Species interactions have long been predicted to increase in intensity toward the tropics and low elevations because of gradients in climate, productivity, or biodiversity. Despite their importance for understanding global ecological and evolutionary processes, plant-animal interaction gradients are particularly difficult to test systematically across large geographic gradients, and evidence from smaller, disparate studies is inconclusive. By systematically measuring postdispersal seed predation using 6995 standardized seed depots along 18 mountains in the Pacific cordillera, we found that seed predation increases by 17% from the Arctic to the Equator and by 17% from 4000 meters above sea level to sea level. Clines in total predation, likely driven by invertebrates, were consistent across treeline ecotones and within continuous forest and were better explained by climate seasonality than by productivity, biodiversity, or latitude. These results suggest that species interactions play predictably greater ecological and evolutionary roles in tropical, lowland, and other less seasonal ecosystems.
Bibliographical noteFunding Information:
We thank C. Muir, V. Vásquez Reyes, S. Tremor, C. García-Jiménez, L. Bolin, J. Godlee, S. Angers-Blondin, J. Tolome, P. Sierra, H. Branch, J. Boyle, S. Lehtonen, C. Cosgrove, M. Little, L. Dyer, H. Barios, J. Assmann, and S. Chimbolema for help with fieldwork; A. Davis for help with structural equation modeling; and G. Langston for help with mapping. We thank G. Suarez of Fundación Colibrí, who passed away in 2017, for helping us establish sites in La Mesenia-Paramillo Reserve, Colombia. We acknowledge the Kluane First Nation, Gitdumden Clan of the Wet’suwet’en, Squamish Nation, Tsleil-Waututh Nation, Tübatulabal, Kumeyaay, Kiliwa, Voto, Emberá-Chamí, and other indigenous peoples on whose traditional territory this field work was carried out. Funding: This work was supported by a UBC Biodiversity Research Grant (A.L.H.), INECOL (proy. 20030-10796 to K.M.), an Universidad San Francisco de Quito Collaboration Grant (E.S.), NSF (grant DEB-1442103 to L. Dyer), and the San Diego Natural History Museum (to S. Tremor).