Single large or several small? Willingness to pay for conservation in the Río de la Plata grasslands

We use a contingent valuation survey to estimate Uruguayan households’ willingness to pay (WTP) for a payment-for-ecosystem-services (PES) program targeting 320,571 hectares of natural grassland in the Río de la Plata region, combining a national in-person probability sample (n=400) with a larger online non-probability sample (n=2,994). In both samples respondents answer a referendum-style payment question with an assigned price. Within the online sample we vary the spatial configuration of the protected area and the payment horizon (one, five, or ten years). In the former, respondents were randomly assigned to two large zones versus fourteen smaller, dispersed zones covering the same total hectares, bringing the ecological Single Large or Several Small (SLOSS) debate into stated-preference valuation for the first time. Across the full sample we test the sensitivity of WTP estimates to respondents’ self-reported certainty. The median household is willing to pay UYU 371.70 (USD 9) per month in the in-person probability sample. We find no statistically detectable difference in median WTP between the two spatial configurations, across payment horizons, or between the in-person and online samples. The in-person/online comparison confounds sampling design with survey mode, and the horizon tests rest on subsamples of 431–544, so those two nulls are not evidence of equivalence. WTP estimates remain stable under moderate certainty corrections but weaken under more aggressive recoding. These findings point to substantial public support for financing grassland conservation, but limited sensitivity of stated preferences to spatial and temporal design features that were not explained to respondents.