Derek Turner writes . . .
Caterpillars Galore
This summer, parts of central and eastern Connecticut are in the middle of a gypsy moth outbreak. Actually, I want to register some discomfort with the term “gypsy moth.” The Roma or Romani people were targeted during the Holocaust, and historically, have been on the receiving end of a good deal of mistreatment, including, I’ve only recently learned, some apparent mistreatment here in my own hometown of Haddam, CT. [1] To name a pest species after them—especially a pest that people have gone to great lengths to try to exterminate with chemical pesticides—seems somewhat messed up. But the term is so well established that it’s hard to know where to begin to try to change the usage. Anyway, I will use the technical name, Lymantria dispar.
It’s hard to describe an L. dispar outbreak to those who haven’t experienced it. These days, when you walk in the woods, there is a constant background “crickle crackle” that sounds like it’s raining, or like a bowl of rice crispies. That is the sound of gazillions of caterpillars slowly, inexorably, defoliating the landscape. When you walk down the road you must dodge caterpillars that are dangling down from the canopy by little silken threads. There is a steady shower of debris from above—a combination of uneaten leafy confetti and tiny bits of caterpillar poo that collect in your hair. If you look carefully at the ground, you can see a slowly accumulating layer of caterpillar frass. There are caterpillars climbing on (and in) your car, climbing up the windows on your house, climbing on the clothes in your closet. One of my neighbors reports getting a pretty bad poison ivy like rash from the caterpillars, which apparently also is a thing. When you look up, you can see the usually thick leafy canopy slowly dissolving away from one week to the next. One thing you do not see is any local birds taking advantage of what seems like it should be a caterpillar smorgasbord. There’s not much you can do to protect your trees, though some people wrap duct tape around the tree trunks and smear it with Vaseline to keep the caterpillars from climbing up. In short, it’s hard to see anything good about this from either a human or an ecological perspective, though as a biological phenomenon, an L. dispar outbreak is intrinsically fascinating.
What they do to leaves.
Caterpillar frass
L. dispar is a classic example of an invasive species. They are not native to New England, but were introduced from Europe in the 19th century by someone with a get-rich-quick scheme that involved interbreeding gypsy moths with silkworms. In a classic case of introducing yet another a new species to control one that was introduced earlier and has few predators, people now seem to place their faith in a Japanese fungus that apparently kills the caterpillars. I am not holding my breath, and I wonder what other things the fungus might kill.
The spread of L. disbar. Source: wikipedia.org
Will Invasive Species Be “Nature’s Salvation”?
In his recent book, The New Wild, Fred Pearce takes issue with the idea that invasive species are generally bad, while native species are generally good.[2] He points to cases, like Ascension Island, in the South Atlantic, where introduced species have given rise to new ecosystems with more biological diversity than anything that existed previously. He argues that native species sometimes do lots of ecological damage (think pine bark beetles in the Rocky Mountains), while some invasives are just fine (think earthworms in North America). Pearce also argues that invasive species often do best in environments that human activities have already damaged. Alien species move in, and we scapegoat them, blaming them for biodiversity loss that we ourselves already caused.
Pearce’s book has also rubbed some biologists the wrong way. Critics (such as Liam Heneghan, writing here) have complained that Pearce’s enthusiasm for invasive species goes too far. The subtitle of the book is “Why Invasive Species Will Be Nature’s Salvation.” That suggests that we should welcome and even facilitate biological invasions. Instead of uprooting invasives, we should root for them. But what about the empirical evidence that many such invasions have reduced biological diversity? Here, surprisingly, paleontology has something to say!
First, though, I need to register some additional discomfort with the very concepts of invasive and native species. Others, such as Brendon Larson, in his wonderful book, Metaphors for Environmental Sustainability, have noted that anti-invasive-species attitudes in environmental contexts resonate in disturbing ways with the feeling that people immigrating from other places are somehow threatening.[3] There's no conceptual firewall that separates talk about "illegal aliens" in the context of immigration policy from talk about "eradicating alien species" in the context of restoration ecology. In my last post, I wrote a bit about the unnecessary use of war as a metaphor in biology. I think we need to be extremely mindful of the source domains for metaphors like “invasive species.” Countries invade one another, but species ranges are not like political boundaries. In deciding to use the term “invasive species,” biologists are deciding to see things in nature as being rather like warfare. I see no compelling reason for doing that. Why not just talk in a more neutral way about species’ shifting geographic ranges? The “invasion” language also invites us to see ourselves as being at war with invasive species—a way of seeing things which I think is distinctly unhelpful.
My own skepticism about the whole idea of invasive species disposes me very favorably toward Pearce’s critique of the conventional wisdom that invasive species are bad. But I’m also not on board with the claim that invasive species will be the planet’s “salvation.”
Lessons from the Devonian Fossil Record
Back to the empirical issues. When a species (like, say, L. Dispar) rapidly expands its range into a new place, what does that mean for biodiversity? Pearce is too quick to assume that everything will be hunky dory. He writes:
“One of the most common charges against alien species is that they almost inevitably cause extinctions among their new hosts. They cause a decline in biodiversity” (Pearce, p. 108).
Pearce tries to rebut this charge in a variety of ways, and he tries to identify some problems with the literature that purports to show that invasive species typically threaten biodiversity. I won’t try to cover the arguments in any detail here. One thing that Pearce does not discuss is evidence from the fossil record.
Alycia Stigall, a paleontologist at Ohio University, has written (e.g., here) about prehistoric cases of biological “invasions,” including the Late Devonian Biodiversity Crisis and the Richmondian Invasion of the Late Ordovician.[4] Her work actually suggests that Pearce, in the lines quoted above, is making a conceptual mistake. When a number of species rapidly expand their geographic ranges, that could lead to a decline in biodiversity even if it doesn’t cause more extinctions or lead to an increase in extinction rates. The reason for that is that the invasion could also depress speciation rates.
Image courtesy of Wikimedia commons.
The marine invertebrate fossil record of the late Devonian (ca. 385 – 360 million years ago) tells a fascinating story. This was a time when plants and insects had moved out onto dry land, but much of the action was in the earth's oceans. Stigall notes that the biodiversity crisis of the Late Devonian is a bit of a head scratcher. It is usually classed as one of the "Big Five" mass extinction events in Earth's history even though--puzzlingly--extinction rates did not spike above background rates. However, biodiversity loss can occur even while extinction rates remain constant, if speciation slows down. Sometimes, even if extinction rates do not increase, which species go extinct can matter a lot. In the Devonian, it seems that endemic ecological specialists will smaller geographic ranges started going extinct. These were replaced by “cosmopolitan” species with larger geographic ranges, that were probably also ecological generalists. (This pattern fits with the Plus ça change model that I discussed in an earlier post.) These cosmopolitan species dramatically increased their ranges—Stigall herself uses the term ‘invasion’—and speciation rates dropped, leading to a decline in overall biodiversity. One reason for the decline in speciation, Stigall suggests, is that when you have cosmopolitan generalist species, you are also less likely to have sustained geographic isolation, which is required for allopatric speciation.
Phacops rana, a trilobite from the Middle Devonian of upstate New York. Before the Late Devonian Biodiversity Crisis, the oceans were full of diversity, with huge coral reef systems and lots of local variation. Image courtesy of Wikimedia commons.
The marine animals of the Late Devonian—the trilobites, brachiopods, and such—were obviously not being transported around in the bilge tanks of freighters. Changes in sea level and other geological events were the likely causes of the range expansions. But Stigall also hints that the episode might nevertheless serve as an illuminating model for our current biodiversity crisis—an example of conservation paleobiology. We tend to fixate on extinction, and the challenge of protecting endangered species. But that fixation might lead us to miss some other important possible consequences of the human activity that Pearce is so interested in. When we move species around the planet, we might be helping lots of generalists, or cosmopolitan species expand their ranges, at the expense of lots of ecological specialists. If the fossil record is any guide here, then we could be in for an episode of reduced speciation rates—and that is in addition to the more intensively studied issue of human activity causing higher extinction rates.
So while the yucky L. dispar caterpillars nosh their way through the forests of the Connecticut River Valley, we should also think about the larger patterns. This is just one of many instances in the world today where cosmopolitan generalist species are massively expanding their ranges. The fossil record can help us to understand what this means. Even if the L. dispar caterpillars are not causing extinctions in North America, the fossil record warns us that this sort of biological phenomenon has in past cases been associated with a decline in speciation. And even if extinction rates were constant (they're not), that could spell trouble for global biodiversity.
[1] Apparently on June 6, 1917, century ago, the local Penny Press in my hometown of Haddam, CT, reported that “The band of gypsies camping in R.W. Butler’s lot were ordered out of town yesterday by the selectmen, assisted by John Rich, constable. They consisted of about 75 people, with nine automobiles, a number of horses, and five wagons.” Haddam News, June 8, 2017, p. 3.
[2] F. Pearce, The New Wild: Why Invasive Species Will Be Nature’s Salvation. Beacon Press, 2015. See also Emma Maris, The Rambunctious Garden: Saving Nature in a Post-Wild World. Bloomsbury, 2013.
[3] B. Larson, Metaphors for Environmental Sustainability: Redefining Our Relationship With Nature. Yale University Press, 2014.
[4] A.L. Stigall, “The Impact of Invasive Species on Speciation: Lessons from the Fossil Record,” in W.D. Allmon and M.M. Yacobucci, eds., Species and Speciation in the the Fossil Record. University of Chicago Press, 2016, pp. 340-365. See also, A.L. Stigall, (2010) “Invasive Species and Biodiversity Crises: Testing the Link in the Late Devonian,” PLoS One 5(12): e15584.