Most people in the UK will be familiar with our favourite household parasites: head lice, fleas, and ticks. But did you know that parasites come in all forms, from tiny microscopic organisms to larger worms? In fact, almost every animal, including humans, can host parasites at some point, and some estimates suggest that parasites could account for anywhere between 50% to 80% of all species, including more than 100,000 known species of parasitic worms alone!
We often associate these parasites with something negative; an organism that leeches off another, causing harm for its own benefit. This idea clashes with our human values of empathy and kindness, making it easy to assume that a world with fewer parasites would be better.
But removing this layer of complexity from our ecosystems may come at a cost, with consequences that are far greater than we might expect…
A quiet disappearance
Puget Sound is like nature’s hidden treasure chest, tucked away in the Pacific Northwest of the United States. This vast, stunning estuary is bursting with life, from playful seals and soaring birds to all kinds of fish and shellfish. But deep beneath the surface, a quiet disappearance has been unfolding. One that tells us something profound about the state of our changing planet.
Scientists at the University of Washington have discovered that over the past century, the abundance of certain marine parasites has dropped by nearly half.
While parasites rarely win any popularity contests, their loss could signal the unravelling of the delicate ecological threads that hold marine ecosystems together. To understand the scale of parasite loss in the modern world, and why it matters, the researchers needed to reconstruct the historical parasite diversity of Puget Sound. But doing this would need a time machine…
Preserved specimens in museums offer a window into the past. Image: Brooke Balentine, Unsplash.
A century in a jar
To look into the past of Puget Sound, the researchers came up with an ingenious solution: the drawers and jars of natural history museums across North America. These glass jars, some over a hundred years old, hold preserved fish specimens collected long before climate change became a concern. Hidden inside the fish were tiny, long-dead parasites, offering clues about past ecosystems. By dissecting hundreds of specimens from 1880 to 2019, the researchers reconstructed how parasite populations in Puget Sound have shifted over time.
Having been stored for decades, these fish became an unexpected archive of ecological data, revealing how factors like temperature changes, pollution, and other environmental shifts affected marine parasites. It was like opening a jar of worms, except this time, the worms told the story of our changing planet.
Complex parasites struggle to adapt to a warming world
After analysing trends in parasite population against environmental data, the researchers were able to pin down the culprit of population changes: temperature. Over the past century, Puget Sound has warmed by about 1°C. At the same time, the abundance of parasites which rely on multiple hosts – like some tapeworms, flukes, and other marine worms – has declined by a shocking 11% per decade.
Complex parasites like these depend on an intricate chain of interactions to survive. For example, a parasite might need to pass from a clam to a fish and then to a seabird to complete its cycle. If any of these hosts are affected by climate change – perhaps shifting their migration patterns, breeding seasons, or population numbers – the parasite’s life cycle can break down. This makes them especially vulnerable to climate change.
In contrast, parasites with simpler life cycles, relying on just one or two hosts, have been much more stable. They don’t depend on a perfect chain of ecological coincidences. So, as the climate warms and ecosystems shift, they have more built-in resilience to a changing climate.
A parasite makes its home on the the eye of a sprat. Image: Hans Hillewaert, Wikimedia Commons.
Why do vanishing parasites matter?
We often think of parasites as troublesome creatures that make people, pets, or wildlife sick. But in natural ecosystems, parasites play complex and often beneficial roles. They help regulate populations of hosts by killing weak or sick individuals, help to transfer energy through food webs, and even influence animal behaviour in ways that shape whole ecosystems. The parasitic liver fluke, for instance, causes infected ants to attach themselves to the tops of blades of grass: easy prey for a grazing herbivore. This is exactly what the parasite wants! When the herbivore eats the ant, the parasite moves into the herbivores digestive system, not only enabling it to reproduce, but influencing the entire food chain.
So, removing parasites from the picture isn’t the same as removing a nuisance; it’s more like pulling threads from a web. Research suggests that losing parasites can weaken the very structure of food webs, making ecosystems less able to withstand other threats such as pollution, overfishing, or drought1. Without these crucial species, ecosystems are less equipped to cope with environmental changes, leading to greater instability and vulnerability.
Lessons from the past, warnings for the future
The implications of this study reach far beyond the Pacific Northwest. Parasites are among the most diverse forms of life on Earth – yet they are rarely included in conservation plans. If similar patterns are happening in other marine environments, the world could be losing parasite biodiversity at rates comparable to or even greater than those occurring in birds, mammals, and insects. And if a 1°C change can wipe out nearly 40% of certain parasite populations, what will another degree or two mean for the complex tapestry of marine life? As the University of Washington researchers, noted: “If parasites are disappearing, we’re losing an invisible layer of biodiversity that we didn’t even know to protect”.
The Puget Sound study is also a reminder of the hidden value of museum collections. Those jars of fish carefully preserved by generations of curators have become archives of ecological history, allowing us to see patterns that no one could have measured in real time. Without them, this century long decline might have gone completely unnoticed.
What can we do?
Protecting the ecosystems that sustain parasites safeguards all life. By reducing greenhouse gas emissions, restoring habitats, and supporting biodiversity, we help maintain the stability that even parasites rely on. Shifting our perception of these creatures from pests to partners in natures web is also essential. Every species, even the ones that make us squirm, play a role in the symphony of life – now at risk of fading into silence.
In peril: the diversity of copepod parasites. Image: K Norris, London Natural History Museum, Wikimedia Commons.
Further exploration
If this article has piqued your interest in the fascinating world of parasites, there’s so much more to learn! Here are some great resources for anyone eager to dive deeper:
- Carl Zimmer’s Parasite Rex is a must read. It takes you on a deep dive into the world of parasites, explaining how these often overlooked creatures play an essential role in ecosystems.
- If you’re looking for something lighter but equally engaging and don’t want to read , Ed Yong’s Ted talks on parasites offers a mix of humour and science, making it a fun engagement for newcomers.
- For a bit of fiction with a touch of parasitism, Bernard Werber’s The Empire of the Ants explores how parasites influence animal behaviour.
Article written by Saeed Yonis
Primary research paper:
Wood, C. L., Welicky, R. L., Preisser, W. C., et al. (2023). A reconstruction of parasite burden reveals one century of climate-associated parasite decline. Proceedings of the National Academy of Sciences, 120(3), e2211903120. https://doi.org/10.1073/pnas.2211903120
Other references:
- Brian, J.I. (2023). Parasites in biodiversity conservation: friend or foe? Trends in Parasitology, 39(8), pp.618–621. doi:https://doi.org/10.1016/j.pt.2023.05.005.
