Incompetent Invaders Dilute Parasites and Alter Disease Dynamics

Guest post by Paula Tierney

Invasive freshwater fish (Leuciscus leuciscus) acts as a sink for a parasite of native brown trout Salmo trutta (2020) Tierney et al. Biological Invasions. https://doi.org/10.1007/s10530-020-02253-1

The Crux

From house cats to cane toads, invasive species are one of the biggest threats worldwide to native plants and wildlife, second only to habitat destruction. There are a few different definitions of an invasive species, but two consistent tenets are a) that they are a living organism spreading and forming new populations outside of their native range and b) causing some kind of damage to the native ecosystem, economy or human health. As humans move around the globe with increasing ease (these last two months aside), the spreading of invasive species is increasingly common in our globalised world.

The spread of invasive species creates new ecological interactions between native and invasive species that can impact how our native ecosystems function, including disease dynamics. One key set of interactions that can be completely changed by the introduction of the invader are that of parasites and their hosts. If development and transmission of native parasites is different in invasive hosts compared to their usual native hosts, the parasite dynamics of the whole system can be altered.

Did You Know: The Power of Parasites

Parasites make up a huge but desperately understudied component of biodiversity. An estimated half of all living organisms are parasites and almost all free-living organisms are infected with parasites. Parasites are admittedly hard to study; they are hidden on or in other organisms and this means they have been chronically underestimated in biodiversity studies.

Read More: Where is the Love for Parasites?

When we do add parasites to a food web, it quite literally turns it on its head. The number of parasites added flips the classic trophic pyramid upside-down, and the number of trophic connections quadruples. Parasites are powerful players in ecological systems – they regulate food networks, control the abundance host populations and act as bioaccumulators for toxins in the environment – so it’s important we don’t forget them when thinking about impacts on ecosystems.

What We Did

We set out to study differences in parasite infection between a native and invasive host using two invaded river systems in Ireland as a kind of natural experiment. The Munster Blackwater has been invaded by the freshwater fish dace (Leuciscus leuciscus) for over 130 years, while the Upper River Barrow has been invaded for less than five years. We studied infection of the widespread acanthocephalan parasite (AKA thorny-headed worm) Pomphorhynchus tereticollis in dace in these long-established and recent invasions.

We compared invasive dace to the usual native host of the worm, the brown trout, which we sampled from the same sites. We wanted to know about differences in host competency: the capacity of a host to allow the growth and development of a parasite. Using the relative proportions of adult and immature subadult parasites and the development of eggs inside the female parasites, we were able to assess whether invasive dace are suitable and competent hosts for our native parasite.

What We Found

We found that invasive dace were generally less infected than native brown trout, especially at the recently-invaded sites. In brown trout, almost all the parasites were adults (as we would expect from a preferred host) but in dace, 88% of the P. tereticollis worms were immature subadults. These subadult forms tend to be found in unsuitable host species; they represent a bit of a developmental dead end for the parasite and probably don’t contribute to ongoing parasite transmission. Parasites infecting invasive dace were also smaller than those from brown trout. From looking at the parasite eggs, we discovered that none of the female parasites in dace reached sexual maturity, meaning that dace were not transmitting infectious parasite stages. The combination of low infection, lots of subadults, small parasites and no mature females in dace all point to the conclusion that dace are incompetent hosts for the thorny-headed worm.

But does this affect the disease dynamics? If dace are taking up infectious stages but not contributing to the cycle of infection, they could act as sinks for infection and dilute infection in native hosts. We homed in on our native fish species and compared infection in brown trout between the long-invaded sites where infection in dace was high and the recently invaded sites where infection in dace is low. We found that native brown trout in the long-invaded sites had a lower abundance of parasites than their counterparts in the recently invaded site. So whilst dace might not be great hosts for our worm, given enough time they seem to lower parasite rates in the native brown trout.

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The thorny-headed worm, Pomphorhynchus tereticollis. Whilst they may not thrive in dace, their continual appearance in the invasive host mean the native brown trout now have low parasite levels (Image Credit: Paula Tierney, CC BY 2.0)

Problems

We love that our study takes place in the natural ecosystem, but it is missing the holy grail of host competency studies which is experimental infection. When researchers experimentally infect hosts in the lab, they have control over things like the dose of parasites a host is exposed to, host diet and the timing of infections. This is impossible in field surveys. This allows you to disentangle finer aspects of host-parasite patterns like the effects of host behaviour, exposure and immunity. Unfortunately for us, since robust procedures for experimental infections have never been developed for our specific study system, it could be impossible to distinguish negative results from bad protocol. It is a weakness that crops up in many parasite studies and it is one reason why a disproportionate amount of parasite ecology research focusses on the same host-parasite systems that already have well-developed protocols.

So What?

Invasive dace in Ireland become infected with the native parasite but do not transmit them, resulting in reduced infection in the co-occurring native host, brown trout. However, this is only apparent at the long-invaded sites where dace have been present for over a century. So, while the presence of invasive species can certainly affect disease dynamics, the impact may not be apparent until long after the invader is established. We can’t yet be certain whether this dilution of parasite infection is good or bad news for native brown trout since the role of parasites in ecosystems can be complex and multi-facetted. However, our study does provide some much needed real-world evidence of altered parasite dynamics by an invasive species.

Paula Tierney is a parasite ecologist currently completing her PhD in the Zoology Department of Trinity College Dublin. When she’s not studying parasites, biological invasions and freshwater fish, she is a scuba diver and a connoisseur of trashy TV. You can follow her on Twitter @_ptierney.

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