Counting Sparrows

A wide-ranging international study involving a USF biologist sheds new light on how invasive species thrive.


Nico Nalianya, a research assistant, installs a bird house in Kenya as part of a USF research project studying sparrows. Photo: Courtesy of Andrea L. Liebl, Martin Lab.


By Vickie Chachere

USF News


TAMPA, Fla. (Aug. 4, 2011) – The adorable but destructive house sparrow – considered one of the world’s most successful invasive species – is able to proliferate in new lands by shedding their homeland parasites rather than unleashing new parasites on native birds, a major international study has concluded.


The study, published recently in the journal PLoS ONE, includes contributions from USF Integrative Biologist Lynn “Marty” Martin as part of its international team of researchers examining how the sparrows have managed to invade environments worldwide and thrive at a cost to native birds.


The study was conducted to better understand how animals invade new habitats, a process becoming all the more important as people routinely spread organisms all over the world through commerce.


Ultimately, this work may lead to better methods of predicting and preventing the loss of native species to invaders, a phenomenon that comes with huge environmental and economic costs. Battling invasive plant and animal species costs the U.S. $120 billion a year in crop damage and biodiversity loss, Cornell University researchers report.


Scientists have long focused on diseases carried by invasive species as a way invaders can come to dominate new environments, and the new study zeroed in on one parasite commonly carried by the sparrows: avian malaria. Avian malaria, related to but distinct from the parasite that infects humans, is the most common blood parasite affecting birds and is transmitted by biting insects.


House sparrows, also known as English sparrows, spread rapidly across the globe since they were introduced from Western Europe about 150 years ago, often by hitching rides on ships or being purposely introduced by humans through naturalization societies – groups intending to re-populate their gardens in the New World with animals and plants from their homelands.


Aggressive and often crowding out native species, the small but charismatic sparrow is both adored (it was mentioned in Shakespeare’s sonnets) and reviled for its voracious appetite for grain (it has been the target of organized eradication programs).


Researchers from 18 universities in the United States and Europe fanned out across the world to collect blood samples from sparrows from 58 locations on six continents to examine what role the malaria carried by sparrows might have played in their incredible global success.


The scientists considered two competing hypothesis: the Novel Weapon Hypothesis, which suggests that colonizing birds may facilitate their own success by bringing new diseases and negatively impacting native species; and the Enemy Release Hypothesis, which suggests that colonizers succeed because they leave their parasites behind.


Using small blood samples, the researchers compared genetic variation in the parasites to determine that Enemy Release was more reasonable than Novel Weapons: house sparrows in North and South America as well as Africa and Australia , areas where the birds were introduced, were less likely to be infected and carried fewer types of parasites when infected, compared to birds from Europe and Asia, the native range of the species.


“House sparrows lose their parasites and don’t get them back,” said Martin, who with Courtney Coon and Andrea Liebl, two graduate students in his laboratory, have been studying the invasion of house sparrows in Kenya.  There, the birds have rapidly moved from the port city of Mombasa inland to the Ugandan border in just a few decades.


“A lot of the parasites that sparrows carry are not relevant to humans, but they do spread to other birds, so understanding how animal invaders colonize new areas can be important for the emergence of novel diseases in wildlife.”


The study was funded by the National Science Foundation, the Swedish Research Council, the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning, the Spanish Ministry of Education and Science, the Norwegian Research Council, the British Ecological Society, and a Romanian Ministry of Education and Science grant.


More on the Martin Lab’s work in ecological immunity and evolutionary medicine can be found here.


Vickie Chachere can be reached at 813-974-6251.