Post by Greg Mayer: More thoughts on E.O. Wilson

January 8, 2022 • 1:20 pm

by Greg Mayer

As WEIT readers well know by now, E.O. Wilson died last month at the age of 92. Jerry knew Ed better than I did, but my scientific interests were actually much closer to his than were Jerry’s. I was greatly influenced by Ed and Robert MacArthur‘s theory of island biogeography. The Harvard Gazette just republished an interview with Ed from 2014, and the interviewer asked what he thought his most important contribution was. Ed said there were several, but his work with MacArthur is the first thing he mentioned.

My well-worn copy of MacArthur and Wilson, purchased in 1977, when I was an undergraduate.

I had learned about their ideas–especially the idea that the number of species on an island was the result of a dynamic equilibrium between ongoing colonization and extinction–in my undergraduate classes. I got a copy of their book and began reading it between my sophomore and junior years of college. When I went off to graduate school to work on island lizards with Ernest Williams, I expected that I would wind up applying and exemplifying the principles of the theory.

The title page of my copy, signed by Ed, with an added sketch of an ant; inscribed on 6 October 2007, at the “Island Biogeography at 40” symposium held at the Museum of Comparative Zoology (MCZ). I had brought my copy with me; Ed and I chatted, and he graciously signed it, adding the ant unbidden. The symposium resulted in a book edited by Jon Losos and Bob Ricklefs (reference below).

Ed’s work in island biogeography included theoretical formulation (though the mathematics was primarily MacArthur’s), analysis of faunal lists and distribution patterns, and, very ambitiously, experimental manipulation of species numbers on mangrove islets in Florida. One of his contributions to island biogeography that has gone largely unremarked, but which I regard as of real significance, are his studies of Cenozoic fossil ant faunas in amber from the Dominican Republic. He found that the ancient amber ant fauna was more characteristic of a continental fauna, rather than that of an oceanic island. I was very impressed by this work; at the time it was, and remains to this day, among the strongest evidence that the Greater Antilles are old continental islands– islands once attached to or closely adjacent to the mainland, from which their fauna derives.  (Several of his papers on the amber ants are open access, and are linked below.)

Ed gave a seminar about his amber fossil work while I was a grad student, and we talked afterwards, and he suggested we continue the discussion. Knowing how busy Ed was, I went to make an appointment to see him with his secretary, rather than rely on us passing in the hallway to set a time (Ed was not often in the hallway). His secretary was very suspicious about who I was and what I wanted, and said I would need to write a letter requesting a meeting. The demand seemed to me beyond the pale, considering that I was a grad student in the MCZ and that Ed wanted the meeting. I never wrote the letter, and that conversation never continued.

A few years later, in 1988, James D. ‘Skip’ Lazell and I wrote that the discovery of bolitoglossine salamanders (a mainland group, with little capacity to cross a salt water barrier) in the Dominican amber would be powerful evidence from the vertebrate fossil record for an old continental origin of the Greater Antillean fauna (Wilson’s work having provided invertebrate evidence). In 2015 George Poinar and Dave Wake reported exactly that– a well-preserved bolitoglossine salamander from the Dominican amber!

Many people (including Ed) have commented on Ed’s rather sour relationship with Dick Lewontin. What is sometimes overlooked in these discussions is that both of them were involved with a group known as the “Marlboro Circle”. This group, which sought to conceptually unify ecology and evolutionary biology, included, among others, Ed’s key collaborator, Robert MacArthur, and also Richard Levins. (Levins was a Marxist who made important contributions to island biogeography, collaborated on publications with MacArthur and Lewontin, and visited Wilson’s experimental mangrove islands.)

I find the activities of this group fascinating– in many ways they set the agenda for ecology and evolutionary biology for the second half of the 20th century. It is one of the most important episodes in the biology of that century–that the group was marked by tragedy (MacArthur’s death at 42) and a falling out among its members lends pathos as well. The episode has, unfortunately, been little studied or remarked upon by historians of science. My last communication with Ed (in 2020) was about some of the sources for the data that went into the origination of his and MacArthur’s formulation of their theory.

I mentioned above that I had expected my studies to apply and exemplify the theory of island biogeography, but as I learned more about reptile distribution and did my own studies in the West Indies, I found that species number on islands was not the result of a dynamic equilibrium between ongoing colonization and extinction. Rather medium (10^4 to 10^5 ka, especially ice age changes in sea level) and long term (10^6 to 10^7 ka; the origin and diversification of major lineages) geological and evolutionary events were much more important in shaping the fauna.

Nonetheless, MacArthur and Wilson, like Darwin in the Origin and Mendel in his seminal paper, had limned many pathways for further progress, and despite failing to find evidence of their equilibrium process, I had, and continue to have, the greatest respect for their work. For my thesis defense, I planned to bring two past members of the Marlboro Circle– Dick Lewontin, my advisor, and Ed Wilson, original island biogeographer– back together, at least briefly.

My thesis defense talk was entitled “A theory of island biogeography, with especial reference to the amphibians and reptiles of the West Indies”. (This was a deliberate mashup of the title of Ed’s book and the title of a monograph from 1914 by Thomas Barbour, an early curator of herpetology at the MCZ, “A contribution to the zoogeography of the West Indies, with especial reference to amphibians and reptiles“.) I gave the talk at Dick’s weekly Population Biology Seminar on the 3rd floor of the MCZ Labs, one floor below Ed’s lab.

I tried to make sure that Ed would attend, despite its location, by placing a copy of my seminar announcement in his mailbox. He did come, and sat at the middle of one side of the great table that sat in the middle of Dick’s lab (Dick was further back in the room). I argued in my talk that the equilibrium theory was a special case of a more general theory, and that the equilibrium theory per se didn’t apply very well to West Indian amphibians and reptiles.

After my talk, among the questions were one or two from Ed. He defended the applicability of his and MacArthur’s theory to broader situations than the ones where it fit best, and, indeed, I concur that they had anticipated modifications, expansions, and refinements that would improve it– that’s why I had said there was a more general theory of which theirs could be a special case. After the questions, Ed left.

Although much of Ed’s public reputation rests, rightly, on Ed’s advocacy for biodiversity, and on the controversy over sociobiology (which accounts for essentially all the negative bits), in remembering Wilson we should not lose sight of his other accomplishments.

Losos, J.B. and R.E. Ricklefs, eds. 2010. The Theory of Island Biogeography Revisited. Princeton University Press, Princeton, N.J. publisher

MacArthur, R.H. and E.O. Wilson. 1967. The Theory of Island Biogeography. Princeton University Press, Princeton, New Jersey. publisher

Wilson, E.O. 1985. Ants of the Dominican amber (Hymenoptera: Formicidae). 1. Two new myrmicine genera and an aberrant Pheidole. Psyche 92:1-9. pdf

Wilson, E.O. 1985. Ants of the Dominican amber (Hymenoptera: Formicidae). 2. The first fossil army ants. Psyche 92:11-16. pdf

Wilson, E.O. 1985. Ants of the Dominican amber (Hymenoptera: Formicidae). 3. The subfamily Dolichoderinae. Psyche 92:17-37. pdf

The “red hen” of Mauritius

March 22, 2021 • 10:15 am

by Greg Mayer

Yesterday’s Readers’ wildlife photos featured photos of birds with “Red Feathers” by frequent contributor and evolutionary biologist, John Avise. Coincidentally, I had been reading the previous day Lost Land of the Dodo, by Anthony Cheke and Julian Hume, and one of the birds they feature is the “red hen” of Mauritius. This red bird, called a “hen” or some equivalent by Dutch colonists and English and French visitors, is actually a rail, Aphanapteryx bonasia (Selys) 1848, and is also known by the English name Red Rail.

The red hen, Aphanapteryx bonasia, plate 29 from Rothschild, 1907 (Biodiversity Heritage Library).

Sadly, it is now extinct, known only from descriptions, drawings, and subfossil remains; a stuffed specimen, now lost, was recorded in Prague in the early 1600s.

Mauritius was known to earlier Arab and Portuguese sailors, but not documented until the Dutch arrived in 1598. The early sailors and colonists found a pristine land, untouched by humans, and among its many wonderful inhabitants were the dodo (Raphus cucullatus) and the red hen, both flightless and easy to kill, and thus to eat. Red hens were last reported in 1693.

The 400 year history of the ecological degradation of Mauritius (and of the two other Mascarene Islands, Reunion and Rodrigues) is well told by Cheke and Hume in their meticulously researched volume, which skillfully weaves together history, zoology, paleontology, and ecology to tell the tale. The book is enhanced by numerous color plates by Hume, which beautifully and expertly depict the islands’ aboriginal biota; you can see some of Hume’s paintings, including of the red hen, here. (My only quibble with this book is that it is encumbered by a numbered endnote reference and note system. One is thus forced to continually turn to the back of the book to find out who the authors being cited are, and to consult the many worthy notes– nearly 100 pages!– accompanying the citations. The book would have greatly benefited from a true footnote system.)

Both the red hen and the dodo quickly went extinct, although the red hen persisted in appreciable numbers for longer. Cheke and Hume indicate that it was the introduction of cats to Mauritius in the late 17th century that did the red hens in.

The dodo, Raphus cucullatus, plate 24 from Rothschild,1907 (Biodiversity Heritage Library).

Interestingly, there is a debate about whether the name dodo might have been transferred to the red hen late in Dutch colonial history, after the true dodo had become extinct. That the name transfer had occurred was first suggested by Alfred Newton in 1868, and in a recent paper, Cheke and Jolyon Parish (2020) argue strongly that such did indeed occur. They suggest that the Dutch, returning to the island in the later 17th century after a hiatus in colonization, and finding the only flightless bird to be Aphanapteryx, called it by the name of what they expected to find– dodaers being their word for what had been the most well-known flightless bird on Mauritius.

The name transfer hypothesis implies that the true dodo was extinct, or nearly so, by the time of these accounts from the second Dutch settlement (1666). The latest dodo record that is generally accepted is of birds on an off shore islet, Isle d’Ambre, reported by shipwreck survivors in 1662. Under this hypothesis, later accounts (up to 1688) that use the name “dodo”, but are unaccompanied by a description of the bird seen, cannot be relied upon. Hume, Cheke’s erstwhile coauthor, though, seems to disagree with the hypothesis that all late uses of the word “dodo” (or equivalent) apply to the red hen (see references in Cheke and Parish, 2020).

Cheke, A. and J. Hume. 2008. Lost Land of the Dodo: An Ecological History of Mauritius, Réunion & Rodrigues. Yale University Press, New Haven, Conn. US UK

Cheke, A.S. and J.C. Parish. 2020. The dodo and the red hen, a saga of extinction, misunderstanding, and name transfer: a review. Quaternary 3(3,4), 15 pp. pdf

Newton, A. 1868. Recent ornithological publications. Ibis 4(New Series): 472–486. BHL

Parish, J.C. 2013. The Dodo and the Solitaire: A Natural History. Indiana University Press, Bloomington. (most of a review)

Rothschild, L.W.R. 1907. Extinct Birds. Hutchinson, London. BHL

The number of species on islands

July 27, 2020 • 1:00 pm

by Greg Mayer

[The following is a trivial, and speculative, discussion about a small part of an important paper in the history of ecology and evolutionary biology.]

There are fewer species of any given group of plants or animals on an island than on an equivalent area of the mainland; and the larger the island, the more species there will be. These two general rules of natural history have been known since at least the 19th century, and are known under the rubrics that island biota are depauperate, and the species-area relationship, respectively. There have been many, not always mutually exclusive, explanations for these phenomena, and in the early 1960s they were on the minds of at least several biologists.

The most important resulting paper was Robert MacArthur and Ed Wilson‘s classic “An equilibrium theory of insular zoogeography.” This paper, together with the expansion of the ideas contained within it into a book-length monograph, The Theory of Island Biogeography (1967), were extremely influential in setting out the questions to be asked, and how to go about answering them, across many areas of ecology and evolutionary biology, not just the phenomena of island life.

MacArthur and Wilson (1963).

In the paper, MacArthur and Wilson proposed (among other things) that the number of species on an island resulted from a dynamic equilibrium between ongoing immigration and extinction of species living on the island, and that the relation of these demographic processes to various physical properties of islands led to the species-area relationship. They illustrated this relationship in two figures showing the relationship between the area of an island or island group and the number of species of land and freshwater birds occurring on that island or island group. One figure was for islands in and near the Sunda group, the other for the Moluccas, Melanesia, and Polynesia. Both figures are interesting, but the second one concerns us today.

Figure 2 of MacArthur and Wilson (1963).

For reasons relating to some research projects I’m pursuing during the pandemic, I was led to look closely at this figure, including the sources of the data as indicated in the legend of the figure: three papers by Ernst Mayr, and a book by James C. Greenway. I have a copy of Greenway’s book (both editions, actually), and Mayr’s 1943 paper is readily available online to anyone from SORA (a wonderful free repository containing vast swaths of the ornithological literature).

Neither of the two earlier papers are readily available, but the University of Wisconsin, Madison, Library scanned a copy of the earliest, and sent me a pdf (kudos to the staff of the Library there for working very hard during the pandemic to keep the scholarly literature available); and I happened to have a reprint of the other, a paper from the modestly obscure Proceedings of the Sixth Pacific Science Congress. Mayr published two papers in the proceedings, and I have reprints of both; the second is the one cited by MacArthur and Wilson.

Mayr (1940a).


Mayr (1940b).

The provenance of my copies is of interest. By looking at the stamps and annotations, you can see that the copies were originally in the library of the Bird Department of the Museum of Comparative Zoology, which stamped them, and added the author and date notation at upper left. (Departments of major museums usually maintain libraries physically separate from the main library of the institution.) Reprints were the principal way in which scientific literature circulated in the 1940s, and it is most likely that the Bird Department obtained the copies shortly after publication, not when Mayr moved to the MCZ in 1953. As duplicates (and Mayr’s arrival may be why the Department had extra copies), they then passed into the possession of my friend and colleague Bob O’Hara; we were graduate students together in the 1980s, and he worked closely with Mayr. At some point, also in the 1980s, Bob gave them to me, and I penciled my name and a correction to the date of publication on them. (See Note on the date of publication below.)

So now we come to the matter at hand. I was trying to track down the actual numbers that went into MacArthur and Wilson’s Figure 2, and some of the data came from Mayr (1940b), the second of the proceedings papers. It’s a 20 page paper, with no table of species numbers by islands, so I was reading it carefully to find what numerical data I could. It’s all on page 202– species numbers for 11 islands or island groups that were included in Figure 2. And a little later, while copying the numbers on to a data sheet, I noticed a pencil mark next to the paragraph with the data. (It is the only mark in the paper, aside from those on the first page.)

A pencil mark highlighting species number data in Mayr (1940b).

The mark was not mine; and it would not be Bob O’Hara’s, who would not have been interested in the particular questions addressed by these data. So who would have been using the MCZ Bird Department Library’s copy prior to the 1980s to highlight data included in MacArthur and Wilson (1963)? It occurred to me, could it have been Ed Wilson himself?

Wilson was at the MCZ with Mayr, and would have had access to this copy. In their paper, he and MacArthur thank Mayr (and a few others) for “material aid and advice during the course of the study.” There is thus no question that they consulted Mayr, and used data from a number of Mayr’s papers and books (in Figure 1, as well as Figure 2). If they looked up the data themselves (as opposed to Mayr directly telling them the species numbers), then this particular copy, now in my possession, is a likely copy for them to have used, and Wilson is the likely person to have made the mark highlighting the data that was used. This then, is my speculative (and trivial) suggestion: that Wilson used this copy in the preparation of his and Robert Mac Arthur’s classic and influential paper.

MacArthur and Wilson’s (1963) Figure 2 plots data for 26 islands or island groups. In the cited references, I can find data for only 21 of them; I do not know where they got the data for Tonga, Kei, Tanimbar, Buru, and Ceram. Jurgen Haffer (2007:163), Mayr’s biographer (and himself an accomplished contributor to ornithological science) records the following interesting tidbit:

Mayr (pers. comm.) had copious data on island sizes, distances from mainlands or other islands, number of species, etc. When he tried to determine relations among all these figures he got into mathematical problems and turned this material over to a graduate student with mathematical abilities. However this student got sidetracked into other problems and this material was never exploited.

This account was recalled to Haffer by Mayr decades later, and what data was compiled, when it was compiled, and who the student was, isn’t known.

Haffer (2007: 170) also states that in the 1933 and 1940b papers Mayr “clearly discussed what became later known as the equilibrium theory of insular biogeography”, a claim that has been echoed by other authors. This is not quite correct. Mayr discussed a number of relevant factors contributing to the characteristics of island faunas in 1940b, and the paper is well worth reading and studying today. But he did not formulate in any clear way, even verbally, MacArthur and Wilson’s later theory.  (My very limited German does not allow me to properly assess the 1933 paper, but Mayr’s two 1940 papers in English seem to parallel closely the earlier paper in German.)

For example, Mayr (1940b:215) does note that on a smaller island a species will have a smaller “effective breeding population” and thus be vulnerable to extinction; this is a striking formulation, obviously influenced by theoretical population genetics (likely gotten from Theodosius Dobzhansky). But in discussing extinction on New Caledonia he is clearly discussing evolutionary events stretching over much of the Tertiary (i.e. tens of millions of years), and not the turnover in ecological time of insular populations contemplated by MacArthur and Wilson’s equilibrium theory. The latter has been demonstrated to occur over annual and decadal scales on, for example, smaller islands in the British Isles.

While not excluding the possibility of a MacArthur and Wilson style ecological equilibrium (because he does not consider the situation), Mayr is clearly discussing the origin and persistence (or not) of endemic forms (species, genera, even families) over evolutionary time. Mayr himself (quoted in Haffer, 2007: 163) offers a much more nearly accurate appreciation of his views, stating that his own ideas embraced the “basic thesis” of MacArthur and Wilson; he did not “clearly discuss” the equilibrium theory.

Note on the date of publication. The Sixth Pacific Science Congress was held in 1939, but the Proceedings were published later, from 1940-1943.  Greenway (1958), who knew Mayr well, cited them as 1941, and this may have been the source of my handwritten correction, but I cannot now recall. Haffer’s (2007) definitive list, based on Mayr’s own lists, cites them as 1940.

Greenway, J.C. 1958. Extinct and Vanishing Birds of the World. American Committee for International Wild Life Protection, New York.

Haffer, J. 2007. Ornithology, Evolution, and Philosophy: The Life and Science of Ernst Mayr 1904–2005. Springer, Berlin.

MacArthur, R.H. and E.O. Wilson. 1963. An equilibrium theory of insular zoogeography. Evolution 17:373-387.  pdf

MacArthur, R.H. and E.O. Wilson. 1967. The Theory of Island Biogeography. Princeton University Press, Princeton, N.J.

Mayr, E. 1933. Die Vogelwelt Polynesiens. Mitteilungen aus dem Zoologischen Museum in Berlin 19:306-323.

Mayr, E. 1940a. Borders and subdivisions of the Polynesian Region as based on our knowledge of the distribution of birds. Proceedings of the Sixth Pacific Science Congress 4:191-195.

Mayr, E. 1940b. The origin and history of the bird fauna of Polynesia. Proceedings of the Sixth Pacific Science Congress 4:197-216. (This paper was reprinted in Mayr (1976) but Mayr sometimes updated papers in that collection, and for my purposes I needed to see the original.)

Mayr, E. 1976. Evolution and the Diversity of Life. Harvard University Press, Cambridge, Mass.

Mayr, E. 1943. The zoogeographic position of the Hawaiian Islands. Condor 45:45-48. pdf

The Iriomote Cat

June 29, 2016 • 10:30 am

by Greg Mayer

The Iriomote cat (Prionailurus benegalensis iriomotensis) is a critically endangered subspecies (sometimes ranked as a full species) of the Leopard cat, a species of small cat distributed widely across Asia from Afghanistan to eastern Siberia and south to the Philippines and Greater Sundas. The Iriomote cat is a dark form, made known to science only in the 1960s, and endemic to the small (112 square miles) island of Iriomote in the Ryukyu Islands of southern Japan. It is not known from the larger islands in the Ryukyus.

"Yon", an Iriomote cat that was hit by a car in 1996, and rehabilitated at the Iriomote Wildlife Conservation Center, where he remained till his death in 2011. He was mounted after he died. Photo by Purplepumpkins via Wikipedia.
“Yon”, an Iriomote cat that was hit by a car in 1996, and rehabilitated at the Iriomote Wildlife Conservation Center, where he remained till his death in 2011. He was mounted after he died. Photo by Purplepumpkins via Wikipedia.

One threat to the cats is being hit by cars, as shown by the survivor, Yon, in the photo above. My Okinawa correspondent sends the following sad news concerning this continuing threat to the cats.

Japan News Iriomote cat.jpg Japan News Iriomote cat 2.jpg

The media attention, extending even to the English language press, shows an admirable public awareness of the cat’s conservation needs.

The Leopard cat and its various subspecies present an interesting biogeographic puzzle, because in addition to occurring on the land bridge islands of the Sunda Shelf (e.g. Borneo), which were connected by dry land to the mainland of Asia during times of glacial sea-level lowering, some also occur on islands that rise from deep water off the continental shelves, such as Iriomote (separated by depths of over 500 m), and some of the Philippines. Except for rats and bats, mammals are generally not very good at over water dispersal (an issue we’ve mentioned here at WEIT before), so the question is, how did they get there? There are at least three possibilities. First, they may have been able to disperse across water by what Darwin called “occasional means of transport”: rafts of floating vegetation (most likely for these cats), ice bridges (not in this case, but suspected for some arctic/antarctic forms), powered swimming (known in elephants), or just floating in the currents (known to occur in tortoises). Second, more substantial geological/geographic changes than glacial sea level drop may have provided a dry shod path to the island in the past; such changes can also narrow water barriers, increasing the chance of successful water crossing. Third, they may have been carried to the islands by man– most or all of the varied West Indian raccoons have turned out to be early introductions. There are various ways of examining these possibilities (fossil/archeological record, paleogeographic reconstructions, genetic divergences among populations, etc.), but I don’t know that the question of dispersal of leopard cats has been directly addressed.

Izawa, M., T. Doi, N. Nakanishi and A. Teranishi. 2009. Ecology and conservation of two endangered subspecies of the leopard cat (Prionailurus bengalensis) on Japanese islands. Biological Conservation 142:1884-1890.

Tamada, T., B. Siriaroonrat, V. Subramaniam, M. Hamachi, L-K. Lin, T. Oshida, W. Rerkamnuaychoke, and R. Masuda. 2008. Molecular diversity and phylogeography of the Asian leopard cat, Felis bengalensis, inferred from mitochondrial and Y-chromosomal DNA sequences. Zoological Science 25:154-163.

More on swimming tortoises

May 8, 2015 • 1:30 pm

by Greg Mayer

Dennis Hansen, our Aldabra correspondent, sent Jerry a very nice video of a swimming Aldabra giant tortoise (Aldabrachelys gigantea). This immediately brought to mind what is, in my view, one of the most important recent papers in biogeography, “The first substantiated case of trans-oceanic tortoise dispersal” by Justin Gerlach, Catharine Muir, and Matthew Richmond.

In the paper, they report an Aldabra tortoise that came ashore on a beach in Kimbiji, Tanzania in 2004. After considering several possibilities, they conclude that the tortoise had floated in from Aldabra, over 700 km away across the Indian Ocean. The copious growth of large barnacles on the limbs and lower parts of the carapace certainly suggested that the tortoise had spent a considerable amount of time in the sea.

The Aldabra tortoise at Kimbiji, shortly after its discovery in December 2004. Photograph: C. Muir. Figure 1 of Gerlach et al. (2006).
The Aldabra tortoise at Kimbiji, shortly after its discovery in December 2004. Photograph: C. Muir. Figure 1 of Gerlach et al. (2006).

Dennis’s video shows that the tortoises enter water, and how they move about in the shallows. The Kimbiji tortoise, despite the ability to swim, could not have swum to the main, but was rather mostly carried by the currents, and presumably spent its time at sea keeping its head, and most of the dome of its carapace, above water. It was, however, walking ashore, apparently intentionally, when found.

So why is this important? It has long been supposed that animals and plants get to oceanic islands by what Darwin called “occasional means of transport“: carried along on logs, masses of vegetation, ice floes, attached to birds, or floating by themselves in the water. (Darwin carried out a series of experiments on the ability of various plant seeds to float in sea water, and their ability to germinate after varying periods of immersion.) Tortoises have usually been thought to float by themselves because of the difficulty they would have in clinging to vegetation, and also because the practice of mariners of earlier centuries of putting giant tortoises in the holds of their ships as a living food supply had shown that tortoises could live for many months without food or water.

Although Darwin and many subsequent zoogeographers (e.g., P.J. Darlington) invoked such occasional means of transport, there has always been a school of thought arguing that such crossings of the ocean by land animals were nigh impossible, and that the presence of non-flying land animals on an island implied a past land connection. In the first half of the 20th century, this school constructed speculative land bridges crisscrossing the oceans, in order for every island animal to have had a dry-shod passage to the island from its home of origin. In the later 20th century, with the development of plate tectonics, the land bridge builders were succeeded by drift enthusiasts, who thought drifting crustal plates could serve to bring oceanic islands into juxtaposition with continents, so that, again, animals might get to islands without having to cross water (or at least not much). (The drift enthusiasts had the advantage that continental drift actually does occur, even if not in the exact plate configurations they hoped for, whereas the land bridge builders’ long, thin isthmuses crossing abyssal oceans have not been borne out by geology.) So, the more or less direct observation of transoceanic crossing by an occasional means of transport provides a crucial link—a vera causa—in the argument for the occurrence and importance of such means in the colonization of islands.

Some younger biologists, raised (and properly so!) on plate tectonics, and perhaps lacking acquaintance with older literature and island organisms in the field, had taken the drift enthusiasts’ claims too much to heart, and seemed to be unaware of the importance of transoceanic dispersal. Now that molecular phylogenies often bolster the argument for the importance of occasional means of transport, they seem a bit surprised to find out that there indeed has been a lot of ocean crossing, not just to oceanic islands, but between continents and continental islands as well. We discussed one such case here on WEIT, the ratite birds, where what had seemed to actually be a good case for continental drift seems to actually involve a fair amount of oceanic crossing. The zoologist Alan de Queiroz has written a popular book on the biology of oceanic dispersal, and the sociology of its rediscovery by some biologists.

de Queiroz, A. 2014. The Monkey’s Voyage: How Improbable Journeys Shaped the History of Life. Basic Books, New York.

Gerlach, J, C. Muir and M.D. Richmond. 2006 The first substantiated case of trans-oceanic tortoise dispersal. Journal of Natural History 40(41–43): 2403–2408. pdf

Island faunas and the Falkland Islands fox

March 22, 2013 • 4:09 am

by Greg Mayer

The evidence from biogeography is arguably the most important evidence for evolution. P.J. Darlington, perhaps the greatest zoogeographer of the 20th century, said that zoogeography showed Darwin evolution. And Jerry has long insisted that biogeography is at least among, if not the, most persuasive evidence for evolution.

Canis antarcticus, by George Waterhouse, from the Zoology of the Beagle.

It was thus with great pleasure that I read a recent paper by Jeremy Austin and colleagues (ref below, news piece here) on the Falkland Islands fox or wolf (Dusicyon australis), a species which had intrigued Charles Darwin, and which he wrote about in the Zoology of the Beagle, the Voyage of the Beagle, and The Origin.

But first, let’s recap the features of island faunas that Darwin thought cried out for an evolutionary explanation. In examining island faunas, Darwin distinguished between continental islands, which had had a connection to a mainland in the recent past (e.g. Great Britain, which was connected to France by Ice Age sea-level lowering as recently as about 12,000 years ago), and oceanic islands, which had never had a connection to the main (e.g. mid-ocean volcanic islands like the Galapagos).

Darwin identified four characteristics of oceanic islands, which I like to call the “four D’s”. First, island faunas are depauperate— they hold fewer species than did comparable areas of mainland habitat. Second, they are disharmonious— they are inhabited by an unusual concatenation of taxa, rather than the usual combinations of predators, herbivores, and omnivores. Instead of cattle and deer, the large herbivores of islands were things like giant tortoises (as in the Galapagos) or giant geese (as in Hawaii). And large predators, such as cats and dogs, were usually lacking altogether (although some islands had very large birds of prey). Third, island faunas showed signs of dispersal— the animals that were there showed the ability to cross salt water. So birds and bats were usually present, but large land mammals and amphibians were usually absent. And finally, there was a strong effect of distance on the character of the fauna– the Galapagos fauna, for example shows clear affinity to the Americas, while the fauna of the similarly situated but Atlantic archipelago of Cape Verde  shows affinity to Africa.

Darwin argued that all of these features can be explained if the inhabitants of oceanic islands are the modified descendants of animals that had been able to disperse there. These animals would need be susceptible to occasional means of transport (dispersal), come from the most accessible mainland (distance), and would be a small, non-representative sample of what occurred on the mainland (disharmonious, depauperate). Darwin contrasted this with what we might expect under an hypothesis of special creation. Why were the island faunas “undercreated” relative to the mainland, and why would they bear the plain stamp of affinity to the nearest mainland, rather than being related to the faunas of other similar islands?

Although we all associate Darwin with the Galapagos, Darwin also visited the Falklands, and they supplied, I believe, an important bit of evidence in his thinking about islands. Darwin was a bit perplexed about the Falklands. In many ways they seemed like oceanic islands. There was only a single species of land mammal, the Falkland Islands fox, which was clearly related to South American foxes (South America has a modest radiation of canids, which are  variously called dogs, foxes, or wolves in English). The mammal fauna thus shows 3 of the four D’s: depauperate, disharmonious, and distance.

Bathymetry between the Falklands and the main. Level III corresponds to the usual estimate of maximum glacial sea-level lowering (120 m), while level IV (140 m) is preferred by Austin et al. (from whom the figure is modified).

An alternative explanation for island faunas being depauperate and disharmonious is that the ecological conditions on the islands are unsuitable, despite seemingly appropriate physical environmental conditions. This alternative explanation is easily tested by introducing exotic species to the island, and seeing how they fare. If they become established, then the cause of their absence is a failure of dispersal, not a failure of environmental suitability. This is where the Falklands helped Darwin, I think. The Galapagos in the 1830s were still nearly pristine, but the Falklands showed him the fauna of an island with little direct habitat disturbance and a small human population, but whose population had brought their animals with them. At the time of his visit, Darwin recorded wild populations of cattle, horses, pigs, rabbits, rats, and mice, with feral cats and at least domestic dogs and sheep coming later. The Falklands were thus quite capable of supporting a diverse and harmonious mammalian fauna; the mammals just needed help getting there. (The increasing human population, and consequent increased disturbance and hunting, led to the extinction of the Falklands fox by the late 1800s.)

But how did the fox get there? Carnivores, in general, are not known to be good at dispersing across sea barriers, and a fox is unlikely to have been able to cross several hundred kilometers of open sea. This is what puzzled Darwin, and led him to suggest that the islands had been connected to the continent, despite the lack of all other animals that might have been expected to cross over on a land bridge. In later years, it was even suggested that the fox was semi-domesticated, and had been brought to the islands by Indians. This is where the latest paper by Austin et al. comes in.

As I noted above, South America is home to a modest radiation of canids, but the closest living relative of the Falklands fox, the maned wolf (Chrysocyon brachyurus), is not very close, with an estimated divergence time of 7 million years ago. What Austin and colleagues have done is extract DNA from fossils of Dusicyon avus, a very recently extinct canid (ca. 3ooo years ago) that was widespread in southern South America. Comparing their DNA to that from skins of the even more recently extinct Falklands fox, they found that the two are very closely related. Indeed, in their best phylogenetic estimate, the mainland avus is paraphyletic with respect to australis, and this is exactly what we would expect if the Falklands had been colonized by mainland foxes from the part of South America nearest to the islands. Furthermore they were able to date the divergence to 16,000 years ago– the height of the last glacial maximum. As is well known, the glaciers withdrew massive amounts of water from the sea, lowering sea levels by about 120 m. Looking at the seabed between the Falklands and the main, we can see that a 120 m lowering would substantially reduce the distance between them. Austin and colleagues favor an even greater lowering, further reducing the distance, but either lowering would reduce the distance to the order of tens, rather than hundreds, of kilometers.

But could a fox cross even tens of kilometers of sea? Yes– on ice floes or sea ice. How do we know? The arctic fox (Vulpes lagopus) ventures way out on to the sea ice, and can float out on ice floes, having been recorded as turning up occasionally in eastern Canada, far to the south of its native range. It is also the only native land mammal of Iceland, an island which has never had a continental land connection, and which it must have reached on ice floes. Darwin himself noted the possibility of ice transport, writing in the Origin “icebergs formerly brought boulders to its [Falklands] western shores, and they may have formerly transported foxes, as so frequently now happens in the arctic regions.”

So Darwin’s dilemma is solved. Glacial sea-level lowering and sea ice provide an “occasional means of transport”, and the fossil record and DNA analysis lead to an identification of the ancestor, and dating of the event to the precise time when such means were most available.


Austin, J.J. et al. 2013. The origins of the enigmatic Falkland Islands wolf. Nature Communications 4(1552).  (pdf, subscription required)

Darwin, C.R. 1859. On the Origin of Species.  London: John Murray.  (DOL)

Darwin, C.R. 1860. Journal of Researches into the Natural History and Geology of the Countries Visited During the Voyage of H.M.S. Beagle Round the World. Last revised edition. London: John Murray. (DOL)

Matias, R. and P. Catry. 2008. The diet of feral cats at New Island, Falkland Islands, and impact on breeding seabirds. Polar Biology 31:609-616. (pdf)

Waterhouse, G.R. 1838. Mammalia. The Zoology of the Voyage of H.M.S. Beagle, under the Command of Captain Fitzroy, During the Years 1832 to 1836. Part 2, No. 1. (BHL, DOL)

House cats as predators

January 29, 2013 • 3:01 pm

by Greg Mayer

It’s long been known that house cats, which are introduced to most of the places they occur (the wild members of the species are found in Europe, North Africa, and western Asia), can wreak havoc on native wildlife, perhaps the most infamous case being that of the Stephens Island Wren (Xenicus lyalli). It has often been said that the wren was exterminated by the lighthouse keeper’s cat, but the story is both a bit more complex, and much more tragic: many cats were involved, not just one, and not just the Wren, but the entire Stephens Island land bird fauna was decimated.

Stephens Island Wren (from Ibis, 1895).
Stephens Island Wren (from Ibis, 1895).

A new study by Scott Loss, Tom Will and Peter Marra in Nature Communications makes new estimates of total mortality of wildlife due to house cats, and they are quite high: median estimates of 2.4 billion birds and 12.3 billion mammals annually in the United States. Money quote:

We estimate that free-ranging domestic cats kill 1.4–3.7 billion birds and 6.9–20.7 billion mammals annually. Un-owned cats, as opposed to owned pets, cause the majority of this mortality. Our findings suggest that free-ranging cats cause substantially greater wildlife mortality than previously thought and are likely the single greatest source of anthropogenic mortality for US birds and mammals. Scientifically sound conservation and policy intervention is needed to reduce this impact.

They are particularly incensed by programs that trap feral cats, but then return them to the wild after neutering them. I must say this seems to be a crazy idea– why in the world would you put the offending predators back into the ecosystem?

The most striking thing to me was their estimate that well over 2/3 of the mortality was due to “un-owned” (i.e. feral or some slight variation thereof) cats, so that cat owners taking steps to insure that their pets do not become destructive predators, while helpful, would leave most of the problem unaddressed.

Media coverage of the study can be found at the New York Times and the BBC.


Buller, W.L. 1895. On a new species of Xenicus form an island off the coast of New Zealand. Ibis 7:236-237.

Galbreath, R. & D. Brown. 2004. The tale of the lighthouse-keeper’s cat: Discovery and extinction of the Stephens Island wren (Traversia lyalli). Notornis 51:193-200. (pdf)

Loss, S.R., T. Will & P.P. Marra. 2013. The impact of free-ranging domestic cats on wildlife of the United States. Nature Communications (pdf)

Medway, D.G. 2004. The land bird fauna of Stephens Island, New Zealand in the early1890s, and the cause of its demise. Notornis 51:201-211. (pdf)