Here is a utilitarian argument for all your eco-communist friends:
if personal self-interest is more reliable than altruism at motivating personal labor, and
if personal self-interest will generate a higher quality of labor than coercion, and
if personal self-interest cannot be relied upon to motivate people to provide the labor needed to maintain communal areas, and
if the Earth would benefit by being carefully and lovingly maintained,
then, it’s in the best interest of the Earth itself that all property be private.
I think all of the premises can be fairly easily proven.
I am fairly new to ancap and have not had an opportunity to read much more than Rothbard’s In Defense of Liberty, and I find that the justice of a societal system derived from the notion of the sovereignty of the individual is hard to denounce.
However, I did have a problem with the notion that all of the land would be privately owned. Initially I felt that it was somewhat unjust for future generations that some would be born as heirs to great estates and others would be born as renters. Plus, the notion that this world belongs to all of us equally, in the same way as air and water, seems to be ingrained in me as a default position.
But, I cannot help but think that the argument above, while utilitarian for the current inhabitants, also protects the rights of future inhabitants to be born on a healthy planet. Do we not owe future generations an obligation to take care of the planet the best way we can? And if private ownership of all land is the best way to ensure that Earth will receive the best care, and I am convinced that it is, then does private ownership then not become the only morally correct answer? I suppose an argument could be made that the individual has no obligation to any future generation, but personally, I feel a great obligation to my offspring, so even if such an argument proved to be more valid, I would still reject it personally.