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.