When any given habitat is overrun by an introduced species, it is sound logic to predict that other species sharing the same space will have a tough time of it.
A team of biologists from the Utah State University in the US, however, has found that cause and effect is not always quite as linear.
The team, led by Robyn Smith from the university’s Ecology Centre, set out to determine the impact of an introduced species of frog – called the coqui – on native bird species in Hawaii.
The coqui (Eleutherodactylus coqui) is native to Puerto Rico and was accidentally introduced to Hawaii in the 1980s. Since then its population has exploded, reaching as high as 91,000 frogs per hectare in some parts of the island chain.
The frogs eat insects, and the researchers’ primary question was whether this reduced the availability of food for the island’s insect-eating birds, many of which are listed as endangered.
From another perspective, they noted that the frogs made an abundant food source for predatory birds, such as the Hawaiian Hawk (Buteo solitarius) and Hawaiian Short-eared Owl (Asio flammeus sandwichensis). Was the end result of this more hawks and owls to prey on nestlings?
The answers, as it turns out, are surprising.
To test their hypotheses, the team identified 15 sites on the island of Hawaii itself, and estimated frog and bird density for each. Frog population was estimated through listening for calls, followed up by visual searchers. There were 20 species of birds logged at each site, with only five of them native.
Surprisingly, the team found that the native bird numbers were unaffected by the relative density of frogs in their vicinity. The researchers suggest that this may be because the frogs forage in leaf litter, while the birds hunt for insects in branches high above the ground. Thus, there is no competition for food.
Introduced bird species, however, such as Common Mynas (Acridotheres tristis) showed a definite relationship to frog numbers. The greater the density of coquis, the more exotic birds.
Smith and colleagues think this might be because of three factors. First, some of the non-native birds may be eating the frogs. Second, the abundance of dead frogs is in turn providing a boost to rotting tissue in the leaf litter, fuelling an explosion in the fly population – providing another food resource for the birds.
Third, the increased biomass the frogs constitute might be boosting nutrient cycling through some habitats. This might explain the increased numbers of birds such as finches that don’t eat frogs or flies.
“I was very surprised with the results for birds. It had been hypothesised before our study that coquis would compete with birds, particularly natives, because we know that coquis reduce insects where they invade,” says 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 researchers found no evidence to suggest well-fed hawks and owls were eating above-normal levels of native birds.