World population needs to decline gradually, though. Developed countries with low fertility should compensate through immigration.

"World population needs to decline gradually, though."

Why does world population "need to decline"?

We're eating resources faster than we're replacing them.

I suppose the alternative is to eat resources at the replacement rate - this is what sustainability is supposed to be about.

Or perhaps some new technology will fix it.

Edit: this was in response to your earlier message, which you've apparently edited.

No, we aren't (running out of resources).

The price of oil today is moderately higher (in constant dollars) than it was in 1946, but only moderately so.

http://inflationdata.com/articles/charts/inflation-adjusted-...

Pretty much every jump in the intervening years is directly attributable to political turmoil (e.g., war) rather than any real decline in the amount of oil available. Note that today's moderately higher prices are in an environment with active war going on in Syria and Iraq, historically two of the largest producers. If it weren't for that, oil prices might well have dropped below 1946 prices (again, in constant dollars).

Similar graphs hold for other commodities.

According to the pundits in the 1970s, we were supposed to be at the "Soylent Green" stage long before now. We aren't.

Old Man Malthus claimed that we were on the verge of the Soylent Green world (though of course he didn't call it that) way back in 1798.

Oh, I was thinking more like trees, biodiversity, and water honestly. Sharks, plankton, etc. Even if oil became scarce, energy doesn't look like an issue in my lifetime, there's always coal and nuclear.

But like rainforests are going away real quick.

1) There are more trees in the United States today than there were 100 years ago. No reason that can't happen elsewhere.

2) Water doesn't actually get used up.

1) Can't is different from unlikely to.

2) Aquifers do.

1) You have absolutely no basis for that statement 2) Fossil aquifers aren't the only sources of water. The aren't even the primary source of water, other than in specific geographically-limited regions.

In Kenya the fertility rate is 4.46, and that's after a period of decline and not especially high by regional standards. I don't think you want to drive that up...

(that's part of a wider problem with trying to draw any inferences about how a basic income might work in a developed country from this. It's perfectly reasonable to believe the results and the appropriate design might be precisely opposite)