Private lands contain much of the world's biodiversity. Conservation of private land, especially agricultural land, is urgent yet challenging because of the diverse priorities of landowners. Local effects of farmland conservation programmes have been evaluated thoroughly, but population-level response to these programmes may depend on effects that extend beyond targeted land parcels. We investigated the landscape-scale effects of a grassland conservation initiative, the Conservation Reserve Enhancement Program (CREP), on a socially and economically important gamebird, the Northern Bobwhite (Colinus virginianus). Barriers to assessing population-level response to conservation include determining the spatial scale at which a species responds to environmental change (the scale of effect) and untangling density-dependent processes. We performed point counts over 6 years at 247 sites with similar local CREP density but varying landscape-scale CREP density. We used an open-population distance sampling model to evaluate population response to landscape-level CREP density and to forecast population densities under differing re-enrollment scenarios. Our model included kernel smoothing techniques to estimate scale of effect and an estimator of the strength of density dependence. Density dependence moderated the effectiveness of the CREP, but overall populations responded positively to increasing landscape-scale CREP density. We estimated that at least 5% of the landscape needs to be in CREP to meet population density goals of 0.25 bobwhite/ha. Conservatively, we recommend 10% of the landscape to be in CREP. Our percent cover recommendations are based on a distance-weighted average of CREP around focal sites. Landscape-scale effects diminished with distance. For example, assuming all else is equal, a CREP field 3000 m away had 88% less of an effect on local abundance than a field 1000 m away. Fields farther than 5000 m away had no effect on local abundance. Synthesis and applications. Our study underscores the importance of a landscape-scale approach to farmland conservation. Benefits of these programmes can extend beyond the local scale, but their importance to local populations diminishes with distance. Estimating the relationship between increasing distance and the utility of conservation land to local populations could provide land managers a practical framework in which to target land enrollment to maximize biodiversity outcomes.

JAPPL-2017-01144_DataIncluded is the data that went in to each of our models described in the manuscript. Included in data are: counts of bobwhite, aggregated densities of conservation fields without specific spatial information (i.e., no x y coordinates), and distance sampling metrics needed to run the model.