Why not more or less than 50%? Why an arbitrary checkerboard pattern instead of reasoned selection of preservation zones based on scientific study? While 50% may be more than needed, our current level of preservation is clearly less than we need if we want to preserve a functioning natural ecosystem.
50% is mostly about the simplicity, there is a certain fairness in the arbitrary nature of the division. The desire is to shift focus of the debate on how to achieve the goal rather than what the goal is. If we can shift out of 20 or 30% of the land, we can shift out of 50. At 50%, arguments about critical habitats become less important – some critical habitats that miss the preservation zones can be preserved by exchange, but with over 12,000 preservation areas covering half the planet, most habitat types will be automatically selected without the need for scientific study or argument.
If humankind cannot survive on 50% of the Earth, it certainly cannot survive on 99% of the Earth with the natural world shoved into a tiny piece of the available space, plus whatever desert, swamps, mountain peaks, ice fields and other wastelands that humans just don’t want.
As a Floridian, the idea of preserving 50% sounds almost absurd, looking at Florida’s desirable sand beaches, hardly 5% are preserved from the most intense development – but places like Brazil are already acknowledging the need to preserve 20 to 30% of privately held lands with their laws, even if enforcement of those laws is lacking. 50% is taking it “up a notch” and acknowledging that nature needs space just as much as people do.