Malaria is not a “privilege” of humans. Though mostly known for its devastating impacts on people worldwide, bird populations have also suffered the consequences of infections by this group of parasites. On the early 20th century, the mosquito Culex quinquefasciatus and the parasite Plasmodium relictum were introduced to Hawaii, and by the end of the century had caused the extinction and population decline of several forest birds, in especial the endemic honeycreepers (subfamily Drepanidinae). Since then, the surviving honeycreeper populations have shifted their range to higher and cooler elevations in the Hawaiian archipelago. More recently, some populations have been colonizing lower elevation areas, and microsatellite data suggests that birds from high elevations, which are genetically resistant to malaria, are the ones dispersing back to the original species range. The fact Hawaiian honeycreepers dispersed and re-colonized lower elevation areas was only possible because populations in the cooler and malaria-free environment of higher elevations could grow back in numbers again. Nonetheless, the recovery process of honeycreepers populations is being affected by global warming. Capture-mark-recapture studies with C. quinquefasciatus show that elevated temperatures not only increase the number of infected mosquitoes, but also allow them to disperse to higher elevations. In addition, the main reservoir of avian haemosporidian parasites in Hawaii, the Omahi elepaio (Chasiempis sandwichensis) has also expanded its range towards higher elevations, an event also associated with warmer atmospheric temperatures.
The Hawaiian problem with bird malaria, and its recent re-emergence due to climate change, has been serving as a piece of advice for conservation biologists and environmental policy makers in order to avoid that the same happens elsewhere. In birds, malaria is just one of the diseases caused by parasites of the order Haemosporida. The devastating example of Hawaiian birds is perhaps the most well documented effects of Plasmodium sp. parasites on birds populations. However, it’s important to keep in mind that contrary to most of the birds that carry haemosporidian infections worldwide, Hawaiian birds have only been exposed to these parasites more recently, and did not evolve to deal with such a disease. Therefore, the effects of haemosporidian parasites on bird populations that have been dealing with such threat over thousand of years remains poorly investigated. In a recent paper published by our group, we look at the effects of these parasites on bird populations of the West Indies, where hosts have been exposed to parasites over longer periods of time. We looked at the correlation between the frequency of 12 genetically distinct strains of haemosporidian parasites and the abundance of 9 bird populations across 13 islands of the Lesser Antilles.
We found a negative relationship between the frequency of parasite strains and the abundance of birds on Lesser Antillean islands. This is groundbreaking because the size of bird populations vary across islands with no obvious ecological explanation. For instance, Lesser Antillean bullfinches are highly abundant in most islands of the archipelago, except for Carriacou in the Grenadines, where these birds are completely absent, despite the suitable ecological conditions for their occurrence. We believe that mortality caused by haemosporidian parasites explain the negative relationships found between pathogen strains and bird abundance. Remarkably, some parasite strains were positively related to the size of bird populations of species that we never recorded infected with such strains. This suggests that some bird species might benefit from the presence of the parasites if they decrease the population size of competing bird species. This is the classic “the enemy of my enemy is my friend” type of situation.
Check out our paper here: