Study helps explain island populations’ susceptibility to exotic diseases

Researchers have shown that Darwin’s finches on smaller islands in the Galapagos archipelago have weaker immune responses to disease and foreign pathogens—findings that could help explain why island populations worldwide are particularly susceptible to disease.

A paper, written by University of Michigan researcher Johannes Foufopoulos, an assistant professor at the School of Natural Resources and Environment who specializes in disease ecology, and collaborators from Princeton University and the University of Upsalla, investigates the relationship between immunological investment (how developed is the body’s immune system), native parasite abundance, and island size. The findings were published online June 8 in the Proceedings of the Royal Society of London.

The paper helps scientists understand how island populations respond to invasive parasite species. The introduction of exotic parasites and diseases through travel, commerce and domestic animals and the resulting destruction in native wildlife populations is a worldwide problem, Foufopoulos said, but it’s even more serious for species that have evolved on islands.

For example, in the Hawaiian islands, many native bird species have gone extinct after the introduction of avian malaria, he said. The Galapagos authorities are now realizing that the greatest danger to the islands’ wildlife comes from exotic species, such as invasive pathogens, accidentally introduced by humans.

The study shows that people on islands have different immune systems “and this may be the explanation for their susceptibility to invasive diseases,” Foufopoulos said.

The team found that larger islands with larger bird populations harbor more native parasites and diseases, because the number of parasites is directly dependent on the size of the population. Island size and parasite richness then influenced the strength of the immune response of the hosts.

The researchers tested two types of immune responses—cell-mediated responses and production of antibodies–in four populations of Darwin’s finches. By challenging the birds immune systems with foreign proteins, they measured the average immune response of each island population.

Finches on smaller islands with fewer parasites had a weaker immune response, Foufopoulos said. For these birds, Foufopoulos said, “maintaining a strong immune system is a little bit like house insurance: you don’t want to spend too much on an expensive policy if you live in an area with no earthquakes, fires or floods.”

Similarly, if parasites are scarce, the birds don’t need to invest in an “expensive” immune system, he said.

When new parasites are then accidentally introduced by humans to these islands, the birds are ill-prepared to resist infection.