Jun 18 2024

Rats!

Published by under General
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What killed off the dodo? Humans first arrived at Mauritius island in the late 1500s. They found on this island fat flightless birds who nested on the ground and were a convenient way to restock their ship’s food supply. Within 80 years the dodo went extinct. But hunting was not the only, and maybe not even the primary, cause of their extinction. Rather it was likely something that the humans brought with them – invasive species. One species in particular tends to follow humans every where we go, causing havoc on any local ecosystem not already adapted to them – rats.

Dodos nested on the ground and typically had one large egg per nest. This was also a convenient food source for invasive rats, who quickly multiplied and decimated the dodo population. There were other invasive species as well (monkeys and pigs), and there was also habitat loss due to human activity – all of these factors put stress on the dodo population that was unsustainable.

It is an unfortunate unintended consequence of human history that as we explored and populated the world, rats followed us. Rats now exist on every continent except Antarctica, and they are likely the most populous mammal species in the world. They are a clever and adaptive species, and are very good at stowing away. They also reproduce very quickly, with a single female able to produce 50 pups per year. They also have a low mortality rate because they live in communities and protect their young.

But perhaps their most successful feature is that they are well adapted to living off human civilization. They go wherever people go and feed on our refuse. They hide away on ships which means they can get anywhere, including previously isolated island ecosystems. They are good climbers, jumpers, and swimmers and find lots of ways to get on board ships. It is estimated that 40-60% of island bird and reptile extinctions are due to rats.

To be clear, I am not saying rats are somehow inherently bad or to demonize them. They are just really good and what they do, and they have “a particular set of skills” that happened to be optimized for exploiting human civilization. We took a species from Northern China and made it into a globally dominant species (actually there are many species of rat, mostly from Asia originally). The actual culprit here is humans. We are responsible for the rat phenomenon.

There are things we can do about it, however. One thing we can do is eradicate rats from isolated islands so that native species can repopulate. This may require that they be reintroduced (when possible). Obviously we can’t reverse extinction (not yet) but we can reintroduce cousins or species that still exist in other locations or captivity. Once eradicated it is feasible to keep rats from being reintroduced.

Also, seabirds can make a significant come back on these islands once invasive species like rats are removed and native vegetation is restored. There is enough fish to sustain a large seabird population, if they can nest on the island (without rats eating their eggs).

Perhaps the largest island restoration project is Zealandia – which is a project to restore a park in New Zealand to its pre-human habitat. To accomplish this it had to be walled off so that no invasive species could enter. New Zealand is a great example of how the isolation of islands allows them to evolve unique, but vulnerable. ecosystems. New Zealand had almost not mammals and no mammalian predators. This meant the native species had no natural defenses against such predators. The main mammalian predators there now are rats, possum, and stoats, all introduced by humans. They have devastated the local wildlife resulting in an estimated 51 species extinctions.

Islands were most vulnerable to rats and other invasive species, but they also represent a unique opportunity, since it is feasible to eradicate rats there and keep them from being reintroduced. One measure that is important to this effort is good anti-rat standards on all shipping. This means leaving no edible waste around, placing rat barriers on any ropes tying a ship to a dock, setting traps, and being vigilant.

What about on continents and in cities? It is generally believed that once a rat population is established in such a location it is essentially impossible to eliminate it. New York City, for example, has a large and well established rat population. In fact, there is estimated to be over 3 million rats in NYC, with distinct genetic populations in uptown and downtown Manhattan. I visit NYC often, and every time I do you can see rats just walking and and down the sidewalk.

There is something else you see in the city – garbage. Catch and kill mitigation strategies simply will not work in NYC. Rats breed too quickly and there are too many places for them to hide. The number of rats in NYC, some experts argue, is essentially a function of how much food there is in the city for them to eat. The rat problem is a sanitation problem. As with shipping, there needs to be universal and tightly enforced regulations to minimize the availability of garbage for rats to eat. This is happening to some degree, requiring metal garbage containers, for example. But again, anyone who has visited NYC can see quite plainly that garbage remains a serious problem. I get that it is challenging to manage the waste of a city as large and dense and NYC, but it’s not an unsolvable problem. It just requires massive investment and a culture change.

Rats as an invasive species is a problem of our own making (like so many things). But it is also a problem that we have the ability to mitigate.

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