Saturday, September 08, 2012

Macro and Micro Predators, Territorial Behavior and the Tragedy of the Commons

There are no large organisms that support themselves primarily by preying on humans; so far as I know, there have been none for several thousand years. There are lots of microscopic organisms that do so. Why the difference?

One possible answer is that macro predators face a tragedy of the commons—the deer I don't eat today will not be around, and fatter, next season because someone else will have eaten it. Micro-predators, on the other hand, have an "incentive" to preserve their food supply, both because the bacteria or viruses on me are all close kin to each other and so face evolutionary pressure to act in their common interest and because I am much larger and much longer lived than they are, so that many generations of them are dependent on a single me. From which it follows that a lethal disease is a mistake. From an evolutionary standpoint, diseases want to live off me while doing as little damage as possible.

When I made this point to my wife, she pointed out that some macro-predators solve the problem the same way humans do—via property rights. Their version is territorial behavior. If a single tiger succeeds in monopolizing his chunk of jungle, it is in his interest to let the fawn grow up today to be a better meal next year. 

Which leads to an interesting conjecture. Territorial behavior solves the tragedy of the commons only if the prey species is not too mobile—if the fawn spared today is likely, as an adult deer, to still be within the range of the tiger that spared it. It would be interesting to know whether there is an inverse relation between the probability that a species is territorial and the mobility of its prey.

All of which gets me some distance from my original point, which was about humans, not deer. One special reasons macro-predators have reason not to choose humans as prey is that humans fight back. Until recently, we have had no similar ability with regard to micro-predators.

With regard to fighting back, the tragedy of the commons point is still relevant. A prudent macro-predator might hunt humans in a fashion sufficiently selective not to provoke any major retaliation, tiger hunts or the equivalent. But doing so means giving up today's meal for a benefit shared with the rest of his species, making the incentive for prudence a weak one.

21 Comments:

At 12:54 AM, September 09, 2012, Blogger Ilíon said...

Don't you think it the least bit odd that you cannot even begin to think these thoughts without seeming to ascribe rational deliberation to tigers, and bacteria … and to “evolution”?

 
At 1:01 AM, September 09, 2012, Blogger Patri Friedman said...

It would be interesting to know whether there is an inverse relation between the probability that a species is territorial and the mobility of its prey.

Thus, competitive government :). (Crypto-anarchy, etc...)

 
At 1:28 AM, September 09, 2012, Blogger Ilíon said...

Lion are territorial, and their prey are generally very mobile.

 
At 7:49 AM, September 09, 2012, Blogger dWj said...

Even if we can fight back equally well against small predators and large predators, I think it's easier for small predators to sustain themselves in a way that doesn't trigger our inclination to fight back than for large predators. If I and some of my neighbors get a cold, we're not going to put as much energy into exterminating it as if one of my neighbors gets eaten by a tiger.

 
At 10:13 AM, September 09, 2012, Blogger David Friedman said...

To Ilion:

I don't think it odd--it's the obvious metaphor for the implications of Darwinian evolution.

 
At 10:37 AM, September 09, 2012, Blogger Nancy Lebovitz said...

Biology is complicated-- so far as I know, rabies hasn't evolved to become less dangerous for humans.

 
At 11:22 AM, September 09, 2012, Blogger Ilíon said...

Ilíon: "Don't you think it the least bit odd that you cannot even begin to think these thoughts without seeming to ascribe rational deliberation to tigers, and bacteria … and to “evolution”?"

D.Friedman: "I don't think it odd--it's the obvious metaphor for the implications of Darwinian evolution."

Interesting. So, Darwinian evolution is obviously *like* an intelligent, deliberate, rational agent?

 
At 11:29 AM, September 09, 2012, Blogger Ilíon said...

"Even if we can fight back equally well against small predators and large predators, I think it's easier for small predators to sustain themselves in a way that doesn't trigger our inclination to fight back than for large predators."

Mice and rats can be seen as "small predators", and we certainly fight against them at least as much as we do tions, tigers and bears. Yet, even today, we only just keep them in check.

I don't think mice and rats are still preying off us because we aren't motivated to kill them.

Mice and rats can hide from us far easier than lions can. And they breed far faster.

 
At 11:58 AM, September 09, 2012, Anonymous Tim said...

But mice and rats don't normally prey off us. They prey off our food supplies, and our refuse, but not to many people are killed and eaten by mice or rats.

 
At 12:53 PM, September 09, 2012, Blogger Ilíon said...

"But mice and rats don't normally prey off us. They prey off our food supplies, and our refuse, but not to many people are killed and eaten by mice or rats."

That's why I said that "Mice and rats can be seen as "small predators"" ... they prey on us indirectly.

But, nor are many people killed and eaten by lions and tigers and bears. And, in fact, mice and rats do far more total damage to us than lions and tigers and bears could ever imagine doing.

 
At 2:33 PM, September 09, 2012, Blogger Joseph Miller said...

David,

I like this a lot. You can summarize by saying that the difference between a parasite and a predator is that a parasite has much more well-defined property rights. You will rarely find one person infected by more than 1 strain of bacteria because of competitive exclusion and the fact that the initially colonizing strain has a probabilistic advantage because of its numbers. The colony's immediate interests are best served by by keeping the host alive.

I think this explains why the short-term interests of the parasite and the host are generally aligned. However, the parasite still has to spread to other hosts, or else it will die when its host dies.

This is where some parasites may not fit your prediction. There are cases in which lethal strains may be encouraged to evolve. When an infection kills or debilitates its host, the host is usually brought to a hospital where there are many more people around to infect. It may be in the reproductive interest of the parasite to kill or weaken its host in order for the colony to spread.

This may be part of the answer to Nancy's point about rabies not becoming less lethal.

Ilion,

Natural selection, is pretty much exactly like a rational agent, except one who can only make revisions that improve its predecessor. That is, it can't in general, plan more than one generation ahead. This is how odd things like runaway sexual selection (Fisherian runaway) can occur. What's your problem with this analogy?

 
At 3:35 PM, September 09, 2012, Blogger Ilíon said...

"What's your problem with this analogy?"

I have enought experience with uncritical Darwinists to know that if you cannot/will not already see my point with respect to what Mr Friedman said ... as with what you said ... then no amount of further explanation on my part will get the point across.
I explained already, in clear English, the point. I do not play "Prove It Again!"

 
At 10:13 PM, September 09, 2012, Anonymous Anonymous said...

"Why has rabies not evolved to be non-lethal?" is like "Why is $species not perfect?"

 
At 12:33 AM, September 10, 2012, Blogger DR said...

I've had this discussion many times before, particularly after reading Plagues and People.

Another limiting factor is that macro-predators evolve at an order of magnitude slower rate than micro-parasites.

When humans fight back against macro-predators the result is virtual extinction. When they fight back against micro-predators, the result is quick and hardy resistance to humans' weapons.

Hence the reason we have MRSA, but no bullet-proof tigers.

 
At 2:13 PM, September 10, 2012, Anonymous jim rogers agriculture said...

I like the last point. Tigers are not stupid, they know humans carry guns!

 
At 5:38 PM, September 10, 2012, Blogger Joseph Miller said...

DR:

Most of the problem with eliminating parasites is that they are harder to find and that it's fundamentally harder to make weapons that kill them. Researchers have to design these drugs blind, and the drugs also have to be non-toxic to the host's cells. Evolution makes this a worse problem because the majority of the targets of antivirals/antibiotics are superficial enough to quickly evolve.

If we could get a drug that was as selective and lethal as a gun, it could exterminate a parasite. But that would be like inventing a smart bomb that only detonates on impact with Al Queda members.

 
At 4:55 AM, September 11, 2012, Blogger Ilíon said...

"Tigers are not stupid, they know humans carry guns!"

They also seem to be cowards, or at least extreme opportunists, since when they do attack humans, they like to do it when we don't see them (such that wearing a mask on the back of your head supposedly reduces attacks).

 
At 3:55 PM, September 12, 2012, Blogger Joe said...

Illion:

You wrote:

Interesting. So, Darwinian evolution is obviously *like* an intelligent, deliberate, rational agent?


When someone replied with roughly "yes, the results of evolution are similer to a rational agent" you replied:

no amount of further explanation on my part will get the point across.
I explained already, in clear English, the point. I do not play "Prove It Again!"


I can't find any explanation from you about whether evolution acts like a rational agent or not, just discussion about rats.

If you really think I have no hope of explaining such high ideas, perhaps another reader could try pointing out what I missed.

 
At 7:40 AM, September 13, 2012, Anonymous Vic said...

http://lucite.org/lucite/archive/atheism_-_dawkins_articles/why%20don’t%20animals%20have%20wheels.pdf

Richard Dawkins in his article "Why Don't Animals Have Wheels" described animals' territorial behavior and the problem of developing roads in nature.

 
At 6:57 AM, September 14, 2012, Anonymous Anonymous said...

man hunts man.

 
At 6:39 PM, September 16, 2012, Anonymous RKN said...

One special reasons macro-predators have reason not to choose humans as prey is that humans fight back. Until recently, we have had no similar ability with regard to micro-predators.

Not sure what you mean by "recently." Organisms, including humans, have been fighting back against bacteria and viruses since the evolution of innate immunity.

 

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