While scientists aren’t exactly sure how many species of microbes inhabit the planet, it’s likely in the tens of millions, the vast majority of which remain entirely undescribed. As a consequence, many researchers, including conservation biologists, carry out their work in ignorance of the viruses, protozoans, and bacteria may be accompanying their target species in relocation projects.
A policy perspective published April 13 in Conservation Letters discusses the scientific rationale for moving species from one place to another, the likelihood that such efforts may also be transporting pathogens, and the factors that dictate whether these microbes become established once released into a new system.
Joshua Brian, a PhD student and community ecologist at the University of Cambridge who coauthored the perspective, researches this issue as part of his dissertation work on the parasites of freshwater mussels, which are among the most threatened animal groups worldwide. Fifty percent of species are listed as near threatened or threatened, and efforts to bolster their numbers have undoubtedly led to exchanges of pathogens between populations, Brian tells The Scientist. His piece, which includes actionable steps for biologists to adopt, such as quarantining species prior to translocation and filling in existing knowledge gaps, uses freshwater mussels as a model system, but he says it applies equally well to any organism being considered for conservation action.
The Scientist: What was the impetus for this?
Joshua Brian: My PhD is broadly based around freshwater mussels and their parasites. At the start, I went around and looked at different mussels and rivers, and saw that, in fact, sites within the same river system often had quite different parasite communities. I thought, ‘Oh, this is super interesting, I wonder why this is?’ I was thinking about some experiments to test this. Could I move mussels around . . . to see what’s causing this? But then I thought, ‘I probably can’t do that,’ because some sites have parasites and some don’t. I can’t be willy nilly moving mussels around because that could move parasites from one [site] to the other.
I had that discussion with my supervisor, and we realized that people actually move mussels around all the time during translocations for conservation and experiments. They don’t seem to ever consider the risk of moving parasites around. We thought this could be a major threat, so that’s where the paper really came from. We realized that there’s a gap in the conservation of freshwater mussels—and we think in conservation more broadly—that people move animals around all the time without really considering what might live inside them.
TS: Are there existing laws that control for this, and if so, what’s lacking?
JB: I haven’t looked into it too much for species more generally, but for mussels, I don’t think there’s anything at all. It seems to be because they’re invertebrates. There are no ethical or legal implications, so I can go out and sample as many as I want. I think both the UK and the US . . . seem to be fine moving mussels around. There are clear quarantine regulations for zebra mussels, but that’s all that’s been thought about, and even there, there’s nothing about what might be living inside zebra mussels themselves.
TS: What are some of the factors you identified as being important for the likelihood of pathogens spreading?
JB: The first one that comes out as being really important is the prevalence of the parasite in the source population. If a pathogen is present at very low levels, even if you make quite a large translocation, there might not be much of a risk. But if the pathogen is prevalent at very high levels, you don’t need to take more than a few mussels before it’s very likely that you’ll move the pathogen along with the mussel. Similarly, there’s also the host density. Once the pathogen is in a population, how fast can it spread? That’s determined by how dense the mussels are. If the mussels are really dense, really tightly packed, it’s very likely that [a pathogen will] get sucked into a mussel next door and spread that way.
The other key thing, which we think is really understudied, is pathogen life history. Some pathogens have multiple hosts in their lifecycle. If all three hosts are in the new ecosystem where they’re being moved to, a pathogen might very easily spread through the population. But if a mussel gets moved to a new site where the other hosts aren’t present, that pathogen will die very quickly.
The final thing is just mussel immune systems. We actually understand almost nothing about freshwater mussel immune systems. If they’re immune, or there’s not much immune variation between populations, then the chances are better that mussels at one site will be able to deal with pathogens as mussels at the other site can. But if there’s some genetic differentiation, and one population has been isolated for a long time, it could be absolutely devastating.
Dangers will vary to a greater or lesser extent depending on how much we know about the parasite, but in all cases, you can’t guard against something if you don’t know that it’s there.
TS: How can pathogens interact with or be exacerbated by other environmental stressors?
JB: There’s two interesting ways in which this can happen. There’s good evidence that environmental variables, especially warming temperatures, actually enhance parasite survival. If it’s warmer, the parasites are more likely to produce more babies, and those babies can then survive longer in the water column.
From the host perspective as well, there’s some really good studies which have shown that mussels will be okay with a particular parasite and it won’t affect survival that much, but if there’s a sudden warming pulse or starvation or anything like that, the mussels with parasites will die a lot quicker than the mussels without parasites. If we combine multiple stresses, then it seems to add this really negative effect.
TS: This paper uses freshwater mussels as a model to look at these dynamics, but how broadly do you think these findings can be applied to other species?
JB: I think they can be applied very broadly. The principles that we talked about—the parasite prevalence, the host density, the parasite life-history strategy—those things are relevant regardless of what organism you’re thinking about. Dangers will vary to a greater or lesser extent depending on how much we know about the parasite, but in all cases, you can’t guard against something if you don’t know that it’s there. I think in general, parasites are still underappreciated and understudied.
The key point, and what I’d like people to take away not just from this paper, but from my PhD generally, is that an individual isn’t actually just an individual. It’s a whole community of all the parasites, viruses, and bacteria that live on it. Until we recognize the community level nature of a single organism, we’re going to keep having problems with parasites and disease spread, in my opinion.
J. Brian et al., “Don’t move a mussel? Parasite and disease risk in conservation action,” Conserv Lett, doi: 10.1111/conl.12799, 2021.
Editor’s note: This interview was edited for brevity.