Researchers have discovered an unexpected relationship between non-native frogs and non-native birds in Hawaii. It turns out that one thriving, invasive frog species has boosted the numbers of several invasive bird species.
In the 1980s, Puerto Rican coqui frogs were accidentally introduced to Hawaii. Today, there are as many as 91,000 frogs per hectare in some areas.
Scientists from Utah State University were concerned about the impact of these abundant frogs competing with birds for food. The researchers set up 15 observation sites on the island of Hawaii to investigate.
At each site, the experts determined if coquis were present by listening for their vocalizations. A visual search revealed the numbers of the frog populations at these sites.
In addition, the scientists found a total of 20 species of birds present, only 5 of which were native. To their surprise, the researchers discovered that the native birds were unaffected by the presence and density of coquis, and that three non-native species of birds were actually thriving in areas where the coquis were residing.
In sites where no coquis were present, there was an average of 57 non-native birds. In sites where coquis density was highest, there was an average of 97 invasive birds.
The researchers explained that the coquis may not be in direct competition with birds for food because they tend to forage in leaf litter as opposed to areas surrounding the canopy where birds forage. The results of the study also suggested that non-native birds may be directly benefiting from coquis by eating the frogs, or may be indirectly benefiting from larger fly populations which are drawn to biomass created by the frogs.
“I was very surprised with the results for birds. It had been hypothesized before our study that coquis would compete with birds, particularly natives, because we know that coquis reduce insects where they invade,” said study co-author Karen Beard.
“In retrospect, I guess it’s not too surprising that predation is a more important interaction than competition–that is a common finding in invaded systems–but it was definitely not what we went in to test.”
The study, which is published in The Condor: Ornithological Applications, may lead to future experiments to reveal details underlying the unexpected dynamic between the non-native frog and bird species.
Image Credit: R. Choi