Image: From the Entomological Society of America’s position statement on insects and biodiversity.
The Entomological Society of America released another great position statement last week. The statement addresses biodiversity and the causes and consequences of species decline. While interacting with legislative staff in Washington, I’ve been told that these sorts of statements are broadly seen as very useful by policy makers and elected officials. The ability to quickly draw on information from a respected scientific body is super helpful for legislative staff when arguing in support of certain policies, or even in drafting the text of legislation.
While I value this new position statement, and I’ve already hyped it to legislative staff, one thing in it caught my eye:
In describing the harm to biodiversity caused by invasive species, the position statement reads, “Approximately 42% of threatened or endangered species are at risk primarily due to the introduction of invasive species…” [italics added by me].
I’m definitely a preacher of the ecological and socioeconomic harms caused by invasive species. However, if for no other reason than it’s difficult to demonstrate the primary cause of any species’ decline, 42% seems high to me. Plus, if I were just guessing, I’d think habitat loss and fragmentation through direct human activities (e.g. deforestation, agricultural production, etc.) would be the primary factor like 90% of the time.
So, I decided to track down where this 42% estimate came from. Coincidentally, one of the three citations that are included in the position statement is for the 42% sentence. Unfortunately, the reference is just a link that leads to a page on the National Wildlife Federation website. The webpage itself says nothing about endangered or invasive species. However, after searching around the site a bit, I did find a reference to 42%, but the sources they give are just a bunch of institutions (e.g. PennState College of Agricultural Sciences, USDA, etc.), no actual research publication.
Still curious, I googled some keywords – something like “42% endangered species invasive” – to see what else I could find. It turns out that the number is mentioned all over the place. The first reference in the peer-reviewed literature that I found was Pimentel et al. (2005) Ecological Economics. This is a very useful paper that catalogs the harms associated with invasive species in the U.S. I’ve actually discussed the paper on this blog before, so I’m kind of embarrassed that I’d forgotten the 42% figure is mentioned in their abstract. But that’s not the end of the story…
Pimentel et al. (2005) arrive at 42% by dividing 400 by 958. They say there are 958 species listed under the U.S. Endangered Species Act as threatened or endangered, and 400 of those are said to be “at risk primarily because of competition with or predation by non-indigenous species (Wilcove et al., 1998)” [again, italics added by me].
So 1) the number is based on what has made it onto the Endangered Species Act list (but see below); and 2) the link to invasive species is from a paper published in 1998, 19 years before the ESA position statement was written.
What do Wilcove et al. have to say?
While somewhat dated, Wilcove et al. (1998) BioScience is a nice review and synthesis that compares the relative importance of different threats (i.e. “habitat destruction, alien species, pollution, overexploitation, and disease”) to biodiversity in the U.S. It turns out Pimentel et al. (2005) may not have been accurate in saying that 42% comes from Endangered Species Act listings. Maybe they were, but I don’t see where they get the number.
Wilcove et al. (1998) actually get there data by combining species classified as “imperilied” by The Nature Conservancy, plus species (and subspecies and distinct populations of vertebrates) listed as endangered or threatened by the Endangered Species Act or formally proposed for listing as of January 1996.
Therefore, the species list used to make the 42% estimate may be a bit broader than just the Endangered Species Act.
After running through various data filters, Wilcove et al. were able to get information for 1880 species, subspecies, and populations.
What about determining the contribution of individual threats? Wilcove et al. point out the following:
This was exactly my concern. Drawing causal connections between invasive species or habitat destruction and species decline is very difficult, and data are often sparse. Still, Wilcove et al.’s analysis, for all its limitations, is super valuable.
I was glad to see that my intuition about habitat destruction was not toooo far off, and they ranked this factor as the #1 threat. Based on Wilcove et al.’s analysis, habitat destruction contributes to the decline of 85% of the species/subspecies/populations they looked at. Invasive species came in second and contributed to the decline of 49%.
In the end, however, it’s not clear to me from Wilcove et al. (1998) where exactly Pimentel et al. (2005) got their numbers. Wilcove et al. found that invasive species contributed to the decline of 49%, but they have a whole section in the paper where they point out that contributing to decline is much different than being the primary cause of decline. Perhaps Pimentel et al. were able to get the raw data set from Wilcove et al., and do their own analysis…? Maybe I’m missing something.
Now, my interpretation is that the information used in the ESA position statement is, at youngest, 21 years old. Plus, it includes subspecies and distinct populations of vertebrates. And it’s more likely an estimate of the percentage of cases where invasive species contribute to biodiversity decline, but are not necessarily the primary driver.
At any rate, I’m not going to be citing 42% any time soon. (Interestingly, there is no mention of the 42% figure in ESA’s position statement on invasive species, which was approved in April of last year.) I’d be willing to say something like this: Invasive species are thought to be a major factor contributing to the decline of nearly half of the endangered or threatened species, subspecies, and distinct populations in the U.S.
That’s still a pretty strong statement and warning about the threats posed by invasive species.