This distributed experiment was designed to maximize the potential to detect large geographic gradients in interaction intensity, if they existed, by standardizing as many components as possible and replicating in time and space. Each collaborator established a transect of four to five sites spanning at least 1000 m of elevation, or as much elevation as possible given the terrain (table S1). Site locations were occasionally adjusted between experimental runs (1 run = a 24-hour assay of seed predation along one transect); in these cases, a transect consisted of four sites during each run with five to six sites in total (table S1 and fig. S2). All sites were on the continental Americas within 300 km of the Pacific coast (Fig. 2A). This latitudinal gradient includes many protected areas along an unbroken mountain chain, minimizing differences in seed predator communities due to large-scale dispersal barriers (e.g., oceans). Sites were in natural areas, although most had experienced light human disturbance (e.g., logging >100 years ago, nearby park roads or trails). Experiments were conducted when some plant species were dispersing seed at each site along the transect; hence, predation rates on experimental seeds incorporate the background effect of differing seed productivity among sites. Experiments were conducted during a snow-free, rainless period or (at sites with daily rain) in typical weather for the transect. We could not fully standardize site phenology as tropical and temperate sites differ in seasonality, and phenology varies among elevations on any given date. To better capture “average” seed predation intensity, we ran the experiment multiple times at 15 of 18 transects (median replicates per transect = 4) from 2015 to 2017. Replicates were separated by at least 2 weeks and usually several months.

We used agricultural seed species to ensure that seeds were not local to any site, breaking potential coevolved or learned associations between seeds and granivores. Seeds bred for human consumption should have minimal chemical defenses, and our seeds had no (sunflowers without shells) or minimal (oats with thin husks) physical defenses, ensuring that seeds were edible by as wide a range of potential granivores as possible. Seeds weighed 61 mg (sunflower) and 46 mg (oat) on average, within the range of natural seed sizes from 7° to 60° absolute latitude (41) and the size range consumed by the smallest to largest vertebrate granivores globally [(42); size per se should not be an issue for invertebrates as they consume seeds in situ]. We bulk-purchased organic seeds from the same suppliers throughout the experiment: sunflower seeds from Community Natural Foods and oats from West Coast Seeds. To ensure that seeds would not germinate if dispersed by granivores, seeds were heat-sterilized at 110°C for 1 hour [modified from (23)]—a low enough temperature to prevent noticeable changes in color or smell. Sterilized seeds were mailed to collaborators and stored in a cool place in odor-proof containers until use.

Collaborators used a consistent protocol to quantify seed predation intensity. For each experimental run, we set out 10 depots of five oat seeds and 20 depots of eight sunflower seeds, alternating between species. We only used intact seeds, so that damage was unambiguously attributable to granivores. Depots were placed ≥5 m from walking trails, and ≥5 m apart in 2015 and ≥10 m apart in 2016 and 2017; otherwise, protocols did not change among years. Seeds were placed on bare ground in a shallow depression (depth, 0.5 to 3 cm; diameter, 5 to 10 cm), either natural or made by the experimenter. Care was taken not to disturb vegetation outside the depot. Depots were marked with a popsicle stick at the depot edge and green flagging tape 1 to 2 m from the depot (Fig. 1).

Beginning in 2016, during 25 of the 56 runs (60 of 79 sites along 14 of 18 transects), we excluded vertebrates from three to four sunflower depots per site, spaced evenly across the site (we used sunflower seeds only because oat seeds rarely showed signs of invertebrate predation). Conical exclusion cages (height, ~12 cm; diameter, 15 cm; 2.54 cm by 2.54 cm wire mesh) were secured over depots using metal pins (Fig. 1C). If a cage was found compromised (e.g., pins pulled out or cage dug under, moved, or trampled), then data from that depot were excluded.

We quantified predation 24 hours after depots were set out. Sites along a transect were generally set up in 1 day and checked 24 hours later in the same order, but three less accessible transects (in Mexico, Colombia, and Ecuador) were split into two groups of sites and the experiment was run over >2 days. After 24 hours, we photographed each depot and any seed remnants. We counted intact and partially eaten seeds and then removed all materials.

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