Hawaiian honeycreeper conservation
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Hawaiian honeycreepers (Fringillidae), of the subfamily Drepanidinae, were once quite abundant in all forests throughout Hawai'i.[1] Only less than half of Hawaii's previously extant species of honeycreeper still exist. The subfamily Drepanidinae historically consisted of at least 51 species.[1]
The honeycreepers are threatened by recently introduced predation, competition, parasitism, degradation of habitat, and infectious disease including mosquito-borne avian malaria.[2] One of the consequences of the invasive birds is the introduction of avian malaria. The pathogen is primarily transmitted via female mosquitoes who will pass on the disease by biting a susceptible individual after having bitten an infected individual. The main mosquito vector (Culex quinquefasciatus) was introduced over a hundred years before the pathogen (Plasmodium r. capistranoae), mostly hosted by the Blue-breasted Quail (Coturnix chinensis).[2] Later, two other mosquito species, the Asian tiger mosquito (Aedes albopictus) and Wyeomyia mitchelii were introduced to the islands.[2] The immune system of the honeycreepers had not been exposed to avian malaria since it’s common ancestor existed 4 to 5 million years ago.[2][1] Thus, the honeycreepers had not co-evolved with the pathogen to develop resistance as those birds on the mainland did.
In the 1970s, a comprehensive survey (Hawai’i Forest Bird Survey) found native birds had retreated from mid or low elevation forest and had been replaced by exotic species but competition was not documented between them and the native species.[2] During the same time, the elevational limit of malaria was established to be approximately 1500m.[2] Above this elevation the mosquito vector could not subsist due to the low temperatures. Due to malaria, those species highly susceptible must reside between 1500m and 1900m but climate warming may move that line higher until these species no longer have a refuge.
Degradation of habitat for the Hawaiian honeycreepers has also been a main cause for the radical decrease in their population numbers. Colonization of the Hawaiian islands has led to extensive deforestation to make way for agriculture, ranching, and other development. Furthermore, where the forests are still intact, introduced pigs and goats have done considerable damage to habitat. Other destructive invasive species include cats, who feed on birds, especially those who are naive to predators (such as Hawaiian honeycreepers).
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[edit] Hawaiian honeycreeper conservation
Efforts to conserve the remaining species are of great interest and a couple of different methods have been described.
[edit] Remove mosquito vector
There are a few strategies for mosquito removal which include the reduction of mosquito breeding sites by: chemical and biological control agents, genetic manipulation of the population, and removal of feral ungulates from critical forest habitats.[4] The goal is to eliminate the mosquito populations using herd immunity, which does not require the unfeasible eradication of every individual mosquito. Another strategy requires releasing genetically manipulated sterile mosquito males into the wild every generation and as a consequence the mosquito populations diminish over time.
[edit] Captive breeding
In many cases habitat protection is not occurring fast enough for critically endangered Hawaiian honeycreeper species to keep their populations afloat. The ZSSD and TPF have established management programs aimed at breeding these vulnerable bird species in captivity and releasing them back into the wild.[5] As reported in 2000, the major challenge for the program did not include successfully breeding the birds in captivity but finding suitable habitat to release them.[5] Thus, habitat management and restoration must be rigorously ensured before this breeding program can be secured.
[edit] Clearing Habitat of invasive species
The Hawaiian honeycreepers are generally specialists both in diet and in habitat. This has left them very vulnerable directly and indirectly to the generalist invaders that have been introduced to the islands.[6] Other birds have provided direct competition for resources with the honeycreepers as well as brought disease (such as avian malaria). However removing the introduced birds is difficult due to their inaccessibility to humans and high dispersal ability. Introduced ungulates include pigs and goats. Removal of large vertebrates requires both fencing and direct removal of the animals. In places where pigs have been removed, vegetation has begun to recover.[7] However, Hawaiian honeycreeper numbers are still in decline and may be due to introduced predators: feral cats, mongooses, and 3 species of rats.[7]
[edit] Hope: the 'Amakihi
Hawai’i ‘Amakihi (Hemignathus virens) is one of seven extant honeycreeper species on Hawai’i Island.[1] It’s a small generalist that has historically shown high mortality rates due to malaria infection.[1] Surprisingly, they have been found at low altitudes (below 400m) despite their exposure to the pathogen. 90% of these birds showed they had contracted and survived the disease.[1] This finding has raised the possibility that the species may be evolving resistance to avian malaria, however this may be only a localized event.[1]
[edit] See also
[edit] References
- ^ a b c d e f g Spiegel, Caleb S.; Patrick J. Hart, Bethany L. Woodworth, Erik J. Tweed, and Jaymi J. LeBrun (2006). "Distribution and abundance of forest birds in low-altitude habitat on Hawai'i Island: evidence for range expansion of native species" (PDF). Bird Conservation International 16: 175–185. doi:.
- ^ a b c d e f Freed, Leonard A.; Rebecca L. Cann, M. Lee Goff, Wendy A. Kuntz, Gustav R. Bodner (November 2005). "Increase In Avian Malaria At Upper Elevation In Hawai'i" (PDF). The Condor 107 (4): 753–764. doi:.
- ^ a b c d e f "2006 IUCN Red List of Threatened Species", IUCN, 2006. Retrieved on 2007-04-26.
- ^ Jacobi, James D.; Carter T. Atkinson. "Hawaii's Endemic Birds", U.S. Department of the Interior, September 28, 2000. Retrieved on 2007-04-26.
- ^ a b 2000 Hawaiian Endangered Bird Conservation Program, Annual Report to: USFAW/DOFAW/KSBE/BRD/ZSSD/TPF
- ^ Amarasekare, Priyanga (Feb., 1994). "Ecology of Introduced Small Mammals on Western Mauna Kea, Hawaii" (PDF). Journal of Mammalogy 75 (1): 24–38. doi:.
- ^ a b Rosa, Karen; Dave Hopper, Sharon Reilly. "Draft Environmental Assessment for Possible Management Actions to Save the Po`ouli", US Fish and Wildlife Service, September 1998, pp. 1-76. Retrieved on 2007-05-16.
