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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An ancient invasion

Just about anywhere you are in the world, you are confronted constantly with invasive species.

Some are so ubiquitous we've stopped even noticing them.  Here in the United States, for example, most lawn grasses are non-natives (including, amusingly, Kentucky bluegrass), as are dandelions, daisies, burdock, garlic mustard, multiflora rose, bush honeysuckle, and thistle.  None of our domesticated animals are native to North America, but neither are such ridiculously common creatures as house mice, the various species of rats, Japanese beetles, pigeons, starlings, house sparrows, and goldfish.  

It's tempting to lump all these species together and say "exotic = bad," but that's a vast, and inaccurate, oversimplification.  Some have clearly had devastating effects on native species; feral and owned-but-outdoor cats, for example, kill an estimated two billion birds a year in the United States alone.  (Yes, that's billion, not million.  Cats are responsible for more bird deaths than any other single cause.)  Other exotics have had far less impact; dandelions may be in every lawn in North America, for example, but they don't seem to do much in the way of outcompeting other species.  (And, as I said earlier, lawn grasses are exotics themselves anyhow.)

A lot of effort by environmental agencies has been put into eradication of exotics, to varying levels of success.  Rats and mice, for example, are generally a lost cause, given their fast reproductive rate and ability to survive on damn near any kind of food; but some isolated islands have done pretty well, most notably South Georgia, which wiped out their rat and mouse infestation in 2018 in order to save endangered birds that nest there.

The southeastern United States, however, has had almost zero success controlling kudzu, also called "mile-a-minute vine" because of its stupendous growth rate.  Introduced in 1876, and hailed as a source of browse for cattle and starch-rich roots that could be used in place of potatoes, the vine went on to cover trees, barns, and slow-moving individuals, and to this day blankets acres during its growing season.

Kudzu in Atlanta, Georgia [Image is in the Public Domain]

Where it gets interesting is the observation by one of my AP Environmental Science students a while back, who said, "But if you go back far enough, isn't everything exotic?"  It's a point well taken.  Species move around, and introductions happen by accident pretty much continuously.  (In fact, there's a whole mathematical model called island biogeography that has to do with the effects of such factors as island size and distance from the mainland on immigration rate and stable biodiversity.)  Our own deliberate and accidental introductions are only continuing a process that has been going on for a long time.

A very long time, to judge by the research of Ian Forsythe (of the University of Cincinnati) and Alycia Stigall (of the University of Tennessee - Knoxville).  They've been studying the "Richmondian Invasion" -- a sudden influx of new species into the shallow sea that covered what is now northern Kentucky, southwestern Ohio, and southeastern Indiana that occurred during the Late Ordovician, 450 million years ago.

The invasion was surprisingly rapid.  Due to exceptionally well-preserved strata, they were able to show that the new species were introduced from the north, as rising seas allowed them to cross what had been a low ridge of dry land, over only a few thousand years.  And what Forsythe and Stigall found was despite the magnitude of the invasion, and the speed with which it occurred, it didn't have very much effect on the recipient ecosystem's pre-existing species.

The reason, Forsythe and Stigall say, is that most of the invaders were low on the trophic ladder -- they were filter-feeders and grazers on phytoplankton.  It'd have been a different story if the invaders had been high-trophic-level predators.

All of this should inform our decisions on where to put our limited resources for environmental management.  High-impact, high-trophic-level invaders -- feral cats, rats, and the like -- are more critical to control than low-level herbivores like pigeons and house sparrows.  (It bears mention, though, that just being a herbivore doesn't mean "harmless;" here in the northeastern United States, whole forests of ash trees are being killed by the emerald ash borer, and farmers and viticulturists are rightly flipping out about the wildfire-spread of the spotted lanternfly.)

So it's a complex subject.  But it's fascinating that an analysis of an exotic invasion 450 million years ago might inform our decisions about how to manage exotics today.  Yet another indication of the value of pure research -- it can give us an angle on real-world problems that we wouldn't have arrived at otherwise.

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Floral invaders

I used to ask a question to my biology classes, during the unit on ecology: what are the only two commonly-eaten fruits that are native to North America?

Some of the most frequent wrong answers -- and where those plants are actually native:

• Apples ("American as apple pie," right?)  Nope, native to Europe, brought over in the early seventeenth century by the French settlers of eastern Canada and now naturalized across the continent.
• Peaches, apricots, and pears -- native to central Asia.
• Plums -- native to China (although there are a few wild North American plum varieties, they're not the ones you ever see in the grocery store).
• Kiwi fruit -- native to east Asia.
• Cherries, strawberries, blackberries, and raspberries -- native to Europe.
• Citrus fruits -- native to southern Asia and Australia.
• Pineapples -- native to South America.
• Bananas -- native to southeast Asia, Papua-New Guinea, and Australia.

Some students -- knowing their botany -- thought I was being tricky and had in mind plants whose product are fruits in the botanical sense, but not to be found in the fruit section of the grocery store, like cucumbers (south Asia) and tomatoes (Central and South America), but no.

The only two commonly-consumed fruits that are native to North America are blueberries and cranberries.  (Squash is also an example, if you count introductions that preceded European colonization; they were widely used by Indigenous Americans, but even they originally came from Mexico and Central America.)

It might be especially hard to believe this apropos especially of blackberries and raspberries, which have gone wild and in many places (like my back yard) form nearly impenetrable thorny thickets of vines.  We have the birds to thank for this; birds consume the berries and then carry the seeds far and wide, a dispersal strategy that is effective enough that both species are now found in every state in the continental United States and every province of Canada.

[Image is in the Public Domain]

Well, so what?  Why does this matter?  The problem is the degree to which non-native (or exotic) species have infiltrated ecosystems -- and changed them.  I could just as well used garden flowers as my example group, but most high school students know fruits way better than flowers.  And I'm ignoring what might be the single most common group of exotic plants in the United States, so ubiquitous that we hardly even think about them; the various common species of lawn grasses.

There are two commonly-cited problems with non-natives. Certainly the best known is that when exotic organisms take hold, they can outcompete and replace native species.  The most successful exotics are the ones that are ecological generalists, able to utilize a wide variety of resources and habitats, and those have especially taken hold in the disturbed ecosystems of cities; consider where you are most likely to find dandelions, burdock, pigeons, house sparrows, and rats, for example.  A second is the accidental introduction of pests that end up destroying native organisms -- three we're constantly fighting here in the northeastern United States are Japanese beetles, the spotted lanternfly, and the emerald ash borer.  (Once again, there's another example in this category you may not have thought about -- feral cats, which take a tremendous toll on native birds.  But I'm guessing the cat lovers in my readership won't appreciate my labeling cats as "exotic pests...")

A third, and less-explored, aspect of the transport of species into new regions is homogenization.  Enough new introductions, and previously diverse and unique ecosystems start looking very much alike.  This was the subject of a paper last week in Nature Communications by a team led by Qiang Yang of the University of Konstanz (Germany), detailing a way to quantify this loss of uniqueness.  

The authors write:

Regional species assemblages have been shaped by colonization, speciation and extinction over millions of years.  Humans have altered biogeography by introducing species to new ranges.  However, an analysis of how strongly naturalized plant species (i.e. alien plants that have established self-sustaining populations) affect the taxonomic and phylogenetic uniqueness of regional floras globally is still missing.  Here, we present such an analysis with data from native and naturalized alien floras in 658 regions around the world.  We find strong taxonomic and phylogenetic floristic homogenization overall, and that the natural decline in floristic similarity with increasing geographic distance is weakened by naturalized species.  Floristic homogenization increases with climatic similarity, which emphasizes the importance of climate matching in plant naturalization.  Moreover, floristic homogenization is greater between regions with current or past administrative relationships, indicating that being part of the same country as well as historical colonial ties facilitate floristic exchange, most likely due to more intensive trade and transport between such regions.  Our findings show that naturalization of alien plants threatens taxonomic and phylogenetic uniqueness of regional floras globally.  Unless more effective biosecurity measures are implemented, it is likely that with ongoing globalization, even the most distant regions will lose their floristic uniqueness.

The problem is, halting this trend is going to be tough.  In a lot of ways, that ship has already sailed.  We can act on local scales -- like my wife's and my effort to convert a section of our property into a native wildflower meadow -- but there has already been too much pot-stirring to have a chance of separating the mixture back to its original configuration of ingredients.  It may be that the best we can do is to mitigate the damage to the extent we can; replacing lawn, choosing to plant natives, removing unwanted exotics when you find them -- and keeping your cats indoors. 

And, of course, remember the somewhat encouraging truth that even introduced species can eventually come into equilibrium with the natives.  European Starlings, introduced into North America in the late nineteenth century, had multiplied into such enormous numbers that in many regions they were the most common bird around, but in the last fifty years have declined to more reasonable (and stable) numbers.  (The only scary thing about this is that we don't have a clear idea of why they've declined -- by some estimates, to fifty percent of the total population in 1970 -- and scarier still, there's been a commensurate decline in native species during the same time frame.)

But the harsh fact is that we've already made irreparable changes to the world's ecosystems, and that's not going to stop any time soon.  The important thing now is to learn from past mistakes -- and do what we can to protect what's still left of our beautiful and unique biodiversity.

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One of my favorite writers is the inimitable Mary Roach, who has blended her insatiable curiosity, her knowledge of science, and her wonderfully irreverent sense of humor into books like Stiff (about death), Bonk (about sex), Spook (about beliefs in the afterlife), and Packing for Mars (about what we'd need to prepare for if we made a long space journey and/or tried to colonize another planet).  Her most recent book, Fuzz: When Nature Breaks the Law, is another brilliant look at a feature of humanity's place in the natural world -- this time, what happens when humans and other species come into conflict.

Roach looks at how we deal with garbage-raiding bears, moose wandering the roads, voracious gulls and rats, and the potentially dangerous troops of monkeys that regularly run into humans in many places in the tropics -- and how, even with our superior brains, we often find ourselves on the losing end of the battle.

Mary Roach's style makes for wonderfully fun reading, and this is no exception.  If you're interested in our role in the natural world, love to find out more about animals, or just want a good laugh -- put Fuzz on your to-read list.  You won't be disappointed.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

Kite flying

A few years ago, I wrote a post here at Skeptophilia called "Grass, gulls, mosquitoes, and mice," in which I laid out the argument that while evolution is usually slow, sometimes it's so fast we can see it happening before our very eyes.  And when that happens, the anti-evolutionists amongst us have some explaining to do.

It's always nice to have another arrow in your quiver, and that came in a recent paper in Nature called, "Rapid Morphological Change of a Top Predator With the Invasion of a Novel Prey," by Christopher E. Cattau, Robert J. Fletcher Jr, Rebecca T. Kimball, Christine W. Miller, and Wiley M. Kitchens, all biologists at the University of Florida, who have been studying Snail Kites, a rare bird of prey found in the Everglades (and, as you'll see, in a few other places).

The Snail Kite, as you might expect from the name, is a specialist predator that feeds only on apple snails, a large species of freshwater gastropod found in the Everglades.  They have hooked beaks for removing the meat from the snail, and taloned feet for holding onto the shell -- well adapted for their niche.

The problem started with the accidental introduction into Florida of the island apple snail (Pomacea maculata), a larger, heavier species native to Argentina.  The native species, the Florida apple snail (Pomacea paludosa), was quickly outcompeted in areas where they both occurred, which concerned not only fans of the Snail Kite but rice farmers, as the island apple snail is a voracious pest on rice crops.

[Image licensed under the Creative Commons, photograph by Andy Morffew]

When an exotic species replaces a native species upon which other animals depend for food, the usual result is a drastic blow to the pre-existing food chain.  Here, though, we have a different result -- an eye-opening response by the Snail Kites that recalls evolutionary biologist Alan Grant's comment in Jurassic Park that "nature finds a way."

As is, the Snail Kites in Florida were not equipped to prey upon the island apple snails -- their feet were too small to hold onto the shells, not surprising as the snails are five times larger than the native Florida apple snails.  But the expected drop in the bird's numbers didn't happen.  Instead, in only a couple of generations, selection was so powerful on the population that the average talon size and bill size increased measurably, and the alterations were reflected by changes in their DNA.

"Nobody would believe me," said Robert Fletcher, co-author of the study, when the findings were announced. "They said, 'No, that cannot be. It's too quick.'"  But even the naysayers were convinced when the introduced snail species showed up in huge numbers in one part of the Snail Kite's range, and instead of leaving the premises, nearly all of the nearby kites converged on the spot.

I guess birds like an all-you-can-eat buffet as much as the rest of us do.

The authors write:

[T]rends in predicted breeding values emphasize that recent morphological changes have been driven primarily by phenotypic plasticity rather than micro-evolutionary change.  Our findings suggest that evolutionary change may be imminent and underscore that even long-lived vertebrates can respond quickly to invasive species.  Furthermore, these results highlight that phenotypic plasticity may provide a crucial role for predators experiencing rapid environmental change.

It's good news for the kites, but it bears mention that a lot of times, the introduction of an exotic species can spell disaster for native ones.  The kites were lucky in that there was already a range of bill sizes because of spontaneous mutations, and the new prey acted as a selecting agent, favoring the largest-billed and largest-footed individuals.

The most interesting part is that once you set this in motion, it ultimately will split the population from related populations elsewhere.  I first saw Snail Kites in Belize, where there are no island apple snails, so the pressure to cope with bigger prey doesn't exist.  Given time -- and, apparently, less time than anyone thought -- the population in Belize and the one in Florida will diverge genetically to the point that they will be, by anyone's definition, different species.

So there you have it: another example of evolution in action.  Cool enough for anyone to appreciate, but for evolutionary biologists, this is nothing short of spectacular.  We can add this to the list of times we've actually observed species evolving quickly enough to see it happen -- which is one more nail in the coffin of strict creationism, not that we particularly needed another one.

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Ever wonder why we evolved to have muscles that can only pull, not push?  How about why the proportions of an animals' legs change as you look at progressively larger and larger species -- why, in other words, insects can get by with skinny little legs, while elephants need the equivalent of Grecian marble columns?  Why there are dozens of different takes on locomotion in the animal world, but no animal has ever evolved wheels?

If so, you need to read Steven Vogel's brilliant book Cats' Paws and Catapults.  Vogel is a bioengineer -- he looks at the mechanical engineering of animals, analyzing how things move, support their weight, and resist such catastrophes as cracking, buckling, crumbling, or breaking.  It's a delightful read, only skirting some of the more technical details (almost no math needed to understand his main points), and will give you a new perspective on the various solutions that natural selection has happened upon in the 4-billion-odd years life's been around on planet Earth.

[If you purchase the book from Amazon using the image/link below, part of the proceeds goes to supporting Skeptophilia!]