The Potential of Parasites

Featured Image Caption: The parasites found in fish can be used to track their movements, even as far back as their natal origins. (Source: Gordon Firestein, licensed by CC BY-SA 3.0, via Wikimedia Commons.)

Source Article: Gagne, R. B., Crooks, K. R., Craft, M. E., Chiu, E. S., Fountain-Jones, N. M., Malmberg, J. L., Carver, S., Funk, W. C., & VandeWoude, S. (2022). Parasites as conservation tools. Conservation Biology (Vol. 36, Issue 1). https://doi.org/10.1111/cobi.13719

What are Parasites?

When most people think of parasites, they think of the common suspects: lice in someone’s hair, ticks in the tall grass, or leeches sticking themselves to an unsuspecting swimmer. In truth, the world of parasites is far more diverse, including bacteria, viruses, and macroparasites such as ticks or nematodes. Some parasites live within a single host for the entirety of their life cycle, while some require multiple organisms of different species. Some grow and evolve incredibly quickly, while some take it slow. Some cause lasting harm to their hosts, while others have relatively benign effects. They are varied, often disliked, and, as it turns out, a potential tool for conservation.

By definition, a parasite is any organism that lives within or on a host of a different species, taking nutrients from the host. Over half of all living organisms are parasites, but many parasite species have likely not been discovered or described. Their diversity means that different parasites are suited for a variety of applications in environmental conservation. For instance, they can help us evaluate issues related to overexploitation of harvested species, habitat fragmentation, invasive species, and climate change.

Unexpected Traveling Companions

To sustainably manage wild populations of harvested species, such as through hunting or fishing, it is important to identify how populations move across land or through water. This can be especially difficult to do with species that have large ranges, such as marine fishes. However, parasites pose a potential solution!

When a host organism is born, it acquires parasites from the location where it originates. Even when it leaves this location, certain parasites that cause lifelong infection remain. Researchers can map the specific types of parasites that a host has to the locations where this combination of parasites can be found, allowing us to identify where populations originally came from. This is valuable for identifying breeding or spawning locations, common migration routes, and other places that are important to protect.

Parasites that occupy a host for a short time period (‘transient parasites’), can be used to identify the host’s recent movement. In this case, the parasite community within the host is again linked to a location where those specific parasites can be found. Knowledge of the parasites’ host colonization times allows researchers to identify a time range during which the host must have traveled through these areas. For animals like fish, this can even help identify illegal trade when the parasite community doesn’t match what would be expected from the claimed fishing site.

Parasite Genetics: A Tool for Conservation

Parasites can also be used to determine how connected a habitat is. Through processes like deforestation, habitats can be fragmented (broken into smaller pieces). Fragmentation isolates small groups of a larger population, which often reduces genetic diversity. If the population density of a fragment is high, this can increase disease transmission. Additionally, animals may be injured or killed when trying to cross between patches of habitat. In order to reduce these problems, efforts are being made to improve habitat connectivity.

One way to determine whether two areas of habitat are adequately connected is to see whether organisms that live in each area have been in contact with each other. When parasites evolve relatively quickly, scientists can compare the genetic information of the parasite populations from multiple hosts. If the hosts have been in contact and exchanged parasites, the populations’ genetic information will be more similar than if the hosts were never in contact.

The origin of recently introduced invasive species can be assessed with similar methods. When researchers want to know where an invasive species spread from, they might compare the genetic information of the new population with the genetic information from multiple potential source populations. If the information from one of the potential source populations closely matches that of the new population, it is likely that the new population came from that source. However, sometimes the host species evolves too slowly for the genetic information of the potential source populations to significantly differ. In this case, researchers may compare genetic data from the invasive species’ parasites instead, as many parasites evolve very quickly.

For example, researchers could not determine the origin of the fire ant invasion in Taiwan just by comparing the Taiwanese population’s genetic information with the information of two potential origin populations (from the United States and Argentina). Despite being geographically separated, the US and Argentinian populations were too genetically similar, because fire ants evolve relatively slowly. However, a virus (S. invicta virus 1) carried by these ants does evolve quickly. Genetic analysis of the virus in all three populations identified the US population as the source of the invasion, because the genetic information more closely matched that of the Taiwanese population.  

Parasites are also a potential biological control method for invasive species, and have been used against feral rabbits and cats. However, it is important to ensure that introduced parasites would not unintentionally affect non-target species by using them as hosts.

Canaries in a Coal Mine

Finally, parasites are often sensitive to environmental changes, and thus can serve as indicator species: species that reflect the state of their environment. For instance, if the population of a host species declines, the population of the parasite may be noticeably affected. Parasites which require multiple host species throughout their life cycles are especially useful for this, as a drop in the parasite population may indicate that one of several species is in trouble.  

Some parasites have free-living stages in their life cycles, where they do not live in or on a host. During these periods, they are especially vulnerable to environmental changes. Parasitic abundance has been linked with changes in ocean acidity, pollutants such as heavy metals, and temperature changes. Choosing an appropriate parasite species and monitoring it may allow researchers to notice environmental changes long before more obvious indicators appear.

Far from just being irritating creepy-crawlies, parasites play a role in assessing conservation issues. Certain parasites are better for different purposes — for instance, ones that permanently inhabit their host are useful for identifying a host’s origin, while quickly-evolving ones can indicate habitat connectivity or host movement through genetic analysis. Researchers already use parasites in certain contexts, such as to monitor fisheries, but there are many opportunities to expand to other uses. Further research into new parasite species and their specific roles will only open up new applications for their use in conservation.

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