## Chesterton’s fence, ecological sensitivity, and the disruption of ecological services

Chesterton’s fence is the principle that reforms should not be made until the reasoning behind the existing state of affairs is understood. …

In the matter of reforming things, as distinct from deforming them, there is one plain and simple principle; a principle which will probably be called a paradox. There exists in such a case a certain institution or law; let us say, for the sake of simplicity, a fence or gate erected across a road. The more modern type of reformer goes gaily up to it and says, “I don’t see the use of this; let us clear it away.” To which the more intelligent type of reformer will do well to answer: “If you don’t see the use of it, I certainly won’t let you clear it away. Go away and think. Then, when you can come back and tell me that you do see the use of it, I may allow you to destroy it.”

[from G. K. Chesterton‘s 1929 book The Thing, in the chapter entitled “The Drift from Domesticity”].

That’s from Wikipedia. I think Mr Levine’s use of it is the flip of what it actually means.

To summarize: Put very simply: don’t destroy what you don’t understand.”

This sounds good, almost like the Precautionary Principle, but there are two points worth quibbling about:

• What is the standard to achieve “understanding” sufficient to enable destruction of the fence? How does that get judged? Clearly, the answer is that it needs to be quantitatively done. That’s what math is for. I don’t think asking a bunch of people their opinion is a good way, which is why I dislike justifying climate change science using an opinion poll among scientists. Climate disruption is real because it’s very basic physics. Period.
• The parable presumes that an overt action is the only way something can be destroyed. If water is slowly but constantly added to a tub, even if the tub has a huge capacity, it will someday overtop and flood, and the repercussions of that flood are not directly attributable to an event or action or decision to achieve those repercussions.

There’s a section I like from a textbook by M. W. Hirsch and S. Smale (Differential Equations, Dynamical Systems, and Linear Algebra, Academic Press, 1974), in their discussion of dynamical systems relating to competing species (Chapter 12, Section 3):

Note that both populations are positive at $p$. Suppose that some unusual event occurs, not accounted for by our model, and the state of the ecology changes suddenly from $v_{0}$ to $v_{1}$. Such an event might be introduction of a new pesticide, importation of additional members of one of the species, a forest fire, or the like. Mathematically the event is a jump from the basin of $p$ to that of $(0, b)$.

Such a change, even though quite small, is an ecological catastrophe. For the trajectory of $v_{1}$ has quite a different fate: it goes to $(0, b)$ and the $x$ species is wiped out!

Of course in practical ecology one rarely has Fig. H to work with. Without it, the change from $v_{0}$ to $v_{1}$ does not seem very different from the insignificant change from $v_{0}$ to a state near $v_{2}$, which also goes to $p$. The moral is clear: in the absence of comprehensive knowledge, a deliberate change in the ecology, even an apparently minor one, is a very risky proposition.