The silent invasion

My office window looks out over our raised-bed gardens and into our front yard.  It's still chilly early spring here in upstate New York -- things won't really start greening up for another couple of weeks -- but we're seeing signs of the coming explosion of growth that tell us warm weather will soon be here.  We actually got out and did some yard work this past weekend, despite clouds and a high of 45 F.  Mostly clean-up that never got done last fall, but we did plant the early peas and lettuce, transplanted some clumps of chives that were taking over one corner of the vegetable garden, and moved a yucca plant that was getting a little too enthusiastic.

From where I sit right now, I can see our bit of grassy lawn, but also the bare branches of a purple lilac, a couple of still-leafless roses, the gnarled branches of a sawtooth oak, the reddish buds of peonies just starting to unfurl, the bright green spikes of daylily leaves, the stubble of the ornamental Miscanthus grass that by midsummer will be taller than I am.  Clumps of brilliant daffodils, crocus, scilla, and chionodoxa already in full flower.

All cool stuff, promising lots of beauty to come.  But you know what all of the plants I've mentioned have in common?

Not one is native to the United States.

Not even the grass.  Just about all the lawn grasses grown in North America are European natives.  Chances are, unless you have deliberately set out to do natives-only landscaping, the vast majority of the plants in your yard are imports as well.  Of everything I can see from my window, only one is native to upstate New York -- a hedge of ninebark (Physocarpus opulifolius).  Two others are eastern natives but originally from a good deal farther south, the Carolina silverbell (Halesia carolina) and black locust (Robinia pseudoacacia).

Thing is, like everything, the situation with exotics is complex.  Not all exotics are a problem.  The charming little bright-blue scilla (Scilla siberica) that pop up everywhere in the very early spring, including all over my lawn, are pretty harmless.  (Contrary to the name, they're not native to Siberia, but to southwestern Russia, the Caucasus, and northern Turkey.)  Garlic mustard (Alliaria petiolata), on the other hand, is an unmitigated thug -- since its introduction in the 1800s, it has spread like wildfire, each plant producing hundreds of seeds, and in many areas has crowded out native plant species.  It's also toxic to a lot of native herbivores, including several species of butterflies.  We've tried for years to rid our yard of this nuisance, without much success.

And don't get me started about multiflora rose (Rosa multiflora).  The Wikipedia page says it's native to Asia, but I'm convinced it was imported directly from hell.  It has pretty white flowers, but more than makes up for that by razor-sharp thorns borne on long, tough, wiry stems that seem to have a deliberate vicious streak.  In general I love roses, but this one is an absolute hazard.

Of course, here in New York, we still have a great many native species that are doing well.  Consider the situation in Hawaii, though -- where on the more populated islands, there are barely any native species left.

Oh, it looks good.  On O'ahu, there are lush forests -- guava, plumeria, cinnamon, peppertree, Kahili ginger, several species of acacia and eucalyptus, banyan, satinleaf -- and flocks of showy birds like the red-billed leiothrix, red-whiskered bulbul, zebra dove, common mynah, and red-crested cardinal.  But not one is native.  The Hawaiian lowland ecosystems were completely destroyed for agriculture and settlement; accidental introduction of the southern house mosquito (Culex quinquefasciatus), and the avian malaria it carried, wiped out nearly all of the birds living below five hundred meters of elevation.  If you want to see native Hawaiian species -- what's left of them -- you have to go up into the mountains, and even there, they're struggling to hang on.

Aarhus University ecologist Jens-Christian Svenning, who has been studying Hawaii's ecology for almost a decade, calls the current situation a "freakosystem."  What's interesting, Svenning says, is that the situation has re-established a healthy, interactive community of species -- just not the ones that were there only two hundred years ago.

"These are wild but changed ecosystems," Svenning said.  "They have passed some critical threshold which means they are unlikely to ever go back to how they were before.  If you removed all people from the planet, Hawaii would be on a different evolutionary ecological trajectory going forward."

Hawaii's iconic plumeria trees, whose flowers are used to make leis, were introduced from the Caribbean in the 1800s  [Image licensed under the Creative Commons Varun Pabrai, Plumeria rubra-4, CC BY-SA 4.0]

Hawaii, however, is just the canary in the coal mine.  A study by Svenning and his colleagues indicates that between thirty and forty percent of all terrestrial ecosystems have "transformed into novel states;" that percentage is projected to rise to fifty by 2100.

It's not, of course, that these kinds of changes can't happen through natural processes.  Three years ago I wrote a piece about the effect that continental collisions can have on the species that live there; and, after all, even less dramatic events than that can lead to extinction.  What strikes me here is the speed with which it's happening.  We've tampered with ordinary ecological succession with no forethought, and as a result, triggered what (to judge by the rates) will rank up there with the "Big Five" mass extinctions -- the Ordovician, Devonian, Permian-Triassic, Late Triassic, and Cretaceous.

So maybe it's time to start thinking about this.

It's too late to undo the silent invasion of exotic species; here in upstate New York, I'm afraid lawn grass is here to stay, as are garlic mustard and multiflora rose, and lilacs, peonies, and daffodils.  At least the last three are pretty and don't seem to be especially harmful.  But we'd better wise up about what we're doing, and fast.  Because remember that as prideful as we get sometimes, to the biosphere we're just another animal species.  We're no more guaranteed survival than anything else.

Let's hope we learn that lesson before it's too late.

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Hot times

In today's contribution from the Completely Useless Advice department: if you own property in southern Africa, you might want to consider selling it some time in the next ten million years or so.

The reason I say this is because of a paper published a couple of months ago in Nature Geoscience that was once again thrown my way by my pal Gil Miller, who seems to have an inordinate talent at ferreting out truly fascinating stuff I hadn't heard about.  The paper is entitled "A Tree of Indo-African Mantle Plumes Imaged by Seismic Tomography," by Maria Tsekhmistrenko, Karin Sigloch, and Kasra Hosseini (of Oxford University), and Guilhem Barruol (of the Université de Paris), and describes the structure of the mysterious "hotspots" -- upwelling of extremely hot magma from deep in the mantle -- that are responsible for such volcanically-active regions as Hawaii, Yellowstone, and Réunion Island.

These hotspots have long puzzled geologists, because they are quite distant from tectonic plate boundaries, where most of the world's seismic and volcanic activity occurs.  Hawaii is the best-studied hotspot; it was one of the most powerful pieces of evidence of plate movement, back in the 1960s when the theory of plate tectonics was first being studied.  The Big Island of Hawaii is just the easternmost point in a chain that extends way beyond what we usually think of as the Hawaiian Islands; even the westernmost island that pokes up above sea level, Kure Atoll, isn't the end of it.  It continues into the Emperor Seamount Chain, which extends underwater all the way to the Kamchatka Peninsula of Siberia. 

My long-ago geology professor described it as being like pulling a piece of fabric (the Pacific Plate) through an upside-down sewing machine (the Hawaiian Hotspot); the needle of the sewing machine punches regular holes upward through the fabric as it moves through, but the sewing machine itself stays in the same place.  The plates are moving; the hotspot isn't.  (And the angle in the chain of seamounts indicates that at some point in the past, the Pacific Plate changed direction, probably because of jostling against other plates.)

The Pacific Ocean floor, showing the Hawaiian-Emperor Seamount Chain [Image is in the Public Domain courtesy of NOAA]

What is still mysterious about hotspots is why they happen at all.  We have a pretty decent idea of why the activity along plate margins occurs -- strike-slip faults like the famous San Andreas, where two plates are moving along each other in opposite directions; trenches/subduction zones like Indonesia, where you get both powerful quakes and huge volcanoes; and mid-ocean ridges/divergent zones like the Mid-Atlantic Ridge, where plates are moving apart and new magma upwells to fill the gaps.  But why would there be a persistent chain of volcanoes out in the middle of a stable plate?

The current paper describes blobs of extremely hot magma originating from the lower parts of the mantle, which rise and then diverge into branches.  The authors write:

Mantle plumes were conceived as thin, vertical conduits in which buoyant, hot rock from the lowermost mantle rises to Earth’s surface, manifesting as hotspot-type volcanism far from plate boundaries.  Spatially correlated with hotspots are two vast provinces of slow seismic wave propagation in the lowermost mantle, probably representing the heat reservoirs that feed plumes...  Using seismic waves that sample the deepest mantle extensively, we show that mantle upwellings are arranged in a tree-like structure.  From a central, compact trunk below ~1,500 km depth, three branches tilt outwards and up towards various Indo-Austral hotspots.  We propose that each tilting branch represents an alignment of vertically rising blobs or proto-plumes, which detached in a linear staggered sequence from their underlying low-velocity corridor at the core–mantle boundary.  Once a blob reaches the viscosity discontinuity between lower and upper mantle, it spawns a ‘classical’ plume-head/plume-tail sequence.

So the Réunion Hotspot is apparently connected to the East African Rift Zone, three-thousand-odd kilometers away.  The EARZ is a developing rift that is ultimately going to shear off the "Horn of Africa," opening a new ocean and creating a new "microcontinent" made up Somalia and bits of Ethiopia, Kenya, and Tanzania.  (As an aside, it's also the site of Olduvai Gorge, where some of the earliest hominin fossils were found.)

[Image is in the Public Domain courtesy of the USGS]

"From looking at the core-mantle boundary, you can maybe predict where the oceans will open,” said study co-author Karin Sigloch.  "If the new models are accurate, a few tens of millions of years from now, you may not want to be in South Africa — or, perhaps, on planet Earth at all."

The reason Sigloch says this is that the team's analysis of the "tree" of magma that underlies both Réunion and the EARZ suggests that it's in the process of forming another branch -- another mantle plume -- that will ultimately end up underneath what is now South Africa.  "In tens of millions of years, a blob of nightmarishly gargantuan proportions will pinch off from the central cusp," Sigloch said, in an interview with Quanta magazine.  "This would produce cataclysmic eruptions.  The Deccan Traps [one of the largest volcanic eruptions ever, and which probably contributed to the extinction of the non-avian dinosaurs 66 million years ago] were caused by what we would think of as a solitary mantle plume.  This future mega-blob, though, would be capable of producing volcanism so prolific and extensive that the Deccan Traps would be a firecracker in comparison."

Pretty scary.  But like I said, if you want to visit South Africa, or if you live there, you still have a ten-million-year window to take care of business.  What's interesting from a geological perspective is that up till now, South Africa has been very stable tectonically.  The majority of the country is made of extremely old rock, what geologists call a "craton" -- a chunk of some of the oldest continents on Earth.  A massive flood basalt eruption, like the Deccan Traps, the Columbia River Flood Basalts, and the largest of them all -- the Siberian Traps, implicated in the cataclysmic Permian-Triassic Extinction -- would (literally) overturn three billion years of stable geology, with catastrophic results for the entire planet.

So yeah.  That's cheerful.  But since we have ten million years before we have anything serious to worry about, it'd be better if to turn your attention to more pressing concerns, even if you live in Johannesburg.  Like what we're doing to destroy the global ecosystem our own selves by our seeming commitment to burn every last gallon of fossil fuels out there, damn the climate, full speed ahead, and which could make the Earth pretty close to uninhabitable a great deal sooner. 

Which now that I think of it, isn't all that reassuring.

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