There aren't a million ways to protect open land from being developed: It can be mitigation for development; it can be exchanged for permits to build elsewhere; it can be donated for federal tax credits; or it can be bought. The source of funds can be federal, state, county, city, corporations, private foundations or a few individuals.
The only question is: Who pays the tab?
For 25 years I lived in a deep side canyon in Laguna Canyon. Instead of a grid pattern, houses followed the creek bed, or were built on small, perched, flatter areas on the slopes. This meant that houses often confronted each other in unpredictable ways. Old parcel maps showed a nonexistent little cul-de-sac right behind our property. Like most of the old city maps, lines were drawn without allowing for the steep terrain.
Access for the cul-de-sac was drawn boldly across the creek and up a steep slope to a possible building pad just above us, with a commanding view of our house and patio. The city would not allow that route, so would-be builders tried to gain access from the top off a narrow private lane — actually a shared driveway — just above our garden. Their only hope was to tempt someone with an easement to sell them access. Every time this happened, we held our breaths until everyone finally turned down the offer.