Dragons are the best (or worst) Invasive Species

I’ve recently I fell down a hole and became obsessed with The Memoirs of Lady Trent by Marie Brennan. If you haven’t read the series, I highly recommend it. Especially if you’re a woman in STEM, since the main character, Isabella, is a woman making her way as a scientist in a fictional version of the 1800’s(ish). Also the science is pretty spot on.

So then why am I here? Pretty much all the science in the books are fairly well explained. There aren’t any glaring errors. Well, true to time period, while Isabella manages very well looking at the specific natural history of the organism of interest (Dragons) she doesn’t really look at how they fit into their environments, she doesn’t ask ecological questions. And given that I’m an ecology nerd, I’m here to add some additional depth to some very specific events in these books.

There aren’t any spoilers from the first book in the series in here, but heavy spoilers for the second book ahead, so be warned. And yes, the actual science is important to the plots of these books. I know squeaked on the inside when I realized that too. Please go read them.

Okay, now that I’ve gotten that out of my system: Ecology.

In the second book of the series, The Tropic of Serpents, Isabella is in a fictional version of Africa researching Swamp Wyrms in well…a swamp. Shocking, I know. Through various plot events, it is revealed to her that the piranhas living in the waters of the swamp are actually an early developmental stage of the adult dragons. Additionally, to create female dragons the people living in the swamp move the eggs into another area, changing the conditions enough to create females. If the eggs are not moved they become males. The males are significantly smaller than the females and are morphologically distinct (they look different). Long story short, in order to prevent a dam from ruining the way of life of the people in the swamp, it is agreed that the larval-stage Dragons will be transplanted to the waters of the nation state that controls the territory, Bayembe. They will then develop into dragons, which will not be able to mate since they are only male or only female because they all hatched in the same conditions, and will guard the city from an impending invasion from the south. In return, the rivers leading up to the large jungle swamp/rain forest will not be dammed.

How would these introduced dragons impact the aquatic ecosystem outside of Bayembe?

Well let’s look at the real root of the problem here: these are big animals that have to eat with no natural predators in their ecosystem (or any ecosystem as it seems likely that Swamp Wyrms are apex predators). How big of a problem is this? Well, it’s hard to say. It isn’t really noted in the series how often Swamp Wyrms have to eat, but other species of dragon have a single large meal every few days. This is much closer to a snake or another ectotherm than to mammals and birds, which have to eat continuously to be able to maintain their internal body temperature. In that case, these specific dragons might be closest to a crocodile or an alligator.

American alligators have been found to consume: giant water bugs, apple snails, crayfish, round-tailed muskrats, marsh rabbits, red-bellied turtles, peninsular cooters, stinkpots, gizzard shad, and Florida gar. That’s a large variety. But the little bugs and snails and crayfish were mostly food for adolescents, while the larger prey was eaten by adults. Additionally, females ate more mammals while males ate more reptiles, in particular turtles.[1] Isabella thinks that the invasive Wyrms may be developing into females, so in that case we are talking about a mammal population that is most at risk. Additionally, crocodiles in Australia have been found to eat crabs, shrimps, and fish in addition to mammals and birds; the type of food eaten appears to be largely impacted by the type of habitat and by the salinity of the environment. This change in diet based on environmental facts also lines up with Swamp Wyrms physiological changes in changing conditions.[2] A juvenile saltwater crocodile needs 4% of its body weight in food a week to continue its rate of growth.[3] So, let’s assume that the growth rate is the same for our dragons. Let’s also assume that their minimum weight is the same as a male crocodile, although that’s probably way too low. So a male saltwater crocodile weighs in at a maximum of about 1000kg (about 2200lbs). So that individual would need to eat 40kg (that’s 88lbs) of food every day. That’s the size of a not-very-small child.

That’s a lot of food. Swamp Wyrms are native to rain forests, an area with some of the highest biodiversity on the planet and plenty of food to support them. The area that they have been moved to is, in comparison, a literal desert. Sure, this is a watery ecosystem, but they have also been moved from a delta to a much more salty bay that is surrounded by desert and grass lands. Now, the water next to the desert my actually be fairly nutrient rich thanks to all the minerals and salts from the sand that would blow into the water. Microbes like to eat this stuff, and then bigger stuff eats the microbes, and so up the chain. However, this would not compare to the rain forest that the Swamp Wyrms are native to.  Although the soils in rain forests are pretty nutrient poor because of constant erosion, there are so many other large animals to pull into the water and consume that they would have a much easier time getting to that 40kg minimum than an open ocean ecosystem. Additionally, this rain forest happens to be in the middle of a delta, which is a double win for biodiversity since deltas are areas where salt and freshwater mix. This leads to both salt and freshwater organisms being able to live in different parts of the delta along the salt gradient, rather than just the salt-loving species of the open ocean that would live next to Bayembe.

There is also the Swamp Wyrms hunting style to take into account. They release a noxious gas that disorients their prey, then leap out of the water and eat the prey. That means that while they may supplement their diet with aquatic organisms, their primary food source is land mammals and birds.  These are going to be in much shorter supply than in the rain forest, where water ways passed under low hanging trees and vines that a mammal could be snatched out of. Plus, mammal are unlikely to go near the salty water consistently since they would be seeking freshwater resources. Birds may hunt for fish in salt water, which would provide the Swamp Wyrms with some food, but the birds would quickly learn to avoid the area and definitely would not get the dragons to their 40kg minimum of food.

Honestly, the animal that is most at risk is the very animals that the dragons were put there to protect, humans. Humans use the ocean for economic and recreational purposes, and fishing is doubtlessly an industry. It would take far less effort to eat a child that has been stunned by noxious gas and is not a particularly strong swimmer than it is to, say, catch a bird mid-air. Sure, they might also go after seals and sea lions, but both of these animals are far harder to catch in the water than a human would be. And considering in this fictional nation, the poorer people live by the water while the nobles live in a compound slightly inland, it seems likely that the water would be used not only for recreation and fishing, but also for washing clothing and bodies as well as defecating. Sure, it is salt water and not ideal for those sort of things, but these people are very poor and live in an arid climate where fresh water would be reserved for drinking. This puts these populations even more at risk, since they would be constantly by the water and if anything were to happen it is unlikely that the nobility would be held accountable. In the end, introducing Swamp Wyrms to Bayembe may help defend the nobility against invaders, but far more likely to kill large numbers of common people compared to the numbers that would be killed in an invasion.

So there are few things that could happen in this scenario:

1.The Swamp Wyrms die could out before they are large enough to take down a person/before they get out of their larval fish phase. This leads to the collapse of the population. Things return to normal unless more Swamp Wyrms are introduced.

2.The Swamp Wyrms could grow to a size that they can take down a person. To get to this size they probably have to eat through a decent amount of the native species. Then, if more Swamp Wyrms are continually introduced, many people would die to support the adult population and the bird and fish population would be continually preyed upon by the juveniles. Over time this would lead to a decline in diversity and may cause ecological collapse.If there is ecological collapse of the marine community then the local economy based on fishing collapses AND the Swamp Wyrms die from lack of food since they will not be able to grow large enough to prey on humans. Or, additional food sources could be introduced to the bay where the Swamp Wyrms live. This could prevent the adult dragons from preying upon humans entirely, but it also could upset other parts of the ecosystem.

Either it seems like humans have to actively manage the problem, just as they have to continuously bring in new dragons, or they have to accept that humans are likely to be the primary food source for this animal. In the long term it will likely hurt the economy of Bayembe simply because of the inevitable decline in the number of fish, in addition to the potential of staggering human loss. However, it makes sense that Isabella didn’t consider these factors when she used Swamp Wyrms as a bargaining chip. Ecology is a fairly new study, one that does not appear to have been developed sooner in her fictional world. Additionally, just as in the real world, global trade appears to have kicked off in the past century with industrialization. Many invasive species are a result of greater global interconnections. Every invasive species has a native habitat. Isabella lives in a world where this is not yet considered to be a major problem, although if her world’s history follows our own it soon will be and the consequences will likely be very severe.

TLDR: Don’t introduce giant lizards into new waters unless you’ve really thought this through.
[1] Delany, Michael F., and C. L. Abercrombie. “American alligator food habits in northcentral Florida.” The Journal of Wildlife Management (1986): 348-353.

[2] Taylor, Janet Ann. “The foods and feeding habits of subadult Crocodylus porosus Schneider in northern Australia.” Wildlife Research 6.3 (1979): 347-359.

[3] Webb, Grahame JW, Gregory J. Hollis, and S. Charlie Manolis. “Feeding, growth, and food conversion rates of wild juvenile saltwater crocodiles (Crocodylus porosus).” Journal of Herpetology (1991): 462-473.


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