12 March 2008

Polynesian rats of Mokapu: is there a statute of limitations for introduced species?

I am all for the prevention of the introductions of species to areas where they are not native and for the elimination of those that were introduced recently, provided that the latter process can be accomplished without harming the coexisting native wildlife*.

A recent post over at Raising Islands--Hawai'i science and environment was about the eradication of the non-native Polynesian rat, Rattus exulans, on a small island of the Hawaiian Archipelago. Detailed information is given in the Final Environmental Assessment (EA) prepared by the U.S. Fish and Wildlife Service and Hawai'i Department of Land and Natural Resources.

Mokapu Island. Picture from the EA.

The subject island is Mokapu, an uninhabited 4-hectare (10-acre) piece of rock with steep sides 1 kilometer off the northern coast of the much larger Moloka'i Island. The justification for the eradication program was the known propensity of the Polynesian rat to prey on ground-nesting seabirds, although as far as I can tell from the speculative information in the EA (see section 1.2.4), it was never demonstrated if such predation actually took place on Mokapu and if it did whether it was threatening the nesting bird populations.

The eradication was accomplished by dropping food pellets containing a rodenticide called diphacinone from helicopters. I will leave aside the thorny issue of introducing a poison into the environment to save a habitat and the wildlife that live there; the EA discusses the potential effects of diphacinone in the environment.

Reading about this raised more of a philosophical question in my mind: how long does an introduced species have to exist continuously at a location before it can be considered “native”? The EA mentions that the Polynesian rat was introduced to the Hawaiian Archipelago about 1500 years ago. By now, the rats are certainly "naturalized" members of the Hawaiian fauna and they are there to stay. And if the native Hawaiian species and the rats have coexisted for so long, how likely is it that the rat could have recently turned into a menace to the native wildlife? I don’t think that is a likely scenario at all. The EA mentions this issue only in passing:
Of the four rodent species, the Polynesian rat arrived with the early Polynesian settlers and is found throughout the main Hawaiian Islands (Hess et al. in press, Tomich 1986). Because the introduction of rats to Hawai'i perhaps as early as 1,500 years ago, its major influences on the native plants and animals are assumed to have occurred long before Europeans arrived at the archipelago.
During so many years of mutual struggle, one may even expect natural selection to have given the native wildlife some ability to resist the rats. Of course, it is possible that the rats invaded Mokapu much recently and therefore, they may still be in the stage where they would be most harmful to the native wildlife of Mokapu. However, according to the EA, people are not known to visit Mokapu, although fishing is done around it. It is, therefore, not clear how the rats ever reached the island. One possible dispersion route is via logs and other debris carried by waves and currents from nearby larger islands. If that has been the most likely way of transfer, then the rats may have reached Mokapu a long time ago.

There may be a deeper issue here. Are we turning the rat into a scapegoat to bear the blame of what everyone darn well knows to be the real cause of extinctions: habitat loss to never-ending development?

*I also support eliminations of goats, those destructive rascals.

1 comment:


Carl Christensen of Hawaii e-mailed this comment:

"I haven't seen the EIS, but in my opinion eradication of R. exulans on Mokapu would be well worth the effort if it can be accomplished. The role of R. exulans in pre-Contact extinctions in the Pacific Islands has only recently been recognized. About a year ago, Dr. Terry Hunt and others at the University of Hawaii held a symposium on its effects, and it was a real eye-opener for me. And that role is continuing. David Burney showed that on Kauai, rats are still playing an important role in the gradual extirpation of our native palm (Pritchardia), a few of which persist on Mokapu. The rats eat the seeds, thereby preventing reproduction, and as mature palms die of old age, they are not replaced. If R. exulans is the only rat on Mokapu, then eradicating it may well lead to a resurgence in the island's Pritchardia population. It has even been suggested that it was the Polynesian rat, not tree-cutting Polynesians, that drove Easter Island's endemic palm to extinction (though the two factors may both have had an effect). And even if rat predation on nesting seabirds hasn't been demonstrated on Mokapu itself, I think the bird-killing propensities of the species are well-enough known in general that its eradication, if possible, would be well worthwhile. I certainly don't downgrade the role of over-development in Hawaii, but it is in otherwise-protected places like Mokapu that eradication of aliens is most likely to be successful and, if successful, to show real benefits for native species. New Zealand is way ahead of Hawaii in this respect, and the Kiwis have demonstrated that kiwis (and tuataras and other native species) do much better on offshore islands if alien predators can be eradicated."