We compared the relative ability of climate, productivity, and biodiversity to explain spatial variation in seed predation using data from gridded databases extracted for each site location.

Climate. We tested the following climate parameters used in Orrock et al.’s (23) analysis of abiotic correlates of oat predation in North American grasslands: mean annual temperature, temperature annual range (maximum temperature of warmest month − minimum temperature of coldest month), annual precipitation, and precipitation seasonality (the coefficient of variation). Long-term averages (1950–2000) were downloaded from the WorldClim website (worldclim.org; accessed 2017) for each site at a resolution of 1 km × 1 km.

Productivity. We tested two measures of productivity: annual AET (the water entering the atmosphere via plant respiration and evaporation from soils) as in Orrock et al. (23) and NPP (biomass per unit area). AET and NPP data are for 2000–2013 at a resolution of 1 km × 1 km, downloaded from NASA’s MODIS Land Science Team (modis-land.gsfc.nasa.gov) and the NASA/USGS EOSDIS database (lpdaac.usgs.gov), respectively. For the four sites where NPP was unavailable (one site per transect on each California transect and the Jalisco transect), we used NPP from the nearest adjacent pixel (0.5 to 1.8 km away).

Biodiversity. To assess diversity, we extracted species richness data for 10 km by 10 km grid cells from Biodiversitymapping.org, a compilation of vertebrate range maps from the International Union for Conservation of Nature and BirdLife International (43). For sunflower seeds, we used vertebrate species richness as a proxy for total seed predator diversity because vertebrate ranges are well mapped and richness data are readily available. Many vertebrates do not consume seeds and many seed predators are not vertebrates, but for our study area—the Pacific coast of the Americas—vertebrate richness varies similarly to country- or state-level richness of ants (44), the most commonly noted invertebrate seed predator in our data. For oats, which are predominantly eaten by small mammals (45), we used species richness of rodents and shrew (Rodentia and Eulipotyphla).

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