I don't believe that purchased development rights will stand the test of time. If the value of the land goes up, then the rights sold 30 years ago by mom and dad when they were bamboozled by a big-city lawyer don't cover the cost of not building now. The current owners may want more money or may build and deal with the lawsuits, buying the rights back as part of the settlement.
Yet an outright purchase by any organization is no guarantee of preservation either. A government entity or conservation group may sell the land to serve some greater purpose -- saving a better parcel or ensuring the survival of the group by fighting a nasty lawsuit. But undeveloped land will always be in danger of destruction as the population keeps climbing.
The main problem is that putting a price on a piece of land devalues its intrinsic worth. Now the dollar value of open space or species habitat has to be computed and thrown into competition against office buildings, roads, and single family trophy homes. The free market economy skews values because of the immediacy of gratification. Look at corporations more worried about this quarter's returns than about next year's product line or a ten-year strategic plan. CEOs and Boards of Directors satisfy the immediate needs of stockholders before worrying about the long-term health of the company. A healthy ecosystem is a long range need yet we'll judge the value of land based on current amenities such as open space or habitat for certain species. The free market cannot adequately shape our future because it is controlled by the daily impulses of buyers and sellers.
That doesn't mean I want a government controlled economy -- those never work (TDR is a government inspired market). I suspect we need something beyond economics. I own land and don't want to lose my money or my interest in it. But the animals and plants also "own" the land in their own manner. Development rights apply to human owners. What sort of rights should we award panthers or pines or promontories of granite? Hard questions and I don't have answers.
But pricing an ecosystem seems suicidal. Someone will buy it out from under us. Perhaps we need to develop some sort of principle similar to antislavery making it as illegal, immoral, and unjustifiable to purchase and exploit vital parts of nature as it is to buy humans, to realize that putting a price on land is akin to putting a price on someone's head: Wanted. 35 acres. Dead or Alive.
Or maybe we need an ethic similar to animal ownership where you can cage and even eat them but you can't be cruel or neglectful. The current zoning laws which judge value on "highest and best use" economically could then be considered cruel and inhumane. Pricing a piece of earth turns it from wilderness -- land worthy in and of itself -- into property worth only what humans value.
We cannot give up trying to preserve land by using the best tools available. But a tool can alter perceptions. If all you have is a hammer, everything looks like a nail. We've pounded mother earth enough. Maybe it's time to look beyond economics for some new tools.