Biological Invasions & The Witcher

Image Credit: The Witcher, 2020

Science and movies often don’t go well together*. It’s no-one’s fault. Science can often be boring and riddled with uncertainties, and movies and TV require plot advancement and definitive results.

But you know what’s a scientific fact? That Henry Cavill’s chin can cut diamond, and if you thrust him into a cosplay outift he probably already had at home and send him out to slaughter a bunch of CGI monsters you’ll get something that is at the very least mildly enjoyable. And if you’re an invasion ecologist who runs a podcast looking at the ecology of movie monsters, mildly enjoyable monsters are enough to dedicate a blog post to.

If you hadn’t already guessed at this point, I’m talking about the recent Netflix series The Witcher. The story as presented in the series isn’t overly complex. A mutated human with superhuman abilities goes about his business, killing off monsters which are causing townsfolk a variety of problems. Nothing odd about that, standard practice historically. Pre-industrial revolution societies weren’t massive on conservation. What I find especially interesting about this world is that thanks to an event which is only referred to briefly in this series, most of the fantastical creatures we see here are non-native species that have been quite recently introduced to the world they exist in.

In brief, this all stems from an event called the Conjunction of the Orbs, in which a bunch of creatures from another dimension got shoved into the main dimension of the story. That main dimension seems rather European in its ecology, with a character at one point listing a number of fish species, which were essentially a list of common European freshwater fish. So for this article, let’s assume that these fantasy creatures were all thrust into a land analagous to mainland Europe. What would happen?

Some ground rules: I’m assuming all new species that we see are a product of this collision with the other dimension, and that all familiar species we see (which are pretty scarce) are not. And while I realise that many of these creatures are magical in nature and therefore play by different rules, for the sake of argument I’m going to assume that they behave and evolve as non-magical beings.

Invasive vs. Non-Native Species

The first thing to point out is that in this series, (with one exception) we only see the exotic creatures that are causing trouble for the locals. According to many modern governments, a non-native species which causes negative effects on the local ecology, economy, or human health is termed invasive. We have no idea how many non-invasive species there are in this world. Where are they then, the species that simply came across during the Conjunction of the Balls and just went about their business, slotting into whatever ecological niche they can find?

Probably EVERYWHERE. In a recent interview with Dr. Helen Roy, she guessed that in the UK, only about 15% of non-native species are causing problems. I would love to see more of those species which are just peacefully co-existing with the native fauna and flora (though I’m aware that CGI budgets only stretch so far).

And that’s not to say that all the new beasties we see are having a negative impact on the novel ecosystems they’ve moved into. Take the big old spider monster we see in the very first scene with Henry Cavill’s Geralt. Whilst it may appear aggressive, it could for all we know be a generalist, snacking on whatever large vertebrate comes into its territory before resting for a week. This might mean that it doesn’t reduce any one species’ population enough to damage the local ecosystem significantly. Although to be fair, if the local mayor is keeping it alive for ‘population control’, I’d say that probably constitutes enough of a threat to human health to classify them as invasive.

Screenshot from 2020-01-31 09-30-44

I know it’s a dragon, but it doesn’t seem to be doing much damage during the show. Maybe leave it alone. They’ll die out from too much inbreeding anyway (Image Credit: The Witcher, 2020)


One of the issues with new species is that they inevitably bring in a bunch of new diseases and parasites with them. Depending on how different the physiology of the new species is to the locals, they could bring over a range of diseases or parasites capable of infecting local populations. When this happens, there are a couple of interesting outcomes that are possible.

The first is called the “dilution effect”, and though there isn’t much evidence for it outside of specific systems, this is where a host with parasites lives somewhere where these parasites can infect other hosts. Because these parasites are infecting new hosts that may not be ideal for the parasites, the parasite population will decrease. This LOWERS the overall risk of infection for the original host, “diluting” infection.

The second outcome is called the “spillback effect”, and this works in the opposite way of the dilution effect. Host with parasites come in, these parasites are able to successfully infect new hosts, increasing the parasite population, which in turn INCREASES the risk of infect for the original host.

So in the world of The Witcher, you can imagine a scenario in which vampires carry some disease that is transmitted through saliva to blood. If these vampires aren’t killing their victims, but biting and only feeding on them (and transmitting this disease) then this raises the risk of another vampire acquiring this infection when it bites that human (spillback effect).

The Treatment of Invaders

The stigmatisation of non-native species is a very real thing (again, see our recent interview with Helen Roy). We do have a tendency to treat non-natives as a problem if they mess with our version of how an ecosystem should look. But our baselines for how an ecosystem should look vary wildly. Studies on sediment cores from Scandinavian lakes are beginning to reveal that fish species that some freshwater fish species that we think of as non-native are actually the original inhabitants of those lakes. Perhaps they were replaced centuries ago by local human activity. Yet they’ve come back into these lakes recently, and as the present inhabitants grew up with different local fish species, they treat these returning fish as invaders.

How would baselines in the Witcher work? Apparently the Conjunction of the Roundnesses occurred 1500 years before the world we’re introduced to in the show. Would locals have adapted to the presence of more of these creatures, at least the non-humanoid ones, and accepted them as native species? It could be that the killing of the monsters has been limited to those we’ve seen that have been terrifying the locals, but the reaction of Sir Crapshispants to the oversized Aye Aye our cast meets in episode six leads me to believe this isn’t the case.

Perhaps the fact that many beings – like Geralt’s Witchers or the manifold magicians we meet throughout the show – can apparently live for extended periods of time means that the public’s baselines stretch back further than ours do. Or perhaps the fact that certain humanoid species like the elves came with them means that these species get lumped in under the “discriminate against” category this world’s humans seem to have drawn up.

Then again, maybe the locals just don’t like dragons and monsters which eat humans.


Ultimately, I don’t think that any of these species would pose much of a threat in the long run, particularly if the Conjunction of the Globes only happened once. I doubt that in one very sudden migration event, large enough populations immigrated to the show’s dimension. Genetic variation would be low, and I do think that over a period of time, the build up of deleterious alleles (basically bad genetics) would be enough to run most species in this world into the ground. Perhaps that’s what we’re seeing the tail-end of in this world.

If you’re keen for more information, the podcast I run with fellow EcoMass author Adam Hasik and our vet friend Dave has an episode on the physiology of the monsters of the Witcher, which I’ve linked below. And as always, if you’ve got any thoughts on the biology in shows like this, please leave a comment below.

Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.

*Or at least, they don’t from the science perspective. Jurassic Park may have benefited from their liberal use of genetics, but boy did that send a bunch of real-life scientists into a tizzy.

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