In this post back in March, I wrote about the efforts to eradicate the non-native Polynesian rat, Rattus exulans on Mokapu, a small island of the Hawaiian Archipelago. The Polynesian rat was introduced to Hawaii about 1500 years ago. An issue I raised in that post was whether a "naturalized" alien species could still be held responsible for current extinctions of native wildlife with which it has presumably coexisted for so long.
Who is better qualified to respond to my arguments on Hawaiian fauna that were highlighted with a legal phrase than Carl C. Christensen, a native Hawaiian trained both as a malacologist and a lawyer? Carl, with whom I had the pleasure of going on a field trip several years ago (not in Hawaii unfortunately, but in Maryland), e-mailed his response almost 3 weeks ago, but only yesterday did I get a chance to read it carefully.
Here is then Carl’s extended comments, posted here with his permission. His 1st sentence refers directly to my arguments in the previous post.
With the land birds, that's probably true to a considerable extent. There was a major extinction wave that began when the Hawaiians first arrived, probably largely as a result of the efforts of the Polynesian (or Pacific) rat, Rattus exulans. But it probably took quite a while--the first European explorers, if I remember correctly, reported the presence of flightless rails on at least one of the islands, and a number of species (such as the native goose) had become restricted to relatively high elevations, even though the fossil record shows that they were originally common at sea level. Similarly, the native Pritchardia palms were (and are) still surviving in a few places, but in greatly reduced numbers from their original status (pollen analysis and other evidence indicates that they were once abundant in the lowlands, where they now exist, if at all, in very small numbers). So extinction due to R. exulans probably wasn't an instantaneous event, but rather a process that was (and is) still continuing; each generation, fewer Pritchardia seeds escape the rats and thus fewer reach maturity to replace their forebears. With offshore islands, though, it is a rather different story; if you eradicate rats, the seabirds that formerly nested there can and will return. So getting rid of the rats serves a very definite purpose in such cases. In fact, Rob Cowie and I recently met with one of the local USFWS biologists to talk about possible reintroductions of locally extirpated land snail species to Lehua Island, a small island just north of Niihau. If rats can be eradicated there (apparently quite feasible), then native plants can be reintroduced, followed by snails (if not the same species that no doubt once inhabited the island, then perhaps congenerics from Nihoa 150 miles or so to the northwest (Nihoa, a steep rock about 900 feet high, has two native endodontids that are surviving quite well--virtually the only remnants of original fauna of maybe 200 species statewide--as well as Philopoa, an endemic monotypic genus of Achatinellidae and 3 endemic species of Tornatellides, another achatinellid genus) (Nihoa is on my list of special places; I visited the island in 1980, and was the first malacologist to look for endodontids there since a Bishop Museum expedition visited the island in 1923—by separate message I'll try to send you some pictures).
Island of Nihoa
R. exulans wasn't the whole story, of course. Indeed, ever since the first humans arrived, Hawaii's native flora and fauna have been subject to an ever-increasing series of threats. It is likely that hunting (especially for the larger flightless species) was a factor, and human-caused habitat modification. The ship rat reached Hawaii in the 1870s or so, setting off a new round of bird extinctions (R. exulans apparently doesn't climb trees as much, thus tree-nesting birds were relatively safe until the tree-climbing ship rat reached here). In the 1980s and 1990s, biologists studying the fast-disappearing 'alala (Hawaiian crow) discovered that it is unique among corvids in that ours have totally lost the ability to defend their nests against rat predation, a problem they didn’t have until climbing rats reached here. But even before that, the 'alala had been decreasing in range; historically, it was known only from parts of the Big Island, whereas fossils of the 'alala and 2-3 other extinct species of corvids are common in fossil deposits at sea level on several of the islands.
Our Achatinella tree snails are another example of how multiple threats are at work, and species that have largely escaped the ravages of earlier introductions may suddenly be overcome by new threats. Achatinella was subject to extremely heavy "predation" by shell collectors during the period of, say, 1870 to 1940; collecting tree snails was a common hobby among high school boys, and some of the collections amassed during that period included 10s of thousands of live-collected shells. Even so, however, when I was first introduced to Achatinella in the 1960s, you could still find dozens in a couple of hours work in areas that were close to town and that had been heavily harvested by earlier generations of shell collectors (they had of course disappeared from other areas, chiefly those where the native forest had disappeared over the previous 100 years or so; indeed, by 1900 or so, local collectors were noting the disappearance of species that Pease and Gulick had collected 50 years earlier). But now, after the introduction of Euglandina [rosea], they have essentially disappeared from many of the places I saw them in the 1960s. They had survived everything up to then, but a new threat (Euglandina) was simply too much for them to handle (there are those who doubt the adverse effect of Euglandina, but they don't have any proof of any other new influence that can account for the post-1960 die-off). The same thing has happened in French Polynesia: the endemic Partulidae had been doing fairly well until the 1970s and 1980s, and were in fact common enough to be the subject of significant research on land snail genetics by Murray, Clarke, and others. Euglandina was introduced in the (vain) hope that it would control the African snail Achatina, and Murray et al. were able to document the extinction of their experimental animals as Euglandina spread across the islands, leaving us with what is probably the nest-documented case of a biocontrol program gone tragically astray.
It's a pretty depressing picture here; I tell people that to be a biologist in Hawaii is to live in constant pain, because you are watching your favorite critters die off before your eyes. Maybe that's why I've switched the focus of my research (such as it is) to fossil and subfossil material--it's already dead, so you don't have to suffer as you watch it go extinct.
Hope the above makes some sense—I've rambled on for far too long as it is.