Black rats: the bad, and, oddly, the good.

Tree climbing, omniverous and fast-reproducing black rats are among the scourges of the Hawaiian natural landscape.

They destroy nesting native birds and their eggs, they eat native plant seedlings, they can eat a native loulu palm's entire season's production of seeds—and there's evidence that when they are removed, much of the native habitat can recover.

(Image: We couldn't quickly put our hands on a black rat photo. This is Hawai'i's first rat, the Pacific or Polynesian rat, which arrived with Polynesian settlers. Credit: New Zealand government.)

But there's another side to this story. Rats, though invasive themselves, can also keep invasive species at bay—and in some conditions, removing rats can create new problems.

University of Hawai'i zoologist Wallace M. Meyer III and University of Hawai'i botanist Aaron B. Shiels review some of the complicated issues in a new paper in Pacific Science, “Black Rat (Rattus rattus) Predation on Nonindigenous Snails in Hawai‘i: Complex Management Implications.” (Pacific Science (2009), vol. 63, no. 3:339–347: 2009 by University of Hawai‘i Press)

The paper, as its title suggests, is focused on snails.

Black rats are considered a major threat to Hawai'i's gorgeous native tree snails, which were once common but are now fadingly rare, and many species are extinct.

But rats also eat the cannibal snail, Euglandina rosea, which also preys on the tree snails. Furthermore, they eat the common garden and forest pest, the giant African snail, Achatina fulica. In tests conducted by the authors, black rats chowed down aggressively on both species. Even quite large snails have their shells readily crushed by rats.

What does this mean for rat control as a conservation tool? It means it ain't simple.

“We hypothesize that reduction or eradication of R. rattus populations may cause an ecological release of some nonindigenous snail species where these groups coexist. As such, effective restoration for native snails and plants may not be realized after R. rattus removal in forest ecosystems as a consequence of the complex interactions that currently exist among rats, nonindigenous snails, and the remaining food web,” the authors write

What remains clear is that rats—not only the black rat, but also the Pacific or Polynesian rat and the Norwegian rat—have had significant impacts on the Hawaiian natural environment.

“Introductions of rats and terrestrial snails have been implicated in the decline of native Hawaiian flora and fauna. All three rat species were introduced to the Hawaiian Islands by people and are among the most noxious invasive species on islands worldwide,” the authors write.

But at this point, in the case of native tree snails, it is not clear whether the rat or the cannibal snail is the greater threat.

“It is unknown if E. rosea predation on other mollusk species would equal or exceed that of R. rattus,” the authors write.

So the research isn't saying rat control is a bad thing. It's urging caution, and as so many scientific papers do, it argues for more research.

© Jan TenBruggencate 2009