I recently finished the book Unseen City by Nathanael Johnson and rather than write a formal review (the upshot is that if you're knowledgeable about natural history, especially urban natural history you likely won't learn a ton but it's a good read, I would recommend it) I wanted to address a specific few sentences in the book about invasive species. Johnson writes: "Immigrant species often do reduce native populations, sometimes significantly. But the ecologist Mark Davis has pointed out that they rarely cause extinctions, and when they do it's of populations in isolated habitats like lakes or islands. All this mixing may yield more biodiversity by producing more combinations, hybridization, and new species." And in the book's conclusion: "These invasive species are not nature's destroyers, but rather its creators. They begin setting up food webs, they evolve and diverge into new species."
While it's true that you can find examples, as Johnson sites, of stories where invasions are followed by an increased biodiversity I found it odd that he brings up this incredibly complex issue but only writes a little more than two sentences about it. One of the big issues here is that we don't all agree on what an invasive species actually is. I'll illustrate with three marine invertebrates that have been introduced to New England through human activity.
One: Hemigrapsus sanguineus, the asian shore crab. Introduced in 1988, this crab is considered a significant concern to local ecosystems primarily due to its voracious feeding. This has two major impacts, one, it can easily out-compete local species like Cancer irroratus and borealis, two, it specifically has been correlated with a decline in juvenile lobsters (presumably it eats the post-larvals).
Two: Carcinus maenas, the green crab. Introduced in the early 1800s this larger crab species purportedly has an "insatiable" appetite and, unfortunately for us, seems to like many of the same animals we like such as clams and mussels. It's also theorized that it may be contributing to salt marsh decline. Which, I will point out right now, we think is bad. I could ramble all day about salt marshes but the two highlights are that they are more productive than rain forests and sequester more CO2 than rain forests (which is why we should have been learning about them growing up in the 80s).
Three: Littorina littorea, the common periwinkle. We have no idea when the common peri got to New England, and it could have been as long ago as circa 1000 AD when Vikings first came to North America.
Here's the rub: almost no one considers L. littorea an invasive. Indeed, a NY Times article describing the research of Prof. Mark Bertness who has done some of the most robust work on the impact of periwinkles on New England coasts as far as I can tell, describes them as a keystone species. You might call that the opposite of invasive.
And yet, when you put the present day ecological data on how periwinkles feed together with historic maps that show where there used to be salt marsh it seems very likely that periwinkles ate through a huge amount of algae and reduced much of that historic marsh to rocky shore. And remember, we agreed above that salt marshes are good.
What's the difference between these three species? Time. There are a couple ways an invasive can be defined as such. It's always non-native but to be invasive it's generally agreed that it has to have some negative impact on ecosystems and/or the economy. For H. sanguineus and C. maenas it's easy to check those boxes but as soon as you look at the whole story for L. littorea you are forced to check the same boxes. Struggling to define invasive (vs non-native, and why we don't call periwinkles invasive) I've often said "well, no one is suggesting any kind of management for periwinkles." But no one is suggesting that for the others either. At least when it comes to marine inverts, the plan is monitoring, education and the hopeful prevention of new invasives but there is no plan to manage the established invaders. And while there are management plans in place for invasive terrestrial plants they will likely need to continue indefinitely. So as far as I can tell, really the only difference is that periwinkles have been around for about a thousand years and no one remembers a New England ecology without them. Heck, they might even be a keystone species!
So the weird thing is that when it comes down to it I do agree with Johnson that we shouldn't be losing our hair over invasives. They are, in a lot of ways, just another example of the biological world moving and evolving and changing. However, I'm hesitant to say, with such simplicity, that invasives (writ large) generate biodiversity. Sure, you can find examples of that happening, but you can also find examples of invasives out-competing local species, close to extinction or completely altering huge swaths of ecosystems (and again, I'm going to make a value claim that salt marsh is more important than rocky shore, we can debate that in the comments if you like).
Through writing this I've been thinking about another book: Out of Eden by Alan Burdick. In this book he doesn't so much argue that invasives are a good thing but that, well, there's not a heck of a lot we can do about them so...why freak out? Again, I think that's where I land. That's also a whole book, not a few sentences in a book that's not really about this complex and often really interesting issue. They might be a problem but our relationship with invasive species is a huge part of the story of how we humans are forever changing the biosphere. Maybe it's not great, I'm still not sure how to make a value claim about such a big, complex issue, but there's some really neat science going on there.
Also, kudzu stinks.
Johnson, Nathanael. (2016). Unseen City. New York, NY: Rodale Inc.