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Once in a math congress someone presented a paper on a mathematical model used to predict the impact of certain measures that could be taken in order to control some pest/plague. A guy in the audience interrupted the speaker and asked why not to "kill them all"(referring to the specimens of the pest/plague) and put a stop to the problem. The speaker observed that we were thinking that kind of things because almost all of us were mathematicians and so we were not aware that it would be a big problem to do that.

From my point of view, he was suggesting that "the right way to control a plague is not to kill them all" is a standard, uncontroversial fact that all biologists would agree. My question is about this last point.

Is that a scientific thesis or an ideological one? I mean, do biologists have theories that allow them to know that the "right" way to control a plague is not to kill every specimen of the species?

Why is it the case that we can assure that exterminating a species is necessarily going to affect, directly or not, humankind?

Is it a matter of chance and, as we are not completely certain of what may happen after exterminating the species, then we don't want to take a risk?

Thanks! And feel free to edit tags :)

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  • $\begingroup$ There are multiple things to consider. Eradicating a specific pest which we could have controlled might make it possible for another pest to take over the old ones ecological nice. Maybe that one than is not so easy to control or kill. But that is just one idea. $\endgroup$ – skymningen Jan 21 '15 at 7:31
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    $\begingroup$ This is outside of my field of molecular biology, but I have a few musings. Just because humans consider something a pest doesn't mean that it doesn't play a role in maintaining ecological equilibrium. The effect of pest control on an ecosystem can often be predicted, as the speaker was apparently doing, and/or modelled on previous experiences. When the effects are unknown, the prudent course is certainly restraint. $\endgroup$ – canadianer Jan 21 '15 at 7:36
  • $\begingroup$ What do you mean by "kill them all". All infected persons or all transmitters? $\endgroup$ – Chris Jan 21 '15 at 8:14
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    $\begingroup$ Just to clarify; you are referring to a pest species in a general sense right (e.g. an agricultural pest species), not plague as in the disease? $\endgroup$ – fileunderwater Jan 21 '15 at 8:43
  • $\begingroup$ You might consider the old (probably apocryphal) story about how persecuting witches led to epidemics of the black plague: Witches keep cats, therefore cats are evil, so kill all the cats (and witches) you can. But cats kill rats, so without the cats the rats have a population explosion. Rats carry fleas, and fleas are the vector for the plague... $\endgroup$ – jamesqf Jan 21 '15 at 18:11
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Biologists are certainly not afraid to exterminate pests and plagues in the wild. See: smallpox and polio. However, such exterminations are nearly impossible and very expensive. See: smallpox and polio. In both cases extinction is through extensive vaccination, preventing the disease from having any hosts to live in. To achieve this you need to vaccinate a huge fraction of the human population for quite some time.

Practically, how would you go about exterminating an agricultural pest? A beetle, for example? Perhaps an army of men, forming a cordon the width of the united states, turning over every rock and every tree from the west coast to the east coast. With a high net or something to prevent beetles from flying over the cordon. And some method for dealing with cities and roads and tunnels and things, because if you miss only five or six they'll be back soon, depending on life cycle and offspring numbers. Or if you forget Canada.

And to use a biological solution to a biological problem, consider myxomatosis and the Australian rabbit problem. Rabbits were(and are) a pest, myxomatosis is a rabbit plague that has a 99.99% fatality rate and is fiercely infectious. Sure enough, myxomatosis wiped out the rabbit population. A few years later the rabbits were back but now the survivors were resistant to myxomatosis.

I would argue that it's a scientific thesis based on hard lessons biologists have learned about doing 100% of anything. There are many biologists with moral precepts against the extinction of pest species. It's a bit of a moot point, however. The only reliable method of extinction is habitat destruction. Whether that's immunizing all the humans or cutting down the rainforests, it does work. It's just usually impractical.

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First, lets define a pest as a species that lives in places we don't want it to live.

To explain why exterminating a species is counterproductive without using moral arguments I need to explain what will happen when you exterminate a species. To explain that, I will need to explain how a species 'decides' where to exist.

Every species lives in their niche. A niche is the n-dimensional hypervolume where a species could live. (Wikipedia is a good starting point for this subject https://en.wikipedia.org/wiki/Ecological_niche#Hutchinsonian_niche )

Their niche is a subset of their adaptive zone. Species could, and would, live in their entire adaptive zone, if only those pesky other species would stop interfering. This means that the limits of almost all species are mostly determined by the presence or absence of other species.

We are getting closer to an answer. If you change one of the N dimensions that make the place the pest lives a good niche, then they will live there in lower numbers. Lower numbers means they don't bother us and that means they are no longer a pest according to the definition at the top. Problem solved!

Practically, this could mean something like keeping the humidity low to keep mold from taking over your bathroom ceiling.

On the other hand, if you use some kind of one-time intervention to destroy the species, the niche will still be there. And you can be absolutely sure that the niche is part of another species' adaptive zone. So after all the effort and poisoned pets (You have no chance of killing a species without serious side effects), all you will have done is exchange one pest for another.

This is a best case scenario, all other scenarios include extinction cascades or severe ecological destruction.

For further reading, here is a very hopeful paper exploring ways to reduce the impact of cascade effects. https://www.nature.com/articles/ncomms1163 It requires killing more species.

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