The researchers who did this genetic work speculated that the snails could have been transported alive in the guts of birds. They pointed in particular to a study published in 1993 showing that common shorebirds called Willets often eat Cerithideopsis snails. Of course, Willets don’t swallow small shelled creatures for the purpose of moving them from one ocean to another; the usual outcome is that the snails are ground to pieces in the birds’ gizzards, with the broken bits of their shells and other undigested remains being regurgitated as pellets. However, death by grinding is not the fate of every ingested snail. In that 1993 paper, the author reported placing four Willet pellets in dishes of seawater, whereupon a total of twenty-nine snails came out of their shells and began to crawl about the dishes.
Chapter Ten
THE LONG, STRANGE HISTORY OF THE GONDWANAN ISLANDS
THE LOST WORLDS OF THE SOUTHERN SEAS
Madagascar. New Zealand. The Falklands. The Seychelles. New Caledonia. The Chatham Islands. From my vantage point in landlocked Nevada—at this particular moment, sitting in a Starbucks in Reno with a coffee, an apple bran muffin, and a view of the somewhat shabby Circus Circus casino—they all sound like wildly exotic destinations, these islands of the southern seas. Images pass through my mind, unfortunately most of them daydreams rather than memories: a giant, mottled gecko camouflaged against a tree trunk in New Caledonia, ring-tailed lemurs bounding through the forests of Madagascar, a kiwi picking out sandhoppers from the seaweed on a beach in New Zealand, flightless steamer-ducks bobbing in the waves off the rocky coast of the Falklands. More than just backdrops for a parade of exotic creatures, however, these islands have come to represent a pivotal point in the debate over vicariance and dispersal, about why living things are found where they are.
All of these islands were once part of Gondwana and all have been disconnected from other major landmasses for many millions of years, thus making them natural test cases for comparing the importance of ancient vicariance and long-distance dispersal. Nearly all have been pointed to, at one time or another, as supporting the vicariance worldview, in the sense of harboring relict Gondwanan biotas. However, as we have seen, that view clearly does not hold up for the flora of New Zealand, which is overwhelmingly made up of the descendants of overwater colonists. Explain New Zealand, Gary Nelson had said, and the world would fall into place around it, and, for plants at least, the world did, a world in which the effects of continental drift have been obscured by a web of ocean crossings.
Nonetheless, my focus to this point on taxa—specifically, plants and various kinds of land vertebrates—leaves open the question of whether the biotas of certain areas might still be best viewed as products of Gondwanan vicariance. Specifically, are the floras and faunas of some or most of the Gondwanan islands dominated by groups that have persisted in place since the breakup of the supercontinent? Are these islands, as some would have us believe, “lost worlds,” where the evolutionary histories of ancient lineages parallel the ancient geologic connections? We have already seen that Gondwanan breakup is not the great general explanation for Southern Hemisphere distributions. But might it at least be paramount for the ancient continental islands?
AN ARK TO THE END OF THE EARTH
The Falkland Islands, or Islas Malvinas, lie some 350 miles east of southern South America, at roughly the same latitude as the Straits of Magellan, which is to say, pretty close to the end of the Earth. By all accounts it is a harsh place: bleak, treeless, and more or less unrelentingly cold, damp, and windy, the muted landscape dissected by great rivers of quartzite boulders called stone runs. Some people find the Falklands beautiful, but it is, at best, a severe beauty.
Biologically, the archipelago is a kind of poor man’s Galápagos, with many unique species, including that flightless steamer-duck and, until the 1870s, a native canid called the Falkland Islands wolf (or warrah), but nothing as bizarre as, say, a seagoing iguana or a tool-using finch to capture the imagination. The Falklands are remembered, of course, for the three-month-long war in 1982 between Great Britain and Argentina, but these days, they’re probably best known as a place where cruise ships stop for the dolphins, seals, and gigantic colonies of Black-Browed Albatrosses and Rockhopper Penguins. In the history of biogeography, these islands are just a couple of footnotes, one for the excessively tame and hence extinct wolf, and the other for the fact that Darwin visited the archipelago during the Beagle voyage and left with a bad impression, finding it a “wretched place,” where the country was all “more or less an elastic peat bog.”
Bob McDowall visited the Falklands in 1999 for the first and, as it would turn out, only time. As with Darwin, who had wondered if the wolf might have reached the islands on an iceberg, the Falklands made McDowall think about long-distance dispersal. That wasn’t unusual; wherever McDowall happened to be, that subject was often on his mind.
This was the same Bob McDowall who, as a young ichthyologist and biogeographer in the 1970s, had baldly challenged the vicariance scientists, arguing, among other things, that we know that long-distance dispersal occurs because people have actually seen it happen. It would be a significant understatement to say he had drawn some ire for his views: the American Museum of Natural History ichthyologist Donn Rosen, after reading one of McDowall’s manuscripts, had vowed to “blast him out of the water,” and an anonymous reviewer of the same paper had threatened to “destroy this person [McDowall] as a scientifically minded naturalist.” (I’m not exactly sure what that threat means, but it doesn’t sound good.) Croizat was even more blunt: he said he wanted to “execute” McDowall.
It wasn’t easy being a dispersalist in the mid-1970s, and even in the late 1990s it wasn’t exactly fashionable. But McDowall had never wavered from his old-fashioned, dispersalist viewpoint. After getting his PhD at Harvard, he had returned to his native New Zealand and had fashioned an impressive career out of research on fishes called galaxioids, combining “pure” science with studies that were important for the country’s fisheries. Galaxioids spend the early part of their lives at sea—in fact, they’re harvested as immature “whitebait”46 as they return to the rivers where they will live as adults—and this makes them good candidates for dispersing across oceans. It only takes one life stage to disperse, and that stage doesn’t have to be, and often isn’t, the adult. Indeed, McDowall had argued throughout his career that these fish, in the whitebait stage, had crossed oceans not just once or a few times, but over and over again. Freshwater fishes are not supposed to be able to cross ocean barriers, since they quickly die in salt water, but galaxioids, McDowall had repeatedly pointed out, are not exactly freshwater fishes. It was this kind of thinking—damned benighted dispersalism filled with assumptions about what kinds of creatures can and cannot cross oceans—that had gotten him into hot water with the vicariance biogeographers back in the 1970s. Never mind that he made a lot of sense.
10.1 Bob McDowall seining for fish on the Chatham Islands, east of New Zealand. McDowall thought the vicariance viewpoint ignored biological realities. Photo by Brian Stephenson.
McDowall had been thinking about the Falklands for a long time. Given his pointed criticisms of vicariance biogeography over the years, one might have thought that he saw the archipelago as a testing ground for his dispersalist views. At first, however, that wasn’t on his mind at all. McDowall was a “fish guy,” and he was dreaming about the Falklands because he had heard that there were galaxioids in the islands’ streams (Darwin had even collected some), and that nobody had ever done a survey to figure out exactly what species were there. He wanted to find out. Sometimes the motivations of biologists are pretty simple. Or, at least, they’re simple before they get complicated.
McDowall got some funding from the National Geographic Society and the Falkland Islands government and took off for the archipelago. Unfortunately, his luggage didn’t reach the islands with him, but that didn’t prove too much of an obstacle. He bought some new clothes,
and the locals helped him out at every turn, even replacing a lost electric-fishing electrode—used to stun fish so they could be collected—with a jury-rigged device partly made from an old golf club.
For three weeks, at first by himself and then with two assistants, McDowall raced around collecting fish on the two main islands, East Falkland and West Falkland, initially using the golf-club device (“We were terrified of getting electrocuted,” he said) and, later, once the lost luggage caught up with them, with the bona fide electric-fishing unit. They collected electroshocked fish at a breakneck pace, hitting 151 sites, mostly in pebbly or gravelly streams with water stained dark from the peat. Not surprisingly, they found just a few species, including one kind of galaxioid. The three biologists ended up writing a book, which they straightforwardly called Falkland Islands Freshwater Fishes: A Natural History. With only a few native and introduced species to write about, the book was very short. (“My wee book,” McDowall called it, betraying his Scottish ancestry.) Nothing they found out would set the scientific world on fire, not even the very small world of galaxioid fish biologists.
However, while on the islands McDowall talked with Tom Eggeling, an environmental officer working for the Falklands government, who said something about the geology of the archipelago that got McDowall thinking about the deep biogeographic history of the place. The Falklands, Eggeling had heard, were once part of Africa. Somehow they had broken away from that continent and wandered, over many millions of years, to their current position off the east coast of southern South America. More specifically, as McDowall later found out, the small tectonic plate that contained the Falklands had been attached to southeastern Africa back in the Jurassic, back when Gondwana was still Gondwana (see Figure 10.2). The main evidence for this ancient connection is that various geological strata of the Falklands—Devonian sandstones, Permian shales, and Jurassic basaltic volcanics, among others—match those of southeastern Africa so closely that a former connection seems inescapable. (It’s the same kind of evidence that Alfred Wegener had emphasized in his original formulation of the theory of continental drift, and it still makes perfect sense.) As the pieces of the supercontinent drifted apart, the Falklands microplate broke off on its own, made a right turn around the tip of Africa, and headed out into the newly forming Atlantic Ocean, rotating 180 degrees in the process and eventually ending up entrenched in the continental shelf of South America. In essence, the bedrock of the Falklands had started out as a piece of Africa and wound up riding on South America’s submerged coattails.
10.2 Movement of the Falklands. Upper: position of the Falkland Plateau before the Middle Jurassic. Lower: present position of the Falkland Islands. Jurassic reconstruction redrawn and modified from Thomson (1998).
Wheels started turning in McDowall’s head. On the one hand, if the Falklands had been connected to Gondwanan Africa, then they must at one time have had plants and animals from that area. On the other hand, the archipelago had been sitting off the coast of South America for a long time now, long enough to have received many immigrant species from that continent. Did the modern biota still show a strong signature of its ancient Gondwanan connection, as a vicariance view might suggest, or was it dominated by more recent colonists from South America? Under the Gondwanan vicariance hypothesis, you would expect Falklands’ species to be most closely related to groups that were found in Africa, or spread widely among the fragments of Gondwana. (The related groups didn’t have to be confined to Africa, because, when the Falklands were part of the supercontinent, Africa was still connected to other Gondwanan landmasses, potentially allowing easy movement of land organisms between Africa and those other areas.) Also, these evolutionary relationships would be distant ones, because the Falklands had been separated from the rest of Gondwana for something like 150 million years; for instance, you wouldn’t expect to find many of the same species or even the same genera in the Falklands and elsewhere. Under the overseas immigrant hypothesis, you would expect most of the island species to have their nearest relatives in South America, and many of these relationships would be very close, reflecting recent dispersal from the continent. McDowall knew that the native galaxioid, Galaxias maculatus, favored the overseas immigrant hypothesis—this same species is found in South America, but not in Africa. But what about the rest of the biota? He had a good idea what the answer was, but he needed to hit the library and make the case concrete.
The case, as it turned out, was very clear. Consider, for instance, the islands’ only native mammal, that extinct wolf, Dusicyon australis. The consensus among taxonomists was that it was closely related to some South American canids and not to any African (or more widespread Gondwanan) group of carnivores (see Figure 10.3). (When McDowall wrote his paper, the best guess was that its nearest relative was a living species called the culpeo. Recent DNA analyses using museum specimens place D. australis as the sister to D. avus, another extinct South American canid.) So count one case for the overseas immigrant hypothesis.47
10.3 The extinct Falkland Islands wolf, a recent immigrant from South America, not a Gondwanan relict. Drawing by George R. Waterhouse.
Or take a group on the other end of the spectrum in terms of diversity, the insects, with a few hundred species on the Falklands. The great majority of them have their closest relatives—either populations of the same species or species in the same genus—in Patagonia or on various sub-Antarctic islands. The Falklands insects have no obvious connection to Africa or to Gondwanan landmasses in general. Jumping over to another large group, flowering plants, McDowall found that there are about 140 known Falklands species and, again, their relatives are overwhelmingly in South America and/or on sub-Antarctic islands, with no clear ties to Africa or Gondwana as a whole.
In his paper, McDowall ran through all the evidence he could find—for birds, spiders, crustaceans, snails, annelid worms, mosses, and lichens, among other groups. Taken together, the relationships left no doubt that the main evolutionary connections of the Falklands are to Patagonia and other cool-temperate lands of the Southern Hemisphere. The relationships are close, too, with many Falklands species and almost all the genera shared with Patagonia or other areas, indicating relatively recent arrival on the islands. Potential African or Gondwanan connections are few and far between, and even most of these are debatable. In McDowall’s words, “There are only, at best, fragmentary residues in the Falklands biota that may point to an African connection.” In short, during the Falklands’ long journey to the edge of South America, the original African/Gondwanan biota had disappeared and been replaced, for the most part, by immigrants from nearby Patagonia. McDowall imagined the history of the archipelago as two converging trajectories, the islands themselves drifting westward to eventually intercept the eastward dispersal of species coming from South America. One can picture the second trajectory as a combination flotilla and cloud of organisms variously swimming, rafting, flying, and drifting on the wind, with the ancestors of the wolf and the flightless duck as the flagships.
On one level, McDowall’s paper was not surprising at all. Almost everything that had ever been written about the flora and fauna of the Falklands had indicated links to South America and not to Africa. For instance, Darwin, who had hardly made a thorough study of the subject, wrote to his friend Joseph Hooker, “The Falkland Isld. flora seems to combine the Patagonian with the Fuegian,” the latter referring to Tierra del Fuego, at the southern tip of South America. Similarly, it wouldn’t take a deep investigation to figure out that Falklands birds were mostly connected to South America rather than Africa; a casual comparison of field guides to the birds of South America and Africa would tell you that. Even hard-core vicariance scientists—including Léon Croizat himself, the originator of panbiogeography—had linked the Falklands biota to Patagonia.48
The significance of McDowall’s study then wasn’t really in the concrete results, which would have been anticipated by anyone with even a passing knowled
ge of the natural history of the Falklands, but in how he placed the results within the deep history of the islands. Anyone who understood vicariance biogeography and knew that the Falklands had once been part of Africa might have expected to find a significant African/Gondwanan element in the islands’ biota. From that perspective, it was a bit of a shock to learn that there was nothing or next to nothing left from those Gondwanan beginnings. By emphasizing the African roots of the Falklands as a landmass, McDowall was setting up the contrast between the ancient geologic history of the islands and their current, obviously South American biota, not to mention taking another jab at his old nemeses, the vicariance scientists.
MANY PATHS TO OBLIVION
In the biblical story, Noah brought onto the ark the cattle and beasts, every bird and creeping thing, and, after five months on the ocean, these animals, apparently none the worse for wear, disembarked on Mount Ararat. By analogy, New Zealand has been dubbed “Moa’s Ark,” and the Gondwanan fragments in general have been called “life-rafts.” However, the history of the Falklands calls into question the suitability of that analogy. In particular, Noah and his animals were on the ocean for 150 days, whereas the Falklands separated from southeastern Africa 150 million years ago, a rather striking difference. For our purposes, the important point is that a lot can happen in 150 million years and, from the perspective of the original passengers on the “ark,” most of it is not good.
The Monkey's Voyage Page 27
