The Fate of Islands - Forbidden and Otherwise

This summer, I introduced my granddaughter to the cooperative board game Forbidden Island (Gamewright Games), an engrossing game in which the players work together to gather certain treasures on an island before it, the players, and the treasures sink into the depths.  The island’s geography is different for each game but the peril is constant:  randomly, parts of the island will sink and ultimately disappear.  She took to it completely, quickly mastering the game’s rules and nuances.  The many times I played it over the past month may partly explain the present post.  That, and the fact that the population dynamics on islands are of great interest to biologists and figure prominently in evolutionary theory – species finding themselves on islands, isolated from their counterparts elsewhere, have the opportunity to evolve differently.

This post is somewhat stream-of-consciousness, segueing as it does from one island to another, leading to no real twist or hook at the end. 

I was mesmerized by an article in the most recent issue of LivingBird titled Garden of the Gulls and written by Hugh Powell.  (Powell, 2022.  Full citations and links to references are listed at the end of this post.)  His article describes some of the population dynamics of species on the volcanic island of Surtsey which came into being beginning in 1963, rising violently some 10 miles off the coast of Iceland.  The three stamps below issued by Iceland in 1965 depict Surtsey at three different stages of its initial development:  November 1963, April 1964, and September 1964.

Not surprisingly, given that LivingBird is a quarterly publication of the Cornell Lab of Ornithology, Powell’s article highlights the role that birds, particularly seagulls, have played in fostering the diversity of the flora and fauna population on the island.  This emphasis is completely justified because birds have been critical to Surtsey.  Overall, this is a story of species immigration and the struggle for survival.  The trajectory of the island’s floral and faunal species population has been dictated by a complex interplay of biology, geography, and geology.

The image below of contemporary Surtsey is from Google Maps.

Scientists took full advantage of the opportunity offered by the island’s birth to study in detail the process by which it, initially completely devoid of life, became populated.    Even amid ongoing eruptions, molds, bacteria, and fungi appeared, as did the first vascular plants (1965).  In the ensuing decades, new species came to Surtsey via ocean currents, winds, and birds.  The document prepared in 2007 as part of the successful effort to add Surtsey to UNESCO’s World Heritage List (Baldursson, 2007) identified three periods in the populating of the island:  an initial decade with a burst of new species appearing and some, but not all, becoming established, followed by a decade of “stagnation” with little growth in diversity, followed by a plant and animal population boom, fueled largely by a large community of breeding seagulls.  As Hugh Powell writes in his LivingBird article:  

Plant diversity had plateaued by the 1970s with fewer than 20 species established, and it didn’t take off again until a gull colony developed in 1986.  The gulls, mostly Lesser Black-backed along with Great Black-based, Herring, and Glaucous – carried with them not just new kinds of seeds but also fertilizer in the form of nitrogen-rich guano.  (Powell, 2022, p. 44.)

The guano nurtured a “lush green meadow” that, by 2013, according to Powell, covered roughly 30 acres on the island, “supporting twice as much plant diversity and five times more biomass than the rest of the island.”  (Powell, 2022, p. 44.)

Where have the species found on Surtsey come from?  Those established on the island reflect those on nearby islands in the Westman archipelago of which it is a part and on mainland Iceland.  The UNESCO nomination document notes that the vascular plant species on the island are mostly ones common in the archipelago, supporting the notion that those islands are the primary source of colonizing species.  Further, all of the Surtsey plant species are found on the Icelandic mainland, and “there has been no indication of species colonising the island from distant sources.”  (Baldursson, 2007, p. 28.)

The history of other islands in the archipelago are a window to Surtsey’s future.  Geologically, Surtsey has spent six decades being eroded by wind and water.  In its first forty years, the island shrank by half.  At its maximum size in 1967, the island was approximately 1 square mile in area; by 2004, it was down to just 0.5 square mile.  Powell describes the fate that awaits Surtsey.  Erosion will continue to eat away at the island:

the low-lying plains will disappear (including the present gull colony), and the hard crater walls will become steep-sided seacliffs where murres and Razorbills will nest alongside the fulmars and kittiwakes already present.  Atop the island, a thick turf will develop dominated by just a few grass species.  (Powell, 2022, p. 46.)

As he reports, the plant population boom has already ended and a downward trajectory has taken hold.  The UNESCO nomination document notes that the shrinking of the island’s above-sea landmass has followed a relatively steady pattern.  As a result, the authors conclude the island will come to resemble, in particular, two other islands in the archipelago formed in a similar fashion some 6,000 years ago and long since eroded to the core of the volcanos which created them.  They predict that Surtsey in that form “will survive for a long time, probably for thousands of years.”  (Baldursson, 2007, p. 56.) 

The Surtsey article dredged up from the deep recesses of my memory a different, but very much related, island story, one focused on an experiment conducted in the 1960s by the renowned naturalist Edward O. Wilson and his student Daniel S. Simberloff.  I certainly wasn’t aware of the experiment when it was ongoing or when the initial research articles were published in 1969.  (Simberloff, 1969a; Simberloff, 1969b.)  Rather, it was Wilson’s excellent memoir published a quarter of a century later (Wilson, 1994) that handled that introduction.

As Wilson recounts it, in the early 1960s, he and ecologist Robert MacArthur had developed a mathematically-based hypothesis of island biogeography.  He writes in his memoir,

We had conjured a plausible image of the dynamic equilibrium of species, with new colonists balancing the old residents that become extinct, but we could offer very little direct evidence.  There are few places in the world where biologists can study the approach to equilibrium on a large scale.  (Wilson, 1994, p. 260.)

A recent assessment of the continuing importance of island-based biological research defines MacArthur and Wilson’s “equilibrium theory of island biogeography” as

a theoretical model . . . that postulates that the number of species present on an island will be determined by the dynamic relationship between immigration and extinction rates.  In turn, immigration rates depend greatly on the island isolation, while extinction rates are mainly associated with island area.  (Santos, 2016, p. 753.)

That same assessment considers the MacArthur/Wilson theory to have been highly significant, shifting the paradigm of island research and fostering development of conservation theory.

That lack of “direct evidence” prompted Wilson and Simberloff to fashion in the early 1960s an experiment that could generate data to test the equilibrium theory of island biogeography.  Persuaded that natural experiments in which islands were denuded of species – such as through volcanic explosions or devastating hurricane hits – offered little prospect of generating the necessary data in a reasonable length of time (Wilson suggested 10 years) showing how life returned to such areas, Wilson and Simberloff elect to fumigate six small mangrove islands in the Florida keys, eliminating all insect life on those islands.  In addition to the total mortality of all insects on this tiny islands, some of mangrove trees suffered damage from which, Wilson and Simberloff report, they mostly recovered.  The authors don’t note that any vertebrate animals were adversely affected by the fumigation.

Of the results of this experiment, Wilson writes that “the cruder predictions of the theory had been met.”  (Wilson, 1994, p. 280.)  In the paper on the initial results, Simberloff and Wilson posit that the strongest evidence from this experiment that a specific dynamic equilibrium of species exists for any island was that the number of species on each island post-fumigation returned “approximately” to its pre-fumigation number with a “rough oscillation about this number.”  (Simberloff, 1969b, p. 285.)  Species turnover was speedy as the theory posited it would be on small islands.

I remember my initial reaction to reading Wilson’s account of the experiment – dismay.  It seemed to be going down the slippery slope of the ends justifying the means, distressed as I was by rendering each of these mangrove islands into a killing field.  Admittedly, I had no idea of the importance of the experiment and, even now, I still don’t.  How critical were these data to the significant role subsequently played by the MacArthur/Wilson theory in island research and conservation?  I do suspect that “permission to wipe out animal populations on federally protected land” wouldn’t be granted today.  Back then, permission was readily forthcoming.  (Wilson, 1994, p. 269.)

It's striking that, at the time when Wilson and MacArthur were fashioning and publishing their theory, Surtsey was being born.  What did Wilson think of the role that research on Surtsey might play?  In the methodology paper that he and Simberloff published in 1969, they acknowledge the emergence of this new island and the research then underway to track its population, but immediately discount its utility because of “the infrequent natural occurrence of such events . . . .”  (Simberloff, 1969a, p. 268.)  Rather than taking advantage of such a natural experiment, the authors consider two alternatives.  The first was, in my opinion, a straw man – “produc[e] new islands similar to natural ones.”  The second was the only viable option in their eyes – “sterilizing preexisting islands.”

I would have voted for natural experiments, but I think I’ll stick to Forbidden Island where I understand the dynamics of the game and there’s not much at stake even as the island disappears under my feet.

References

Baldursson, Snorri, and Álfheiður Ingadóttir, Nomination of Surtsey for the UNESCO World Heritage List, 2007.

Powell, Hugh, Garden of the Gulls, LivingBird, Summer 2022, Volume 41, Number 3.

Santos, Ana M.C., et al., New Directions in Island Biogeography, Global Ecology and Biogeography, Volume 25, Number 7/8, 2016.

Simberloff, Daniel S. and Edward O. Wilson, Experimental zoogeography of Islands:  Defaunation and Monitoring Techniques, Ecology, Volume 50, Number 2, 1969a.

Simberloff, Daniel S. and Edward O. Wilson, Experimental Zoogeography of Islands:  The Colonization of Empty Islands, Ecology, Volume 50, Number 2, 1969b.  []

Wilson, Edward O., Naturalist, 1994.

Simple and Obvious Truths

In which the blogger begins with invasive Burmese pythons and ends up with beached Viking funeral ships.

English ivy, an invasiveI am somewhat suspicious of the simple truth or the obvious truth (not necessarily mutually exclusive adjectives for truth) because, from my perspective, things in general, nature, life don’t usually work simply or obviously. (I will avoid the endless loop and not suggest that this is a simple and obvious truth.) I also have a tendency to accumulate trees, lots of trees, and, as a result, end up often deep in details, missing the forest entirely.

I was prompted to think about simple truths and obvious truths when I read a fascinating (and, in true New Yorker style, lengthy) article by Burkhard Bilger in which he describes the exotic or invasive fauna, most prominently the Burmese python, that runs rampant in Florida and considered what might and might not be done to arrest its spread (Swamp Things: Florida’s Uninvited Predators, The New Yorker, April 20, 2009). Though the article seemed to strike a death knell for native Florida fauna (and all small dogs), I was brought up short when Bilger wrote the following:

The one statistic that seems to hold true is the rule of ten, described by an English biologist in 1993: one in ten exotics escape into the wild; one in ten of those become established; one in ten established exotics become pests. . . . Invasives can undoubtedly drive natives to extinction, . . . . But most of the time an ecosystem isn't a game of musical chairs. When a new species arrives, it rarely takes another's place. It just finds another spot to sit.

Well, in that single paragraph, there’s a example of the simple truth and also a challenge to a commonly accepted obvious truth.

A Simple Truth

First, a simple truth. A statistical rule of ten is clearly a macro view of the forest, the trees are indistinguishable. Given, say, 1000 invasive species, 10% or 100 will escape; of the escapees, 10% or 10 will become established; and of those species 10% or 1 will become a pest – 0.1% of the original non-native population. Is that a lot or a little? I don’t know. But, it’s so simple, and so easy.

But it’s not quite so simple – there are some trees in this forest. Biologist Mark Williamson, who first calculated it, calls it the tens rule, and he stresses that it needs to be approached with caution. In fact, the tens are really fives to twenties. In other words, the percentages may range from 5 to 20% at each stage. (The Varying Success of Invaders, Mark Williamson and Alastair Fitter, Ecology, Vol. 77, No. 6, p. 1662) Not only does the rule have a fairly broad range of wiggle room, but it also has a number of exceptions (with much higher percentages at some of the different stages – escape, establishment, or becoming a pest) even within the handful of organisms studied and reported on by Williamson and Fitter.

An Obvious (and Simple) Truth

The challenge to an accepted obvious truth in the paragraph quoted above is this argument: An ecosystem may have room, and invasive species that establish themselves may occupy vacant niches, without upsetting native species.

Wait, wait, wait. Isn’t it obviously (and simply) true that invasive species are bad, and natives are good? I mean, just consider that kudzu tidal wave engulfing the woods just down the road. That’s certainly what the Alien Plant Working Group, sponsored by the National Park Service, posits: "Invasive non-native organisms are one of the greatest threats to the natural ecosystems of the U.S. and are destroying America's natural history and identity.” The APWG states, “[E]ach alien plant is one less native host plant for our native insects, vertebrates and other organisms that are dependent upon them.”

And this view is the one promoted by a local group that works to maintain and protect a park close to my home – the park straddles a creek for many miles of its run. This group describes the possible fate of flora in the park in jeremiadic terms similar to those of the APWG – if action isn’t taken now, the several hundred plant species native to this area will be completely displaced by a limited number of invasive non-native species.

Well, this obvious truth may not be so true. This forest is made up of many, many trees. Turns out there’s an ongoing scientific battle over the consequences of species invasion, and whether we should be in continual crisis mode about it, indeed, whether we might have let our emotions override some necessary scientific distance. The debate supports my wariness of the obvious.

Biologists James H. Brown and Dov F. Sax are in the vanguard of the revisionists cautioning about that obvious conclusion concerning invasive species. At the outset, they suggest jettisoning the idea that direct or indirect human agency is a sine qua non for defining an invasive/non-native/exotic/what-have-you species. They question whether we can or should look to a “pristine” past when things were natural or native, because the invasion of alien species “should be viewed in the context of ‘natural’ colonization events that have occurred without human intervention throughout the Earth’s history.” (An Essay on Some Topics Concerning Invasive Species, Austral Ecology, 2004, p. 530) Invasion isn’t a new story, it’s a very old one.

The consequences Brown and Sax suggest are certainly much more complicated than the APWG’s assertion that every invading alien plant species displaces a native species. They review the arguments about whether different ecosystems are capable of absorbing more animal and plant species, whether and how invasions might lead to increased biodiversity in a system, etc. The answers aren’t clear cut and don’t seem to fall uniformly on one side or the other. The consequences of a species invasion can be devastating, but not necessarily so. The villain in the piece isn’t always a villain. Indeed, a New York Times article on the revisionists in this debate was entitled “Friendly Invaders.” (Article by Carl Zimmer, The New York Times, September 8, 2008)

Getting Paleontology into the Mix

When I read about the tens rule, I initially wondered whether what the fossil record might say about it. I recognize that this is a quintessentially naive question. There’s a degree of precision (even within its fudge factors) to the tens rule unlikely to be reflected in the fossil record (that is, if the rule itself is really a reflection of nature).

Still, as Brown and Sax write, “The fossil record shows many episodes of massive invasions and extinctions . . . .” Further, there have been invasions of ecosystems by non-native species throughout the history of life on Earth. And it need not be on a massive scale. “[T]hroughout the past, individual species dispersed across existing biogeographical barriers to invade new continents and islands, oceans and lakes. These independent colonization events were sporadic and infrequent, but over the millennia of the Earth’s history, their numbers were large and their impact on global biodiversity was great.” (p. 531) In the final analysis, species invasion hasn’t been the exclusive work of human agents, and this period’s natives aren’t the previous period’s natives.

With invasive species, the truth may not be quite so obvious or simple.

Beached Viking Funeral Ships

I have read a little bit of the biogeographical literature on the dispersion of species over the Earth’s history – where species end up and why – and am quite taken by some of its extremely clever terminology. Among the avenues through which fauna might move into new areas are corridors (paths offering little obstruction to dispersion), filter bridges (avenues that don’t result in free flow, some species move across these bridges, others don’t), and sweepstakes routes (dispersal where, like the lottery, the odds are very long against success – think islands in the middle of an ocean). (See Bringing Fossils to Life: An Introduction to Paleobiology, by Donald Prothero, 1998, Chapter 9)

But, for me, the best names are those coined by paleontologist Malcolm C. McKenna in describing how plate tectonics can account for some of the dispersal of species. There are the Noah’s arks created when pieces of land masses break off and move across water to join different land masses. Species catch a ride.

My favorite describes a mechanism that’s actually a cheat when it comes to species dispersal. McKenna described beached Viking funeral ships (how glorious) occurring when traveling land masses, carrying their fossil record, collide with other land masses and deposit that fossil record some place unexpected. These are invasions of the long dead, the fossilized. According to Prothero (on whose descriptions of McKenna’s terms I’ve relied), one example of this process at work is the presence in North America of Cambrian European trilobite fossils, and the presence in Scotland of North American trilobite fossils.

Malcolm McKenna, who died last year, had been curator of vertebrate paleontology at the American Museum of Natural History and geosciences professor at Columbia University. The publication in 1997 of Classification of Mammals capped a lengthy career studying fossil mammals. According to his New York Times obituary, McKenna’s legacies include the one that is left by most fossil collectors: “Until almost the end, he was still bringing home fossils, his wife said. Another scientist is classifying the mammal fossils piled up in the basement of their house.” (Malcolm McKenna, 77, Fossil Seeker, Dies, by John Noble Wilford, The New York Times, March 10, 2008)


Yellow-crowned Night Heron chicks born at the extreme northern edge of their range -- not invasives . . . yet.