing a toe with an exposed nerve, indicating an active infection. This bird was shivering, which is symptomatic of malaria. Unfortunately, the bird's peripheral blood circulation was inactive. We were unable to obtain a blood sample to diagnose malaria. In the iiwi, a Hawaiian Honeycreeper considered a species at risk, we documented malaria in 40 percent of 20 individuals sampled. This compares with 0 percent of 206 individuals sampled between 1988-1991. This is a strong indication that climate warming is increasing the exposure of birds to mosquitoes that are infectious (meaning the sexual phase of the malarial parasite is completed). In addition, 29.0 percent of apapane (another Hawaiian Honeycreeper) sampled tested positive. This compares with 10 percent that tested positive during 1988-1991 in the same site. There was an increase in prevalence in Hawaii amakihi from 2.6 percent to 14.3 percent. We had the highest proportion of birds with active pox virus infections in 14 years of study, including some of the most virulent infections yet documented.
We sampled mosquitoes at upper elevations using buckets filled with an infusium of water and dead grass, or an infusion of water with rabbit alfalfa and brewer's yeast. During the time of the epizootic, a Culex mosquito laid eggs that hatched into larvae in a bucket filled with water and dead grasses. This is the first record of the vector breeding in the core population of endangered birds. We did not detect mosquitoes in the buckets during 2002, so disease at upper elevations is still episodic.
The good news in this year of study is that the male akepa and two of the iiwis that tested positive were known to survive. All these cases occurred during the fall of 2001. All three birds, with unique combinations of color bands on their legs, were known to be alive (observed through binoculars) during the late spring of 2002. The Hawaii akepa is the first record of a sick endangered bird, with pox virus and probably malaria, surviving. The hatch year iiwi is the first record of a young iiwi with malaria known to have survived. This is because previous challenge experiments performed with juvenile iiwi had 100 percent mortality. The adult iiwi is the first record of tolerance of malaria in a wild iiwi.
During the second year of this research project, we will study birds, forest, mosquitoes, and pathogens at a study site at an accessible elevation of 5,000 ft. Preliminary indications are that the birds exist below carrying capacity; and disease is the most likely factor. This is the best spot to discover if tolerance or resistance to malaria is evolving. We will use compressed elevational gradient to document gene flow. In addition, we will screen all blood samples collected since 1987 for malaria, making this the longest term study of disease in birds. We will be providing artificial nesting cavities at the 5,000-ft site to establish carrying capacity experimentally and to compare nestling death with that at upper elevation site.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
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|| Freed LA, Cann RL. On polymerase chain reaction tests for estimating prevalence of malaria in birds. Journal of Parasitology 2003;89(6):1261-1264.