"We all have a need to decorate Mother Nature because it belongs to all us," Chilean-born Danish artist Marco Evaristti said Thursday. "This is my iceberg; it belongs to me."

All too often, I've seen the results of people who have that kind of attitude. I've seen the graffiti and trash they leave behind. I've seen where they've taken things from parks, leaving damage in their wake. It's all part of the "tradegy of the commons."

The National Parks present another instance of the working out of the tragedy of the commons. At present, they are open to all, without limit. The parks themselves are limited in extent -- there is only one Yosemite Valley -- whereas population seems to grow without limit. The values that visitors seek in the parks are steadily eroded. Plainly, we must soon cease to treat the parks as commons or they will be of no value to anyone.

What shall we do? We have several options. We might sell them off as private property. We might keep them as public property, but allocate the right to enter them. The allocation might be on the basis of wealth, by the use of an auction system. It might be on the basis of merit, as defined by some agreed­upon standards. It might be by lottery. Or it might be on a first-come, first-served basis, administered to long queues. These, I think, are all objectionable. But we must choose -- or acquiesce in the destruction of the commons that we call our National Parks.

In this particular case, he probably didn't do any harm. Still, I can't agree with his attitude.