A Global Plan for Parasite Conservation

Why should we conserve parasites?

If you’re a long-time follower, you probably already know why we should conserve parasites. But for those of you who are new, welcome, and please enjoy this short journey into posts from the past!

Parasitism is a common consumer strategy in the natural world; so much so that 40-50% of all animals might be parasites! That’s perhaps millions of parasitic animal species spread across 15 phyla, including animals as diverse as ticks, intestinal worms, and bot flies. There are also parasitic plants and fungi. Parasites might have especially high extinction risks, because they are at risk from both primary extinction pressures, like the direct effects of climate change, and secondary extinction, or co-extinction, when their host species decline or disappear. If conservation efforts are supposed to conserve all species based on their intrinsic value, then parasite species should be a large target for conservation activities.

But maybe you’re more of a utilitarian, and you want to know what parasites do for ecosystems and for us. The short answer? A lot, and probably a lot more than we know. We know the most about parasite species that harm people, harm our domestic species, and threaten wildlife species, but those parasite species are just drop in the bucket of global parasite biodiversity. We haven’t discovered and described most of those other, relatively benign parasite species, even in groups that we know provide important ecosystem services, like the parasitoid wasps that provide pest control. And some parasite species have already gone extinct due to human activities—science didn’t even give them a name before we didn’t have them any moa. All of this is to say that we do not know everything about parasites, so we do not know exactly what a world without parasites would look like.

But we do know that parasites play important roles in ecosystems. For example, parasite biomass is a large and important part of food webs. Within food webs, parasites link many species together in ways that we might not even expect, like the nematomorphs that cause crickets to jump into streams, where the crickets are eaten by endangered Japanese trout. Every non-parasitic species that you can think of evolved with parasites and interacts with parasites, which is why sex and immune systems evolved. In humans, immune systems might totally freak out in the absence of parasites, leading to auto-immune disorders. While no one wants to conserve detrimental human parasites, a few relatively benign parasites might be good for people and other species, too. Parasites are so central to the biology and ecology of non-parasitic species that some question whether we can even conserve hosts without their parasites: if we brought back mammoths from extinction, but couldn’t bring back mammoth parasites, would we really have brought back mammoths?      

What steps do we need to take to conserve parasites?

There are strong arguments for conserving parasites, but unfortunately, we are not conserving parasites yet. In fact, in some cases, we are driving parasites to extinction when we try to conserve other species, like when we delouse or deworm host species brought into captivity. Given how little we know about most parasite species and how little we are currently doing to conserve them, what immediate steps can we take to conserve parasite biodiversity?

We suggest that 12 steps should be taken in the next decade to conserve parasite biodiversity. Some of these steps will appeal most to researchers interested in fundamental science and people who want to participate in community science programs, because they involve data collection and synthesis. For instance, we need more research about how parasite biodiversity responds to changes in host biodiversity. Other steps are geared more towards practitioners, because they involve risk assessment and prioritization and conservation practice, like creating ways to assess parasites’ extinction risks and building red lists of threatened parasite species. And everyone can enjoy and be involved with the steps related to education and outreach, like including parasite-themed lessons in K-12 and college education.

If you’re interested in learning more about the 12 steps in The Global Parasite Conservation Plan, check out our recently published paper! This was a wonderful group effort from an international team of researchers, many of whom you might have seen at our ESA Organized Oral Session in 2018. And for a bunch of new papers about parasite conservation, check out our whole “Parasite Conservation in a Changing World” special issue that was just published in Biological Conservation!

This was, of course, a shameless plug for my own research, but it was for a good cause. Let’s save the parasites.

Parasites and de-extinction

[We’re still taking a break from the “how to become a successful parasite ecologist” post series. More on that in a few weeks!]

Sometime during my undergraduate education, I was required to prepare for and participate in a class debate exercise regarding whether we should bring animals like the woolly mammoth back from extinction. In the years since, I haven’t kept up with that literature at all, so I was quite surprised to read this opening line in a recent paper: “De-extinction is rapidly transitioning from scientific aspiration to inevitability.” Wow!

But that wasn’t even the most exciting part of the paper. Wood et al. (2017) went on to point out that to successfully ‘resurrect’ extinct species, we would need to ensure that the appropriate abiotic and biotic environments exist to sustain those resurrected species. You know what that means, don’t you? Parasites. If we’re going to resurrect extinct species, we need to give them parasites.

Here’s a quote from the paper. I hope it makes you ponder things… I certainly did.

“Would it be possible to genetically manufacture a parasite fauna and microbiota to suit the resurrected species, perhaps using palaeoecological data as a guide? Or would a mixture of extant parasites and microbiota, from species with a similar ecological niche, be sufficient? What implications would there be of a failure to adequately reconstruct these obligate microbiotic communities for the resurrected species and the ecosystem within which it is to be embedded?”

mammoth

Reference:

Wood, J. R., Perry, G. L. W. and Wilmshurst, J. M. 2017. Using palaeoecology to determine baseline ecological requirements and interaction networks for de-extinction candidate species. Funct Ecol, 31: 1012–1020. doi:10.1111/1365-2435.12773

 

The coextinction of parasites, commensals, and mutualists – a call for more natural history studies!

By definition, mutualists, commensals, and parasites (hereafter “affiliates” in this post) depend on their hosts for resources or services. Therefore, if a host species goes extinct, the affiliates associated with that host may go extinct, too. And in fact, coextinction events like this should be as common as – or even more common than – extinctions of hosts, because we know that every host species has many mutualists, commensals, and parasites. Just think about the mites living in your eyebrows, the bacteria living in your intestines, and that one time that you had lice in third grade. If humans disappeared, all of those affiliate species might also go extinct!

Of course, you might not believe in the intrinsic value of all species; you might be wondering why you should care if some tiny species that you’ve never heard of goes extinct. Extinctions of mutualistic species – such as the gut microbes that help you digest food and the pollinators that keep our agricultural systems running – have obvious implications for our economy and health. But parasites, too, play important roles in our lives. For instance, they regulate populations of wildlife host species, and they may prevent you from having allergic reactions to things that you shouldn’t be allergic to. And of course, species exist in intricate webs of interactions, and by accidentally (or purposely!) adding or removing species from ecosystems, we have often learned that one species can have huge impacts on ecological communities.

So, coextinctions of affiliates are important, and these coextinctions should be common. That means that we have documented tons of these coextinction events, right? Actually, we haven’t! There are very few examples of documented coextinctions (Dunn 2009, Colwell et al. 2012), and some of those are not entirely open and shut cases. But why?

Say that you document the extinction of a particular host species: Host A. Should every affiliate associated with Host A also go extinct? Because some affiliates likely use multiple host species, some of the affiliates of Host A probably survived on other host species. Also, even an affiliate that historically only used Host A might be able to continue existing if it can switch to a new host species. For instance, maybe Host B, a close relative of Host A, is a suitable alternative host.

Now imagine that you’re trying to document affiliate coextinctions as Host A disappears. What evidence might you use to figure out which affiliates have also disappeared? There might be published accounts of some of the affiliates of Host A, but there are very few host species (if any) for which every affiliate species has been documented. Therefore, the loss of one host species means that several unnamed and undescribed invertebrate species will be lost before ever being documented by humans. Even if you had a perfect list of every affiliate species, it might be really difficult to confirm whether each affiliate was now extinct. That’s because we rarely (if ever) have perfect lists of every host species used by a given affiliate species. So, if one affiliate species frequently uses three host species, but you think it is a specialist on Host A, you might think the affiliate has gone extinct, only to find it happily hanging out on Hosts B and C when you survey those species three decades later.

To summarize, we predict many coextinctions of affiliates to occur as hosts go extinct, but we have hardly documented any such coextinctions. It may be that that affiliate species are much less vulnerable than we expect due to the use of multiple host species or host species switching as a primary host goes extinct, and/or it may be that we are just very poorly equipped to observe and document these coexinctions. Clearly, if we’re going to get better estimates of affiliate coextinction rates, we need more data! Specifically, we need:

  • Better understanding of the natural histories of these systems. We need complete lists of affiliates for each host species, complete lists of host species for each affiliate species, preserved specimens of affiliates for genetic identification, and information on the strengths of the interactions between each affiliate and host species.
  • Better estimates of how frequently affiliates shift host species, and whether jumps to new host species are associated with declines in the availability of the current species. In other words, how often do we expect affiliates to sink with the ship versus swimming to safety? (For further reading about this, see Kiers et al. 2010.)

(It’s been too long since my last pirate worm cartoon….)

Extinction

Some related reading:

Conservation – save the parasites along with the hosts?

Are pubic lice going extinct?

References:

Colwell, R.K., R.R. Dunn, N.C. Harris, and D.J. Futuyme. 2012. Coextinction and Persistence of Dependent Species in a Changing World. Annual Review of Ecology Evolution and Systematics 43: 183-203.

Dunn, R.R., N.C. Harris, R.K. Colwell, L.P. Koh, and N.S. Sodhi. 2009. The sixth mass coextinction: are most endangered species parasites and mutualists? Proc. R. Soc. B 276: 3037–3045.

Kiers, E.T., T.M. Palmer, A.R. Ives, J.F. Bruno, and J.L. Bronstein. 2010. Mutualisms in a changing world: an evolutionary perspective. Ecology Letters 13(12): 1459-1474.

Will Tasmanian devils soon go extinct?

Last week, we talked about Tasmanian devil facial tumor disease, which is caused by an infectious cancer that is transmitted after a susceptible devil bites a tumor on an infected devil. After becoming infected, devils almost always die within several months. As the disease has spread through their range, devil populations have drastically declined, and there is great concern that the devils will go extinct in the near future. But not all infectious diseases that cause high host mortality will lead to host extinction, so what is special about Tasmanian devil facial tumor disease?

A while ago, I posted about the difference between pathogens with density dependent and frequency dependent transmission dynamics. To recap: when transmission is density dependent, the rate of host contacts (and thus pathogen transmission) increases with host density. When transmission is frequency dependent, the rate of host contacts (and thus pathogen transmission) does not change with host density. This means that as a host population crashes due to high mortality from an infectious disease, the transmission rates of pathogens with density dependent transmission will decline, but the transmission rates of pathogens with frequency dependent transmission will not change. Therefore, it isn’t possible for pathogens with density dependent transmission to be the cause of host extinction, because the pathogen will go extinct due to low transmission rates before the host goes extinct. (Note, however, that pathogens with density dependent transmission might cause declines in a host population that make the host population more susceptible to local extinctions due to stochastic events, like bad breeding years.)

Historically, only sexually transmitted pathogens and vector transmitted pathogens were thought to have frequency dependent transmission. And we know that Tasmanian devil facial tumor disease is not sexually transmitted or vector transmitted. However, McCallum et al. (2009) found that models with frequency dependent transmission fit mark-recapture data for devil disease dynamics better than models with density dependent transmission. How can that be? Well, it might that the number of aggressive encounters between individuals is not dependent on host density; for instance, if confrontations at carcasses continue to be likely to occur even as populations decline.

When pathogens utilize frequency dependent transmission, we know that unselective culling to reduce the number/density of susceptible hosts won’t stop a pathogen from invading a naïve host population. That’s because no matter how many individuals you cull, you won’t reduce the actual pathogen transmission rate, which is independent of host density. But what if instead of unselective culling, we try to stop the spread of Tasmanian devil facial tumor disease by selectively culling infected individuals? A culling program was undertaken to try this, but it was not effective (Lachish et al. 2010). Beeton and McCallum (2011) used epidemiological models to show that while selective culling might be effective, the rate of culling that would be necessary is just too high to be logistically possible given current resources.

Tasmanian devils have been the largest extant marsupial carnivore since the thylacine went extinct. What will happen if the Tasmanian devil goes extinct? Well, there are already documented changes in the animal communities in Tasmania that might be caused by devil declines, where the abundances of some species (e.g., feral cats) have increased and the abundances of other species (e.g., eastern quoll – so cute!) have decreased (Hollings et al. 2014). But the long term changes that will result from devil population declines (or full extinction) are hard to predict.

It’s a rough time to be a marsupial.

References:

Beeton, N. and H. McCallum. 2011. Models predict that culling is not a feasible strategy to prevent extinction of Tasmanian devils from facial tumour disease. Journal of Applied Ecology, 48: 1315–1323.

Hollings, T., M. Jones, N. Mooney, and H. McCallum. 2014. Trophic cascades following the disease-induced decline of an apex predator, the Tasmanian devil. Conservation Biology 28(1): 63-75.

Lachish, S., H. McCallum, D. Mann, C.E. Pukk, and M.E. Jones. 2010. Evaluation of selective culling of infected individuals to control tasmanian devil facial tumor disease. Conservation Biology 24(3): 841-851.

McCallum, H., M. Jones, C. Hawkins, R. Hamede, S. Lachish, D. Sinn, N. Beeton, and B. Lazenby. 2009. Transmission dynamics of Tasmanian devil facial tumor disease may lead to disease-induced extinction. Ecology 90(12): 3379–3392.