EARTH AND LIFE EVOLVE TOGETHER—EXCEPT WHEN THEY DON’T
On West Falkland Island, swarms of igneous dikes cut across the exposed geological strata, like dark walls running through the paler rock. These dikes were formed by magma pushing into older strata in the Early Jurassic, about 190 million years ago. Similar dikes are found in southern Africa and are part of the argument that the Falklands microplate was once attached to that region. These rocks record a piece of the deep history of the place. They tell us that this is a fragment of old Gondwana.
Rocks are not living organisms, though. The volcanic dikes have persisted in many places while the plants and animals around them have disappeared, to be replaced by other, unrelated species. The rock of the Falklands may be ancient, but the biota, as a continuous entity, definitively is not. From a biological perspective, “Gondwanan” is a misnomer for the Falklands and other small fragments of the supercontinent.
Instead, these continental islands are converging on the history of oceanic islands like Hawaii and the Galápagos. In the case of the Chatham Islands (and perhaps the Falklands and New Caledonia), the transformation is complete; none of the original Gondwanan lineages have made it to the present. In others, some relict groups probably remain, intriguing but no longer part of the main theme: the leiopelmatid frogs and pettalid mite harvestmen on New Zealand, the caecilians and blob-like sooglossid frogs of the Seychelles (an archipelago I haven’t discussed here that tells much the same story as the other islands54), and perhaps the boas and iguanian lizards of Madagascar, among others. You might think of these islands as the Gondwanan fragments that, because of their small size, have been especially prone to extinctions, perhaps especially local mass extinctions, such as those caused by the inundations of the Chathams and New Caledonia. Because of extinction, the histories of how their modern biotas came into being are mostly or entirely histories of overwater colonization and subsequent speciation. Biologists who study these islands, if they are not died-in-the-wool vicariance scientists, increasingly speak of them in these terms: “New Caledonia must be considered as a very old Darwinian [i.e., oceanic] island,” one set of authors writes. “Extinction, colonization and speciation have yielded a biota in New Zealand which is, in most respects, more like that of an oceanic archipelago than a continent,” says another group.
In hindsight, this whole business of the Gondwanan islands might seem obvious. These areas were isolated for 50 or 100 or 150 million years. Maybe we should have expected them to get hammered by mass extinctions (both global and local) and to absorb an influx of species from overseas. Why did we ever believe that their floras and faunas would still strongly reflect their ancient geologic origins? The fact that many biogeographers did believe this is a reflection of the power of the myth of tectonic-driven vicariance in general and Gondwanan breakup in particular. The Gondwanan story is almost irresistibly simple and elegant—the wandering fragments of a former world, carrying their cargoes of species into the present. And there’s something especially appealing about the myth applied to the Gondwanan islands, from Madagascar and the Seychelles to New Zealand and New Caledonia. These are remote, exotic places, where one can almost imagine (forgetting such things as the ubiquitous sheep pastures of New Zealand or the gargantuan nickel-mining operations on New Caledonia) that Jurassic dinosaurs still lurk in some dark and inaccessible corner. In our minds, at least, they are the lands that time forgot. I know that I believed the myth, without giving much thought to what the evidence actually revealed.
Bob McDowall was one of the few who never believed the myth, even when vicariance biogeography was at its height. From the beginning, however, he seems to have intuited that the story of Gondwana had a great power over others and that it needed to be countered head-on; the myth had to be identified and, if not shattered, at least drastically tempered. In the final paragraph of his Falklands paper, McDowall included a statement about the origins of the archipelago’s biota that looks to me like an attempt at myth-breaking. He focused, in particular, on the iconic phrase coined by Léon Croizat that captured the essence of the vicariance worldview, a phrase that I imagine McDowall found doubly galling because of the vicious criticism he had absorbed from Croizat. “For me,” McDowall wrote, in a voice that gains power from restraint, “the particularly interesting aspect of these patterns, and for biogeography generally, is the demonstration that the Croizatian dictum . . . that ‘earth and life evolve together’, does not have general application in the way that some believe it has.” In other words, it isn’t all about the fragmentation of the Earth generating a matching pattern in the living world. For the Falklands and other Gondwanan islands, the relevance of the Croizatian dictum had all but vanished, figuratively speaking, in the waves; in waves of extinction that erased the original inhabitants and in waves of immigrant species arriving from beyond the sea.
46In New Zealand, whitebait are usually mixed with beaten eggs, milk, and flour, and served as patties. The patties are said to be better if there’s more whitebait than batter.
47A complication here is that, according to the latest molecular dating analyses, the ancestors of D. australis reached the Falklands during the most recent glacial period, when sea level was considerably lower than it is today and the islands were separated from the mainland by a strait that may have been as little as twelve miles wide (Austin et al. 2013). That strait could have been filled at times with sea ice or glacial ice from the nearby continent, which means that these animals might have made it to the islands under their own power. It’s unclear whether a dispersal event of that sort would qualify as normal or long-distance dispersal. An alternative is that D. australis might have colonized the islands as passengers on drifting ice, the same idea that Darwin entertained when he encountered the living wolves on his Beagle voyage. (The wolves became extinct in 1876, some forty years after Darwin’s visit.)
48In a 2005 paper, Juan Morrone and Paula Posadas, two vicariance biogeographers (although not extreme ones), suggested that the Falklands have been connected by land to South America for most of the past 130 million years. However, the plants and animals of the Falklands argue against such an extended connection. In particular, the biota has a very island-like character, with relatively few insects, no amphibians, and only the one native mammal, the extinct wolf. This depauperate biota indicates colonization mostly or entirely by overwater dispersal.
49Some scientists have claimed that even the greatest extinctions are not really outliers, because they are predicted by a simple power law describing the frequency of extinction events of different magnitudes. For at least some compilations, extinctions twice as large as some baseline are about four times less frequent, extinctions four times as large are roughly sixteen times less frequent, and so on up to the most extreme events on record. The fit of extinction data to a simple power law has been used to argue that even the largest mass extinctions have the same underlying causes as background extinction (Buchanan 2001). The notion here is that small perturbations can have consequences ranging from very small (few extinctions in a time period) to very large (mass extinctions), the latter occurring when small perturbations cascade unpredictably to produce disproportionately large effects. A common analogy is that dropping a single grain of sand on a sandpile can cause just a few other grains to move or can set off a massive avalanche involving millions of grains. However, making this connection between the power law and underlying cause is something of a leap of faith. In particular, the power-law distribution of extinction frequencies could be caused by a parallel distribution of underlying causes; that is, it could be that the perturbations are not all small, but instead have a great range of magnitudes—from single grains of sand to giant boulders, so to speak—that fit the power law. Along with most paleontologists, I am following this latter notion in assuming that mass extinctions have unusual causes.
50The volcanic island that formed around 6 milli
on years ago may have completely eroded away some 2 million years later, producing an even more recent submergence (Heenan et al. 2010). However, the evidence for this later submergence is less clear.
51It has been claimed that the earliest branching point within the troglosironid harvestmen predates the reemergence of New Caledonia. However, the range of estimates for that branching point (28 to 102 million years ago) extends to after the hypothesized time of reemergence (37 million years ago).
52Some geologists and biologists have suggested that New Zealand might have been entirely submerged during the Oligocene, making its history parallel to those of the Chathams and New Caledonia. However, the divergence ages within New Zealand for the mite harvestmen, as well as for leiopelmatid frogs, refute this drowning hypothesis. Mike Pole has also argued that the fossil plant record does not indicate the kind of mass extinction required by this “drowning” hypothesis. For cogent arguments against the idea that New Zealand was completely submerged, see Pole et al. (2010) and Giribet and Boyer (2010).
53A proposed, but still controversial, Late Cretaceous connection between Madagascar and South America via Antarctica (Noonan and Chippindale 2006) would not change the general conclusions here, because there are relatively few connections of Madagascan to South American taxa.
54The Seychelles, as part of a larger landmass called the Mascarene Plateau, detached from India about 65 million years ago (McLoughlin 2001). Several groups on the Seychelles—including the sooglossid frogs and caecilians—are most closely related to Indian lineages and are estimated to have diverged from the latter before the two landmasses separated (Zhang and Wake 2009; Biju and Bossuyt 2003). These findings are consistent with the notion that these groups have existed on the Seychelles since before that tectonic fragmentation event. However, most Seychellian species are in the same genera or even the same species as taxa found elsewhere, indicating that they arrived fairly recently by overwater dispersal (Stoddart 1984).
In September 1995, hurricanes Luis and Marilyn swept through the Lesser Antilles, traveling northwest. In early October of that year, local fishermen watched a natural raft made up of logs and uprooted trees wash up in a bay on the eastern shore of the island of Anguilla, near the path of both hurricanes. On the raft were at least fifteen green iguanas (Iguana iguana), a species that did not previously occur on the island. From the tracks of the hurricanes as well as prevailing currents in the area, researchers conjectured that the iguanas probably came from Guadeloupe, 175 miles to the southeast. (Suggestively, a sign from the Parc National de Guadeloupe washed ashore on Anguilla at about the same time.) In the wake of the hurricanes, green iguanas also showed up on two other islands, Antigua and Barbuda, where they had not been seen before.
As of 2011, a population of green iguanas remained on Anguilla, descendants of the rafting lizards, mixed with some escaped pets.
Section Four
TRANSFORMATIONS
Chapter Eleven
THE STRUCTURE OF BIOGEOGRAPHIC “REVOLUTIONS”
HAWAIIAN ANOMALIES
Steve Montgomery stopped on the muddy slope and pointed out a hard, thorny “house” constructed by a moth caterpillar. I had been scrambling to keep up, forcing aside tangles of branches and executing involuntary, spastic pirouettes in the rust-colored volcanic mud. I came up alongside him and briefly examined the insect’s case, which I probably would have passed off as detritus. Steve identified it as being from a species in the genus Hyposmocoma, one of the many radiations of Hawaiian insects each resulting from a single colonization of the archipelago. Hyposmocoma is one of the biggest of these radiations, with several hundred species, and possibly the most bizarre, especially the larvae; some of the caterpillars are amphibious, living perfectly happily on land or in the water, and others eat snails after tying them down with silk strands, like a spider. They’re some of the seemingly endless examples of weird island life forms.
We were in the Koolau Mountains on Oahu, in a forest that managed to look lush and scruffy at the same time, and unnaturally patchy, like many forests filled with nonnative plants. It was not the Hawaii of dreams. My longtime friends and current scientific collaborators, John Gatesy and Cheryl Hayashi, were a little ways behind, sliding around just like I was. In contrast, Steve, despite being the oldest of the four of us by at least a decade, moved quickly and seemingly effortlessly through the forest and kept up a running natural history commentary as he went. His reservoir of knowledge about the Hawaiian flora and fauna seemed bottomless. As we passed through a patch of ginger plants, with their verdant, footlong leaves, he showed me how to squeeze something like rosewater out of the flower buds, but said the rhizomes (underground stems) from this particular species weren’t as good to eat as the usual marketplace ginger. He talked about how cattle had long ago trashed the native vegetation and, moving on both literally and figuratively, handed me some leaves that he said smelled like camphor when crushed. Finding introduced fiddlewoods growing next to native saplings, he turned into a one-man ecological restoration crew, taking out a small handsaw and cutting down the invaders to give the natives more light.
This was in August 2011, off the Pali Highway, which runs from Honolulu northeast to Oahu’s Windward Coast. John, Cheryl, and I had met up with Steve at a place called the Nuuanu Pali (“Cool Heights Cliff”) Wayside Park, where in 1795 the army of King Kamehameha I won a decisive battle for control of the island, driving hundreds of the men fighting for the Oahu chief, Kalanikupule, over the thousand-foot-high precipice. That shallow history wasn’t on our minds, though. We had come to Hawaii to look for insects called jumping bristletails, and Steve—a freelance entomologist who, among many other things, provided the bugs for the TV show Lost and is an authority on another group of carnivorous Hawaiian caterpillars—was one of the very few people who knew where to find them. I had set up the rendezvous—look for an Asian guy (me), a big white guy (John), and a small Asian woman (Cheryl), I told him, and this abbreviated description apparently worked, because Steve found us as soon as he got out of his car at the Nuuanu Pali parking lot. Even without a description to go on, we probably would have picked him out, too; in jeans, T-shirt, and a baseball cap, with wild gray hair and beard, and glasses tilted askew, he stood out as the potential freelance field entomologist among the tourists.
After we introduced ourselves, Steve pulled out a glass vial and handed it to me. It was filled with bristletails preserved in alcohol that he had collected for us while visiting his sister in Indiana. Definitely a bug guy, I thought. Then he looked down at our feet, asked somewhat incredulously if we were going to wear those “tennis shoes” (actually we were all wearing lightweight hiking boots), and showed us his preferred footwear for Hawaiian fieldwork, sock-like Japanese tabi with an indentation between the big toe and the others, and metal spikes on the soles. They looked like good mud shoes, so I borrowed an extra pair that he had (regretting it later because they pinched my feet). Then, after getting our gear together, the four of us headed roughly east from the parking lot into the forest, at first following a faint trail, but quickly veering off into the pathless mud and brush.
No more than half a mile from the parking lot, past the ginger patch and the Hyposmocoma caterpillar in its thorny house, we reached a kind of dell, noticeably darker than what we had come through. This was the spot where Steve had seen bristletails on the trunks of a native tree called Pisonia. He pointed out the Pisonia trees, with pale bark and whorls of large leaves at the ends of branches, and showed us what he thought would be a good collecting technique, brushing tree trunks and boulders over a white bedsheet placed on the ground.
11.1 Close-up of a jumping bristletail, Neomachilis halophila, from coastal California. Bristletails broke a biogeographic “rule” by crossing the ocean between North America and Hawaii. Photo by Merrill Peterson.
After setting our packs down and stuffing some plastic vials into our
pockets, we fanned out, quickly immersing ourselves in the shared emotions of all collectors—the anticipation at each stone overturned, or trap checked, or, in this case, every sweep of a tree trunk or moss-covered boulder, and then the at-first-barely-perceptible-but-ever-increasing disappointment accompanying each small failure to find the thing looked for. But this time, we didn’t have to wait long for success. Within minutes, Steve yelled that he’d found one, and we converged around him to peer at a small, dark form in a plastic vial.
Bristletails are more or less all of a piece: they all have a humped thorax (which they flex as part of the jumping mechanism); big compound eyes that meet at the midline like skiing goggles; long abdomens with short, spine-like appendages on each segment, which may or may not be vestiges of real legs; and three long bristles that stick out from their tail end. In gestalt, they look like tiny land shrimp. Once the basic form is learned, any bristletail is instantly recognizable as a bristletail, but, without a microscope, it’s hard to tell one particular kind from another. The one in Steve’s vial was, to ape Dr. Seuss’s description of the Lorax, biggish and darkish, but beyond that it just looked like a bristletail.
There were a couple of things that had led John, Cheryl, and me to this first encounter with bristletails in Hawaii. One was that, although none of us are entomologists—John is an expert on mammal evolution and Cheryl on the evolution of spider silk—we had become intrigued by the deep history of these jumping, shrimp-like creatures that branched off from the rest of the evolutionary tree of insects in the Devonian or thereabouts, before insects evolved wings and took over the world.55 (The bristletail order, Archaeognatha, with about five hundred known species worldwide, is, at best, a modest, albeit persistent, evolutionary success story in comparison to the winged insects.) Another was that some bristletails, including the Hawaiian ones, have unusually large pseudogenes (genes that have lost their function) that can be used in odd ways to decipher the history of populations. What interested us about the particular shrimp-like insect in Steve’s vial, though, was that it was on a volcanic island in the middle of the ocean, some 2,400 miles from North America, and even farther from any other likely continental source area. Bristletails are supposed to be inept at crossing ocean barriers, because they’re flightless and delicate and have a tendency to jump in a completely random direction when disturbed, presumably not the best thing to do on a raft shaken by waves. However, their presence in Hawaii suggests that their voyaging capabilities have been underestimated, like those of frogs, monkeys, and other creatures. Not only had bristletails managed an improbable journey to some initial landing spot in the archipelago, they had also reached a total of at least six islands within it—Kauai, Oahu, Molokai, Lanai, Maui, and Hawaii—requiring at least three crossings of the channels between islands.56
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