Moa extinctions caused even moa parasite co-extinctions

Happy Valentine’s Day, Everyone! I only ever blog about papers that I love, and today is no exception. But in honor of this holiday, I’m going to break from tradition and blog about a paper that I love and I co-wrote, and I hope that you’ll forgive my shameless plug.

Within the parasite conservation literature, there exists a paradox: we expect that many, many parasite species should be threatened by co-extinction with their threatened host species, yet we have documented few parasite co-extinctions. If parasite species are so vulnerable, why haven’t we seen more co-extinction events? It might be that parasites are not as vulnerable to co-extinction as we originally assumed. (I could write several blog posts about that possibility, and perhaps one day I will.) Another possibility is that many co-extinctions have occurred, but we failed to notice and/or document them. The latter possibility is the topic of today’s post.

In order to document an extinction (or co-extinction), we need to know two things. First, we need to know that the species once existed. To document which species once lived, we usually rely on historical resources: fossilized remains, museum specimens, published scientific studies or surveys, and even old books/diaries/letters. But many species don’t fossilize well, and many more are too small, cryptic, or seemingly important to end up in museum collections, scientific studies, or other historical records. Second, we need to know that the species doesn’t exist anymore. Documenting the absence of a species is incredibly difficult; its only really possible via exhaustive surveying. That’s why we are sometimes pleasantly surprised when a thought-to-be-extinct species is suddenly found living somewhere unexpected, still hanging in there.

For parasites, I like to think of these two documentation steps as compiling/comparing passenger lists. To know that a parasite species once existed on a host species, we need to create a historical passenger list for that host species. It is hardest to (re)create a complete passenger list for a host species that went extinct (=sank) long ago. But it’s also very difficult to document the parasite passenger lists for host species that are currently threatened (=sinking), because they might have already lost some passengers (e.g., if we de-loused the hosts when we brought them into cavity), or the hosts might be too rare or difficult to thoroughly sample for parasites. Once we have recreated the best historical passenger list that we can – which is probably incomplete, but better than nothing – we need to compare it to all of the parasite passenger lists for all extant host species, to see if any extant host species served as “life boats” that carried some parasite species into the present. As I mentioned last week, we are far from having sampled all extant parasite species on all extant host species, so we’re currently working with incomplete present-day parasite passenger lists.

Given all of the difficulties described above, it’s not surprising that we have documented so few parasite co-extinctions! But this week in PNAS, Boast et al. (2018) published a great paper that convincingly documents the co-extinction of a few parasite species that once infected the now-extinct moa (giant flightless birds from ancient New Zealand). They used ancient DNA to reconstruct the best ever ancient passenger list for several moa species, and they were able to show that some parasite species survived to present day on other bird species (e.g., the kiwi), while others did not. You should definitely give their paper a read!

But if you want a shorter, bloggier version of the parasite story, you could also go read the open access commentary about Boast et al.’s paper that I co-wrote. It was super fun to write, because we were allowed to talk about parasite co-extinction and fossil poop at the same time, and it was especially fun because I was invited to co-author the piece by my science hero. (Thanks, Kevin!)

I think those are enough warm, fuzzy, parasite valentine feels for now. Happy reading!