What can we say about the effectiveness of payments for ecosystem services (PES) based on the available scientific literature? To find out, we examined 38 studies that represent the best evidence we could find.The vast majority of the evidence in those 38 studies was still very weak, however. In other words, most of the studies did not compare areas where PES had been implemented with non-PES control areas or some other kind of countervailing example.On average, the more rigorously designed studies showed very modest reductions in deforestation, generally of just a few percentage points. Meanwhile, the majority of the available evidence suggests that payments were often too low to cover the opportunity costs of agricultural development or other profitable activities that the land could have been used for.This is part of a special Mongabay series on “Conservation Effectiveness.” As far as conservation strategies go, payments for ecosystem services (PES) are based on a relatively simple concept — perhaps deceptively simple. The idea behind PES is, essentially, to pay landowners to protect their land in the interest of ensuring the provision of some “service” rendered by nature, such as clean water, habitat for wildlife, or carbon storage in forests.One of the most attractive aspects of PES programs is that they don’t just channel investments into environmental conservation. People also reap the rewards of those investments, literally and figuratively. That means that PES, in theory, can help alleviate poverty and reduce the conflicts that can arise between conservationists and local communities at the same time that it ties conservation funds directly to activities that benefit the planet.The devil, of course, is in the details. PES programs are typically voluntary, meaning landowners must choose to participate by enrolling their land. But if they choose to participate, that might be because there is no other use for their land for which they could be better compensated, meaning the land might not have been at risk to begin with. On the other hand, if the landowner has a genuine interest in keeping their patch of forest standing, how can you be sure that they wouldn’t have protected that forest even without getting paid?Paul Ferraro, a professor at Johns Hopkins University whose research focuses on the design and evaluation of environmental programs, told Mongabay that protected areas are not simple to evaluate either, but they’re more straightforward than PES. The areas are generally selected for conservation by a government, and it is possible to have some understanding of what affects that government’s choice. It is therefore easier to control for the factors that affect that choice when designing research into how effective a protected area is at conserving the land.“But in PES, there’s administrative selection and then there’s the landowner’s or the land user’s choice to participate or not,” Ferraro told Mongabay, “and it’s a lot harder to disentangle the characteristics of the people who participate in PES and the actual effect of the PES program.”For that matter, how do you even decide which regions to make PES programs available in? You’d obviously want to implement your conservation program in the most threatened landscapes — there’s no sense in spending limited conservation funds to protect a forest that’s not at risk of being cut down in the first place. But predicting where deforestation is most likely to occur is a trick in itself.“Let’s say in the Amazon you have had deforestation rates of 0.5 percent per year, something like that. But that actually means that if you had, hypothetically, say 1,000 forest plots, only five of those would disappear every year. So it becomes very important to protect the right ones, and to kind of guess what the right ones are,” Sven Wunder, a senior economist at the Center for International Forestry Research (CIFOR), told Mongabay.Once a site is selected for PES implementation, how do you guarantee that the deforestation the program deters won’t simply pop up in a nearby unprotected forest instead? How do you make sure any of a number of other factors wouldn’t have lowered the deforestation rate whether or not a PES program was implemented?