Thursday, February 1, 2007

"The paragon for prehistoric human induced ecological catastrophe"

No ponderous tome of a blog entry for me this time around--the Muse ain’t down wit’ me today, and I’m having a hard time finding a hook for the piece I want to write. So instead, I’m going to ramble a bit.

You all know (or should know) what an invasive species is. They are “an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health." (That definition, btw, came from Executive Order 13112--signed, naturally, by Bill Clinton rather than the current placeholder). They are a problem--perhaps not as pressing as our disappearing supply of fresh water (see this Popular Science article for some of the depressing details, and this article for China’s possibly dire future), but more important than the worldwide shortage of tungsten.

The example of an invasive species which most people know about is the rabbit invasion of Australia, but there are a number of other examples, such as the house gecko, Hemidactylus frenatus, wiping out the native night gecko Nactus populations on the Mascarene Islands. For more current examples, see the 100 Worst Invasive Alien Species list on the Global Invasive Species Database.

The news doesn’t always have to be bad, nor does every introduction of an invasive species end up being catastrophic. In Arizona, zoologists played around with the drying cycles of local ponds and prevented invasive bullfrogs from wiping out tiger salamanders (Maret, Biological Conservation v127n2, Jan. 2006), and Japanese arthropods are being brought to the U.K. to fight Fallopica japonica (Kurose, Mycologist, 22 Jan. 2007).

And sometimes nature creates its own equilibrium. The effects of invasive species on the Galápagos Islands are well known, and the Galápagos are host to numerous attempts to wipe out invasive species--some successful, others not. Feral pigs in the Galápagos were responsible for wiping out numerous native species, and it took thirty years to kill every feral pig just on Santiago Island. However, Santiago Island also saw something unusual: more-or-less peaceful coexistence between rattus rattus, the black rat, and Nesoryzomys swarthi, the Galápagos rice rat.

Rattus rattus, despite being cute as the dickens--just look at him! Don’t you want to scritch him and cuddle him?--is a ecological threat. Quoting from Harris, “Space Invaders?" (Oecologia v149n2, Nov. 2006), “the introduction and spread of the black believed to have caused the worst decline of any vertebrate taxon in Galápagos," including nearly a dozen native rodent species. But on Santiago Island, the black rat and the rice rat successfully coexist without the rice rat having to (again quoting Harris) “adjust its space use, habitat preferences and activity patterns." (The black rat is larger and more aggressive, but the rice rat is a more efficient consumer of local flora, especially cactus).

So yay for N. swarthi. Unfortunately, disaster is more common. And sometimes worse than disaster.

Most of you know the story of Rapa Nui, a.k.a. Easter Island, and how the local civilization and the island’s trees rapidly declined in the 18th century. And, per Jared Diamond’s Collapse, the most widely-known explanation for the collapse of the ecology is man’s shortsightedness: “the clearest example of a society that destroyed itself by overexploiting its own resources." The natives of Rapa Nui stupidly used up the land and wood too quickly for it to regenerate. Shame on them, such self-annihilation is dumb, we’re so much smarter now, etc etc etc.

However. Terry L. Hunt puts forth an alternate and quite convincing theory in “Rethinking Easter Island’s Ecological Catastrophe" (Journal of Archaeological Science v34n3, Mar. 2007). After extensive research he concludes that what ultimately destroyed the ecology--the cause of the ecocide--was...wait for it...our old friend, the rat, specifically rattus exulans, the Pacific rat. The (well-sourced) theory goes that rats were brought to Rapa Nui as an easy source of protein for new colonists. There were no native predators for the rats, and “an almost unlimited high-quality food supply in millions of palms each producing abundant nuts." Rats, of course, are fecund, and in ideal situations (such as this Rapa Nui) they double their population every 47 days. So one mating pair can produce almost 17 million (17,000,000) rats in about 1128 days, or just over three years.

How bad could the impact of rats have been? Hunt gives a list of field studies where the rats drove both plant and animal species to extinction, one of which notes that substantial amounts of native vegetation on the Hawaiian Islands are common only at higher altitudes, above 1500 meters, which happens to be the elevation range of R. exulans. Words like "deforestation," “extirpation," and “habitat destruction" are used in regard to rats’ effects.

How did the rats do it? “Unlike birds, rats can penetrate hard, thick seed cases (even coconuts)...and destroy the reproductive potential of the majority of seeds they consume...numerous studies have shown that plant materials are the primary food for R. exulans...experimental enclosures on Little Barrier Island show that Pacific rats strongly depress nikau seedlings by destroying seeds, underground stems, and leaves."

The rats wiped out the trees and major foliage, which led to environmental “fragility," which, combined with drought, wind, and soil erosion, led to Rapa Nui the way it is now. And all because the settlers brought some rats to eat. (Just as the rabbits of Australia were brought there to be hunted).

So, remember, you writers of alien invasion novels. What gets humanity may not be the aliens, but the food stock they bring with them.


Anonymous said...

I'd like to point out that a close reading of Jared Diamond suggests the other article (which I've also read) isn't so much a refutation as a question of emphasis.

People introduced the rats, and people cut down the trees... population levels were high, then they were low. Human bone pits show great initial diet diversity... all the way to 'people bones' on top, as I recall.

-- Brian

Library Rat said...

Interesting ideas.

I do, however, feel the need to call shenanigans on the list of 100 worst invasive species. Man should certainly be at the top.

Jess Nevins said...

I read Hunt's article as more critical of Diamond than that, especially in the conclusion. But Diamond's point is that the humans were ultimately responsible for the deforestation, while Hunt's point is that while humans cut down the trees, it was the rats who were ultimately responsible for the collapse of the ecology. I think it's more than just emphasis.

Jess Nevins said...

Library rat--

I thought that was a given. :-)

Library Rat said...

It should be a given, but I got the impression from the intro to the list that the author felt otherwise. Maybe I should look at it again.

Jess Nevins said...

I guess they didn't stress humanity's position as #1 invasive threat, at that. Go figure.

(BTW, I'm also a rat owner).