While we already know how destructive invasive species can be on native populations, when the advancing threat of climate change is added to the picture, we encounter some extraordinarily devastating consequences. Hawaiian honeycreepers are a unique group of birds belonging to the finch family, found exclusively in the Hawaiian Islands. An equivalent to Darwin’s finches in the Galapagos, over 50 distinct species have evolved from the original colonists to fill a variety of ecological niches.
Each honeycreeper is a specialist, meaning that they have specific diet and habitat requirements. The diverse phenotypes are informative as to what the preferred food source is. A slender curved beak indicates an insect and nectar-feeding bird, while a short, broad beak is a sign of a bird that feeds on nuts and seeds.
Before the introduction of mosquitos in 1826, bloodsucking insects did not exist in Hawaii. Mosquitos act as effective vectors for pathogens, transmitting them between hosts as they feed. By the early 1900s, at least two avian diseases were present – avian pox and avian malaria. Ironically, these were most likely introduced by imported birds that were released in an attempt to replace the declining populations of native birds. Songbirds like honeycreepers are especially susceptible to avian malaria, which was spread very quickly via mosquito vectors. Unlike most diseases where pathogens and hosts coevolve, allowing the host to develop some resistance, honeycreepers are naive hosts without any previous exposure to this type of disease. With mortality rates as high as 90% and increased predation pressure by invasive predators, honeycreeper populations have been devastated over the past several decades.
In addition to the disease itself, healthy forest habitat is disappearing and being degraded due to human development and competition with non-native plants, which creates a more stressful environment. This lowers the bird’s immune systems, as energy that would normally be spent investing in defense against disease must be allocated to finding food and appropriate habitat. Invasive wild pigs have contributed to a steep rise in the number of mosquitos by digging up forest floor and creating ideal breeding grounds, further increasing disease transmission.
Since these birds live in such an isolated location and can’t escape the disease by moving elsewhere, their only option is to flee to higher altitudes where mosquitos cannot survive. While this offers temporary refuge, the increasing global temperature is allowing mosquitos to expand their range and move into higher elevations. The “mosquito line” falls around 1500 meters, but is steadily rising as it essentially squeezes the birds into a continually shrinking habitat. Only 19 species of honeycreepers remain in upland forests, and several are listed as critically endangered by the International Union for the Conservation of Nature.
Some creative management plans are currently in the works in an effort to help honeycreepers deal with avian malaria, and aid in population recovery. Genetically manipulated, sterile male mosquitos are being released into forests to hinder reproductive success of the mosquitos, thus reducing their population size over time. Ex situ breeding programs are currently breeding honeycreepers as a way to preserve them in an artificial environment. This strategy is useful as it allows researchers to learn specifics about the physiology of the birds and can provide insight as to what can be done to help wild populations. Additionally, predatory mammals like mongoose, cats, rats and wild pigs are being trapped and removed as a way to keep non-disease related mortality as low as possible.
And finally a bit of good news. It has recently been discovered that at least two species of amakihi honeycreepers have adapted some resistance to avian malaria, and have begun repopulating lowland forests where malaria is rampant. This affirms that these birds do have the ability to evolve and have a chance at fighting the disease, given adequate time. The critical unknown factor is the rate at which globally increasing temperatures will allow mosquitos to advance into higher altitude forests and reach the birds. The hope is that the current management actions will mitigate the disease long enough that the birds will have a chance at developing resistance.