Invasive mynas a disease threat to native birds

Is the Indian myna (Acridotheres tristis) — one the world’s worst avian pests — waging biological warfare to gain a competitive advantage over Australia’s native birds as they spread through eastern Australia?

PhD scholar Nicholas Clark, of Griffith University’s Wildlife Disease Ecology Group, has found that Indian mynas introduced into Australia in the mid-19th century harboured two protozoa blood parasites: Plasmodium, the agent of avian malaria, and Haemoproteus. Both are transmitted by bloodsucking mosquitoes and midges, and parasitise the birds’ red blood cells.

Highly social and aggressive, Indian mynas are now abundant in many urban areas in eastern Australia and are spreading into surrounding rural regions, where they compete with and displace Australia’s native mynas and other native bird species.

Clark saw an opportunity to test two competing hypotheses for the success of invasive species.

The Enemy Release Hypothesis proposes that invaders proliferate because the diversity and numbers of natural predators and pathogens are greatly reduced in the new environments they colonise. The rival Novel Weapons Hypothesis proposes that invaders harbour novel pathogens that spill over and suppress native competitors.

If the latter hypothesis holds true, Clark believes Australian native birds that are immunologically naïve to the Indian myna’s Plasmodium and Haemoproteus blood parasites could be threatened as the mynas spread into rural regions.

The mynas themselves have probably reached an evolutionary truce with their native pathogens and are not significantly affected by the parasites. But both pathogens have the ability to parasitise a potentially broad range of native birds, including Australia’s native parrots — from which mynas diverged in the Late Cretaceous, over 65 million years ago.

Clark and his colleagues analysed the prevalence and diversity of strains of the Plasmodium parasite in Indian myna populations — the Haemoproteus parasite was detected in museum specimens from the Melbourne area, the original site of introduction, but appears to have dropped out when the mynas were transplanted to Sydney and Brisbane.

In contrast, Plasmodium infections were common in Indian mynas and significantly more common than in native mynas — although Australian mynas shared some of the strains found in their Indian cousins.

Clark and his colleagues believe introduced mynas have benefited by escaping from their natural Haemoproteus parasite strains but are important reservoirs for different species or strains of Plasmodium.

Indian mynas are one of only three birds on the International Union for Conservation of Nature’s list of the world’s Top 100 Invasive Species. They were deliberately introduced into Melbourne between 1862 and 1873 in a misguided attempt to control insect pests in the city’s market gardens.

They were subsequently transplanted to Sydney, then to Queensland where, ironically, their failure to control cane beetles and other sugar cane pests led to the later introduction of an even worse invasive species: the cane toad.

Image caption: Nick Clark with an avian friend.