What percent of species extinctions are caused by parasites?

Guess who forgot to write the (second to) last post in the parasite-driven host extinction mini-series until midnight yesterday? This blogger! I hope you guys know how to decipher zombie grammar.

Last week (and the week before), I asked you guys to guess what percentage of species extinctions have been caused (extinct species) or nearly-caused (endangered species) by parasites, pathogens, or viruses in our current mass extinction. As of midnight last night, the top three selections were 10% of extinctions/near extinctions (38% of voters), 0% of extinctions/near extinctions (19% of voters), and 20% of extinctions/near extinctions (17% of voters). The remaining 26% of voters thought that 30-90% of extinctions and near extinctions have been caused by infectious disease, though those votes were skewed more towards the lower estimates.

And now for the moment you’ve all been waiting for! Ladies and gentleman, boys and girls, if you’ll step right up and look behind Curtain #2, you’ll see the answer to the question that I’ve been hyping up for two weeks! DRUM ROLL, PLEASE! BBBBBbbbbrrrrrrrrrrrrrrrrrrrrrrrrrrrr….


Womp womp womp. I don’t actually know! To my knowledge, no one has calculated the answer since 2004 (Smith et al. 2006 – pdf link), and there are many more extinct and endangered species listed now than there were in 2004. Also, the field of disease ecology has grown substantially in the past decade, so (hopefully) we know more now than we did in 2004. But I’m going to report the previous estimates anyways, because that’s what we have:

Of the 3576 plant and animal species that were extinct or critically endangered by 2004, the IUCN listed infectious disease as a contributing threat for 248 species (7%). Most of those species were amphibians, and I would guess that would still be the case if the analysis were repeated today.

There is a caveat. For most of the species where infectious disease was listed as a contributing threat, there were also several other contributing threats listed. So we have to wonder: do infectious diseases only imperil already small or stressed populations? Actually, what I hope you’re wondering is: under what conditions can infectious diseases drive their host populations to extinction? I hope you’re wondering really, really hard, because one day soon, I’m going to write a post about it.

So, our most common vote (10%) was correct! I can’t tell if (1) you guys just have really good intuition about these things, (2) you’ve learned this before, or (3) you cheated! Fess up in the comments, if you dare!

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